Jiang Liu

We present a statistical study of 272 proton isotropy boundary (IB) events, spanning 50 keV up to 2 MeV observed by ELFIN on the nightside magnetosphere, extending the energy range beyond the 120 keV population considered in prior works. Viewed from Low Earth Orbit, the IB is the magnetic latitude poleward of which persistently isotropic pitch‐angle distributions () are first detected. We characterize the IB distribution in local time, energy, geomagnetic activity, and 50 keV precipitation from isotropic protons. We find these IBs primarily exhibit negative energy‐latitude dispersion patterns consistent with equatorial magnetic field‐line curvature (FLC) scattering, with a 20%–30% chance of any particular energy channel exhibiting positive dispersion. The lowest latitude and most energetic IBs were detected in the pre‐midnight sector, consistent with the typical location of maximal cross‐tail current‐sheet thinning. We identify that proton IBs form the lower‐latitude boundary of a region with significant FLC‐caused proton precipitation between the outer ring current and inner edge of the plasma sheet (“PS2RC”), resulting in perpetual loss of protons exceeding typical plasma sheet energies. We show this 50 keV precipitation is often sufficiently intense and distributed to produce ionization enhancements over a range of altitudes at auroral/sub‐auroral latitudes. We also compare proton IB properties to those of electron IBs observed by ELFIN, finding similar trends across local time and activity. These results demonstrate that the IB and precipitation of isotropic particles in their poleward vicinity can be geophysically significant in connecting the magnetosphere and ionosphere.

Recent analyses indicate that relativistic electron injections from the tail plasma sheet are a crucial, and maybe dominant, contributor to outer radiation belt enhancement events. This is based on precise examination of flux evolution along the dual Van Allen Probe trajectories, which, when the spacecraft are in appropriate locations, allow confident identification of flux enhancements from injections. These injections result from tail meso‐scale flow bursts, which map to the ionosphere as meso‐scale flow channels having an equatorward flow component corresponding to their earthward flow component within the plasma sheet. These flow channels are observable by ground‐based radars when there is sufficient spatial coverage. However, they can also often be identified from auroral imagers and ground magnetometers, since ionospheric flow channels are associated with auroral streamers and ground magnetic field depressions. We have analyzed three cases having isolated brief time intervals of streamers and injections, one case with multiple such intervals, and one major radiation belt enhancement event. This analysis indicates that when auroral streamers and associated ground magnetic field depressions extend equatorward to radiation belt invariant latitudes, they are associated with injections that result in a net enhancement in radiation electron fluxes. Thus, ground auroral and magnetic field observations have potential for helping verify the connection of flow bursts to radiation belt enhancements and for helping detect such radiation belt injections. Furthermore, this study indicates that radiation belt physics is intimately related to auroral and tail physics via their direct physical association with meso‐scale flow bursts and thus localized magnetotail reconnection.

Meso‐scale enhanced flow channels often extend across the polar cap from the dayside to the nightside auroral oval, where they enter the oval via localized reconnection and cause auroral oval disturbances. Examples here show such flows associated with azimuthally moving polar cap arcs that develop an equatorward portion that bends in the direction opposite to the arc's motion, becoming nearly parallel to the nightside auroral oval and possibly becoming an oval poleward boundary arc. The arcs then continue to move equatorward, and “lay down” along the auroral oval, leading to an explosive substorm onset (or expansion enhancement) that rapidly expands azimuthally with laying down of the arc, and later expands further with westward traveling surge sliding along the arc. This gives a longitudinally broad region of expansion‐phase aurora, and a similarly broad magnetic depression, indicating large and broad substorm energy release. The surge sliding along the poleward boundary arc occurs at a higher latitude than the laydown portion of onset, so that sliding leads to later magnetic depressions at locations west of the initial onset and at higher latitudes. The polar cap arc appears to be pushed equatorward and westward (for By >0) by the flow of new plasma moving across the polar cap, the arc being like a weather front that demarcates an advancing boundary between plasma regimes along polar cap field lines. The onset and expansion of the activity appear to be driven by the flow of this new plasma from the polar cap to the region of expansion phase activity.

Dawnside auroral polarization streams (DAPS) are fast eastward flows in the dawn convection cell of Earth's ionosphere. With a steep flow gradient near the interface between Region 1 and 2 currents and a peak poleward of it, DAPS were suggested to be responsible for instabilities and dramatic events in the magnetosphere‐ionosphere (M‐I) system. To predict these events, it is important to investigate when and where DAPS prefer to occur and how they are related to other M‐I phenomena. We conduct this investigation statistically using 10 years of Swarm data and find that DAPS under sunlit and dark ionospheric conditions exhibit different dependencies on magnetic local times and geomagnetic activities, reflecting a complicated interplay between magnetotail dynamics and ionospheric conductance. The statistical results also reveal a strong correlation between DAPS and embedded Region 2 currents. These findings provide new insights into the DAPS generation mechanism.

Whistler‐mode chorus and hiss waves play an important role in Earth's radiation belt electron dynamics. Direct measurements of whistler wave‐driven electron precipitation and the resultant pitch angle distribution were previously limited by the insufficient resolution of low Earth orbit satellites. In this study, we use recent measurements from the Electron Losses and Fields INvestigation CubeSats, which provide energy‐ and pitch angle‐resolved electron distributions to statistically evaluate electron scattering properties driven by whistler waves. Our survey indicates that events with increasing precipitating‐to‐trapped flux ratios (evaluated at 63 keV unless otherwise specified) correlate with increasing trapped flux at energies up to ∼750 keV. Weak precipitation events (precipitation ratio 5 over midnight‐to‐noon local times during disturbed geomagnetic conditions. These results are crucial for characterizing the whistler‐mode wave driven electron scattering properties and evaluating its impact on the ionosphere.

Bursty bulk flows (BBFs) play a crucial role in transporting energy, mass, and magnetic flux in the Earth's magnetotail, particularly in the earthward direction. However, their impulsive nature and small spatial scale present significant challenges for in-situ observation, as only a limited number of spacecraft operate within the vast expanse of the magnetotail. Consequently, studying their statistical characteristics is a highly demanding task, and accurately predicting their behavior remains a distant goal. In this study, we analyze key characteristics of BBFs and apply regression-based models to predict their parameter behaviorUsing observational data from the THEMIS mission collected between 2007 and 2023, we conducted a feature analysis on parameters associated with BBFs evolution, including velocity, magnetic field, electric field, temperature, density, pressure, and specific entropy indices. Through statistical techniques, we identified parameters exhibiting predictable patterns during BBF events, distinguishing them from background conditions. Furthermore, we used XGBoost regression model, optimized for different parameter combinations, to forecast BBF duration, physical parameters’ average minimum, and peak intensity. This study also tested combinations of parameter predictions across instruments. When using observed background value in parameter combination, our models achieved Mean Absolute Percentage Errors of under 35% for critical variables, including Bz, Btotal, plasma pressure, and ion temperatures, and ion specific entropy and so on. Additionally, we observed BBF duration’s spatial distribution trends: it peaked at approximately X=-13Re, while decreasing with increasing Z distance from the plasma sheet, showing dawn-dusk asymmetry consistent with prior observations. This work highlights the potential of regression methods in forecasting BBFs characteristics and offers insights into their spatial behavior, supporting enhanced prediction capabilities in magnetospheric studies. Future research will aim to improve accuracy with enriched datasets.

We present multi‐platform observations of plasma cloak, O+ outflows, kinetic Alfven waves (KAWs), and auroral oval for the geomagnetic storm on 17 March 2015. During the storm's main phase, we observed a generally symmetric equatorward motion of the auroral oval in both hemispheres, corresponding to the plasmasphere erosion and inward motion of the plasma sheet. Consequently, Van Allen Probes became immersed within the plasma sheet for extended hours and repeatedly observed correlated KAWs and O+ outflows. The KAWs contain adequate energy flux toward the ionosphere to energize the observed outflow ions. Adiabatic particle tracing suggests that the O+ outflows are directly from the nightside auroral oval and that the energization is through a quasi‐static potential drop. The O+ outflows from the nightside auroral oval were adequate (‐ #/‐s) and prompt (several minutes) to explain the newly formed plasma cloak, suggesting that they were a dominant initial source of plasma cloak during this storm.

Following the auroral substorm onset, the active aurora undergoes expansion, which can vary in spatial and temporal extent. The spatiotemporal development of the expansion phase active aurora is controlled by new auroral intensifications that often follow the initial onset. Using seven examples, we investigate the nature of these new auroral intensifications and address a question: are they new auroral onsets, that is, “successive onsets” or poleward‐boundary intensifications (PBIs) and ensuing auroral streamers? We observed events that included both types of auroral features—successive onsets and PBIs—and their combinations. For multiple‐onset substorms, successive onsets may occur eastward, westward, and poleward of the initial onset, resulting in a diverse range of expansion phase spatial extent and durations. Single‐onset substorms show only one auroral onset, but their spatiotemporal development can resemble that of multiple‐onset substorms. However, the additional activations are mainly PBIs and subsequent streamers. In some cases, PBIs undergo explosion, leading to a rapid poleward and azimuthal expansion of the aurora, resembling the auroral substorm onset. A prolonged sequence of PBIs and its longitudinal extension can contribute significantly to the spatiotemporal development of substorms expansion phase. Results suggest that post‐onset flow channels drive the spatiotemporal development of the substorm expansion phase by (a) triggering successive onsets and (b) inducing bursts of PBIs and their prolonged sequence. We speculate that post‐onset flow channels likely originate from the polar cap, but more evaluation is required. Our findings highlight the significance of examining imager data before solely relying on magnetometers to identify substorm onsets.

The Poynting vector (Poynting flux) from Earth's magnetosphere downward toward its ionosphere carries the energy that powers the Joule heating in the ionosphere and thermosphere. The Joule heating controls fundamental ionospheric properties affecting the entire magnetosphere‐ionosphere‐thermosphere system, so it is necessary to understand when and where the Poynting flux is significant. Taking advantage of new data sets generated from DMSP (Defense Meteorological Satellite Program) observations, we investigate the Poynting flux distribution within and around the auroral zone, where most magnetosphere‐ionosphere (M‐I) dynamics and thus Joule heating occurs. We find that the Poynting flux, which is generally larger under more active conditions, is concentrated in the sunlit cusp and near the interface between Region 1 and 2 currents. The former concentration suggests voltage generators drive the cusp dynamics. The latter concentration shows asymmetries with respect to the interface between the Region 1 and 2 currents. We show that these reflect the controlling impact of subauroral polarization streams and dawnside auroral polarization streams on the Poynting flux.

Magnetotail earthward‐propagating fast plasma flows provide important pathways for magnetosphere‐ionosphere coupling. This study reexamines a flow‐related red‐line diffuse‐like aurora event previously reported by Liang et al. (2011, https://doi.org/10.1029/2010ja015867), utilizing THEMIS and ground‐based auroral observations from Poker Flat. We find that time domain structures (TDSs) within the flow bursts efficiently drive electron precipitation below a few keV, aligning with predominantly red‐line auroral intensifications in this non‐substorm event. The diffuse‐like auroras sometimes coexisted with or potentially evolved from discrete forms. We forward model red‐line diffuse auroras due to TDS‐driven precipitation, employing the time‐dependent TREx‐ATM auroral transport code. The good correlation (∼0.77) between our modeled and observed red line emissions underscores that TDSs are a primary driver of the red‐line diffuse‐like auroras, though whistler‐mode wave contributions are needed to fully explain the most intense red‐line emissions.