Alfvén wings form when a magnetized obstacle, such as a planet with an intrinsic magnetic field, sits embedded within a flowing plasma. The interaction perturbs both the plasma velocity and the ambient magnetic field, launching Alfvénic disturbances that race along magnetic field lines while being simultaneously carried downstream by the plasma flow. Whether these disturbances can outrun the advection depends on a single dimensionless quantity: the Alfvén Mach number, the ratio of the flow speed to the local Alfvén speed. When the Mach number drops below unity, disturbances propagate along the field faster than the plasma can sweep them away, and the combination of Alfvénic propagation and plasma advection produces two stationary, wing-like conduits that channel energy and momentum between the planet and its surroundings.

At Earth, the solar wind is normally strongly super-Alfvénic, with Alfvén Mach numbers around 5 to 8, meaning the flow speed vastly exceeds the Alfvén speed. But during transient solar events such as coronal mass ejections, the Mach number can plummet. Spacecraft have caught Earth in exactly this state twice: during a sub-Alfvénic interval in May 2002 and again during the April 2023 coronal mass ejection, when a magnetic cloud imposed conditions under which our planet’s magnetosphere morphed into a wing-dominated configuration. Similar wing structures have been observed at Io and Ganymede within Jupiter’s magnetosphere, and at Mercury during a sub-Alfvénic interplanetary coronal mass ejection event. For planets orbiting magnetically active stars, especially close-in exoplanets, such conditions may be routine rather than exceptional.

The new study, led by Sakshi Gupta, Arnab Basak, and Dibyendu Nandy of the Center of Excellence in Space Sciences India at IISER Kolkata, employed the Star-Planet Interaction Module built on the PLUTO magnetohydrodynamic solver to simulate an Earth-like planet with realistic mass, radius, and magnetic tilt of 11 degrees. The team varied three parameters independently: stellar wind speed, stellar wind magnetic field strength from 10 to 50 nanotesla, and planetary dipole field strength at half, one, and twice the Earth’s surface equatorial value. Ninety simulations covering all combinations of these parameters revealed how the global magnetospheric configuration responds to the upstream Alfvén Mach number. Crucially, the prescribed upstream flow remains super-Alfvénic in all runs; the wings arise because the plasma decelerates and the magnetic field piles up as it approaches the planetary obstacle, so the local Mach number falls below unity in the near-planet wake.

The simulations show that increasing stellar wind speed leads to a systematic closure and narrowing of the Alfvén wings. At the lowest wind speeds, broad, well-developed wings extend above and below the ecliptic plane with a large opening angle, forming two nearly symmetric current-carrying structures that connect the planetary obstacle to the ambient plasma. Under these conditions, perturbations excited at the planet propagate preferentially along the magnetic field direction, efficiently channeling energy into the wings and inflating them laterally. At the highest wind speeds, however, the wings cannot be distinguished separately: they come close together with negligible opening angle, straighten out, and become almost parallel to the ecliptic plane. The plasma flow velocity is now strong enough to generate ample ram pressure that forces the wings down and aligns the extended magnetosphere along the flow direction.

Stronger stellar magnetic fields, by contrast, facilitate the opening of the wings. A higher stellar field indicates a larger local Alfvén speed, so the interaction remains sub-Alfvénic over a wider region and a wider range of wind speeds. The stronger stellar field pulls the wings outward away from the ecliptic plane through a stronger Lorentz force acting on the plasma and field lines in the magnetotail region, increasing the wing opening angle. A stronger planetary dipole, however, acts in the opposite direction: it creates a more rigid obstacle that resists lateral inflation of the wings, forcing them to remain closer to the ecliptic plane and requiring stronger wind forcing to open them to comparable extents. The relative strength of these three parameters, wind speed, stellar field, and planetary field, therefore determines the steady-state morphology of the wings and the entire planetary environment.

To validate the physics captured by the simulations, the team compared the measured wing opening angles with the analytical prediction of Neubauer from 1980, which assumes a point-source conducting obstacle in a uniform magnetized plasma flow. The simulated angles follow a clear positive correlation with the analytical values across all parameter combinations, confirming that the Alfvén Mach number is the primary controlling parameter for wing geometry. Some simulated angles exceed the prediction because the Neubauer formulation treats the obstacle as a point conductor with zero spatial extent, whereas the simulated planetary magnetosphere acts as a finite, inflated obstacle that deflects the incoming stellar wind over a cross-section significantly larger than the planetary radius, broadening the interaction region and forming wing current sheets at larger angular separations.

Inside the wings themselves, the simulations reveal regions of intense dynamical interaction rather than mere geometric curiosities. Analyzing plasma properties along a fixed downstream location in the night-side magnetotail, the team found a marked reduction in flow speed within the wings compared to the ambient medium, meaning the flow decelerates inside the Alfvén wings. The corresponding magnetic field magnitude profiles show that the field strength inside the wings is consistently higher than the background, a pile-up caused by the clustering of planetary field lines reconnected with the stellar field in both the upper and lower parts of the magnetotail. This enhanced magnetic pressure opposes the incoming plasma flow, leading to the significant velocity reduction. Faster incoming winds advect the magnetotail and compress the magnetic flux more, producing larger field magnitudes inside the wings, while stronger stellar fields produce wider wings.

The study also uncovered a clear interdependence between the day-side magnetopause stand-off distance and the night-side magnetotail current sheet length. As wind speed increases, ram pressure compresses the day-side magnetosphere, lowering the stand-off distance, while the higher wind speed advects and stretches the night-side magnetotail, resulting in a longer current sheet. The team identified a linear scaling between the magnetotail current sheet length and a dimensionless composite forcing parameter that measures the ratio of the wind’s dynamic-pressure driving to its magnetic confinement, with coefficients of determination as high as 0.985. This linearity holds across the full range of regimes explored, suggesting that magnetotail stretching behaves predictably in response to the kinetic driving and magnetic tension of the stellar wind, with the slope determined by the planet’s intrinsic magnetic field.

By bridging solar system observations and exoplanetary systems, the study demonstrates how Earth-like magnetospheres might transform into wing-dominated configurations during extreme stellar events or within the sub-Alfvénic regimes of close-in exoplanets. Young, magnetically active stars produce winds that can be orders of magnitude denser and more magnetized than the present-day solar wind at Earth, and highly active M dwarfs can expose their planets to sustained intervals of low Alfvén Mach number, reshaping planetary magnetospheres and modulating star-planet interactions. The findings can aid the interpretation of Alfvén wing signatures in observational data, from auroral radio emissions of M-dwarf planet hosts to star-planet magnetic interactions, and enhance our understanding of how planetary magnetospheres, the dynamic shields that regulate the transfer of energy, mass, and momentum between stars and planets, respond to the diverse and often violent stellar winds that sweep through the cosmos.

Subject of Research: Space Physics

Article Title: Stellar wind driven Alfvén wing dynamics in planetary and exoplanetary magnetospheres

Article References: Gupta, S., Basak, A., & Nandy, D. (2026). Stellar wind driven Alfvén wing dynamics in planetary and exoplanetary magnetospheres. Frontiers in Astronomy and Space Sciences, 13, Article 1909950. https://doi.org/10.3389/fspas.2026.1909950

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1909950

Keywords: Alfvén wings, magnetosphere, stellar wind, exoplanets, space weather, magnetohydrodynamics, star-planet interaction, Stellar, wind, driven, Alfv, wing

The study, published in Frontiers in Astronomy and Space Sciences, dissects every negative excursion of the SYM-H index, a one-minute-resolution measure of the low-latitude magnetic disturbance, between January 1981 and May 2026. That record spans nearly five complete solar cycles and contains 78,096 separate disturbance events. Rather than measuring each excursion from threshold crossing to recovery as one undivided duration, the researcher split every event at its minimum: the growth lifetime runs from the downward crossing of a threshold to the deepest point of the disturbance, while the recovery lifetime runs from that minimum back up to the recovery crossing.

The split matters because the two halves of a storm are governed by different physics. Growth is driven from outside, as enhanced solar-wind convection injects energetic ions into the ring current and depresses the equatorial magnetic field. Recovery is controlled from inside, as charge exchange, Coulomb collisions, wave-particle scattering, and dayside outflow slowly drain the trapped population. The minimum of the excursion is the single instant at which accumulation gives way to loss, making it the natural dividing line between the two regimes.

When the two duration ensembles were fitted with a finite-range power law — a distribution whose slope is set by an exponent and whose reach is set by a temporal cutoff — the two channels turned out to be strikingly different. Growth lifetimes follow an exponent of 1.079 with a cutoff near 669 minutes, while recovery lifetimes follow a steeper exponent of 1.305 with a cutoff near 3,328 minutes, roughly five times longer. The separation between the two exponents, about 0.226, holds at every threshold tested, from quiet-time ripples at the median disturbance level all the way up to superstorm amplitudes near minus 300 nanotesla.

The asymmetry is not subtle. In the archetypal superstorm of September 1989, the field spent about 10 hours descending to its minimum and more than 63 hours climbing back — a factor of six in a single event. Across all 89 one-year windows in the record, the recovery exponent exceeded the growth exponent in every single window, a consistency that survives even when the analysis is restricted to the 45 non-overlapping windows that share no data.

The most surprising result emerges when the exponents are tracked against solar activity. The growth exponent rises significantly with sunspot number, with a correlation coefficient of about +0.56 in independent windows and a shift of roughly +0.10 from solar minimum to solar maximum. The recovery exponent, by contrast, is statistically flat across all five cycles, with any potential solar-cycle modulation bounded to roughly a tenth of the growth response. The solar cycle therefore imprints itself selectively on disturbance timing: it touches the growth slope and the temporal cutoffs of both channels, but leaves the recovery exponent untouched.

This decoupling resolves a long-standing puzzle. Earlier work had shown that the undivided burst lifetime of SYM-H follows a single power law that is robust to the choice of threshold, which seemed to suggest a unified underlying process. The new analysis shows that this apparent invariance is an emergent property. Because the recovery cutoff is roughly five times the growth cutoff, the tail of the combined burst distribution is dominated by configurations in which nearly the entire burst is recovery. The burst exponent is therefore pinned by the recovery channel, whose exponent is threshold-stable and cycle-flat, while the body of the distribution carries the growth signature. A single power-law fit to the composite returns an effective exponent that falls between the two channel values, reproducing the earlier result without requiring a single physical process.

Peeling back another layer, the recovery channel itself decomposes into a dwell — the contiguous run of samples tied at the minimum value — and a climb back to threshold. The dwell is steep, short-scaled, and flat across the solar cycle, while the climb transit carries a cycle-responsive exponent that mirrors the growth side. The composite recovery exponent is a mixture of these two sub-processes, and its flatness is a consequence of the standard storm-timing convention that assigns the cycle-flat dwell to the recovery channel.

The findings carry practical weight for space weather forecasting. Recovery-timing priors can be treated as cycle-independent provided their range is conditioned on solar phase, while growth-timing priors must be conditioned on cycle phase throughout. For modelers, the decoupling sets a falsifiable target: any stochastic description of SYM-H excursions must now produce two distinct exponents from one time series, route the solar-cycle imprint through the growth channel alone, and reproduce the symmetric cycle response of the underlying transit statistics.

The analysis applies specifically to low-to-mid-latitude disturbances, where the SYM-H index is most representative. Higher-latitude geomagnetic activity is known to follow different and more variable scaling behavior, shaped by complicated ionospheric effects that dominate closer to the poles. Extending the two-channel decomposition to those regions remains an open challenge for future work.

Subject of Research: Statistical scaling of geomagnetic storm growth and recovery timing across five solar cycles

Article Title: Growth–recovery decoupling in the timing of geomagnetic disturbances across five solar cycles

Article References: Wanliss, J. A. (2026). Growth–recovery decoupling in the timing of geomagnetic disturbances across five solar cycles. Frontiers in Astronomy and Space Sciences, 13, Article 1948617. https://doi.org/10.3389/fspas.2026.1948617

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1948617

Keywords: geomagnetic storms, SYM-H index, power-law distributions, solar cycle, ring current, space weather, magnetosphere, burst-lifetime statistics, Dst index, solar wind, Growth, recovery

The plasmasphere is a torus of cold plasma, roughly 1 electron volt in temperature, with densities between about 100 and 10,000 particles per cubic centimeter, extending from the top of the ionosphere out to between 2 and 6 Earth radii in the equatorial plane. It is fed by plasma flowing upward from the ionosphere, and during quiet geomagnetic conditions it co-rotates with Earth and swells to its largest extent. When a solar storm strikes, however, the enhanced convection electric field peels away the outer layers in a process called plasmasphere erosion. Plasma on open drift paths escapes sunward in a flow channel known as the drainage plume, and the plasmapause, the sharp boundary where density drops by more than a factor of five over less than half an Earth radius, contracts earthward.

Why does this matter beyond pure curiosity? The plasmasphere acts as a master switchboard for the radiation belts. Its density distribution controls where very low frequency and ultra low frequency waves can exist in the magnetosphere, and those waves in turn govern the acceleration and loss of energetic electrons. Whistler-mode hiss waves, which drain electrons from the belts, live primarily inside the plasmasphere and in dense plume regions, while chorus waves, which pump electrons up to relativistic energies, thrive outside it. The plasmapause and plume geometry also form resonance cavities for ULF waves, shaping where drift-bounce resonance can energize particles. In short, mapping the plasmasphere’s fine structure is essential for forecasting the radiation environment that threatens satellites, astronauts, and power grids.

Existing techniques each fall short in a different way. In situ satellites make single-point measurements along their trajectories, but a typical inner-magnetospheric orbit lasts about 8 hours while the plasmasphere erodes in only 3 to 4 hours, so each density profile is badly smeared in time. Extreme ultraviolet imagers such as the camera on NASA’s IMAGE satellite could map line-of-sight integrated density at a 10-minute cadence, but they detect only minor ions like helium and oxygen, not the dominant hydrogen, and they require assumptions to deconvolve a three-dimensional region from two-dimensional projections. Radio tomography sidesteps both problems by measuring total electron content directly along many crossing paths simultaneously.

The principle resembles a hospital CT scan. Each spacecraft transmits a pair of phased radio signals at two different frequencies, a primary and a reference, and every other spacecraft in the constellation records the phase difference between them. From that phase difference, the total electron content along the line of sight between the two spacecraft can be computed. With enough crossing ray paths, a tomographic inversion reconstructs a two-dimensional map of plasma density in the orbital plane. The PlaTo team drew on earlier concepts, including the more ambitious PILOT mission concept developed for the 2024 Heliophysics Decadal Survey, but deliberately narrowed the scope to a focused, cost-effective constellation.

The team distilled the mission’s purpose into three science questions. First, what causes the lumpiness in the drainage plume? Density structures in the plume last about 2 hours at geosynchronous distance, flow at roughly 12 kilometers per second, and can modulate whistler-mode wave intensity and drive localized electron precipitation into the atmosphere. Candidate mechanisms include velocity shear between plume flow and the co-rotating plasmasphere, turbulence, ionospheric coupling timescales, and electric field structuring. Second, does the interchange instability operate at Earth’s plasmapause? This process, known to drive radial plasma transport at Jupiter and Saturn, would amplify density gradients at the plasmapause on timescales of roughly 17 minutes for structures near 0.39 Earth radii in size. Third, does the refilling rate of eroded flux tubes stay constant or change over time, as some observations at geosynchronous orbit suggest it does by a factor of two or more?

Those questions translate into demanding measurement requirements. The mission must resolve density changes as small as 3 to 10 particles per cubic centimeter, at spatial scales of about 0.34 Earth radii, on cadences of 5 to 15 minutes, across an observation region spanning roughly 3 to 5.5 Earth radii and several hours of magnetic local time. A back-of-envelope calculation balancing the number of map pixels against the number of possible ray paths suggested about 11 spacecraft would suffice, so the team padded the estimate to 16 and then ran simulated two-year missions using Ansys Government Initiatives’ Systems Tool Kit software, counting how many usable science observations each constellation size would collect against real historical geomagnetic activity indices.

The simulation results settled on 14 spacecraft as the sweet spot. Ten satellites would fly on an outer orbit with altitudes from 300 to 28,702 kilometers, an apogee near 5.5 Earth radii, spaced 5 degrees apart around apogee, while six would occupy an inner orbit from 5,684 to 11,481 kilometers, near 2.8 Earth radii, spaced 9 degrees apart. Both orbits sit nearly in the equatorial plane with inclinations below 15 degrees, and their perigees are chosen so the lines of apsides precess together. Twelve spacecraft could, on average, meet the minimum science goals, but reducing the expected observations by one standard deviation leaves only about 8 plume events, whereas the 14-spacecraft constellation delivers sufficient observations for all three science questions even with margin for losing up to two spacecraft. Reliability analysis based on decades of satellite lifetime data shows the constellation-level reliability remains above 0.999 for these configurations.

The radio payload design involved a careful balancing act between two error sources. Lower frequencies measure smaller density variations more sensitively, but sharp density gradients at the plasmapause, where density can change by about 100 particles per cubic centimeter over a few tenths of an Earth radius, refract the signals and bend them away from the straight paths the inversion assumes. Higher frequencies reduce refraction error but suffer more from background radio noise, dominated by the Sun, whose spectral irradiance near 100 megahertz reaches about 10 to the minus 18 watts per square meter per hertz under active conditions. Minimizing the combined error pointed to broadcast frequencies near 200 and 600 megahertz, with each spacecraft transmitting at 50 watts for 10 seconds in a ping cycle that completes across all 14 spacecraft every 5 minutes. Even in a worst-case refraction scenario with a ray striking a density gradient at 84 degrees from the normal, the resulting TEC error would be only about 0.02 percent.

The PlaTo concept remains provisional, and the authors lay out a validation path that includes flying the proposed constellation through a high-fidelity magnetosphere model, testing tomographic reconstruction techniques such as the multiplicative algebraic reconstruction method, and running observing system simulation experiments with data assimilation against a truth model. Beyond the three headline questions, a working tomographic constellation would also resolve mysterious mesoscale features such as the dawnside shoulder, the nightside notch, and plasmaspheric fingers, whose formation mechanisms remain unexplained. If the concept advances toward flight, it would mark the first time humanity watches its own planet’s inner plasma shell the way radiologists watch the human body: continuously, in cross-section, and in motion.

Subject of Research: A proposed 14-spacecraft radio tomography constellation to image the dynamics of Earth's plasmasphere.

Article Title: The plasmasphere tomography (PlaTo) mission concept

Article References: Thaller, S., Frank, W., Malaspina, D., Andersson, L., & Ergun, R. E. (2026). The plasmasphere tomography (PlaTo) mission concept. Frontiers in Astronomy and Space Sciences, 13, Article 1809242. https://doi.org/10.3389/fspas.2026.1809242

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1809242

Keywords: plasmasphere, radio tomography, magnetosphere, space weather, radiation belts, geomagnetic storms, plasmapause, drainage plume, interchange instability, satellite constellation, mission concept, heliophysics

The study, authored by Academy Research Fellow Lauri Holappa and colleagues, tackles a long-standing puzzle in space physics. Where the fast-moving solar wind streams past the boundary of the magnetosphere, it generates enormous rolling vortices known as Kelvin–Helmholtz waves, the same instability that ruffles wind-blown water surfaces and shapes striking, breaking-wave patterns in clouds. These waves are not decorative curiosities. They are one of the principal mechanisms by which solar wind plasma, momentum and energy can be transferred across a boundary that would otherwise be difficult to penetrate, feeding the dynamics that ultimately light up auroras and disturb technological systems on the ground and in orbit.

What has made this boundary so difficult to study is its sheer inaccessibility. Satellites crossing the magnetopause pass through the most wave-active regions only rarely, and each crossing captures just a fleeting slice of a vast, three-dimensional interface. As a result, researchers have accumulated tantalizing but fragmentary evidence that the waves behave differently on different sides of the magnetosphere, without a clear physical explanation of why. Holappa’s team took a different route: rather than relying on sparse in-situ measurements, they used global computer simulations of the entire magnetosphere, allowing them to watch wave growth evolve everywhere along the boundary simultaneously under controlled solar wind conditions.

The simulations revealed that the key to the asymmetry is the draping of the interplanetary magnetic field, the magnetic field carried outward from the Sun by the solar wind. When this field encounters Earth’s magnetosphere, it cannot pass through; instead it piles up and drapes around the obstacle, much as water flows around a rock in a stream. Crucially, that draping is not symmetric. The orientation of the solar wind’s magnetic field relative to Earth’s own field determines where field lines bend most sharply and where they align most closely with the magnetopause, and this geometry varies from one side of the planet to the other and between the two hemispheres.

According to the study, the draped interplanetary magnetic field acts as a damping agent on the Kelvin–Helmholtz waves, but the damping is unevenly distributed. Where the draped field lines are oriented in a way that suppresses the shear-driven instability, wave growth is choked back; where the geometry is more permissive, the waves flourish. The result is a magnetopause that is far from uniform: wave activity is strong in some sectors and weak in others, following patterns dictated by the direction of the solar wind’s magnetic field and by the seasonal tilt of Earth’s dipole axis relative to the solar wind flow.

One of the most striking findings concerns hemispheric asymmetry. The strongest wave activity was found to develop on the winter hemisphere, the half of the planet tilted away from the Sun. Earth’s magnetic axis is inclined relative to its rotational axis, and as the planet orbits the Sun, the orientation of the magnetosphere with respect to the incoming solar wind shifts through the seasons. This changing geometry, described in the study in terms of the dipole tilt, alters how the draped solar wind field meets the magnetopause in each hemisphere, and under conditions of negative dipole tilt it tips the balance decisively toward the winter side.

The study also resolved a dawn–dusk asymmetry that has intrigued researchers for years. Depending on the direction of the east–west component of the interplanetary magnetic field, the enhanced wave activity concentrates either on the dawn side of the magnetosphere or on the dusk side. In other words, the solar wind’s magnetic field acts something like a switch, selecting which flank of Earth’s magnetic shield bears the brunt of the wave-driven pressure. Holappa described the finding as the culmination of a long process of interpretation that began in 2020, when, during a visit to NASA as a postdoctoral researcher, he noticed unusual simulation images of the magnetosphere and later realized they showed Kelvin–Helmholtz waves behaving differently on different sides of the boundary as part of a single, coherent phenomenon.

Asymmetries in boundary wave motion had been observed before, but the new work goes a step further by demonstrating how they arise. By connecting the wave patterns directly to the draping geometry of the interplanetary magnetic field and the reconnection geometry at the magnetopause, the study turns what had been a set of empirical observations into a mechanistic explanation. This matters because Kelvin–Helmholtz waves are not merely a diagnostic of boundary conditions; they are a transport mechanism. Where the waves are strongest, the solar wind has an enhanced opportunity to find a way through the magnetic shield, carrying charged particles and energy into the inner magnetosphere.

The practical stakes are considerable. Space weather driven by solar activity can disturb satellites, degrade satellite positioning signals, disrupt high-frequency radio communications and induce currents in power grids, with the most severe effects following major solar storms. Improving the fidelity of space weather models requires knowing where, and under what conditions, the magnetopause leaks most readily. The new results provide modelers with a physically grounded rule of thumb: the distribution of wave-driven transport around the magnetopause depends predictably on the solar wind’s magnetic field direction and on Earth’s seasonal dipole tilt, and it should therefore be represented asymmetrically rather than averaged out.

The research also places Earth in a broader cosmic context. Kelvin–Helmholtz instabilities are ubiquitous in nature, appearing in the Sun’s atmosphere, at the boundaries of other planets’ magnetospheres, and even at the interface between the heliosphere and interstellar space. By showing how a draped magnetic field modulates wave growth along such a boundary, the study offers insights that may transfer to these other environments, where the same interplay of shear flow and magnetic orientation governs how energy crosses cosmic boundaries. For Earth, the image evoked by the researchers is a familiar one: waves meeting a breakwater, where the water does not move the same way on both sides. The new study shows that our planet’s magnetic breakwater is similarly uneven, and that the pattern of its hidden turbulence can now be anticipated from the direction of the wind that blows from the Sun.

Subject of Research: Solar wind magnetic field draping and Kelvin–Helmholtz wave asymmetries at Earth's magnetopause

Article Title: Space waves reveal where Earth’s magnetic shield is under pressure

Article References: Space waves reveal where Earth’s magnetic shield is under pressure. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: magnetosphere, Kelvin–Helmholtz waves, solar wind, magnetopause, space weather, interplanetary magnetic field, magnetic reconnection, dipole tilt, auroras, University of Oulu, Geophysical Research Letters, space plasma physics

The rapid brightness changes, which the team calls internal modulation, occur at frequencies of a few hertz and are embedded within the main pulsations that last from several seconds to several tens of seconds. This hierarchical structure is not merely a curiosity of optics. It is widely understood as the ionospheric fingerprint of whistler-mode chorus waves in the inner magnetosphere, which scatter electrons into Earth’s atmosphere. Chorus bursts lasting seconds drive the main pulsations, while individual chorus elements on subsecond timescales drive the internal modulation. A model proposed by Yoshizumi Miyoshi and colleagues in 2015, later confirmed by conjugate observations between the Arase satellite and ground-based instruments, links each level of auroral variation to a corresponding level of wave activity.

Despite this physical understanding, internal modulation has been notoriously difficult to study systematically. Its amplitude is modest; a classic 1977 study by Royrvik and Davis reported that the 3 hertz modulation could reach roughly 20 percent of the relative intensity. Many standard auroral cameras capture images no faster than once per second, far too slow to resolve a few-hertz flicker, while high-time-resolution photometers sacrifice spatial coverage. Moreover, high-speed imaging trades sampling rate against signal-to-noise ratio, and the sheer volume of data produced by rapid cameras demands automated analysis. These constraints left the occurrence statistics of internal modulation, particularly its dependence on magnetic local time and latitude, largely unexplored.

The new study overcame these barriers with a combination of cutting-edge hardware and machine learning. The team deployed ORCA-Quest quantitative CMOS cameras, operated at 20 frames per second, which comfortably satisfies the Nyquist criterion for the targeted modulation frequencies. Equipped with a BG3 glass filter that suppresses the strong forbidden emission lines at 557.7 and 630.0 nanometers while retaining sensitivity to nitrogen band emissions, the cameras improved the signal-to-noise ratio for broadband auroral light. Identical systems operated at Kiruna, Sweden, at roughly 65.1 degrees adjusted geomagnetic latitude, and Skibotn, Norway, at about 66.7 degrees, two sites separated by approximately 170 kilometers at nearly the same longitude, with non-overlapping fields of view projected to 100 kilometers altitude.

The analytical pipeline was equally sophisticated. A pre-screening step based on the Tromsø AI deep-learning classifier, which achieves an average precision of 93.1 percent, filtered color all-sky images for likely auroral activity. For each qualifying minute, the team generated a keogram, a time-space plot built by stacking pixel intensities along a fixed north-south slice of the 20 hertz image sequence. A hierarchical two-stage classifier, built on a ResNet-18 backbone, then first determined whether the keogram contained pulsating aurora and, if so, whether internal modulation was present. The training labels came from 12,429 manually annotated Kiruna keograms spanning 98 nights, of which 8,163 were labeled as containing pulsating aurora and 2,608 as containing internal modulation within it.

Performance testing confirmed the approach was viable. The pulsating aurora classifier achieved a mean test F1 score of 0.9506 across ten training runs, indicating highly stable detection. The internal modulation classifier, inherently harder because the phenomenon is subtle and its definition involves boundary judgments, reached a mean F1 of 0.7472, with precision of 0.6630 and recall of 0.8630. Importantly, a weighted re-evaluation of all 241 misclassifications, in which clear errors counted fully but borderline and label-questionable cases counted less, raised the effective F1 to 0.8290. This error audit showed that many apparent failures were cases near the definitional boundary rather than genuine model mistakes, giving the team confidence to apply the classifier to tens of thousands of unseen keograms.

The statistical results were striking. Combining manual labels and model inference, the fraction of pulsating aurora accompanied by internal modulation increased toward the morning sector of magnetic local time at both sites. More remarkably, when compared at the same local time, the lower-latitude site, Kiruna, consistently showed a higher internal modulation fraction than Skibotn. The difference was most pronounced between 04:00 and 06:00 magnetic local time, where central differences of 0.293, 0.390, and 0.221 were measured at 04:00, 05:00, and 06:00 respectively, and the strict uncertainty ranges excluded zero at the two earlier hours. The authors argue that this latitudinal separation, though only about 1.6 degrees, is large enough for the two sites to sample different regimes of the auroral oval, and that observational biases cannot fully account for the gap.

A detailed single-night case study from Kiruna on 20 to 21 November 2025 illustrated how these statistics play out in real events. After an auroral breakup around 21:40 universal time, the probability of internal modulation did not rise monotonically. It first appeared as modulation embedded within clear main pulsations, then receded as the main pulsation dominated, and finally re-emerged in a state where the rapid component itself became dominant without a distinct slower envelope. A visual review of the test set reinforced this pattern: the fraction of internally modulated events in which the main pulsation was indistinct was 36 percent on the midnight sector but 69 percent in the 04:00 to 06:00 morning window, suggesting that the hierarchical time structure of the underlying chorus waves itself changes with local time.

These patterns carry deep physical implications. The morning-sector enhancement suggests that the excitation conditions and temporal structure of chorus waves depend on magnetic local time. According to nonlinear wave growth theory, the threshold amplitude for chorus excitation depends on the density and temperature anisotropy of energetic electrons; substorm injections lower this threshold, while later decay of the hot-electron population raises it, potentially shifting chorus toward a sparser temporal regime in which internal modulation becomes more prominent. Additionally, some morning-sector cases showed low overall brightness and internal modulation dominance, consistent with precipitation of relatively high-energy electrons with small velocity dispersion, a scenario linked to chorus waves propagating off the magnetic equator, possibly aided by duct propagation along field lines.

The connection may extend even to relativistic electron microbursts, sudden bursts of energetic electrons that represent a significant loss process for the outer radiation belt. Previous work has shown that microbursts favor the morning sector and the L-shell range of roughly 4.5 to 6.0, closely matching the region where the internal modulation fraction peaked in this study. If internal modulation is, in some cases, the optical signature of the high-energy tail of pulsating aurora precipitation, its statistical occurrence could provide ground-based constraints on when and where energetic electron loss is favored. While the authors caution that optical data alone cannot uniquely identify the underlying plasma mechanisms, coordinated observations with low-Earth-orbit particle detectors could soon test this link. What began as an elusive flicker within the northern lights has become a quantifiable window into the wave-particle interactions that shape Earth’s space environment.

Subject of Research: Statistical occurrence of rapid internal modulation in pulsating aurora and its link to chorus wave-driven electron precipitation

Article Title: Statistical properties of internal modulation in pulsating aurora

Article References: Nanjo, S., Taki, T., Hosokawa, K., Brändström, U., Miyoshi, Y., & Johnsen, M. G. (2026). Statistical properties of internal modulation in pulsating aurora. Annales Geophysicae, 44(2), 1023-1042. https://doi.org/10.5194/angeo-44-1023-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-1023-2026

Keywords: pulsating aurora, internal modulation, chorus waves, machine learning, qCMOS camera, magnetosphere, electron precipitation, auroral oval, magnetic local time, space physics, whistler-mode waves, radiation belt

The Ap index, together with its close cousin Kp, was introduced by Julius Bartels in 1949 and 1957 and has since become a standard measure of the strength of magnetospheric convection, the vast circulation of plasma and magnetic flux driven by magnetic reconnection on the day and night sides of the magnetosphere. This convection penetrates deep into the inner magnetosphere, shaping electric fields that influence everything from satellite orbits to the ring current that circles the Earth. Because Ap is derived from three-hourly measurements, it captures a remarkably wide dynamic range, from quiet conditions below 20 nanotesla to disturbed values exceeding 60 or even 70 nanotesla during major storms.

Previous studies had already hinted that the Ap record contains rich periodic structure. Analyses of monthly Ap power spectra going back to 1868 have revealed a period of roughly four years associated with a double-peaked structure in geomagnetic activity, daily spectra dominated by harmonics of a six-month period, and additional signatures tied to the 27-day rotation of the Sun. Other researchers, applying singular spectrum analysis to the Dst index, which tracks the ring current more directly, identified significant components at 6 months and at 47, 22, 11, 10.6, 5.5, and 1 year. What the new study adds is a moving, time-resolved picture: instead of computing a single spectrum over an entire record, the team computed Fourier coefficients continuously, window by window, so that the spectral content of Ap could be watched as it evolves in step with geomagnetic activity.

The method is elegant in its simplicity. The authors treated the Ap index as a function of time and decomposed it into a Fourier sum with a fundamental period of 27 days, corresponding to one solar rotation. Because Ap is reported every three hours, each 27-day window contains 216 data points, and the team computed up to 108 harmonics within each window. Rather than relying on a fast Fourier transform of raw samples, they used linear interpolation between the three-hourly values and evaluated the Fourier integrals analytically over each three-hour segment, effectively summing the areas of trapezoids weighted by sines and cosines. This approach, they note, allows a Fourier series reconstruction that typically outperforms a straightforward Fourier transform, and it yields both amplitudes and phases for every harmonic at every moment in time.

The reconstruction itself is strikingly successful. For February 2001, a relatively quiet month with a maximum Ap of about 40 nanotesla and no abrupt peaks, the original data and the Fourier reconstruction are practically indistinguishable for most of the month. February 2003, by contrast, was a violently disturbed interval, with Ap peaking near 70 nanotesla and rapid transitions from smooth behavior to abrupt jumps; even here the reconstruction tracks the data closely, with only minor deviations. February 2017, a quiet period near solar minimum, rounds out the sample, and again the reconstructed curve hugs the observations. The message is that even a jagged, three-hourly geophysical index can be captured almost perfectly by a modest set of harmonics whose amplitudes and phases drift smoothly in time.

Plotting the harmonic amplitudes as contour maps, with time on the horizontal axis and harmonic number on the vertical axis, revealed a clear hierarchy: the strongest power sits at low frequencies, and amplitudes fall steadily as frequency rises. Among the low-frequency structures, the team identified pronounced lines at harmonic numbers 27 and 54, corresponding to diurnal and semidiurnal periods, the unmistakable fingerprints of atmospheric tides. These tidal signatures were especially sharp in the quiet February 2017 data and in February 2003, but notably weaker in the more active February 2001. The interpretation is intriguing: geomagnetic activity is strong enough to bury the magnetic signatures of atmospheric tides and planetary waves within the Ap time series, so the very disappearance of tidal lines becomes an indicator of a disturbed magnetosphere.

The phases of the harmonics told an equally disciplined story. After making the phases continuous and normalizing them by harmonic number, the researchers found that they decrease nearly monotonically, and almost linearly, with time. During quiet February 2017, most of these normalized phase curves are essentially straight lines, while deviations from linearity appear precisely during intervals of heightened geomagnetic activity. When the team computed the root mean square of the normalized phases in several frequency bands, they found that during the solar-maximum years 2001 and 2003 the lowest band, spanning harmonics 1 through 19, started near 90 degrees, whereas in quiet 2017 it started near 0 degrees, a systematic difference that tracks the phase of the solar cycle.

By constructing a complex counterpart of the Ap index from the amplitudes and phases, the team also plotted hodographs, the looping paths traced by the tip of a rotating complex vector. These loops turned out to encode the level of disturbance in a visually intuitive way. February 2001, at solar maximum, produced large, widely spread loops reflecting intense storm activity; February 2003 yielded more compact trajectories with moderate disturbances; and quiet February 2017 drew fewer, tighter loops spread uniformly over the complex plane. The authors suggest that such concentration patterns near the origin during disturbed periods, versus uniform spreading during quiet times, may be a general property worth exploring further.

The central surprise came when the team tested a natural hypothesis. They expected that during disturbed periods, higher-frequency harmonics would intensify, mirroring the abrupt, jagged changes in the Ap index. To quantify this, they computed the ratio of the root mean square amplitude of harmonics 15 through 20 to that of harmonics 5 through 10 for every time window. No one-to-one correspondence with geomagnetic activity emerged, but one event stood out dramatically: the substorm of 13 to 14 February 2001, in which a Dst decrease was followed by recovery and Ap climbed above 20 nanotesla. During that event, the ratio dropped sharply, meaning the higher harmonics were suppressed relative to the lower ones, the exact opposite of the anticipated behavior.

The authors are careful about what this means. They had hoped that abrupt changes in Ap might be predictable from the behavior of the Fourier coefficients before the changes occur, but the analysis did not support that hope. The suppression of high frequencies during the 2001 substorm might be a general feature of geomagnetic storms, a property of only some class of substorms, or simply a coincidence, and the team argues that it deserves dedicated future analysis. If high-frequency suppression does turn out to be typical of most storms, they note, it would carry a profound implication: the auroral index Ap would be governed mainly by processes intrinsic to the magnetosphere rather than by direct solar forcing. Either way, the study demonstrates that a familiar, half-century-old index still conceals structure that standard analyses have missed, and that watching its harmonics evolve in time offers a new lens on the dynamics of near-Earth space.

Subject of Research: Fourier spectral analysis of the planetary geomagnetic index Ap during disturbed and quiet periods

Article Title: Harmonic content of Ap index

Article References: Vandas, M., Romashets, E., Imam, T., Hasan, T., Majumder, P., & Karki, S. (2026). Harmonic content of Ap index. Annales Geophysicae, 44(2), 689-696. https://doi.org/10.5194/angeo-44-689-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-689-2026

Keywords: geomagnetic index, Ap index, Fourier analysis, space weather, magnetosphere, substorm, Dst index, solar cycle, atmospheric tides, geomagnetic storms, magnetospheric convection, spectral analysis

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 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 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 study, led by Sota Nanjo of the Swedish Institute of Space Physics in Kiruna, together with colleagues from Kyoto University, Nagoya University, and the Okinawa Institute of Science and Technology, relied on a scientific CMOS camera fitted with a diagonal fisheye lens at the Poker Flat Research Range in Alaska. On 8 February 2016, the system recorded 512-by-512-pixel images at a blistering 80 frames per second, covering a full 180-degree field of view. That combination of speed and breadth is what makes the new observations special. Earlier studies of flickering aurora almost always used narrow-field cameras, which resolve the tiny, kilometer-scale flickering patches in exquisite detail but lose all context of how the blinking is organized along an auroral arc that can stretch for hundreds of kilometers.

Flickering aurora was first identified in 1968 as a rapid, roughly 10-hertz fluctuation within bright auroral displays. The modulation is subtle — typically only 10 to 20 percent of the background brightness — and the blinking patches are small, just a few kilometers across. Decades of rocket, radar, and optical work have converged on a compelling explanation: the flickering is driven by electromagnetic ion cyclotron waves, or EMIC waves, generated in the auroral acceleration region at altitudes of roughly 4,000 to 8,000 kilometers. Sounding-rocket measurements have caught field-aligned bursts of electrons oscillating in step with the optical flickering, and the cyclotron frequencies of oxygen-ion EMIC waves at those altitudes match the observed blink rates remarkably well.

The exact mechanism remains debated. One influential model, proposed by Michael Temerin and colleagues in 1986, suggests that EMIC waves create an oscillating electric potential that resonantly accelerates low-energy electrons downward, modulating the electron flux at the wave frequency. A competing picture, advanced by J. P. McFadden and co-workers a year later, holds that the waves periodically perturb and release cold electrons trapped in the acceleration region, letting them fall through the auroral potential drop. More recent theoretical work by K. Sakanoi and colleagues, along with numerical simulations by Björn Gustavsson’s team, has emphasized that the complex spatiotemporal patterns of flickering arise from interference among multiple dispersive Alfvén or EMIC waves rather than from any single wave. The new wide-field observations speak directly to this interference-based framework.

During the Alaskan observing campaign, a faint east–west arc appeared around 06:20 Universal Time and intensified into a classic discrete auroral arc by about 06:30. From that point onward, patch-like blinking regions became detectable within the arc, and the display persisted for roughly an hour before fading as a bright bulge propagated westward. Applying fast Fourier transforms to the pixel-by-pixel brightness time series, the researchers found that flickering occurred intermittently across a frequency range of 3 to 20 hertz, with most of the power concentrated between 4 and 12 hertz — squarely within the band expected for oxygen-ion cyclotron waves generated several thousand kilometers up.

The most striking discovery came from mapping the dominant flickering frequency across the sky. Rather than flickering randomly, the arc organized itself into coherent clusters of similar periodicity, some extending more than 10 kilometers. In one particularly revealing second of data, a region flickering at around 8 hertz coexisted simultaneously with another region pulsing at about 13 hertz, separated by roughly 150 kilometers along the same arc. The team argues that such a large separation is difficult to reconcile with a single localized wave source feeding one resonance cone. Instead, multiple wave sources appear to be distributed along the arc, each shaped by its own local plasma environment — electron density, ion composition, and the field-aligned potentials known as inverted-V structures that power discrete aurora.

Remarkably, some of these frequency clusters moved together with the background arc. Comparing frequency maps separated by just four seconds, the researchers tracked a 7-to-8-hertz cluster drifting eastward in lockstep with the auroral form it accompanied. This co-migration suggests that the wave frequencies are not dictated simply by large-scale parameters such as magnetic latitude, but are tightly bound to the local plasma conditions and potential structures hosted by the arc at any given moment. If the dominant frequency reflects the local oxygen-ion cyclotron frequency, the 8- and 13-hertz bands would correspond to wave sources at altitudes of roughly 6,000 and 3,800 kilometers respectively — neighboring field lines hosting waves born at different heights.

To probe the relationship between patch size and blink rate, the team developed an automated detection scheme based on keograms, time-stacked cross sections of the images. Individual case studies initially hinted at a dispersion-like behavior: during stable intervals, larger patches tended to flicker more slowly, with 7-to-11-kilometer patches pulsing near 4 hertz while 3-to-5-kilometer patches some 20 kilometers away blinked near 14 hertz. But when the researchers compiled statistics on 23,704 detected patches over the full hour, the trend evaporated. The mean flickering frequency held steady at about 8 plus-or-minus 4 hertz regardless of patch size, and the typical apparent north–south patch dimension was 4.4 plus-or-minus 2.4 kilometers when projected to an assumed emission altitude of 110 kilometers. Robustness checks on viewing geometry, detection thresholds, and projection altitude confirmed the null result.

That statistical absence of a frequency–size relation is itself scientifically meaningful. If flickering were produced by a single wave mode, the dispersion relation of obliquely propagating EMIC waves should imprint a clean connection between frequency and perpendicular wavelength. The fact that it does not persist supports the interference picture, in which the optical scale reflects the standing-wave pattern formed by multiple superposed waves — a scale plausibly set by the electron inertial length in the acceleration region. Intriguingly, during moments when the arc was stable and a single frequency dominated, the observed frequency–size combinations did not conflict with the oxygen-EMIC dispersion relation, hinting that the patch scale could even serve as a diagnostic of the modulation altitude, mapping to electron inertial lengths of order 100 to 400 meters in the source region.

The observations also captured when flickering refuses to appear. As the arc narrowed and surged equatorward in the minutes before a substorm onset, the blinking vanished — even as the aurora brightened sharply. Meanwhile, a broader, latitudinally wider discrete aurora on the poleward side continued to flicker. The authors suggest that the growth of EMIC waves, driven by instabilities in the precipitating electron beam, depends sensitively on plasma parameters such as background electron density, which may have shifted unfavorably along the onset arc, while resonance conditions were maintained along the broader polar-side display. Because the study rests on optical imaging alone, without conjugate in-situ wave measurements, the EMIC interpretation remains a consistency argument rather than a direct wave identification. Still, by extending flickering aurora research from narrow keyholes to a mesoscale panorama, the work demonstrates that the visible shimmer of the northern lights encodes the microscopic physics of wave–particle interactions — and that reading it requires nothing more than a fast camera, a fisheye lens, and a polar night.

Subject of Research: Mesoscale structure and EMIC wave generation of flickering aurora observed by wide-field high-speed imaging

Article Title: Mesoscale structure of flickering aurora from wide-field high-speed imaging

Article References: Nanjo, S., Kurita, S., Sergienko, T., Miyoshi, Y., & Kataoka, R. (2026). Mesoscale structure of flickering aurora from wide-field high-speed imaging. Annales Geophysicae, 44(2), 959-975. https://doi.org/10.5194/angeo-44-959-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-959-2026

Keywords: flickering aurora, aurora, EMIC waves, space physics, magnetosphere, auroral acceleration region, high-speed imaging, sCMOS camera, Poker Flat Research Range, electron precipitation, substorm, ionosphere