We present evidence of damping of equatorial noise due to Finite-Larmor-radius (FLR) effect in the inner Van Allen belt. Detail observation of the FLR phenomenon in the inner belt region has not been reported until now. Waves primarily damped by the FLR mechanism can influence the energy dependent proton density structure. We analyze a typical case recorded by the Van Allen probe that involves FLR damping of equatorial noise, which was propagating radially towards the Earth, at L-shell ~1.7. As a result of this damping, protons in the energy range of ~18 – 21 MeV at L-shell ~1.7 – 2 get energized. This kind of wave-particle interaction should be included in the future models of the inner Van Allen belt. This phenomenon may also account for the unknown proton loss mechanism reported in Selesnick and Albert (2019).

This study presents results from magnetic field line conjunctions between the medium-Earth orbiting Demonstration and Science Experiments (DSX) satellite and the low-Earth orbiting VLF Propagation Mapper (VPM) satellite. DSX transmitted at very low frequencies (VLF) towards VPM, which was equipped with a single-axis dipole electric field antenna, when the two spacecraft passed near the same magnetic field line. VPM did not observe DSX signals in any of the 27 attempted conjunction experiments; the goal of this study, therefore, is to explain why DSX signals were not received. Explanations include i) the predicted power at LEO from DSX transmissions was too low for VPM to observe; ii) VPM’s trajectory missed the “spot” of highest intensity due to the focused ray paths reaching LEO; or iii) rays mirrored before reaching VPM. Different combinations of these explanations are found. We present ray-tracing analysis for each conjunction event to predict the distribution of power and wave normal angles in the vicinity of VPM at LEO altitudes. We find that, for low-frequency (below 4kHz) transmissions, nearly all rays mirror before reaching LEO, resulting in low amplitudes at LEO. For mid- and high-frequency transmissions (~8kHz and 28kHz respectively), the power at LEO is above the noise threshold of the VPM receiver (between 0.5µV/m and 1µV/m). We conclude that the antenna efficiency and plasmasphere model are critical in determining the predicted power at LEO, and are also the two most significant sources of uncertainty that could explain the apparent discrepancy between predicted amplitudes and VPM observations.

We report observations of a Bursty Bulk Flow (BBF) penetrating to the outer edge of the radiation belt. The turbulent BBF braking region is characterized by ion velocity fluctuations, magnetic field (B) variations, and intense electric fields (E). In this event, energetic (>100 keV) electron and ion fluxes are appreciably enhanced. Importantly, fluctuations in energetic electrons and ions suggest that they are locally energized. Using correlation distances and other observed characteristics of turbulent E, test-particle simulations support that local energization by E favors higher-energy electrons and leads to an enhanced energetic shoulder and tail in the electron distributions. The energetic shoulder and tail can be amplified to MeV energies by adiabatic transport into the radiation belt where |B| is higher. This analysis suggests that turbulence generated by BBFs can, in part, supply energetic particles to the outer radiation belt and that turbulence can be a significant contributor to particle acceleration.

• Observations of wave damping due to finite Larmor radius effect in the inner belt region • Evidence of wave-particle interaction modifying the proton density profile at L~1.8

Fast-localized electron loss, resulting from interactions with electromagnetic ion cyclotron (EMIC) waves, can produce deepening minima in phase space density (PSD) radial profiles. Here, we perform a statistical analysis of local PSD minima to quantify how readily these are associated with radiation belt depletions. The statistics of PSD minima observed over a year are compared to the Versatile Electron Radiation Belts (VERB) simulations, both including and excluding EMIC waves. The observed minima distribution can only be achieved in the simulation including EMIC waves, indicating their importance in the dynamics of the radiation belts. By analyzing electron flux depletions in conjunction with the observed PSD minima, we show that, in the heart of the outer radiation belt (L*<5), up to 45% of multi-MeV electron depletions are associated with PSD minima, demonstrating that fast localized loss by interactions with EMIC waves are a common and crucial process for ultra-relativistic electron populations.

The turbulent energy cascade that is characteristic of bursty bulk flow (BBF) braking regions in the Earth’s magnetotail has been shown to be the energy source for large-amplitude electric fields (>50 mV/m) which can, in turn, result in local energetic electron acceleration. These pre-energized electrons move inward to stronger magnetic fields being adiabatically energized and can eventually supply an energetic tail to electron distributions in the outer radiation belt. Using wave and plasma measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites during four tail seasons from 2015 to 2019, we study the process of BBF magnetic and kinetic energy being transferred to electrons by turbulent electric fields from a statistical perspective. We identify turbulent BBF regions by the presence of high-amplitude electric fields. We show that the high-amplitude electric fields are associated with an increase in electron temperature by three times compared to quiet times and with a ten-fold increase in temperature fluctuations. They are also associated with strong variations of energetic electron fluxes, indicative of local acceleration. We further discuss the implications of these findings and the role of this pre-energized electron population in supplying the outer radiation belt.

This study presents analysis of very low frequency (VLF) transmitter signal measurements on the Very-Low-Frequency Propagation Mapper (VPM) CubeSat in low-Earth orbit. Six months of satellite operation provided good data coverage, used to build global statistical maps of VLF power distribution. The power distribution above four powerful transmitters is used as input for ray tracing to study signal propagation to the conjugate hemisphere in two plasmaspheric density models. The ray tracing results are further compared with VPM measurements to determine which model provides better agreement with observations. As ray propagation largely depends on the background plasma density distribution, this indirect method can be used for plasmaspheric density model validation as an alternative to multipoint in situ plasma measurements that may not be readily obtainable. In addition, it can be used to investigate Landau damping and ducted vs. non-ducted propagation of VLF signals.

Simultaneous bursts of EMIC and longer-period ULF waves were observed by the GOES-16 magnetometer on the 11 January and the 7 September 2017. No correlation was found between the repeat times of EMIC wavepackets and Pc4 ULF wave periods. However, the repetition times visually correlated with variations in solar wind dynamic pressure (Pdyn). EMIC wavepackets are composed of short duration subpackets and when subpackets last >1/2 the ULF wave period, correlation with the ULF wave cycle was strong, R=0.6. On 11 Jan., observations suggest the Pc5 ULF waves were in-part directly driven by Pdyn. The observed Pc4-5 ULF waves were predominately poloidal. Assuming high azimuthal m-numbers, the observations as a whole imply that the simultaneous repeating EMIC and Pc4-5 wave bursts were formed by mode cross-coupling through a common wave generation free energy source of anisotropic ion distributions created by repetitive magnetospheric compressions driven by small Pdyn fluctuations.

We have observed ~30 Hz EMIC wave on September 8, 2017 at ~10:45 to 10:55 UT. This wave is a 2XH+ EMIC wave. This wave was generated by pitch angle anisotropy of protons with energies in the range of ~40eV - 450eV. This anisotropy is likely resulted from strong background Magnetosonic wave. The high frequency EMIC wave caused precipitation of low energy particles in the ionosphere.