We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or $\Delta L\sim 0.56$ Δ L ∼ 0.56 ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at $L\sim 5-7$ L ∼ 5 − 7 at dusk, while a smaller subset exists at $L\sim 8-12$ L ∼ 8 − 12 at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an $L$ L -shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of $\sim 1.45$ ∼ 1.45 MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

Heavy (O+) ion energization and field‐aligned motion in and near the ionosphere are still not well understood. Based on observations from the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) Enhanced Polar Outflow Probe at altitudes between 325 km and 730 km over 1 year, we present a statistical study (24 events) of ion heating and its relation to field‐aligned ion bulk flow velocity, low‐frequency waves, and field‐aligned currents. The ion temperature and field‐aligned bulk flow velocity are derived from 2‐D ion velocity distribution functions measured by the suprathermal electron imager (SEI) instrument. Consistent ion heating and flow velocity characteristics are observed from both the SEI and the rapid‐scanning ion mass spectrometer instruments. We find that transverse O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to very narrow regions (∼2 km across B), is more likely to occur in the downward current region and is associated with broadband extremely low frequency (BBELF) waves. These waves are interpreted as linearly polarized perpendicular to the magnetic field. The amount of ion heating cannot be explained by frictional heating, and the correlation of ion heating with BBELF waves suggests that significant wave‐ion heating is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Surprisingly, the majority of these heating events (17 out 24) are associated with core ion downflows rather than upflows. This may be explained by a downward pointing electric field in the low‐altitude return current region.

Plasma sheet electron precipitation is critical in magnetosphere‐ionosphere coupling and has long been attributed to electron scattering by whistler‐mode and electron cyclotron harmonic waves. Recent observations have revealed that time domain structures (TDSs) that appear as broadband electrostatic fluctuations may also scatter plasma sheet electrons. However, there has been no observational evidence of TDS scattering electrons into the ionosphere. This study presents potential evidence from conjugate observations between the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission and the low‐altitude Enhanced Polar Outflow Probe (e‐POP) spacecraft. During the five events presented, THEMIS observed intense electron injections accompanied by TDSs, while e‐POP captured precipitation of plasma sheet electrons with energies ∼100–325 eV over a broad pitch angle range. The observed TDSs can efficiently scatter these electrons exceeding the strong diffusion limit. Our results suggest that TDSs may contribute to plasma sheet electron scattering around times of injections.

The diffuse aurora precipitations provide more than 70% of the energy flux from the magnetosphere to the ionosphere (e.g., Newell et al., 2009). Based on the rigorous FAST spacecraft analysis provided by Dombeck et al. (2018), this percentage of the energy coming from the diffuse aurora is potentially a large overstatement. It is, nonetheless, still substantial and likely in the range of ∼30%-70%. These precipitations modify the ionosphere conductance, affecting thereby the convection electric field in the magnetosphere (e.g., Hardy et al., 1989). Therefore, the understanding of the sources and modeling of the fluxes of diffuse aurora precipitations represent the substantial interest for modeling the magnetosphere-ionosphere coupling (e.g., Yu et al., 2016).It is widely accepted that the diffuse aurora is primarily produced by magnetospheric electrons scattered into the loss cone by wave-particle resonant interaction processes, either driven by electron-cyclotron harmonic (ECH) waves at radial distances larger than ∼8 Earth radii (

Realistic electron diffusion rates are computed by incorporating the observed electron hole distributions • Electron hole distributions in velocity and parallel scale and E-field intensities control the scattering efficiency • Realistic lifetime estimates suggest efficient plasma sheet electron losses due to electron holes in plasma injections Supporting Information:

Suprathermal electron bursts (STEBs), characterized by a board energy spectrum and a field-aligned pitch angle distribution, have been well recognized to be associated with electron acceleration by inertial Alfvén waves and are thus conventionally termed as "Alfvénic aurora." In this study, we report joint Enhanced-Polar-Outflow-Probe (e-POP) and ground-based optical observations of Alfvénic auroras. In particular, we highlight the prominence of 630-nm red line emissions under low-energy Alfvénic auroral precipitation. During the event interval, e-POP traverses two arcs. One bright arc dominated by green line emissions is clearly seen by all optical instruments; it is embedded in upward field-aligned currents (FACs) yet leaves little imprint on the e-POP suprathermal electron imager (SEI), likely due to that the precipitation is well above the upper energy limit of SEI. On the other hand, there is a red line arc that is pronounced only in 630-nm images. Such a red-line-only arc is located in a transition from large-scale upward FACs to downward FACs and is associated with a prominent STEB structure detected by e-POP SEI. The STEB features an inverse energy time dispersion, namely, that lower-energy electrons are seen earlier while higher-energy electrons appear later. The red-line-only arc and its separation from the green line arc evolve in a repeatable fashion, each stemming from a poleward auroral intensification (PAI) propagated from higher latitudes. Following each poleward auroral intensification the green line arc progressively moved southward, while the red-line-only arc is quasi-stationary and stayed relatively stable in latitude. We propose tentative interpretations of the above features based upon stationary inertial Alfvén waves.

Energetic electron scattering and precipitation from the Earth's plasma sheet to the ionosphere is an important contributor to magnetosphere–ionosphere coupling. In this study, we investigate the role of one of the most intense wave emissions, kinetic Alfvén waves (KAWs), in energetic electron scattering. We have evaluated the effect of KAWs on energetic electrons within a curved magnetic field configuration exhibiting sharp cross field gradients. The magnetic field in Earth's magnetotail plasma sheet with an embedded dipolarization front is used as a working example. Taking into account electron bounce motion and perpendicular guiding-center drifts, we have shown that electrons with energies of tens to hundreds of keV can be scattered by KAWs in pitch angle and momentum through Doppler-shifted Landau resonance near the magnetic equator. The bounce-averaged pitch-angle diffusion coefficients for near-loss-cone (∼2°) electrons are on the order of 10–7–10–6 rad2/s for a characteristic KAW amplitude of 1 mV/m and approach the strong diffusion limit of ∼10–4 rad2/s for amplitudes of greater than 10 mV/m. These results suggest that under such ambient conditions, KAWs can pitch-angle scatter energetic electron population into the loss cone. In Earth's plasma sheet, this scattering is, thus, very likely to cause significant precipitation during active times. The diffusion coefficients of energetic electrons at large pitch angles (∼45°–∼80°) are more than two orders of magnitude larger than those of electrons near the loss cone, suggesting that KAWs contribute to isotropization of anisotropic electrons due to adiabatic heating should they drift into the vicinity of the magnetic field gradient.

Plasma sheet electron precipitation, much of it driven by whistler mode waves, is critical for magnetosphere-ionosphere coupling. This precipitation leads to a secondary electron population at low altitudes, which moves upward along magnetic field lines to the equatorial plasma sheet. We investigate observational evidence for such electron precipitation and the ionospheric feedback provided by the secondary electron outflows. Using the near-equatorial Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we show that whistler mode wave bursts are accompanied by enhanced field-aligned electrons. The properties of these electrons are not related to the whistler mode waves, as would have been expected from local wave-particle interactions. Thus, it is unlikely that the field-aligned electrons are formed by electron Landau resonance with whistler mode waves. This population can be the secondary electron outflows resulting from plasma sheet electron precipitation to the ionosphere. Combining THEMIS, Cluster, and Fast Auroral Snapshot Explorer (FAST) measurements, we show that this field-aligned electron population is also observed at low altitudes, where it is associated with high-frequency electrostatic waves. These low-altitude waves are likely generated by the secondary electron outflows and, in turn, scatter these electrons outside of the loss cone, allowing them to be observed by the wide field-of-view particle instrument on the near-equatorial THEMIS spacecraft. We discuss how this secondary electron population can subsequently alter whistler mode wave generation and propagation in the magnetosphere.