Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN's Low Altitude Perspective.

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 ΔL∼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∼5-7 at dusk, while a smaller subset exists at 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-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 ∼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.

How a Realistic Magnetosphere Alters the Polarizations of Surface, Fast Magnetosonic, and Alfvén Waves.

System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods that might be applied to spacecraft observations.

Magnetopause ripples going against the flow form azimuthally stationary surface waves.

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth's magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09-15h) the waves' Poynting flux exactly balances the flow's advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.

Direct observations of a surface eigenmode of the dayside magnetopause.

The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.

Long-lasting poloidal ULF waves observed by multiple satellites and high-latitude SuperDARN radars.

Poloidal ultra-low frequency (ULF) waves between 5-10 mHz were observed by multiple satellites and three high-latitude Super Dual Auroral Radar Network (SuperDARN) radars during the recovery phase of a moderate geomagnetic storm on Jan 24-27, 2016. The long-lasting ULF waves were observed in the magnetic field and energetic particle flux perturbations during three successive passes by two Geostationary Operational Environmental Satellites (GOES) through the dayside magnetosphere, during which plasmasphere expansion and refilling were observed by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. The radial magnetic field oscillation was in phase (~ 180° out of phase) with the northward (southward) moving proton flux oscillation at 95 keV, consistent with high-energy drift-bounce resonance signatures of protons with second harmonic poloidal standing Alfvén waves. The longitudinal extent of the waves approached 10 hours in local time on the dayside and gradually decreased with time. High-time resolution (~ 6 s) data from three high-latitude SuperDARN radars show that the wave intensification region was localized in latitude with a radial extent of ~ 135-225 km in the subauroral ionosphere. No signature of these waves were observed by ground-based magnetometers colocated with the GOES satellites suggesting that the poloidal waves were high-m mode and thus screened by the ionosphere. During this interval one of the THEMIS probes observed a bump-on-tail ion distribution at 1-3 keV which we suggest is the source of the long-lasting second harmonic poloidal ULF waves.

Survey of ionospheric Pc3-5 ULF wave signatures in SuperDARN high time resolution data.

Ionospheric signatures of ultra-low frequency (ULF) wave in the Pc3-5 band (1.7-40.0 mHz) were surveyed using ~6 s resolution data from Super Dual Auroral Radar Network (SuperDARN) radars in the northern hemisphere from 2010 to 2016. Numerical experiments were conducted to derive wave period dependent thresholds for automated detection of ULF waves using the Lomb-Scargle periodogram technique. The spatial occurrence distribution, frequency characteristics, seasonal effects, solar wind condition and geomagnetic activity level dependence have been studied. Pc5 wave events were found to dominate at high and polar latitudes with a most probable frequency of 2.08 ± 0.07 mHz while Pc3-4 waves were relatively more common at midlatitudes on the nightside with a most probable frequency of 11.39 ± 0.14 mHz. At high latitudes, the occurrence rate of Pc4-5 waves maximizes in the dusk sector and during winter. These events tend to occur during low geomagnetic activity and northward interplanetary magnetic field (IMF). For the category of radially bounded but longitudinally extended Pc4 events in the duskside ionosphere, an internal driving source is suggested. At midlatitudes, the Pc3-4 occurrence rate maximizes premidnight and during equinox. This tendency becomes more prominent with increasing auroral electrojet (AE) index and during southward IMF, which suggests many of these events are Pi2 and Pc3-4 pulsations associated with magnetotail dynamics during active geomagnetic intervals. The overall occurrence rate of Pc3-5 wave events is lowest in summer, which suggests that the ionospheric conductivity plays a role in controlling ULF wave occurrence.