The jets in question are known as bursty bulk flows, or BBFs. They form when magnetic reconnection snaps stretched, energy-laden field lines in the magnetotail, the long nightside extension of Earth’s magnetic field. The newly reconnected flux tubes contract earthward under magnetic tension, accelerating plasma toward the inner magnetosphere at hundreds of kilometers per second. Each jet typically carries a dipolarization front, a boundary where the magnetic field abruptly snaps back toward a more dipolar, compressed geometry. Together, BBFs and their dipolarization fronts are thought to transport a large fraction of the energy and momentum that ultimately powers auroras and disturbs ground-based technology.

Between 18:00 and 22:00 UT on that December evening, the three inner THEMIS probes, positioned in the near-Earth plasma sheet, detected six distinct BBF intervals, with clear dipolarization signatures in the last four. Crucially, the magnetic footpoints of these flows, traced down the field lines to an altitude of 110 kilometers, mapped into the pre-midnight sector directly over northern Fennoscandia, an exceptionally well-instrumented patch of the auroral zone. There, the IMAGE magnetometer network, all-sky cameras at Skibotn and Kilpisjärvi, and the low-altitude Swarm A and C satellites stood ready to record whatever the tail threw at the ionosphere.

At the start of the interval, the team found the ionospheric current system in a stable configuration known as the Harang discontinuity, a boundary region where eastward and westward currents meet and turn northward. With each successive BBF arrival, this configuration became progressively distorted. The mapped footpoint of each flow landed in a region of counterclockwise vorticity in the horizontal equivalent current, the signature expected above an upward field-aligned current. Current magnitudes shifted within seconds to minutes, an auroral arc brightened in step with the flow detection, and the familiar Harang pattern gradually gave way to a clear, intensifying westward electrojet, the strong westward current channel that characterizes substorm expansion.

The escalation followed a striking progression. The first two dipolarizing intervals produced only auroral pseudo-breakups, modest brightenings of pre-existing arcs that shared some morphology with substorms but never developed the poleward expansion and global current reconfiguration of the real thing. The final two intervals, by contrast, displayed textbook auroral breakups with poleward expansion, classified as genuine substorms by the SOPHIE technique applied to SuperMAG ground data. In the last event, THEMIS measured a flow peaking at 605.5 kilometers per second, the westward electrojet surged and expanded, and the aurora bifurcated, brightened and raced poleward in near-perfect synchrony with the magnetotail signatures.

The scientific centerpiece of the study came during the fourth BBF, when the Swarm A and C satellites, flying side by side at roughly 480 kilometers altitude, crossed the mapped footpoint of the flow within minutes of its detection in the tail. Both spacecraft recorded a sharply localized pair of upward and downward field-aligned currents spanning only 35 to 50 kilometers in latitude, with current densities exceeding 1 microampere per square meter and peaks reaching 6.2 microamperes per square meter. The upward current sat directly beneath a discrete auroral arc, and the arc intensified at the very moment the current was measured. Notably, the dual-satellite method, which resolves only structures larger than about 150 kilometers, smoothed this narrow pair away entirely, showing that the finest structure of the BBF current system can only be captured by single-spacecraft measurements.

A multiscale minimum variance analysis of the Swarm magnetic data added further texture. The downward current proved planar and east-west aligned, while the upward current was distinctly non-planar and inclined away from the east-west direction, exactly the geometry predicted for the field-aligned currents flanking an earthward-moving plasma bubble. The team also tracked enhancements of 9 to 20 kiloelectronvolt electron fluxes in the tail that coincided with each arc brightening, suggesting these electrons, channeled along field lines, were sufficient to produce the auroral light without requiring additional acceleration closer to Earth.

The study also quantified the space weather consequences. Rapid variations of the horizontal ground magnetic field, the quantity that drives geomagnetically induced currents in power grids, spiked simultaneously with each of the last four dipolarizations, with peak amplitudes growing as the sequence progressed. The largest perturbation reached 213 nanotesla per minute at Tromsø, a value comparable to peaks reported in a recent catalog of events associated with actual power-grid disturbances, though the geophysical conditions in this case were milder. Maps of the Rate-Of-TEC-Index, a measure of ionospheric electron-density irregularities that degrade GPS signals, intensified in step with each dipolarization, peaking near 67 to 69 degrees north, right at the BBF footprints.

Taken together, the observations provide the first direct, multi-instrument picture of a coherent, time-aligned chain of coupling signatures linking recurrent magnetotail flows to the high-latitude ionosphere: localized vorticity in ionospheric currents, discrete arc brightenings, field-aligned current pairs on arc scales, and cumulative escalation toward substorm onset. The results lend strong support to the wedgelet picture of the substorm current wedge, in which many small current elements generated by individual flow bursts superpose to build the large-scale current system traditionally attributed to a single monolithic structure. For space weather forecasting, the message is sobering: even in the absence of a full substorm, BBFs with strong dipolarization fronts can generate localized, sharp perturbations capable of stressing technological systems, which means the small, fast events in the tail deserve attention in their own right.

Subject of Research: Magnetotail bursty bulk flows, dipolarization fronts, and their coupling to ionospheric currents and auroras during a substorm sequence

Article Title: Ionospheric currents and auroral signatures during successive earthward bursty bulk flows and dipolarization: a 7 December 2023 case study

Article References: Lanabere, V., Buchert, S., Blagau, A., George, H., Nanjo, S., Juusola, L., Dimmock, A. P., Wharton, S., Vanhamäki, H., Marghitu, O., Richard, L., Cai, L., Wallner, A. V. L., Kotova, D., Jin, Y., Hoppe, T., Carter, J. A., & Aikio, A. (2026). Ionospheric currents and auroral signatures during successive earthward bursty bulk flows and dipolarization: a 7 December 2023 case study. Annales Geophysicae, 44(2), 977-1001. https://doi.org/10.5194/angeo-44-977-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-977-2026

Keywords: bursty bulk flows, dipolarization, magnetotail, ionosphere, field-aligned currents, aurora, substorm, THEMIS, Swarm, space weather, geomagnetically induced currents, Fennoscandia

The study, led by Garima Karki of Kumaun University in Nainital, India, together with Brigitte Schmieder of the Observatoire de Paris and KU Leuven, Ramesh Chandra, Pooja Devi, Pascal Démoulin and Stefaan Poedts, focused on a quiescent filament observed on September 28, 2023. Quiescent filaments are the long-lived, relatively stable cousins of the structures that sometimes erupt as the engines of coronal mass ejections. They consist of dense, cool plasma at chromospheric temperatures, roughly 10,000 kelvin, embedded within the million-degree corona and supported against gravity by magnetic fields. Understanding how plasma enters, circulates within, and drains out of these structures is a central problem in solar physics, because the mass budget of a filament over its lifetime cannot be explained by static conditions alone. Material must be continuously supplied and removed, and the mechanisms responsible remain actively debated.

The key instrument in this investigation was the Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, or THEMIS, located on the Canary Island of Tenerife. THEMIS observed the filament in the H-alpha spectral line, the workhorse diagnostic of cool solar plasma, using its Multi-channel Subtractive Double Pass spectrograph in a scanning mode that delivered both high spatial resolution and high spectral resolution. Crucially, the THEMIS field of view happened to be centred on the northern end of the filament at the precise moment when one of a series of repetitive plasma motions swept through it. That coincidence of timing and pointing allowed the team to capture spectra of an elongated cool blob as it fell, and those spectra revealed H-alpha line profiles that were dramatically broadened and shifted, the spectroscopic fingerprints of extremely fast motion.

To convert those distorted line profiles into a velocity, the researchers employed a technique known as the cloud model, a classical method in solar spectroscopy that treats an individual absorbing structure as a uniform slab of plasma with well-defined properties: a source function, a Doppler width, a line-of-sight velocity, and an optical thickness. By fitting synthetic profiles to the observed ones and minimising the root-mean-square difference between model and measurement, the team derived a downflow speed of 89.8 kilometres per second, with a remarkably tight standard deviation of plus or minus 0.25 kilometres per second. For context, the sound speed in the cool filament plasma is on the order of 10 kilometres per second, so the falling material was moving at roughly nine times the local sound speed, unambiguously supersonic. Such speeds are rarely measured with this degree of confidence in quiescent filament structures, and the precision of the cloud-model fit lends the result considerable weight.

The supersonic measurement alone would be noteworthy, but the broader context of the observation makes it genuinely intriguing. Using data from the Global Oscillation Network Group, or GONG, a worldwide network of instruments that monitors the Sun in H-alpha around the clock, the team tracked plasma moving along the filament body toward its northern end. These repetitive motions suggested a conveyor-bear-like flow of material along the filament’s internal magnetic structure. When the flowing plasma reached the region imaged by THEMIS, it appeared to be falling toward the chromosphere. The researchers first considered a ballistic interpretation: material launched or displaced along the filament’s magnetic dips simply falling under gravity along a curved trajectory. A ballistic path could, in principle, account for the observed acceleration and speed, painting a picture of cool plasma draining from the filament body down into the dense lower atmosphere.

However, the team could not rule out a second and arguably more exotic scenario: coronal rain. Coronal rain occurs when plasma trapped in coronal magnetic loops undergoes a thermal instability. Under certain conditions, the delicate balance between heating and radiative cooling in the corona breaks down, and localised pockets of plasma cool catastrophically from millions of kelvin to chromospheric temperatures in a matter of minutes. The resulting condensations become dense and heavy, lose magnetic support, and plummet along the curved field lines of the hosting loop system, often appearing as shimmering showers of dark blobs in H-alpha and other cool-line observations. Coronal rain is now recognised as a ubiquitous phenomenon in the solar corona, and numerical simulations with modern magnetohydrodynamic codes have reproduced its formation in striking detail, including its connection to cycles of heating and cooling in coronal loops.

A crucial clue linking the falling plasma to the hot corona came from the Atmospheric Imaging Assembly, AIA, aboard NASA’s Solar Dynamics Observatory. In AIA’s hot extreme-ultraviolet channels, which sample plasma at temperatures of a million kelvin and above, the team identified a coronal bright point at exactly the same location as the supersonic downflow, with the temperature there reaching approximately 6 million kelvin. Coronal bright points are small, compact regions of enhanced emission associated with magnetic bipoles and frequent small-scale reconnection events. The spatial coincidence between a 6-million-kelvin bright point and a stream of 10,000-kelvin plasma falling at 90 kilometres per second suggests a physical connection: heating and reconnection at the bright point may destabilise nearby coronal plasma, triggering the thermal condensation that then rains down, or alternatively may inject material that subsequently follows a ballistic descent along the filament’s magnetic architecture.

The THEMIS spectra also confirmed a phenomenon that has fascinated solar physicists for decades: counter-streaming flows. Along the fine internal strands of the filament, which the high-resolution observations resolved to widths of less than one arcsecond, plasma was observed flowing in opposite directions simultaneously. Counter-streaming was first documented in prominences in the late 1990s and has since been attributed to various mechanisms, including intermittent heating events at the filament’s footpoints, waves propagating along the strands, and networks of small-scale jets from the chromosphere below. The fact that THEMIS measured these bidirectional flows in individual sub-arcsecond strands, using both imaging and spectroscopy, provides some of the most direct evidence yet that the filament’s fine structure is a dynamically active web of plasma conduits rather than a static reservoir of gas.

What makes this study compelling is the way it stitches together observations across an enormous range of scales and temperatures. A single arcsecond-scale blob of cool plasma, falling at nine times the sound speed, is connected to a million-degree bright point, to repetitive flows threading a filament tens of thousands of kilometres long, and to the grand question of how the Sun’s atmosphere cycles mass between its coolest and hottest layers. Whether the supersonic downflow proves to be ballistic drainage or coronal rain, the measurement demonstrates that quiescent filaments are anything but quiet: they are sites of continuous, violent mass transport that ground-based spectrometers of the THEMIS class are now capable of quantifying with exquisite precision. As solar physicists refine the interplay between thermal instability, magnetic topology and reconnection in these structures, each such observation brings the field closer to understanding not only the life cycle of filaments themselves but also the precursors of the eruptions that can send space weather hurtling toward Earth.

Subject of Research: Supersonic downflows in a quiescent solar filament observed with THEMIS and their relation to a coronal bright point

Article Title: Supersonic Flows Observed by THEMIS Related to a Coronal Bright Point and Filament

Article References: Karki, G., Schmieder, B., Chandra, R., Devi, P., Démoulin, P., & Poedts, S. (2026). Supersonic Flows Observed by THEMIS Related to a Coronal Bright Point and Filament. Solar Physics, 301(9), Article 148. https://doi.org/10.1007/s11207-026-02744-9

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02744-9

Keywords: solar physics, solar filament, prominence, THEMIS, coronal bright point, coronal rain, H-alpha spectroscopy, cloud model, counter-streaming flows, SDO/AIA, GONG, supersonic downflows