Pitch Angle Scattering of Sub‐MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves

Abstract

Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low‐amplitude short‐wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub‐megaelectron volts (sub‐MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves.

Direct Observation of Subrelativistic Electron Precipitation Potentially Driven by EMIC Waves

“…It is important to note that the simulation results without (with) including hot plasma effects due to EMIC waves can only explain the electron precipitation at energies >~800 keV (>~1 MeV), requiring other mechanisms to explain the low energy electron precipitation down to ~250 keV. For example, nonresonant scattering (e.g., Chen et al, ) and nonlinear effects (Denton et al, ; Hendry et al, ; Kubota et al, ; Omura & Zhao, , ) are suggested to lower the E min down to a few 100s keV. However, the evaluation of these effects is beyond the scope of this case study and left as future investigation.…”

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…Do EMIC waves only act on ultrarelativistic electrons (cf. Denton et al, , in this collection and discussion in section )? Another question that warrants deeper investigation is whether EMIC scattering occurs significantly or not in the plasmasphere and inner zone.…”

“…These waves drive considerable contemporary scientific interest, particularly during the recent Van Allen Probes mission. Many recent studies are dedicated to the loss they cause to ultrarelativistic electrons (e.g., Thorne & Kennel, ; Albert, ; Jordanova et al, ; Miyoshi et al, ; Rodger et al, , Rodger et al, ; Li et al, , 2014; Usanova et al, , ; Kersten et al, ; Blum et al, ; Clilverd et al, ; Woodger et al, , ; Colpitts et al, ; Shprits et al, , , , ; Hendry et al, , ; Zhang et al, ; Aseev et al, ; Drozdov, Shprits, Usanova, et al, ; Capannolo et al, , ; Denton et al, ; Qin et al, ), themselves related to the complex location and duration of these waves. EMIC waves are discrete electromagnetic emissions in multiple frequency bands (e.g., Saikin et al, ), which are observed across a large region of geospace (e.g., Saikin et al, ), including the ring current and the plasmasphere, dayside plumes, and the outer dayside magnetosphere (Engebretson et al, ; Engebretson et al, ; Engebretson et al, ; Tetrick et al, ).…”

“…For instance, Omura et al (2009) provide the comparison between a hybrid and a full computation in which the energetic and cold components of electrons are treated as particles. Hybrid codes are used to investigate the self‐consistent generation of whistler waves in the inner magnetosphere, such as the nonlinear generation and growth mechanisms of chorus waves (e.g., Katoh & Omura, 2004, 2006, , 2013; Wu et al, ; da Silva et al, ) and EMIC waves (e.g., Hu & Denton, ; Hu et al, ; Denton et al, , in this collection). These methods have significant potential.…”

“…These methods have significant potential. For instance, Denton et al () in this collection showed that nonlinear interactions with EMIC waves can cause precipitation of sub–megaelectron volt electrons, while the general assumption based on quasi‐linear resonant interactions is that the dominant interactions occur for >~2‐MeV electrons (e.g., Kersten et al, , and references within). Recent multi‐instrument observations from Hendry et al () corroborate this finding, showing one event of nonlinear EMIC‐driven electron precipitation at sub–megealectron volt energies.…”