A Statistical Study of Spatial Variation of Relativistic Electron Precipitation Energy Spectra With Polar Operational Environmental Satellites

Abstract

The mechanisms that drive relativistic electron precipitation (REP) from the radiation belts can be better understood with a better knowledge of the particle energies involved. National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, being a network of multiple satellites, can provide multiple point spectral data over a long time period, including the Van Allen Probe's era. The number of energy channels is limited, but the particle detectors on Polar Operational Environmental Satellites have a narrow field of view allowing an investigation of bounce loss cone particles. We use the ratio of count rates in the E3 (>300 keV) and the P6 (>700 keV) channels as a parameter to define spectral hardness. Using this parameter, the spatial variation of spectral hardness of REP events was investigated. It was found that very soft events were mostly found in the dusk‐midnight‐early morning magnetic local time sectors and L∼ 5–7 while the hardest events were located in the postnoon sector peaking at L∼ 4–5. The hardest events peaked at lower L shells, and less than 20% were coincident with low‐energy (30–80 keV) proton precipitation. Further, around 70% of nightside REP coincident with proton precipitation was associated with stretched magnetic field lines indicating that curvature scattering may have been an important driver. Around 62% of nightside REP coincident with proton precipitation associated with relaxed magnetic field lines, suggesting a mechanism other than magnetic field curvature scattering, was highly energetic.

Impact of Background Magnetic Field for EMIC Wave‐Driven Electron Precipitation

“…During the time intervals of the presented case studies, the observed trapped electron flux at higher energies ( E > 2.6 MeV) that could be most effectively scattered by EMIC waves was distributed in a single belt and peaked at lower L values well inside the plasmasphere (Goldstein et al, ); thus, no detectable REP occurred at the 3:00 UT time period. The results of this case study are consistent with a recent statistical study by Shekhar et al (), which investigated electron precipitation observed by POES satellites and analyzed event hardness based on precipitation flux ratios. This study showed that a subset of precipitation that appeared to be energy selective had a shift in distribution of events with the highest‐energy events found at lower L values and earlier local times across the dayside magnetosphere and the lower energy events peaked closer to midnight at higher L values.…”

“…Therefore, other scattering mechanisms could explain such outliers. Statistical studies from Smith et al () and Shekhar et al () suggest current sheet scattering as a possible precipitation mechanism that occurs in a region that can overlap with the region of EMIC wave‐driven precipitation. Further work is required to utilize complementary satellite observations such as POES, GOES, and Van Allen Probes to look for EMIC waves associated with the REP observed by balloons and coincident proton precipitation.…”

Energetic Electron Precipitation: Multievent Analysis of Its Spatial Extent During EMIC Wave Activity

“…All the cases occurred during substorms, within the plasmasphere (at least during the periods when RBSP density measurements were available) and near the dusk sector, in agreement with previous studies (Shekhar et al, ; Yahnin et al, ), given the favorable region of EMIC wave excitation due to ring current proton injections (Jordanova et al, ). The precipitation trends at low orbit all show strong precipitation of both ring current protons and relativistic electrons (E4 channel onboard POES/MetOp), with precipitation patterns very similar across all the available LEO satellites located near the conjugate locations of the EMIC wave activity observed by RBSP.…”

“…Since both protons and electrons can interact with EMIC waves via cyclotron resonance, typical signatures of EMIC‐driven precipitation show simultaneous precipitation of tens of keV protons and relativistic electrons (Carson et al, ). Statistical studies reveal that such events occur mainly in the afternoon‐midnight sector (Shekhar et al, ; Yahnin et al, ), in agreement with the favorable location of EMIC wave enhancements in association with ring current injections (Morley et al, ; Pickett et al, ; Usanova et al, ). A recent study by Hirai et al () showed direct evidence of correlation between the rising tone EMIC wave structure and the associated bursts of REP.…”

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

“…Other case studies have also demonstrated a connection between precipitation events and EMIC waves (Blum et al, ; Clilverd et al, ; Clilverd et al, ; Hendry et al, ; Hendry et al, ; Rodger et al, ). It is possible that other mechanisms might be responsible for some of the Hendry et al () events (see, e.g., Shekhar et al, ; Smith et al, ; Yahnin et al, ). But it seems that many of these events probably are caused by EMIC waves.…”