5:15 PM - 7:15 PM
[PEM13-P03] Direct Observations of Loss Cone Electron Input in the Dayside High-Latitude Inner Magnetosphere
Keywords:energetic electron precipitation, wave-particle interaction, dayside magnetosphere, whistler mode chorus waves, Arase
Whistler mode chorus waves are detected primarily outside the plasmapause from the nightside to the dayside in the inner and outer magnetosphere. Nightside chorus waves are frequently observed at geomagnetically disturbed times and are thought to be mostly excited through linear wave growth due to electron temperature anisotropy. On the other hand, dayside chorus waves are much more frequently detected, and both linear and non-linear growth are involved in their generation in the dayside off-equatorial regions. Thus, the dayside chorus waves have different generation mechanisms and wave characteristics from the nightside ones.
The resonance energy tends to become higher in the off-equatorial regions due to larger magnetic field intensity. Additionally, the occurrences of high-latitude chorus waves are more frequent around the dayside than the nightside. Therefore, dayside chorus waves are considered to contribute to higher-energy electron precipitation. Electron measurements with the sounding rockets and numerical simulations have suggested that the higher-energy (~sub-relativistic) electron precipitation is caused by the interaction with dayside chorus waves propagating to higher latitudes (Miyoshi et al., 2010; Namekawa et al., 2023). Furthermore, the previous statistical survey of loss cone electrons by Takahara et al. (2025) also indicates the contribution of high-latitude dayside chorus waves to higher-energy (tens of keV) electron precipitation in the regions of MLT > 3.
Although the role of dayside high-latitude chorus waves in scattering electrons into the loss cone has been discussed for a long time, quantitative analysis of them has just recently been enabled by electron analyzers onboard the Arase satellite that is capable of electron measurement with high angular resolution enough to resolve small loss cone angles in the magnetosphere. Moreover, the Arase satellite performs electron measurements up to |MLAT| = 40°, providing superior spatial coverage compared to other satellites. In this study, we investigated loss cone filling events that occurred at higher latitudes (10° < |MLAT| < 40°) from the dawnside to the dayside (MLT: 3–15) for electrons with energies of 42–88 keV observed by the Arase satellite from March 2017 to March 2022.
We found that chorus waves are the primary cause for loss cone electron input events (Type 1) in these regions, especially at 10° < |MLAT| < 20°. In addition, the occurrence probability of Type1 events decreases as it moves to higher magnetic latitudes and is limited up to |MLAT| < 25°. This spatial distribution may result from the spatial distribution of the resonance energy defined by the ambient magnetic field intensity and electron density and/or the less occurrence of intense chorus waves at higher latitudes, which cause electron scattering into the loss cone. Besides, the loss cone filling events not associated with chorus waves (Type 2) are found to frequently occur around the dawnside (MLT = 3–9) at 20° < |MLAT| < 40°. This presentation also discusses the possible factors for Type 2 events.
The resonance energy tends to become higher in the off-equatorial regions due to larger magnetic field intensity. Additionally, the occurrences of high-latitude chorus waves are more frequent around the dayside than the nightside. Therefore, dayside chorus waves are considered to contribute to higher-energy electron precipitation. Electron measurements with the sounding rockets and numerical simulations have suggested that the higher-energy (~sub-relativistic) electron precipitation is caused by the interaction with dayside chorus waves propagating to higher latitudes (Miyoshi et al., 2010; Namekawa et al., 2023). Furthermore, the previous statistical survey of loss cone electrons by Takahara et al. (2025) also indicates the contribution of high-latitude dayside chorus waves to higher-energy (tens of keV) electron precipitation in the regions of MLT > 3.
Although the role of dayside high-latitude chorus waves in scattering electrons into the loss cone has been discussed for a long time, quantitative analysis of them has just recently been enabled by electron analyzers onboard the Arase satellite that is capable of electron measurement with high angular resolution enough to resolve small loss cone angles in the magnetosphere. Moreover, the Arase satellite performs electron measurements up to |MLAT| = 40°, providing superior spatial coverage compared to other satellites. In this study, we investigated loss cone filling events that occurred at higher latitudes (10° < |MLAT| < 40°) from the dawnside to the dayside (MLT: 3–15) for electrons with energies of 42–88 keV observed by the Arase satellite from March 2017 to March 2022.
We found that chorus waves are the primary cause for loss cone electron input events (Type 1) in these regions, especially at 10° < |MLAT| < 20°. In addition, the occurrence probability of Type1 events decreases as it moves to higher magnetic latitudes and is limited up to |MLAT| < 25°. This spatial distribution may result from the spatial distribution of the resonance energy defined by the ambient magnetic field intensity and electron density and/or the less occurrence of intense chorus waves at higher latitudes, which cause electron scattering into the loss cone. Besides, the loss cone filling events not associated with chorus waves (Type 2) are found to frequently occur around the dawnside (MLT = 3–9) at 20° < |MLAT| < 40°. This presentation also discusses the possible factors for Type 2 events.