Substorms in Earth’s magnetotail drive record ionospheric electric fields

Deep inside Earth’s magnetotail, sudden explosions of magnetic energy called substorms are sending shockwaves of electric field all the way down to the equatorial ionosphere, and the effects are far larger than scientists anticipated. A new analysis of radar measurements from the Jicamarca Radio Observatory in Peru, published in Frontiers in Astronomy and Space Sciences, shows that during recent geomagnetic storms, substorm-driven electric fields lifted the ionospheric plasma at velocities exceeding 300 meters per second, corresponding to an eastward electric field of more than 7 millivolts per meter, the largest ever observed over Jicamarca. The finding suggests that the low-latitude ionosphere responds to magnetospheric substorms in ways that current space weather models completely miss.

The study, led by Bela G. Fejer of Utah State University together with Luis A. Navarro of the University of Colorado Boulder, Sophia R. Laranja of Brazil’s National Institute for Space Research, and Percy Condor of the Jicamarca Radio Observatory, examined the response of equatorial ionospheric electric fields to three recent storms: a strong double-dip storm on 24 April 2023, a severe CME-driven storm on 10 to 11 October 2024, and a moderate storm on 16 to 17 May 2025. The team combined F-region vertical plasma drift measurements from the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere Medium Power radar and the observatory’s full-power incoherent scatter radar with solar wind data from the Advanced Composition Explorer satellite, geomagnetic indices from the SuperMAG network, and ionosonde measurements from the Jicamarca vertical incidence pulsed ionospheric radar.

What the researchers found were bursts of upward vertical plasma drift with amplitudes greater than 100 meters per second that lasted less than an hour, appearing near dusk even when the interplanetary magnetic field was nearly steady and southward. Under those conditions, the standard explanation for ionospheric disturbances, the penetration of solar wind-driven convection electric fields from high latitudes to the equator, should have produced smooth, slowly varying perturbations that follow the solar wind electric field. Instead, the Jicamarca data show sharp, bursty spikes in upward drift that come and go on timescales of tens of minutes, closely correlated with substorm onsets identified in the SuperMAG auroral electrojet indices rather than with changes in the solar wind.

The mechanism the authors propose involves low-entropy magnetospheric bubbles and bursty bulk flows. When magnetic reconnection switches on in the magnetotail, localized transient reconnections create magnetic flux tubes with lower entropy than their surroundings. These bubbles race earthward as bursty bulk flows, and when they reach the inner magnetosphere they cause magnetic dipolarization, particle acceleration, and substantial field-aligned currents flowing into and out of the ionosphere. Those currents, the researchers argue, drive the zonal electric field perturbations that penetrate to equatorial latitudes as inductive prompt penetration electric fields, producing the bursty upward drifts seen by the radar.

The most dramatic event occurred near dusk on 10 October 2024, during a severe geomagnetic storm that began with a sudden storm commencement at 1514 UT. As the solar wind motional electric field increased, the derived vertical drifts rose to about 85 meters per second at 2300 UT, in good agreement with the widely used Prompt Penetration Electric Field Model. But then, starting at about 2320 UT, the upward drifts surged to roughly 340 meters per second at 2345 UT before collapsing sharply. SuperMAG data showed substorm onsets at 2316 UT and 2358 UT, with the first occurring almost exactly at the longitude of Jicamarca, and the asymmetric disturbance index peaked at about 2245 UT, 2310 UT, and 2330 UT. The ionosonde-derived drifts showed the F-layer virtual height excursions of up to 100 kilometers in just five minutes near dusk, a vertical leap the authors interpret as the work of a large-amplitude bursty prompt penetration electric field rather than the smooth undershielding perturbation the model predicted.

The consequences of that single substorm-driven electric field spike were spectacular. The enormous upward drift near dusk drove the F layer to unprecedented heights over Jicamarca, and the resulting very strong equatorial plasma bubbles, channels of depleted plasma density that form when the bottomside ionosphere becomes unstable, extended far beyond their usual tropical domain to mid-latitudes. Independent studies of the same storm documented super equatorial plasma bubbles confined to longitudes between 65 and 75 degrees west, with long-lasting bubble activity to the west of 60 degrees west and suppression to the east, a striking longitudinal variability that earlier work attributed to an undefined storm-time electric field driver. The new results suggest that substorm electric fields, together with slower disturbance dynamo electric fields generated by storm-altered thermospheric winds, played a decisive role in that variability.

The moderate storm of 16 to 17 May 2025 provided an equally surprising result. For the first time, Jicamarca measurements near dusk revealed two large-amplitude, short-lived, substorm-associated bursty prompt penetration upward drifts, with amplitudes of roughly 70 and 90 meters per second, occurring within about two hours and driving strong equatorial plasma irregularities over Brazil. Four substorm onsets occurred during this period, and the second drift peak coincided with increases in the asymmetric disturbance index. The authors note that strong duskside prompt penetration electric fields can occur even during minor geomagnetic activity, and that in this event again the large bursty drifts were not reproduced by the Prompt Penetration Electric Field Model, which instead captured only the smoother components tied to solar wind electric field changes.

The failure of the model is significant for space weather forecasting. The Prompt Penetration Electric Field Model, driven by solar wind electric field changes and widely used to predict equatorial ionospheric perturbations, reproduced the initial drift peak at 2300 UT on 10 October 2024 but not the ensuing sudden decrease and the much larger bursty increase that followed. During the April 2023 storm, the model also failed to reproduce the large-amplitude bursty drifts measured by the Jicamarca radar during the second main phase, even though it had performed reasonably well during the first main phase and recovery phase on 23 April. The authors conclude that the early-night large-amplitude prompt perturbation drifts during these storms arise from a driver that the model does not account for: magnetospheric substorms associated with bubbles and bursty bulk flows.

The new results also connect to a broader pattern of substorm-driven ionospheric effects. Previous work by the same group documented rapidly recurring, relatively small daytime upward drift perturbations of about 20 meters per second during the September 2017 geomagnetic storm, and very large premidnight bursty upward drifts over Jicamarca under nearly stationary southward IMF during the very large December 2006 storm. Very large upward drifts of about 150 meters per associated with a dramatic increase in the F-layer height near dusk during the March 2015 St. Patrick’s Day storm are also indicative of large substorm-driven electric fields. Recent measurements during an isolated substorm with only small changes in the duskward solar wind electric field showed very strong early-night bursty upward drifts up to about 120 meters per second over Jicamarca. During the May 2024 superstorm, the Sanya radar in China measured very large postmidnight downward drifts closely correlated with much smaller daytime upward drifts at Jicamarca, suggesting that substorm-driven large prompt penetration drifts may not be restricted to near dusk.

The authors emphasize that significantly more detailed high time- and spatial-resolution global electrodynamic measurements and complementary modeling studies are needed to better understand the properties of substorm-driven electric fields and their thermospheric effects. Ground-based and satellite equatorial electrojet measurements, they note, are important underutilized sources of global storm-time ionospheric electric fields, as are DMSP equatorial vertical and zonal drift measurements when coordinated with complementary global ground-based observations. As the Sun approaches the peak of its current activity cycle and severe geomagnetic storms become more frequent, the bursty substorm-driven electric fields documented at Jicamarca represent a driver of equatorial space weather that forecasters cannot yet predict, one capable of lifting the ionosphere hundreds of kilometers and sending plasma bubbles to mid-latitudes within minutes.

Subject of Research: Magnetospheric substorm-driven large-amplitude ionospheric bursty prompt penetration electric fields

Article Title: Magnetospheric substorm-driven large-amplitude ionospheric bursty prompt penetration electric fields

Article References: Fejer, B. G., Navarro, L. A., Laranja, S. R., & Condor, P. (2026). Magnetospheric substorm-driven large-amplitude ionospheric bursty prompt penetration electric fields. Frontiers in Astronomy and Space Sciences, 13, 1890579. https://doi.org/10.3389/fspas.2026.1890579

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1890579

Keywords: Magnetospheric, substorm-driven, large-amplitude, ionospheric, bursty, prompt, penetration, electric, fields, scientific research