15:50 〜 16:10
[PEM13-16] 中緯度プラズマ圏であらせ衛星が観測した低域混成共鳴周波数帯の静電的波動の励起源
キーワード：プラズマ圏、プラズマ波動、雷、ホイッスラー波、磁気圏、あらせ
The lower hybrid resonance (LHR) frequency (fLHR) is an important plasma parameter to know, because fLHR controls the reflection of whistler mode waves and is related to the generation of the plasmaspheric hiss in the inner magnetosphere. The lower hybrid resonance appears near fLHR for perpendicularly propagating waves (θWNA ~ 90°, where θWNA is the wave normal angle). The lower hybrid waves, which appear just below fLHR (e.g., Graham et al., 2019), can heat thermal electrons and ions through the Landau resonance (e.g., Lashkul et al., 2001; Cairns & McMillan, 2005) and play a role in particle acceleration at the magnetopause.
Van Allen Probes (Mauk et al., 2014) observations showed that electrostatic emissions at fLHR occur around the plasmapause (Liu et al., 2021; Ouyang et al., 2022). The occurrence of the LHR-band emissions is limited around the magnetic equator (|MLAT| < 3°, where MLAT is the magnetic latitude). The proton ring distributions and the density gradient instability are the possible energy sources of the LHR-band emissions (Liu et al., 2021; Ouyang et al., 2022).
The Arase satellite (Miyoshi et al., 2018) measures plasma waves in regions that Van Allen Probes cannot cover (|MLAT| > 20°), thanks to the high inclination of the orbit. We have found that Arase frequently detects LHR-band emissions in the off-equatorial region inside the plasmapause. To understand the generation mechanism of the LHR-band emissions at mid-latitudes, we conducted a statistical analysis of the LHR-band emissions detected by Arase by analyzing PWE-OFA (Matsuda et al., 2018; Kasahara et al., 2017) and MGF (Matsuoka et al., 2018) data. Our statistics show that most of the LHR-band emissions occur in the off-equator region of |MLAT| > 20°, rather than the equatorial region, inside the plasmasphere. Their occurrence rate reaches ~70% at L = 2-3 and |MLAT| ~ 30° on the dayside. Compared with the result of Liu et al. (2021), this result indicates that the mid-latitude plasmaspheric population dominates in the inner magnetosphere. We also examined MLT, SYM-H, F10.7, and seasonal dependences of the LHR-band emissions. The variations of the occurrence rate and the wave power at L = 1-2 show a different trend from those at L > 3, suggesting a different energy source at lower L-shells. Our observational results were then compared with past statistical studies of lightning whistlers (Oike et al., 2014) and plasmaspheric hiss (Li et al., 2015; Meredith et al., 2021). We find that the MLT and seasonal dependences of lower hybrid waves at L = 1-2 are similar to that of lightning whistlers, while the SYM-H dependence at L > 3 is similar to that of the hiss waves.
To further clarify the relation between the LHR-band emissions and lightning whistlers/hiss waves, we conducted a raytracing of whistler mode waves. The result demonstrates that the latitudinal distribution of the LHR-band emissions can be attributed to the magnetospheric reflection of the whistler mode waves at the off-equator. The E/B ratio of the simulated whistler mode waves further suggests that the electrostatic nature of highly oblique whistler waves around the reflection point cannot explain the latitudinal distribution. We propose that the LHR-band emissions are excited through the mode conversion from the whistler mode to the lower hybrid mode.
The discovery of the mid-latitude plasmaspheric LHR-band emissions prompts us to reassess the acceleration and loss of electrons in a low L-shell region. In addition, the LHR-band emission is the evidence for the connection between the magnetosphere and the atmosphere.
Van Allen Probes (Mauk et al., 2014) observations showed that electrostatic emissions at fLHR occur around the plasmapause (Liu et al., 2021; Ouyang et al., 2022). The occurrence of the LHR-band emissions is limited around the magnetic equator (|MLAT| < 3°, where MLAT is the magnetic latitude). The proton ring distributions and the density gradient instability are the possible energy sources of the LHR-band emissions (Liu et al., 2021; Ouyang et al., 2022).
The Arase satellite (Miyoshi et al., 2018) measures plasma waves in regions that Van Allen Probes cannot cover (|MLAT| > 20°), thanks to the high inclination of the orbit. We have found that Arase frequently detects LHR-band emissions in the off-equatorial region inside the plasmapause. To understand the generation mechanism of the LHR-band emissions at mid-latitudes, we conducted a statistical analysis of the LHR-band emissions detected by Arase by analyzing PWE-OFA (Matsuda et al., 2018; Kasahara et al., 2017) and MGF (Matsuoka et al., 2018) data. Our statistics show that most of the LHR-band emissions occur in the off-equator region of |MLAT| > 20°, rather than the equatorial region, inside the plasmasphere. Their occurrence rate reaches ~70% at L = 2-3 and |MLAT| ~ 30° on the dayside. Compared with the result of Liu et al. (2021), this result indicates that the mid-latitude plasmaspheric population dominates in the inner magnetosphere. We also examined MLT, SYM-H, F10.7, and seasonal dependences of the LHR-band emissions. The variations of the occurrence rate and the wave power at L = 1-2 show a different trend from those at L > 3, suggesting a different energy source at lower L-shells. Our observational results were then compared with past statistical studies of lightning whistlers (Oike et al., 2014) and plasmaspheric hiss (Li et al., 2015; Meredith et al., 2021). We find that the MLT and seasonal dependences of lower hybrid waves at L = 1-2 are similar to that of lightning whistlers, while the SYM-H dependence at L > 3 is similar to that of the hiss waves.
To further clarify the relation between the LHR-band emissions and lightning whistlers/hiss waves, we conducted a raytracing of whistler mode waves. The result demonstrates that the latitudinal distribution of the LHR-band emissions can be attributed to the magnetospheric reflection of the whistler mode waves at the off-equator. The E/B ratio of the simulated whistler mode waves further suggests that the electrostatic nature of highly oblique whistler waves around the reflection point cannot explain the latitudinal distribution. We propose that the LHR-band emissions are excited through the mode conversion from the whistler mode to the lower hybrid mode.
The discovery of the mid-latitude plasmaspheric LHR-band emissions prompts us to reassess the acceleration and loss of electrons in a low L-shell region. In addition, the LHR-band emission is the evidence for the connection between the magnetosphere and the atmosphere.