Abstract
AbstractElectromagnetic ion cyclotron (EMIC) waves are known to typically cause electron losses into Earth's upper atmosphere at >~1 MeV, while the minimum energy of electrons subject to efficient EMIC‐driven precipitation loss is unresolved. This letter reports electron precipitation from subrelativistic energies of ~250 keV up to ~1 MeV observed by the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, while two Polar Operational Environmental Satellites (POES) observed proton precipitation nearby. Van Allen Probe A detected EMIC waves (~0.7–2.0 nT) over the similar L shell extent of electron precipitation observed by FIREBIRD‐II, albeit with a ~1.6 magnetic local time (MLT) difference. Although plasmaspheric hiss and magnetosonic waves were also observed, quasi‐linear calculations indicate that EMIC waves were the most efficient in driving the electron precipitation. Quasi‐linear theory predicts efficient precipitation at >0.8–1 MeV (due to H‐band EMIC waves), suggesting that other mechanisms are required to explain the observed subrelativistic electron precipitation.
Highlights
Electromagnetic ion cyclotron (EMIC) waves occur in the Earth’s magnetosphere, typically below the Hydrogen, Helium and Oxygen ion gyrofrequencies, with frequencies of 0.1–5 Hz (Erlandson & Ukhorskiy, 2001; Fraser et al, 2010)
The region where these phase space density (PSD) minima occurred for this event overall agrees with the location, spatial extent (L*~4.7–4.9), and UT (~2–3 UT) of EMIC waves observed by RBSP-A, suggesting that these waves are important in this event for depleting the outer radiation belt electrons through scattering them into the loss cone
We report a direct observation of sub-relativistic electron precipitation from
Summary
Electromagnetic ion cyclotron (EMIC) waves occur in the Earth’s magnetosphere, typically below the Hydrogen, Helium and Oxygen ion gyrofrequencies (fcH, fcHe, fcO), with frequencies of 0.1–5 Hz (Erlandson & Ukhorskiy, 2001; Fraser et al, 2010). 10s–100s keV (more details in the Supporting Information), roughly consistent with the L shell extent and location of EMIC waves observed by RBSP-A These observations, together with ground-based measurements, provide additional evidence that EMIC waves were active longitudinally and driving scattering at MLTs between RBSP-A and FIREBIRD locations. The region where these PSD minima occurred for this event overall agrees with the location, spatial extent (L*~4.7–4.9), and UT (~2–3 UT) of EMIC waves observed by RBSP-A (and electron precipitation observed by FIREBIRD), suggesting that these waves are important in this event for depleting the outer radiation belt electrons through scattering them into the loss cone. 0.1 DSD, where DSD is the strong diffusion limit With this definition, the minimum energy of electrons subject to efficient pitch angle scattering due to EMIC waves is ~800 keV (~1 MeV). MeV (>4 MeV) into the loss cone, suggesting that other mechanisms are required to precipitate electrons at
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