The team’s approach tackles the linearized gravity-wave equations, which describe small-amplitude waves superimposed on a background atmosphere. Unlike fully nonlinear time-step models, which must march forward through tiny increments of time and can model dramatic phenomena such as wave breaking and secondary wave generation, linear methods solve for the wave field directly. The trade-off is well documented in the literature: in earlier work by other researchers, a time-step model required several hours of computation while a linear method completed the same task in seconds. That speed makes linear methods far better suited for analyzing measured data, where many model runs may be needed to interpret observations.

At the heart of the new model is a multilayer method, a technique first applied to acoustic-gravity waves in 1962. The idea is elegant: the atmosphere is sliced into a stack of thin horizontal layers, and within each layer the complicated equations—with coefficients that vary continuously with altitude—are approximated by a simpler system with constant coefficients. The analytic wave solutions in neighboring layers are then stitched together by requiring continuity of the variables at each interface. The new model uses the numerical variant of this approach, in which the equation coefficients are evaluated at the middle of each layer. Crucially, this variant retains the height derivatives of background atmospheric parameters such as temperature and wind, which the older physical multilayer method discards by assuming those parameters are constant within each layer.

That distinction matters more than it might seem. In 1973, the influential physicist C. O. Hines criticized physical multilayer methods, arguing that it becomes impossible to find physically meaningful state variables when viscosity and wind are both nonzero, or when thermal conductivity and a temperature gradient coexist. As later researchers pointed out, this criticism does not apply to the numerical multilayer method, because in a purely mathematical context it is sufficient to prove that the method converges to the correct solution as the layers become infinitesimally thin. The new model sidesteps the old debate entirely, solving the equations in a way that explicitly accounts for viscosity, thermal conduction, and ion drag—the frictional force that charged ions exert on the neutral air.

The mathematical machinery draws an unexpected inspiration from a seemingly unrelated field: radiative transfer, the study of how light propagates through scattering atmospheres. Solving the linearized gravity-wave equations turns out to be mathematically analogous to solving the equations of radiative transfer, and the authors adapted techniques developed for the latter, including the discrete ordinate method with matrix exponential and the matrix operator method. The solution strategy rests on a matrix-exponential formalism and offers two classes of solvers: global matrix methods and scattering matrix methods. In the global matrix approach, all the layer equations and boundary conditions are assembled into one large banded system of linear equations, which is solved efficiently using standard LAPACK routines for complex band matrices. The scattering matrix approach instead builds up the solution layer by layer, using reflection and transmission matrices borrowed conceptually from seismology.

Both solvers must contend with a notorious numerical pitfall known as swamping. When dissipative wave modes—viscosity waves and thermal conduction waves—are included in the model, certain descending modes grow so explosively in the upward direction that the computation overflows. The new model tames this problem with scaling matrices that rescale the exponential terms depending on whether their growth rates are positive or negative, a technique standard in radiative transfer theory. The researchers also carefully distinguish ascending from descending wave modes using the real parts of the eigenvalues of the propagation matrix, and they incorporate a causality-preserving technique called the imaginary frequency shift, introduced by Harold Knight and colleagues in 2019, which ensures that the computed wave field never responds before its source acts—a fundamental physical requirement.

The model’s flexibility extends to both single-frequency waves and time-dependent wave packets. For a monochromatic source, the problem is solved at a single excitation frequency. For a wave packet—say, a pulse of air disturbance with a Gaussian envelope in time—the equations are transformed into the frequency domain, solved across a band of frequencies, and then reconstructed in time via an inverse Fourier transform. In their demonstration simulations, the team used realistic atmospheric and ionospheric data for the EISCAT Tromsø auroral location in Norway, with an altitude grid stretching from 80 to 500 kilometers. The wave packets produced temperature perturbations of roughly 31 to 33 Kelvin and vertical velocities reaching more than 20 meters per second, depending on the horizontal wavelength.

The numerical experiments delivered a clear verdict on the competing solvers: all methods achieved identical accuracy, with relative errors below one part in a million, but the global matrix method was significantly faster than the scattering matrix method, especially for time-dependent wave packets, which require numerous matrix operations in every layer. The simulations also quantified the impact of ion drag, which moderately attenuates the waves between roughly 180 and 350 kilometers altitude, where the ion number density is relatively high. Interestingly, the model showed that in the altitude range of 150 to 300 kilometers, waves computed with the full general model attain larger amplitudes than those from the simplified model, because including altitude-dependent background properties weakens the effective vertical damping.

The researchers emphasize that this model is only the first component of a larger ambition: a comprehensive tool for analyzing ionospheric gravity waves using satellite measurements. Two extension paths are envisaged. In one, the neutral-atmosphere and ionospheric equations would be solved together as a fully coupled system, with the state vector augmented by perturbed ion density and diffusion velocity. In the other, a two-step strategy would first solve the neutral atmosphere and then feed the wave-induced perturbations into established ionosphere models such as SAMI2 or SAMI3. For now, the implementation is freely available as open-source code on GitHub and archived on Zenodo, an unusual level of accessibility for a specialized numerical tool. By combining decades-old multilayer concepts with modern matrix-exponential techniques from radiative transfer, the DLR team has given atmospheric scientists a fast, rigorous, and transparent instrument for decoding the ripples that connect the weather below to the charged skies above.

Subject of Research: Numerical modeling of linearized atmospheric gravity waves using a multilayer matrix-exponential method

Article Title: A numerical model for solving the linearized gravity-wave equations by a multilayer method

Article References: Doicu, A., Efremenko, D. S., & Trautmann, T. (2026). A numerical model for solving the linearized gravity-wave equations by a multilayer method. Annales Geophysicae, 44(2), 655-688. https://doi.org/10.5194/angeo-44-655-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-655-2026

Keywords: gravity waves, atmospheric modeling, ionosphere, multilayer method, matrix exponential, numerical simulation, ion drag, viscosity, thermal conduction, radiative transfer, open-source code, Annales Geophysicae

The research, led by Charlotte M. van Hazendonk of The University Centre in Svalbard and the University of Bergen, together with Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies, focuses on an event that unfolded between 13:00 and 14:30 Universal Time on 16 November 2021, under modest geomagnetic conditions with a Kp index of just 2. The team identified the event as a sequence of six auroral arcs marching poleward across the sky, visible primarily in the 630.0 nanometer oxygen emission line that glows at altitudes around 250 kilometers. Periodic arc sequences like these are the optical signature of ULF waves — magnetohydrodynamic oscillations with frequencies between one millihertz and one hertz — that transfer energy and momentum along geomagnetic field lines from the magnetosphere down into the ionosphere.

What makes the detection remarkable is the sheer breadth of instrumentation brought to bear on a single event. The team combined the European Incoherent Scatter Svalbard radar, a meridian scanning photometer at the Kjell Henriksen Observatory, and a chain of ground-based magnetometers with data from Defense Meteorological Satellite Program spacecraft, the Iridium constellation, and the Swarm satellites. Two inversion models tied the observations together: the ELSPEC method, which converts radar measurements of electron density into estimates of precipitating electron spectra and field-aligned currents, and the Lompe model, which fuses ground and space magnetometry with convection data to map the large-scale electrodynamics of the polar ionosphere. This multi-scale, multi-instrument strategy proved essential, because the event turned out to be far more complicated than any single dataset suggested.

The radar told the most dramatic part of the story. From 12:55 UT onward, the EISCAT Svalbard radar recorded periodic bursts of particle precipitation arriving roughly every ten minutes, each one heating the electron population and boosting electron densities in the E and F regions of the ionosphere. Crucially, the ion temperature stayed flat — there was no sign of frictional or Joule heating inside the narrow radar beam. That combination, enhanced electron temperatures and densities without ion heating, is the classic signature of what researchers call type 2 ion upflow. In this mechanism, hot electrons create an enhanced ambipolar electric field that drags the much heavier ions upward along the magnetic field, like a thermal elevator lifting atmospheric material toward the magnetosphere.

And the elevator was busy. Applying a standard detection criterion that requires upward ion velocities exceeding 100 meters per second across at least three consecutive altitude bins, the team calculated ion upflow fluxes ranging from 1 times 10 to the 13th to 1 times 10 to the 14th particles per square meter per second, with a median value of about 3.3 times 10 to the 13th. In the established classification scheme for Svalbard observations, that places the event squarely in the low-to-medium flux range — notable, because at such quiet geomagnetic activity one would normally expect only low fluxes. The upflow was real, measurable, and directly tied to the passing auroral arcs, yet satellite passes at 13:27 and 14:38 UT showed no evidence that the upflow had grown strong enough to escape as genuine ion outflow into the magnetosphere.

The energy accounting was equally striking. The ELSPEC inversion revealed field-aligned currents — vertical current systems that couple the ionosphere to the magnetosphere — reaching magnitudes of up to 6 microamperes per square meter during the brightest arcs, while the larger-scale Lompe analysis showed currents around 3 microamperes per square meter with an alternating up-down pattern across the arc region. Total energy fluxes peaked at 12 milliwatts per square meter in localized measurements and about 8 milliwatts per square meter at large scale, values confirmed independently by the DMSP satellites’ ultraviolet imager. Joule heating rates derived from the Lompe model reached 8 to 11 milliwatts per square meter in bands between oppositely directed current sheets. For context, these current and energy values are comparable to those previously reported for large-scale field line resonances — the grand, standing Alfvén waves of the magnetosphere — despite this event appearing, by several measures, to be small-scale.

That contradiction is the scientific heart of the paper. Ground magnetometers showed a phase variation exceeding the 180-degree change expected for a classical field line resonance, and a spectral power pattern inconsistent with a large-scale standing wave. The absence of ion temperature enhancements in the radar beam further argued against the frictional heating that field line resonances typically produce. Yet the current magnitudes and dissipation rates matched those of their large-scale cousins. The authors suggest several possible reconciliations: phase mixing may have broadened the frequency peak of an initially externally driven resonance, giving it small-scale characteristics; the radar beam, only about 3.5 kilometers wide at 250 kilometers altitude, may simply have missed the bulk of the Joule heating occurring to the south; or the wave energy may have been channeled preferentially into particle acceleration rather than resistive heating, a possibility hinted at by earlier work on kinetic Alfvén waves in the storm-time inner magnetosphere.

The study is also candid about the limits of the models. The Lompe inversion depends strongly on the assumed ionospheric conductance, and the team found that plausible variations in the poorly constrained background conductance could swing the inferred convection velocities and Joule heating rates substantially. Magnetometers, moreover, integrate currents over wide areas and are susceptible to ocean-induced signals that can contribute up to half of the measured disturbance, while small-scale ULF waves are often attenuated before they ever reach the ground. The five-minute analysis windows used in the model also smooth over a significant fraction of the wave’s fifteen-minute cycle. The authors therefore weight the direct radar and optical observations most heavily, and they call for future studies with space-based auroral imaging and spatially extended incoherent scatter measurements to pin down the conductance and complete the energy budget.

Why does this matter beyond the physics of pretty lights? Ion outflow is a fundamental source of plasma for the magnetosphere, influencing everything from satellite-damaging storm dynamics to the long-term evolution of planetary atmospheres. Establishing that even small-scale, quiet-time ULF waves can drive significant ion upflow — with energy dissipation rates rivaling their large-scale counterparts — means the contribution of these waves to the global mass and energy budget has likely been underestimated. It also validates the Samson-Rankin framework, in which field-aligned electric fields within the wave structure accelerate electrons downward to paint the arcs while simultaneously setting ions in motion upward. The Svalbard event, captured on closed magnetic field lines in the post-noon sector, demonstrates that the ionosphere and magnetosphere are coupled in both directions at once: energy pours down as accelerated particles, and matter climbs back up in response. For a phenomenon triggered by waves oscillating just once every fifteen minutes, the consequences ripple far beyond the auroral oval.

Subject of Research: First observations of ionospheric ion upflow driven by ULF wave auroral arcs at high latitude

Article Title: High-latitude observations of ULF wave driven ion upflow

Article References: van Hazendonk, C. M., Baddeley, L. J., Laundal, K. M., & Partamies, N. (2026). High-latitude observations of ULF wave driven ion upflow. Annales Geophysicae, 44(2), 697-714. https://doi.org/10.5194/angeo-44-697-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-697-2026

Keywords: ion upflow, ULF waves, aurora, ionosphere, magnetosphere, Svalbard, EISCAT, field-aligned currents, Joule heating, field line resonance, space physics, Annales Geophysicae

The heart of the problem lies in the ionospheric Ohm’s law, the equation that describes how electric currents flow perpendicular to Earth’s magnetic field. In its full, altitude-resolved form, the current density depends on the electric field, the magnetic field, and the local neutral wind, weighted by two key electrical properties of the ionosphere: the Pedersen conductivity, which governs current flow along the electric field, and the Hall conductivity, which governs current flow perpendicular to it. Both conductivities vary strongly with altitude, peaking in the E region between roughly 100 and 130 kilometers. So do the winds. When researchers integrate the Ohm’s law over altitude to obtain the height-integrated version used in countless empirical models and data assimilation schemes, they implicitly assume that the wind does not vary with height — an assumption that decades of rocket measurements have shown to be wildly unrealistic.

Hatch and his team show that when the wind’s altitude dependence is properly retained, the height-integrated Ohm’s law splits into two distinct wind terms: one weighted by the Hall conductivity profile and one weighted by the Pedersen conductivity profile. These are not mathematical curiosities. Because the Hall and Pedersen conductivity profiles peak at different altitudes, and because the wind shears dramatically between 80 and 140 kilometers, the two weighted winds generally point in different directions and have different magnitudes. The distinction only vanishes in idealized cases — when the wind is constant with altitude, or when the two conductivity profiles differ by no more than a constant factor — conditions that real measurements almost never satisfy.

To quantify just how different these two winds can be, the team turned to a rare and precious dataset: horizontal wind profiles derived from trimethylaluminum chemical release experiments carried aboard sounding rockets launched from the Poker Flat Research Range in Alaska between 2007 and 2018. When TMA vapor is released in the upper atmosphere, the glowing trail is tracked by ground cameras, allowing scientists to triangulate the wind with remarkable precision. The researchers combined fifteen such wind profiles with conductivity profiles calculated from measurements by the Poker Flat Incoherent Scatter Radar, supplemented by the NRLMSIS 2.0 empirical atmospheric model and the International Reference Ionosphere 2016 model. For each rocket flight, they computed both the Hall-weighted and the Pedersen-weighted neutral winds.

The results are striking. The magnitudes of both weighted winds fall in the range of roughly 10 to 100 meters per second — equivalent, in terms of their electromagnetic effect, to electric fields of about 0.5 to 5 millivolts per meter, which is far from negligible in auroral electrodynamics. More importantly, the difference between the two winds grows with geomagnetic activity, measured by the Kp index: the more disturbed the space environment, the larger the gap between the Hall-weighted and Pedersen-weighted winds. Interestingly, the two vectors also become more aligned with each other as activity increases, even as their magnitudes diverge. This is the first experimental comparison of the two wind terms, and it makes clear that collapsing them into a single number is not a benign simplification.

The study also delivers a blow to a widely used rule of thumb. In 1995, researchers suggested on the basis of simulations that the so-called effective neutral wind — the wind relevant for height-integrated Joule heating — could be approximated by the wind measured at 160 kilometers altitude, and this convention has been adopted in subsequent observational studies. Hatch’s team tested this idea against their rocket data and found it wanting: for the majority of their fifteen profiles, using the 160-kilometer wind produced larger errors than simply assuming the wind was zero. The statistically best proxy for the Pedersen-weighted wind turned out to be the wind at the altitude where the Pedersen conductivity peaks, typically between 115 and 128 kilometers. For the Hall-weighted wind, ironically, assuming zero wind incurred the lowest median error of any proxy tested. All proxies, however, carried errors of at least several tens of meters per second, suggesting there are no easy fixes for the wind problem.

The consequences extend to one of the most important quantities in space physics: Joule heating, the frictional warming of the upper atmosphere as electric currents push against it. The team proved analytically, using the Cauchy–Bunyakovsky–Schwarz inequality, that the commonly used expression for height-integrated Joule heating in terms of height-averaged quantities is mathematically guaranteed to be a lower bound on the true heating rate. When they compared the lower-bound approximation against the true wind contribution computed from full altitude profiles, they found underestimations ranging from 9 to 96 percent, with the gap narrowing as geomagnetic activity increased. During quiet periods, both the magnitude and the direction of the wind influence total Joule heating; during active periods, the wind matters primarily through its orientation relative to the plasma convection, either reducing or enhancing the total heating depending on geometry.

These findings ripple outward into the broader architecture of space weather modeling. The key equation linking magnetosphere and ionosphere in many global models assumes a neutral wind that is zero or constant with altitude — an assumption the new analysis shows cannot be justified in a rigorous height-integrated framework. More advanced formulations, such as the treatment published by Arthur Richmond in 1995, do account for the three-dimensional wind field and are used in state-of-the-art coupled models like WACCM-X and TIE-GCM, but they remain unused in a large fraction of experimental studies and assimilation techniques, largely because comprehensive three-dimensional measurements of the coupled system are so scarce. The team also verified that their quasi-steady approximation is robust: in the altitude range where conductivities peak, ion inertial effects remain at least an order of magnitude smaller than collisional drag even under extreme conditions.

There is practical hope in the results as well. Because the Pedersen-weighted wind is best approximated by the wind near the Pedersen conductivity peak, measurements confined to a narrow altitude band around 100 to 120 kilometers could substantially improve height-integrated estimates. This points to the utility of Fabry–Perot interferometers tuned to the 557.7-nanometer green line of atomic oxygen, which peaks in emission between roughly 100 and 130 kilometers — in contrast to the red-line instruments that probe the higher F region and are less relevant here. Next-generation incoherent scatter radar facilities such as EISCAT_3D may also contribute wind estimates in this critical altitude range. One caveat is that auroral precipitation can shift the green-line emission layer across a broad span of altitudes, complicating the interpretation of such measurements.

Ultimately, the study is both a warning and an invitation. It warns that the two-dimensional picture of ionospheric electrodynamics, however convenient, hides a genuine ambiguity: there is no single neutral wind that can represent the winds’ effect on height-integrated currents and heating. And it invites the community to invest in the measurements — from sounding rockets, green-line interferometers, and advanced radars — that would allow the ambiguity to be resolved. As researchers increasingly join the chorus calling for more neutral wind observations, this work shows that even a single well-placed wind measurement near the Pedersen conductivity peak could sharpen our understanding of how the Sun’s energy flows into, and heats, the atmosphere we live beneath.

Subject of Research: The role of neutral winds in height-integrated ionosphere-thermosphere electrodynamics

Article Title: What is the neutral wind in height-integrated ionospheric electrodynamics?

Article References: Hatch, S. M., Burchill, J., Vanhamäki, H., de Mesquita, R. L. A., & Laundal, K. M. (2026). What is the neutral wind in height-integrated ionospheric electrodynamics?. Annales Geophysicae, 44(2), 715-729. https://doi.org/10.5194/angeo-44-715-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-715-2026

Keywords: ionosphere, thermosphere, neutral wind, Joule heating, Pedersen conductivity, Hall conductivity, sounding rockets, Poker Flat, space weather, magnetosphere-ionosphere coupling, incoherent scatter radar, Fabry-Perot interferometer

The team coupled two sophisticated numerical models into a single pipeline. At the front end sits EUHFORIA, the European Heliospheric Forecasting Information Asset, a three-dimensional magnetohydrodynamic model that simulates the inner heliosphere from 0.1 to 2 astronomical units. EUHFORIA injects CMEs at its inner boundary as flux-rope structures defined by launch time, position, angular width and initial speed, then propagates them through the ambient solar wind, capturing shock formation, CME-CME interactions and the evolution of the embedded magnetic field en route to Earth. At the Lagrange L1 point, 1.5 million kilometres sunward of our planet, the model outputs time series of proton density, bulk velocity, thermal pressure and the three components of the interplanetary magnetic field, which then serve as the upstream boundary conditions for the second model.

That second model, Gorgon-Space, is a global magnetohydrodynamic code originally developed for high-energy-density plasma physics, including Z-pinches and laser-plasma experiments, and later adapted to planetary magnetospheres. Its defining numerical feature is that it evolves the magnetic vector potential rather than the magnetic field itself, which guarantees that the divergence-free condition on the magnetic field is preserved to machine precision. That property matters enormously during CME impacts, when the magnetosphere is compressed so violently that standard solvers can accumulate spurious magnetic monopoles and degrade the solution. Gorgon-Space runs on a fixed Cartesian grid spanning 90 by 80 by 80 Earth radii with a uniform resolution of half an Earth radius, resolving the bow shock, the magnetopause and a substantial portion of the magnetotail, while a thin-shell ionosphere at roughly 110 kilometres altitude closes the coupled system.

The two test cases were ideally matched. CME1, launched on 21 June 2015 at 05:01 UT with an initial speed of 1250 kilometres per second, arrived at Earth on 23 June at 00:03 UT and ignited a major G4-class geomagnetic storm, with the planetary Kp index peaking at 8 and the SYM-H index plunging to roughly minus 200 nanotesla. CME2, launched on 25 June at 10:51 UT, was actually faster, leaving the Sun at 1450 kilometres per second, and reached Earth on 28 June at 12:52 UT. Yet it produced only a minor disturbance, with Kp hovering near 2 and SYM-H dipping to just about minus 20 nanotesla. The difference, the simulations show, lay entirely in the orientation of the embedded interplanetary magnetic field.

CME1 carried a sustained southward magnetic field component, with Bz below minus 15 nanotelsa for more than six hours. Southward fields are the golden ticket for geomagnetic trouble: they reconnect antiparallel with Earth’s northward-pointing dayside field, tearing open previously closed field lines and allowing solar wind energy and momentum to pour into the magnetosphere. The reconnection electric field, a standard proxy for the rate of energy transfer, exceeded 15 millivolts per metre during CME1, well above the 1 to 2 millivolts per metre threshold for strong driving. CME2, by contrast, kept Bz above minus 5 nanotesla for over 90 percent of its passage, yielding a feeble reconnection field below 3 millivolts per metre and almost no energy transfer, despite its higher speed.

The simulated magnetospheric response to CME1 was dramatic. The dayside magnetopause and bow shock were pushed inward to roughly 10 Earth radii from the planet’s centre, compared with about 20 Earth radii during the weaker event, reflecting the far greater dynamic pressure of the incoming solar wind. Thermal pressure surged throughout the equatorial plane, and the magnetotail stretched and distorted in ways consistent with the onset of nightside reconnection, with the formation of X-lines and the ejection of plasmoids converting magnetic energy into kinetic and thermal energy. Flow velocity visualisations showed energetic, irregular plasma motion around the magnetosphere as solar wind energy was transferred through dayside reconnection, while the closed-field regions on the nightside shrank as open magnetic flux accumulated.

The ionospheric consequences were equally striking. Field-aligned currents, the conduits through which magnetospheric stress is transmitted to the high-latitude ionosphere, surged past 23 million amperes during CME1, compared with less than 8 million amperes of weak, disorganised current during CME2. The simulations reproduced the classic two-phase response to a shock arrival: a preliminary impulse driven by compression of the dayside magnetosphere, followed by a main impulse marking the full engagement of the global Region-1 current system. Crucially, the modelled current patterns matched observations from the AMPERE satellite constellation closely in morphology, timing and integrated magnitude, validating the dynamic conductance treatment that links auroral precipitation to ionospheric conductivity.

The cross-polar cap potential, a key measure of global ionospheric convection, climbed to approximately 160 kilovolts during CME1, a signature of the powerful convection electric field that drives plasma transport across the polar cap and feeds the ring current that defines storm intensity. During CME2 the potential barely moved, physically consistent with the low plasma velocities observed in the magnetosphere for that event. This convection also matters for practical technology: the same current systems that light up auroras can induce geomagnetically induced currents in long conductors such as power lines and pipelines, and ionospheric disturbances can degrade high-frequency radio communications and introduce positioning errors in satellite navigation.

The broader lesson is one that space weather forecasters have long suspected but rarely demonstrated with such quantitative clarity: CME speed alone is a poor predictor of storm intensity. A fast CME with an unfavourable magnetic geometry can pass by almost unnoticed, while a slightly slower one carrying hours of southward field can cripple infrastructure. By resolving the complete causal chain from CME launch at the Sun, through heliospheric propagation, to magnetospheric compression, current intensification and ionospheric electrodynamic response, the EUHFORIA-Gorgon-Space chain offers a physics-based way to reject false alarms from fast but magnetically benign eruptions. The authors note that their framework, with its numerically robust divergence-free formulation, is well suited to operational forecasting of geomagnetically induced current risk, and future work will extend validation to auroral electrojet indices derived from ground magnetometer networks.

Subject of Research: Magnetosphere–ionosphere response to two coronal mass ejections simulated with a coupled EUHFORIA–Gorgon-Space modelling chain

Article Title: Magnetosphere–ionosphere response to the 21 and 25 June 2015 coronal mass ejections

Article References: Sabri, S., & Poedts, S. (2026). Magnetosphere–ionosphere response to the 21 and 25 June 2015 coronal mass ejections. Annales Geophysicae, 44(2), 743-763. https://doi.org/10.5194/angeo-44-743-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-743-2026

Keywords: coronal mass ejection, space weather, geomagnetic storm, magnetosphere, ionosphere, EUHFORIA, Gorgon-Space, magnetic reconnection, field-aligned currents, cross-polar cap potential, interplanetary magnetic field, MHD simulation

The region in question is the lower thermosphere–ionosphere, or LTI, a layer spanning roughly 100 to 200 kilometers in altitude where the neutral gas of the atmosphere and the electrically charged plasma of the ionosphere are so intimately intertwined that they effectively share a single energy budget. During quiet conditions, the dominant energy source is sunlight: extreme ultraviolet radiation absorbed by atomic oxygen, molecular oxygen, ozone and nitrogen heats the gas and creates the ionosphere in the first place. But when a coronal mass ejection compresses Earth’s magnetic shield, the picture changes dramatically. Solar wind energy funnels down magnetic field lines into the polar regions, energetic electrons and protons rain into the atmosphere, and a process called Joule heating takes over as the thermodynamic heavyweight of the entire system.

Joule heating is, in essence, frictional heating on a planetary scale. Ions in the ionosphere are dragged across magnetic field lines by electric fields inherited from the solar wind, while the much denser neutral gas below resists being pushed along. The collisions between these two populations convert electromagnetic energy delivered from the magnetosphere directly into heat. Because the heating rate depends on the square of the electric field, any small-scale structure in the field that a model fails to resolve translates into a systematic underestimate of the deposited energy. That quadratic sensitivity is precisely why Joule heating remains, in the words of the research community, among the least quantified of the major processes in the upper atmosphere, despite being the most significant during disturbed times.

To dissect the storm-time energy flow, the team ran the National Center for Atmospheric Research’s Thermosphere Ionosphere Electrodynamics General Circulation Model, version 2.0, a first-principles simulation that solves the momentum, energy and continuity equations for neutrals, ions and electrons from about 100 up to 600 kilometers altitude. They simulated four days around the St. Patrick’s Day storm, which reached a minimum Dst index of roughly minus 223 nanotesla and a minimum SYM-H of minus 234 nanotesla, and they computed every significant heating and cooling term at each grid point before integrating them over the polar cap above 56 degrees latitude. Crucially, they ran the model twice with two different specifications of the high-latitude electric potential: the empirical Weimer 2005 model, which is a statistical average built from satellite measurements, and the Assimilative Mapping of Ionospheric Electrodynamics technique, known as AMIE, which blends real ground-based and space-based observations into a time-dependent map of the electrodynamics.

The comparison between the two runs is the heart of the study. Before the storm, the two drivers produced reasonably similar energy budgets, with the AMIE run showing only modestly larger Joule heating. At the peak of the storm, however, the AMIE-driven simulation produced more than twice as much Joule heating and more than twice as much associated ion-to-neutral heat transfer as the Weimer-driven run. The authors attribute this gap to several compounding factors. The Weimer model resolves features only down to about 2 to 3 degrees of magnetic latitude and roughly an hour of magnetic local time, while AMIE can typically achieve 1 to 2 degrees and 15 to 30 minutes, capturing more of the small-scale electric field variability that the quadratic heating law converts into heat. Moreover, AMIE incorporates actual observations during the event, whereas Weimer represents a climatological average response that the real storm departed from substantially at its height.

Viewed from the perspective of the neutral gas, the energy ledger shifts dramatically between quiet and storm conditions. One day before the storm peak, the largest heating source was solar extreme ultraviolet radiation combined with energetic particle precipitation, followed by heating from the recombination of molecular oxygen. Joule heating was split roughly evenly between energy deposited promptly into the neutrals and energy first given to the ions and then handed over to the neutrals through collisions. At the storm peak, Joule heating and ion-neutral heating became the dominant sources by a wide margin, while the solar contribution rose only slightly. On the cooling side, carbon dioxide emission at 15 micrometers dominated before the storm, but during the disturbance nitric oxide cooling at 5.3 micrometers surged far past it, behaving as the natural thermostat that helps the thermosphere shed the storm’s excess heat over the following two to three days.

The ion and electron perspectives complete the picture. Ions receive a fraction of the Joule heating directly and a small additional contribution from hotter electrons, then cool almost entirely by transferring heat to the far denser neutral gas; at the storm peak both their heating and their cooling rose sharply in tandem. Electrons, by contrast, are heated mainly by solar radiation and by the thermalization of secondary electrons created when energetic particles ionize the neutral gas, and they cool through elastic and inelastic collisions with neutrals and ions. Notably, the electron energy budget showed only small differences between the two model runs, both before and during the storm, indicating that the driver-dependent uncertainty is concentrated almost entirely in the Joule heating pathway and its ion-mediated branch.

These discrepancies matter far beyond academic bookkeeping. The amount of Joule heating deposited during a storm determines how much the upper atmosphere swells, and a puffier thermosphere means increased drag on the thousands of satellites in low Earth orbit, degrading orbit predictions and collision-avoidance margins. The study also highlights a deeper data problem: the entire archive of simultaneous ion, neutral and electron temperature measurements in the 100 to 200 kilometer altitude range amounts to only about 60 hours of observations, most of it from the Atmosphere Explorer C and E satellites of the late 1970s and early 1980s. Recent re-analyses of that legacy data have even shown that the textbook ordering of temperatures, in which electrons are hotter than ions and ions hotter than neutrals, does not always hold, a fact that current models, including TIE-GCM, are forced to assume away.

The authors argue that closing these gaps requires new, systematic, co-located and co-temporal measurements of all the parameters that enter the Joule heating calculation, ideally from an in situ platform flying through the region where the heating peaks. Mission concepts of exactly this kind are actively being pursued at both ESA and NASA, including the proposed Daedalus low-perigee spacecraft. Meanwhile, the recently released TIE-GCM version 3.0 offers higher spatial resolution that may capture more of the sub-grid electric field variability responsible for the largest uncertainties. Until such measurements and models mature, the study stands as a vivid demonstration that our forecasts of storm-time upper atmospheric response rest on driver choices that can swing the energy budget by a factor of two, a humbling reminder of how much remains unknown about the interface between Earth and space.

Subject of Research: Storm-time energy budget and Joule heating uncertainty in the high-latitude lower thermosphere–ionosphere

Article Title: Storm-time energy budget in the high latitude lower thermosphere–ionosphere: quantification of energy exchange and comparison of different drivers in TIE-GCM

Article References: Tourgaidis, S., Sarris, T., Baloukidis, D., Buchert, S., Pirnaris, P., Papadakis, K., & Balafoutis, A. (2026). Storm-time energy budget in the high latitude lower thermosphere–ionosphere: quantification of energy exchange and comparison of different drivers in TIE-GCM. Annales Geophysicae, 44(2), 773-793. https://doi.org/10.5194/angeo-44-773-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-773-2026

Keywords: geomagnetic storm, Joule heating, lower thermosphere, ionosphere, TIE-GCM, space weather, St. Patrick's Day storm 2015, AMIE, Weimer model, nitric oxide cooling, energetic particle precipitation, satellite drag

The magnetospheric cusp is one of the most dynamic regions of near-Earth space. When the interplanetary magnetic field carried by the solar wind points southward, magnetic reconnection at the subsolar magnetopause opens field lines and allows magnetosheath plasma to penetrate deep into the magnetosphere. This reconnection is pulsed in nature, producing flux transfer events that repeat every few to about fifteen minutes and generate a cascade of ionospheric phenomena: poleward moving auroral forms, reversed flow events, and strong flow channels. Under northward interplanetary conditions, reconnection shifts to the high-latitude lobes, but the cusp remains a site of intense, transient dynamics. All of these processes leave fingerprints in the magnetic field measured on the ground, in the form of rapid fluctuations in the ultra-low frequency, or ULF, band.

To capture these fingerprints, the team processed data from 2020 through 2023 recorded by the West Greenland magnetometer chain, whose stations span magnetic latitudes from the auroral zone deep into the polar cap. They applied a Butterworth bandpass filter to the northward magnetic component, isolating variations with periods between 10 and 600 seconds, a range that encompasses the Pc 3 to Pc 5 pulsation classes as well as Pi 2 irregular pulsations. Rather than trying to classify individual wave events, the researchers computed a broadband metric: they took the absolute value of the filtered signal and smoothed it with a ten-minute boxcar window. This metric, denoted X_ULF, integrates the total variability across the band and therefore responds to transient, impulsive, and wave-like magnetospheric phenomena alike, not just to coherent ULF waves.

The authors are careful about interpretation. Because a bandpass filter preserves the frequency content of any step-like disturbance, a sudden change in the ionospheric current, such as that produced by a flux transfer event, would appear in the metric just as a genuine wave would. Many of the power spectra they examined resemble the spectrum of a Heaviside step function, precisely what an impulsive current change would produce. The team therefore treats their metric as a measure of the ground magnetic response to all rapid dayside dynamics, whether oscillatory or impulsive, rather than as a pure ULF wave proxy. This honesty about what the data can and cannot distinguish strengthens the statistical conclusions that follow.

The results reveal a striking geographic and seasonal structure. At Thule, the northernmost station in the polar cap, enhanced variability appears centred on magnetic local noon during summer, with amplitudes that grow as the F10.7 solar radio flux increases, a clear signature of solar cycle influence. The seasonal variation is anti-correlated with the solar zenith angle, meaning the signal is strongest when the Sun sits highest above the horizon. At Upernavik, slightly further south, a similar but stronger noon-time population appears with a smaller seasonal swing. In contrast, the southern stations in the auroral zone show their dominant activity at night, consistent with substorm processes, and their daytime signals respond strongly to geomagnetic storms as tracked by the ap index.

The pivotal finding emerges when the four years of data are averaged by season and plotted against magnetic latitude and local time. During summer, two separate populations of rapid geomagnetic variation are clearly visible: one poleward of about 75 degrees quasi-dipole latitude, associated with the cusp, and one equatorward of it, associated with auroral oval dynamics on closed field lines. Between them lies a pronounced minimum near 75 degrees, a quiet latitudinal gap that cleanly separates the two source regions. During winter, this minimum vanishes. The poleward population shifts southward, following the cusp as the Earth’s dipole tilt changes, and merges with the equatorward signals, becoming indistinguishable from them.

The physical explanation hinges on solar illumination and the electrical conductivity of the ionosphere. Ground magnetic signals in this frequency band are produced by horizontal ionospheric currents, which obey Ohm’s law and therefore depend on both the electric field and the conductivity of the ionospheric E-region. In the cusp, the electron precipitation streaming in from the magnetosheath is extremely soft, typically below about 220 electronvolts, so it deposits its energy in the F-region above roughly 200 kilometres rather than enhancing E-region conductivity. That means the conductivity overhead must be supplied by solar extreme-ultraviolet radiation, which is only sufficient during summer when the Sun never sets. The team demonstrated this by computing Hall conductances at each station using empirical formulas, and found plots that closely mirror the measured ULF variability at the two northern stations, year after year as solar activity climbed toward maximum.

The dependence on the interplanetary magnetic field further confirms the cusp origin of the high-latitude signal. The noon-time population poleward of 75 degrees appears for all IMF clock angles, indicating that it does not require either purely subsolar or lobe reconnection, but it shifts toward the morning or afternoon side of magnetic noon depending on the sign of the IMF B_y component. This behaviour matches the well-known Svalgaard-Mansurov effect and the documented response of cusp precipitation, poleward moving auroral forms, and the DPY current system to the east-west interplanetary field. Meanwhile, the equatorward dayside population is most pronounced during southward IMF, consistent with enhanced energy transfer through subsolar reconnection, and shows a local minimum at magnetic noon that the authors attribute to travelling convection vortices generated along the flanks of the magnetosphere, away from the bow-shock nose.

The study also resolves a long-standing puzzle in the literature. Earlier work, including analyses of the MACCS search-coil network and SuperDARN radar observations, had reported dayside ULF signatures occurring several degrees equatorward of the expected cusp boundary, leading some researchers to conclude that no distinct cusp wave signature exists on open field lines. The new analysis suggests those winter-time observations captured a mixture of populations: the weak, sun-dependent cusp signal blended with Alfvénic waves bouncing between hemispheres on closed field lines and with signals from more energetic particle precipitation, as documented in comparable Antarctic studies. Only in summer, when the tilted dipole pushes the cusp to its highest latitudes and solar EUV powers the E-region conductivity, does the cusp population crystallise out of the background as a separate, statistically robust entity.

One frustrating asymmetry remains: the optical observations needed to study cusp auroras require darkness, while the geomagnetic signal identified here requires sunlight, so the two cannot be combined in a single campaign. The authors note, however, that the electric fields driving the summer signals almost certainly persist through winter, and propose that dedicated high-cadence radar and satellite measurements could reveal them. For now, the Greenland chain has delivered something no previous array could: a four-year, high-cadence statistical portrait of the dayside high-latitude ionosphere from the auroral oval to the polar cap, and with it, the first unambiguous isolation of the magnetic signature of the polar cusp itself, written in the flickering of Earth’s field whenever the midnight Sun shines over the Arctic.

Subject of Research: Statistical analysis of rapid dayside geomagnetic variations associated with the magnetospheric cusp using Greenland ground-based magnetometers

Article Title: High latitude, dayside rapid geomagnetic variations observed with ground-based magnetometers in Greenland

Article References: Eldor, M. V., Johnsen, M. G., Olsen, N., & Willer, A. N. (2026). High latitude, dayside rapid geomagnetic variations observed with ground-based magnetometers in Greenland. Annales Geophysicae, 44(2), 811-823. https://doi.org/10.5194/angeo-44-811-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-811-2026

Keywords: geomagnetism, polar cusp, ULF waves, magnetometers, Greenland, magnetosphere, solar wind, ionosphere, magnetic reconnection, space weather, auroral oval, ionospheric conductivity

The dunes were first described in the scientific literature in 2020, when a study led by Maria Palmroth used two nearly simultaneous photographs taken by amateur observers to establish that the auroral emission associated with the dunes was produced at an altitude of approximately 100 kilometers. That altitude places the phenomenon in the lower thermosphere, a notoriously difficult region to measure directly. Follow-up work in 2021 revealed the remarkable scale of a single dune event: the wave-like stripes spanned more than 1,500 kilometers of horizontal distance, from western Finland to Scotland, and remained visible for at least four hours. Those studies also found that the dunes were associated with electron precipitation in the 10 to 20 kiloelectronvolt range and with a large temperature inversion layer just below the mesopause, the boundary region roughly 85 to 100 kilometers up where temperatures reach their coldest point in the entire atmosphere.

The leading hypothesis is that the dunes are an optical signature of an atmospheric wave propagating horizontally through the diffuse aurora near the mesopause. As the wave modulates the density of atomic oxygen, the ingredient responsible for the green auroral emission, it imprints its crests and troughs onto the glow, producing the characteristic stripes. The prime candidate is the mesospheric bore, a sharp wavefront followed by a train of trailing crests that can only propagate within a ducting structure, such as a temperature inversion layer or a wind shear. Bores have horizontal wavelengths of 20 to over 100 kilometers, compatible with the 30 to 45 kilometer pseudo-wavelength measured in the few dune events studied so far. But alternatives exist: some researchers have proposed acoustic waves ducted through dusty plasma produced by meteor showers, while others have suggested a purely magnetospheric origin, in which waves near the plasmapause spatially modulate the flux of precipitating particles. Without statistics, the debate could not be settled.

The new survey tackles that gap by mining Skywarden, an observation database created and maintained by the Ursa Astronomical Association in Finland. The researchers searched the archive from January 2000 to December 2025, restricting the search to reports containing at least one photograph showing the dunes. Each candidate observation then passed through a three-step verification process: the observers themselves flagged possible dune structures, the Skywarden moderating team reviewed the auroral forms present in each submission, and finally two of the study’s authors visually inspected every report, discarding cases where clouds or airglow structures could mimic the dunes. The result was a curated set of 308 dune observations, made during 61 distinct events by 183 named citizen scientists from Northern Europe, North America, Australia, and New Zealand. As a baseline for comparison, the team also retrieved nearly 10,000 timestamped aurora reports from the same database.

The statistical patterns that emerged are striking. Ninety-two percent of the dune reports came from observers below 61 degrees geomagnetic latitude, compared with 74 percent for all aurora observations, suggesting that the dunes frequently occur in the equatorward, subauroral part of the auroral oval. The local time distribution is even more distinctive: 89 percent of dune observations were made before magnetic midnight, with a sharp peak between 21:00 and 22:00 magnetic local time, roughly two hours earlier than the peak for aurora in general. Permutation tests with false discovery rate correction confirmed that both the local time difference and the monthly distribution difference are statistically significant, while the solar wind and interplanetary magnetic field parameters showed no significant differences between dune events and typical aurora. In other words, the dunes do not appear to require a special kind of solar wind driving.

What the dunes do require, apparently, is geomagnetic activity. The SuperMAG SME index, a proxy for auroral electrojet intensity derived from ground magnetometers, had a median value of 871 nanotesla during dune observations, significantly higher than the median of 507 nanotesla for all aurora reports. The SMR ring current index was likewise skewed toward more disturbed values, with a median of minus 49 nanotesla, though the dunes do not necessarily demand a full-blown geomagnetic storm. Individual events could span more than 5 degrees of geomagnetic latitude, and combining multiple observers’ reports showed that about a third of dune events lasted more than two hours, with the longest, observed from New Zealand and Australia during the great geomagnetic storm of 11 May 2024, persisting for 6.9 hours. Seasonally, the events peaked in October, with a secondary peak in March, a pattern shaped by darkness, cloudiness over Finland, and the equinoctial enhancement of geomagnetic activity known as the Russell-McPherron effect.

Perhaps the most technically revealing part of the study concerns ionospheric currents. For 15 subsets of dune observations made by multiple photographers in Fennoscandia, the team applied the Spherical Elementary Current System method to data from the IMAGE magnetometer network, reconstructing the equivalent ionospheric currents with a spatial resolution of about 100 kilometers. The analysis showed that the dunes were consistently associated with strong auroral electrojet signatures: in 10 of the 15 cases they occurred within the eastward electrojet, matching the original 2020 discovery, but in one case they appeared within the westward electrojet, and in four cases they persisted through the transition from eastward to westward current. Crucially, every analyzed event took place in the vicinity of the Harang discontinuity, the boundary region on the nightside where the large-scale ionospheric current pattern reverses direction.

These current patterns, combined with the strong pre-midnight preference, point toward an intriguing possibility: that precipitating protons may play a key role in creating the diffuse green background against which the dunes appear. Proton precipitation produces secondary electrons that excite atmospheric constituents, generating diffuse emissions that concentrate on the equatorward side of the auroral oval in the dusk sector, precisely where the dunes are most often reported. The authors caution that confirming this requires detailed case studies with satellite particle measurements or ground-based spectrometers. Meanwhile, the seasonal anomalies, particularly the surplus of October events and the scarcity of September and February events relative to general aurora activity, hint that middle-atmospheric dynamics, including gravity waves, mesospheric bores, and the seasonal behavior of the stratospheric polar vortex, may also shape when the dunes can form. Short-lived events may trace gravity waves, while long-lived displays may require the sustained duct of a mesospheric bore.

Beyond the physics, the study stands as a milestone for citizen science in space physics. The dunes are low-contrast structures in a dim auroral form, best seen at low elevations with the narrow field of view of a commercial camera, conditions under which all-sky imaging networks struggle. The researchers acknowledge biases in their dataset, from the dominance of Finnish observers to uncertainties of up to 463 kilometers in the true location of the dunes relative to the photographers, but they argue that the growing quality of citizen science data, supported by field guides, moderator review, and coordinated international networks, is transforming amateur photographs into a legitimate scientific record. As aurora chasing communities around the world continue to grow, the sky itself is becoming a distributed observatory, and phenomena once invisible to science are stepping into the light.

Subject of Research: Statistical survey of the dune aurora phenomenon using citizen science observations

Article Title: Dune aurora: survey from a citizen science database

Article References: Grandin, M., Juusola, L., Partamies, N., Bruus, E., Rautiainen, J., Lach, D., Jia, J., van de Kamp, M., Karvinen, E., Kauristie, K., & Hoppe, T. (2026). Dune aurora: survey from a citizen science database. Annales Geophysicae, 44(2), 855-880. https://doi.org/10.5194/angeo-44-855-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-855-2026

Keywords: dune aurora, citizen science, aurora, mesospheric bore, ionosphere, thermosphere, geomagnetic storm, auroral electrojet, Harang discontinuity, Skywarden, space physics, atmospheric waves

Reinisch’s life story began amid upheaval. He was born on 26 November 1936 in Beuthen, Silesia, then part of Germany, and the war-related dislocation of his early childhood, including a traumatic escape, shaped his worldview permanently. His family was resettled after the war to Hausach, a village in the Black Forest region of Baden-Württemberg, where he grew up, worked various jobs to help support his family during high school and university summer breaks, and became a celebrated star of the local soccer team. Sports remained a constant throughout his life; he continued playing soccer during his university years, regularly traveling home for weekend matches, and he also excelled at competitive table tennis. Those who knew him often remarked that the same competitive energy and team spirit he displayed on the pitch would later define his approach to running large scientific programs.

His scientific formation took place at the Albert-Ludwigs-Universität Freiburg, where he studied physics and was accepted as a graduate student by Karl Rawer, the director of the Ionosphären Institut Breisach and one of the towering figures of twentieth-century ionospheric research. The Breisach institute was an extraordinary environment for a young scientist. Reinisch worked alongside established researchers including Ewald Harnischmacher, Klaus Bibl, Rudolf Eyfrig, Hannes Hesse, Adolf Paul, and Klaus Jacobs, all of whom had already made significant contributions to the study of the ionosphere, and his fellow graduate students, among them Rolf Kraft, Jürgen Büchau, and Gerhard Schmidtke, became lifelong friends and colleagues. It was during these years that Reinisch developed the lasting passion for ionospheric physics and for the techniques of observing ionospheric plasma that would define his entire career. He was not solely a scientist during this period, however; he was also a member of the Freiburger Bachchor, a renowned choir that once included the future famous soprano Hildegard Behrens. He proudly recalled singing in performances of Bach’s Weihnachtsoratorium and Matthäus Passion, and he especially treasured the occasion when the choir shared the stage with the legendary baritone Dietrich Fischer-Dieskau.

The year 1963 brought two milestones. Reinisch completed his Diploma in Physics and became Ionospheric Station Chief at the Breisach Institute, taking responsibility for the observational work that underpinned the institute’s research. In the same year he married Gerda, whom he had met and fallen in love with during a study semester in Munich and who remained his lifelong partner. His early research interests centered on the lower ionosphere: D- and E-region absorption, the puzzling sporadic-E layers that appear and disappear unpredictably at altitudes around 100 kilometers, and the analysis of whistler data, the electromagnetic waves generated by lightning that travel along magnetic field lines through the magnetosphere. These phenomena may sound arcane, but they are fundamental to understanding how radio energy propagates through and is absorbed by the upper atmosphere, and mastering them gave Reinisch an experimental grounding that few of his contemporaries could match.

In 1965, Bodo and Gerda emigrated to the United States, settling in Bedford, Massachusetts, where several organizations were engaged in ionospheric research. His friend Jürgen Büchau, who had emigrated a few years earlier and worked at the Air Force Geophysics Laboratory, convinced him it was the right career move, and it proved to be exactly that. Reinisch chose the university path, joining the University of Massachusetts Lowell as a research physicist while pursuing his Ph.D. alongside funded research. What followed was a 45-year career at UML during which he rose to Professor and Chair of the Electrical Engineering Department. In 1975 he founded the Center for Atmospheric Research, now the Space Science Laboratory, and served as its director until his retirement in 2010. From this institutional base he made his most celebrated contributions in three areas: the Digisonde, the Radio Plasma Imager, and the International Reference Ionosphere.

The Digisonde is the achievement for which Reinisch is best known, and its impact on space weather research is difficult to overstate. An ionosonde is a ground-based radar that transmits radio pulses vertically into the ionosphere and records the echoes returning from layers where the plasma frequency matches the transmitted frequency; the resulting ionogram is a fingerprint of the electron density profile overhead. Reinisch was the driving force behind transforming this decades-old technique into an advanced, fully automated instrument. Today more than 70 Digisondes operate worldwide, forming a global network that monitors the ionosphere continuously. Their unique autoscaling capability, the ability to interpret ionograms without human intervention, makes ionospheric data rapidly accessible for space weather forecasting and for global assimilative models that blend observations with physics to predict conditions hours and days ahead. In the 1980s, together with his colleague Xueqin Huang, Reinisch published a series of seminal papers describing the mathematical algorithms that convert ionogram traces into electron density profiles. Those papers have become classics, and they remain foundational reading for anyone using radio sounding to probe ionospheric plasma. Applying the same inversion techniques to topside sounder ionograms made it possible to analyze the vast data sets collected by the Alouette and ISIS satellites during the 1960s through 1980s, which in turn stimulated renewed efforts in topside modeling and produced significantly improved models of electron density in the topside ionosphere, the region above the density peak where satellites fly and where much of the practical impact of ionospheric variability is felt.

Reinisch did not stop at probing the ionosphere from the ground; he extended radio sounding into the plasmasphere, the vast reservoir of cold plasma that co-rotates with Earth far above the ionosphere. His Radio Plasma Imager, flown on NASA’s Imager for Magnetopause-to-Auroral Global Exploration satellite, was the first plasmaspheric sounder ever flown, and it provided unprecedented imaging of plasmaspheric densities along magnetic field lines. Working against numerous technical and programmatic challenges, Reinisch led the design and implementation of this unique instrument, and its success stands as a testament to his scientific and technical vision. The RPI measurements demonstrated that field-aligned density distributions cannot be described as constant profiles or as simple diffusive equilibrium, assumptions embedded in many earlier models. Instead, the data revealed a far more dynamic and structured inner magnetosphere, offering unique insights into its architecture and dynamics and stimulating a new generation of plasmasphere and polar cap models, including those developed by Reinisch and his own group. No other data source provides such detailed information about electron density in the plasmasphere, which is why the RPI legacy continues to shape magnetospheric science years after the IMAGE mission.

The third pillar of Reinisch’s legacy is the International Reference Ionosphere, the joint project of the International Union of Radio Science and the Committee on Space Research that serves as the standard model of Earth’s ionosphere, a status formally recognized by the International Standardization Organization. Reinisch’s improvements to the IRI electron density profile in the bottomside and E-region, based on data from the expanding global Digisonde network, were essential for radio wave propagation studies and for the model’s countless practical applications, from over-the-horizon radar planning to GPS correction schemes. In recognition of this leadership he was elected Chair of the IRI Working Group, serving from 2002 to 2010, and he was a regular and energetic participant in the biannual IRI workshops, helping to ensure the success and vitality of a community gathering that continues to this day. He also served as editor for many of the IRI-dedicated issues of Advances in Space Research, curating and disseminating the collective output of the international modeling effort.

After retiring from UML in 2010, Reinisch founded Lowell Digisonde International, a company that continued to build and further develop Digisondes and distribute them worldwide, ensuring that the technology he created would keep evolving rather than stagnate. Throughout his career he mentored generations of students and early-career scientists, offering guidance not only in their scientific work but in their professional and personal lives. His contributions were recognized with the highest honors in his field: the Appleton Prize of URSI in 2011, awarded with a citation crediting him with revolutionizing radio sounding from ground and space through the Digisonde and the IMAGE/RPI instrument, both essential data providers for space weather monitoring and ionospheric modeling, and the Kristian Birkeland Medal for Space Weather and Space Climate in 2014. He served URSI as Vice Chair and then Chair of Commission G, helped found the first ionospheric COST Action in Europe, and acted as Associate Editor of Radio Science from 1987 to 1997 and of IEEE Transactions on Antennas and Propagation from 2003 onward.

Shaped by his childhood experiences during and immediately after the Second World War, Reinisch rejected war and violence and advocated international collaboration and mutual understanding as pathways to a more peaceful world. His commitment to cooperation benefited science in both developing and developed nations, and German-American relations held particular importance for him. He was deeply engaged in the greater Boston community and played a central role in founding the German International School Boston, which has grown into a recognized success. In acknowledgment of these efforts alongside his scientific achievements, he was awarded the Officer’s Cross of Merit First Class, the Bundesverdienstkreuz 1. Klasse, by the President of Germany. Bodo Reinisch leaves behind a global network of instruments still sounding the sky, a standard model still guiding engineers and forecasters, and a community of scientists who lost not only an exceptional researcher but a generous mentor, an inspiration, and a dear friend.

Subject of Research: Life and scientific legacy of ionospheric physicist Bodo Reinisch, developer of the Digisonde and the IMAGE Radio Plasma Imager

Article Title: Obituary: Bodo Reinisch (1936–2025)

Article References: Bilitza, D. (2026). Obituary: Bodo Reinisch (1936–2025). History of Geo- and Space Sciences, 17(1), 61-63. https://doi.org/10.5194/hgss-17-61-2026

Image Credits: AI Generated

DOI: 10.5194/hgss-17-61-2026

Keywords: Bodo Reinisch, ionosphere, Digisonde, space weather, Radio Plasma Imager, IMAGE satellite, International Reference Ionosphere, radio sounding, plasmasphere, URSI, Appleton Prize, obituary

The D region, spanning roughly 60 to 100 kilometers in altitude, is a peculiar place. It is a weakly ionized plasma, meaning that for every free electron there are between a hundred million and a trillion neutral molecules. In such an environment, electrons collide far more often with neutral particles than with each other, and the electron-neutral collision frequency exceeds even the frequency at which electrons spiral around Earth’s magnetic field lines. This makes the D region a realm where individual collisions, not collective plasma behavior, dictate what happens to each electron. When a high-power high-frequency radio wave sweeps through this region, electrons absorb energy from the wave’s oscillating electric field through a process known as Ohmic heating. Elastic collisions scatter the electrons in random directions, converting the ordered energy of the wave into thermal motion, while less frequent inelastic collisions drain energy away by exciting the neutral molecules around them.

It is those inelastic collisions that give the electron distribution its strange and beautiful shape. Myrvang and Gustavsson’s numerical model, a modern re-implementation of a kinetic solver first derived by physicist Peter Stubbe in 1981, tracks how electrons gain energy from the radio wave and then lose it in discrete chunks as they excite vibrational, rotational, and electronic states in molecular nitrogen, molecular oxygen, and atomic oxygen. The most dramatic effect comes from molecular nitrogen, whose vibrational states have large cross sections for electron impact in the energy range between roughly 2 and 3.5 electron volts. When enough electrons reach those energies, they slam into nitrogen molecules and lose about 0.29 electron volts per collision, exciting the molecule’s vibrational modes. The result, visible in the model output, is a sharp cut-off in the electron distribution at approximately 2 electron volts, an effective cliff that truncates the population of energetic electrons far more abruptly than any Maxwellian would allow.

The technical achievement of the new work lies in how meticulously it handles this energy degradation. Where earlier solvers treated electrons as jumping between discrete velocity points, losing exactly the excitation energy in a single step, the Norwegian team used continuous, energy-dependent cross sections for every inelastic process and carefully partitioned degrading electrons across the finite velocity bins of their numerical grid. They also corrected subtle errors in the original formulation, noting that some of the factors in Stubbe’s published equations carried incorrect units. Starting from a Maxwellian guess, their iterative scheme converges, with a damping factor applied for stability, within about sixty iterations under typical ionospheric conditions. Neutral densities come from standard atmospheric models, the neutral temperature is set to 200 kelvin, and the electron density is assumed to be ten billion per cubic meter, values representative of the quiet D region.

To make sure the kinetic solver was trustworthy, the authors built an entirely independent check: a Monte Carlo simulation that follows one million individual electrons as they are accelerated by the oscillating electric field of the heating wave between collisions. Collision times are drawn from exponential random distributions, scattering is assumed isotropic, a good approximation for electrons below about 5 electron volts, and each collision is classified as elastic or inelastic according to the best available laboratory cross sections. The simulation captures the full time evolution of the electron population, from a cold 300-kelvin Maxwellian start to a steady state in which the energy pumped in by the radio wave exactly balances the energy lost to the neutral gas. That steady state arrives remarkably quickly at lower altitudes, in about three milliseconds at 80 kilometers, but takes as long as ten seconds at 120 kilometers, where collisions are rarer and the electrons take longer to settle.

The agreement between the two methods is convincing at most altitudes. At 80 and 100 kilometers, the Boltzmann solver and the Monte Carlo simulation produce energy distributions that match well above 0.1 electron volts, with second-moment temperatures of roughly 1750 and 1230 kelvin respectively in the kinetic calculation. Below 0.1 electron volts the two methods diverge somewhat, a discrepancy the authors attribute to the approximate analytical expressions used for rotational excitation and elastic collisions in the Boltzmann approach, approximations that matter most where the neutral density is highest. At 120 kilometers a more serious problem emerges: the electron-neutral collision frequency drops below the electron gyro-frequency, violating the solver’s assumption that electrons are unmagnetized. There the Boltzmann method overestimates the temperature significantly, predicting 350 kelvin where the Monte Carlo simulation, which fully accounts for the magnetic field, finds only 219 kelvin. The lesson is clear: the kinetic solver is a superb tool for the D region, but it cannot simply be extended upward into the magnetized E and F regions without modification.

Perhaps the most provocative result concerns temperature itself. The authors show that even when a temperature can be defined for the heated electrons, by taking the second moment of the distribution as a measure of total thermal energy, that single number conceals more than it reveals. At 90 kilometers with an electric field of 2.0 volts per meter, the second-moment temperature is a scorching 5124 kelvin, yet the actual distribution bears little resemblance to a Maxwellian at that temperature. An alternative measure, an energy-dependent effective temperature derived from the local slope of the distribution, swings wildly across the energy spectrum. It peaks near 10,000 kelvin around 1.1 electron volts, where electrons degraded from higher energies pile up in a relatively flat region of phase space, then plunges in the 1.8 to 2.7 electron volt range where nitrogen vibrational excitation drains the population, and finally settles toward the neutral gas temperature at the lowest energies. Undulations between 0.1 and 1.2 electron volts trace the resonance peaks in the vibrational cross sections of molecular oxygen.

These findings matter far beyond the niche community of ionospheric heating experiments. Maxwellian assumptions are baked into standard calculations of electron cooling rates, radio wave absorption, refractive indices, and D region chemistry. If the true distribution deviates sharply from Maxwellian whenever electrons are heated above roughly 500 to 600 kelvin, then every one of those derived quantities may need revisiting. The shape of the non-Maxwellian distribution is not fixed either: it changes with altitude and with the strength of the heating field, growing more distorted as more energy is injected. At 70 kilometers, where frequent collisions keep electrons from gaining much energy between impacts, the distribution stays relatively close to Maxwellian even at strong fields, with temperatures rising only from about 400 to 1600 kelvin as the electric field increases from 0.75 to 4.0 volts per meter. Higher up, at 90 to 100 kilometers, the same field range drives temperatures from a few hundred kelvin to between 5000 and 6000 kelvin, and the deviations become dramatic.

This paper is the first of a pair. In the companion study, the authors compute electron cooling rates for vibrational excitation of nitrogen and oxygen and for the excitation of fine structure levels in atomic oxygen, using the non-Maxwellian distributions rather than Maxwellian approximations. Together, the two papers promise to sharpen the tools scientists use to interpret experiments at facilities such as the EISCAT heating facility in Norway, where radio-induced electron heating has been measured in situ since the year 2000. They also serve as a reminder of a deeper truth in physics: temperature is a bookkeeping device, not a description. In the thin, collision-dominated air of the upper atmosphere, the electrons tell a richer story, one that can only be read by solving the kinetic equations in full, one collision at a time.

Subject of Research: Non-Maxwellian electron energy distributions in the ionospheric D region under artificial high-frequency radio wave heating

Article Title: Non-Maxwellian electron distributions in the D region during artificial heating – Part 1: Model development and electron temperature

Article References: Myrvang, M., & Gustavsson, B. (2026). Non-Maxwellian electron distributions in the D region during artificial heating – Part 1: Model development and electron temperature. Annales Geophysicae, 44(2), 921-935. https://doi.org/10.5194/angeo-44-921-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-921-2026

Keywords: ionosphere, D region, artificial heating, electron temperature, non-Maxwellian distribution, Boltzmann equation, Monte Carlo simulation, HF radio waves, vibrational excitation, molecular nitrogen, electron cooling, space physics

The instrument in question is the Jovian Plasma Dynamics and Composition analyzer, or JDC, one of six sensors that make up the Particle Environment Package on Juice. JDC is designed to measure positive ions, negative ions and electrons over an energy range from 1 electron volt per charge up to 35 kiloelectron volts per charge. Its most demanding assignment will be to detect the tenuous, extremely cold ionospheres of Jupiter’s icy moons, Ganymede, Europa and Callisto. These ion populations carry energies of only a few electron volts, which makes them notoriously difficult to observe, and the success of that effort depends on the instrument performing flawlessly at the very bottom of its energy range.

Measuring such cold ions is complicated by the spacecraft itself. In space, a vehicle accumulates electric charge through a balance of currents: sunlight knocks photoelectrons off its surfaces and drives it positive, while the ambient plasma electrons, being far more mobile than the ions, tend to drive it negative. In dense plasma regions, the electron current wins and the spacecraft charges negatively. A charged spacecraft repels ions of the same sign, preventing them from ever reaching the detectors. The outflow of cold hydrogen ions from near-Earth space, for instance, went undiscovered for decades precisely because visiting spacecraft were positively charged, and it was finally revealed only through indirect methods. Conversely, a negatively charged spacecraft can attract positive ions and make the lowest energies observable, but it distorts their trajectories and energies in ways that must be carefully modelled and corrected.

During the lunar-Earth gravity assist on 19 and 20 August 2024, Juice passed within two Earth radii of the planet’s centre, cutting straight through the plasmasphere, a cold, dense cloud of ionised upper atmosphere that co-rotates with Earth. This region, filled with hydrogen, helium, oxygen and nitrogen ions at temperatures below one electron volt, resembles in many respects the ionospheres that JDC will encounter at Ganymede and Europa. As the spacecraft cruised through, JDC recorded energy-time spectrograms of the positive ions around it, and the team immediately noticed something odd. The signal showed artificial banding at different energies, and the positions of the peaks did not match the ratios expected for known ion species such as hydrogen, helium and oxygen.

The culprit turned out to be the way JDC sweeps through its energy range. To cover the spectrum efficiently, the instrument uses a triangular voltage sweep on its electrostatic analyzer, stepping upward through the energies and then downward, with the two halves interleaved so that gaps in coverage are filled in and mid-range energies are sampled twice per sweep, doubling the effective time resolution. The design is clever, but the flyby data showed that the upward and downward halves were not interleaving properly. The intended voltages were not being realised in practice. The energy sweep pattern, it emerged, pushed the instrument’s high-voltage power supply outside its engineering specification, and the supply could not settle fast enough at each new step, particularly when dropping from high voltages to near zero.

Fortunately, the team had a way to check. The flight model of JDC is still on the ground; it was replaced on the spacecraft by the nearly identical flight spare after late-discovered electrical grounding issues. The researchers partially disassembled the laboratory model, soldered a dedicated resistive low-capacitance voltage divider directly onto the output of the high-voltage power supply, and recorded the actual voltage waveform during a sweep with a high-resolution oscilloscope inside a vacuum tank. Averaging more than a thousand individual recordings pushed the effective dynamic range beyond 16 bits, enough to capture the entire sweep in a single measurement. The traces revealed clear voltage spikes and, more importantly, a systematic lag: during many energy steps, especially the first ten of the upward sweep, the voltage never reached its intended value before the measurement window closed.

Armed with these laboratory measurements, the team reconstructed the true energies of every step in the flyby data. The correction was made purely from the ground-based voltage measurements, without any reference to the independently measured spacecraft potential, and when the corrected spectrogram was compared with the potential estimate from Juice’s Radio and Plasma Wave Investigation, the agreement was striking. The artificial banding vanished, the ion signal now varied smoothly with the spacecraft potential exactly as a cold plasma should, and the upward and downward halves of the sweep became mutually consistent. The reconstructed energies carry an uncertainty of roughly half an electron volt, dominated by the voltage measurement precision and by small differences between the laboratory model and the flight spare.

To understand what JDC was actually seeing, the team also built a simple simulation of a negatively charged spacecraft ramming through a cold, corotating oxygen plasma. The model showed that when the spacecraft potential is sufficiently negative, it overcomes the ram velocity of about seven kilometres per second and attracts ions toward the spacecraft from all directions, including from behind, where JDC’s field of view is pointed. This explains why the observed ion energies fell below the magnitude of the spacecraft potential, and why the spectrum showed gaps at the lowest energies for part of the flyby. The simulated spectra reproduced the key features of the corrected observations, confirming that the instrument was behaving as physics dictates once the energy scale was fixed.

Correcting future data would have been possible, but it would have been a wasteful compromise: some energy steps would have to be discarded, the spacing between steps would be irregular, and coverage below 10 electron volts would remain sparse. Instead, the team designed three entirely new energy sweep tables, validated each one on the laboratory model, and uploaded them during a general software update of the Particle Environment Package in February 2026. The new sweeps abandon the triangular interleaving in favour of a long, slow descent through the energies, with three sacrificial steps used to raise the voltage rapidly to kilovolt levels before the measurement begins. The result is a stable voltage during every step and an even distribution of energy bins across the range. One table covers the full 1 electron volt to 35 kiloelectron volt span, a second trades some high-energy coverage for reduced power consumption, and a third is dedicated to ionospheric work, densely sampling just 1 to 500 electron volts, the range suggested by Juno’s 2021 flyby of Ganymede as ideal for studying the moons’ ionospheres.

The episode is a powerful argument for in-flight calibration. Calibrating particle detectors at energies of a few electron volts on the ground is extremely difficult because stable, intense ion beams at such low energies are hard to produce, so JDC’s response at the bottom of its range had only ever been extrapolated. Planetary flybys offer something no laboratory can: access to multiple cold particle populations, and, when the spacecraft potential varies with time, a natural calibration source spanning the lowest energies. The lessons learned from Earth’s plasmasphere, combined with a twin instrument in a European laboratory, have turned a potential scientific disappointment into a success story. When Juice finally reaches Ganymede, the only moon in the solar system with its own magnetic field, its plasma analyzer will be ready to capture the faint, cold ions escaping from the icy surface, with an energy scale it can truly trust.

Subject of Research: In-flight calibration of the Juice spacecraft's JDC plasma analyzer using the 2024 Earth plasmasphere flyby to optimize low-energy ion measurements at Jupiter's icy moons

Article Title: Flying through the plasmasphere to optimize low-energy ion measurements

Article References: Stenberg Wieser, G., Wieser, M., Barabash, S., Wittmann, P., Kalla, L., Fränz, M., Roussos, E., Vorburger, A., Wurz, P., Wahlund, J.-E., Brandt, P. C., Futaana, Y., Shimoyama, M., Pontoni, A., Galli, A., Riedo, A., Ho, G., Mitchell, D. G., Clark, G., … Baláž, J. (2026). Flying through the plasmasphere to optimize low-energy ion measurements. Annales Geophysicae, 44(2), 937-948. https://doi.org/10.5194/angeo-44-937-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-937-2026

Keywords: Juice mission, JDC analyzer, plasmasphere, low-energy ions, spacecraft charging, Earth flyby, Ganymede, ionosphere, electrostatic analyzer, in-flight calibration, Jupiter icy moons, space plasma instrumentation