篠原 育

Abstract Substorm energetic electron injections serve as a significant energy source for chorus wave generation, markedly altering the distribution of energetic electrons. Using the Arase satellite data, we present direct evidence for the nonlinear evolution of chorus waves following a substorm injection. The substorm injection causes the enhancement of energetic electron fluxes (∼20–200 keV) during which chorus waves appear as clear and intense rising‐tone elements. Linear theoretical analysis shows that anisotropic energetic electrons provide free energy for the generation of seed chorus waves and the enhancement of energetic electrons increases the linear growth rate. Furthermore, nonlinear theoretical analysis shows that the increase in energetic electrons reduces the threshold amplitude, which is conducive to the chorus wave entering the nonlinear growth stage. These results indicate that nonlinear growth plays a significant role in the amplification and spectral evolution of chorus waves through a decrease in the threshold amplitudes.

Abstract Ducted propagation of whistler‐mode waves has attracted attention as a process that explains how whistler‐mode waves propagate to high latitudes, resulting in the loss of relativistic electrons to the atmosphere and changes in the upper atmosphere due to electron precipitation. However, few studies have compared the observed density ducts and wave propagation characteristics to theoretical predictions in detail, particularly for low‐density ducts. We present four patterns of ducting modes as electron density increases or decreases, as observed by the Arase satellite. (a) Lower‐band (LB) waves propagating along a high‐density duct with small wave normal angles (WNAs), (b) LB waves propagating along a low‐density duct with a wide distribution of WNAs up to above the Gendrin Angle, (c) LB waves propagating along a low‐density duct with WNAs around the Gendrin Angle, and (d) upper‐band waves propagating along a low‐density duct with small WNAs. We derived the WNAs for these cases, and their characteristics were consistent with the ducting theory. Based on this theory, we calculated the frequency range in which the waves were likely to be trapped in the ducts. We compared this frequency range with the power spectra of the recorded whistler‐mode waves and found consistency between the theory and observations. Furthermore, it is suggested that the WNAs for cases (b) and (c) have azimuthal distributions based on a comparison of the WNA analysis of the simple simulated waveforms and the observed data.

Abstract Whistler‐mode chorus waves play important roles in the development of energetic electron populations in the Earth's inner magnetosphere. We have statistically analyzed rapid changes in the electron flux associated with chorus waves using data from the Arase satellite. The Arase satellite observations obtained from 23 March 2017 to 12 October 2018 show that the rapid changes are concentrated near the magnetic equator from nightside to dawnside. We compared the energy and pitch angle range of the rapid changes in the electron flux with the region bounded by the resonance energy curve of whistler mode waves which are calculated from properties of the observed chorus waves in 46 events. This comparison shows that, for most of the events, the energy and pitch angle range of the rapid changes in the electron flux can be explained by the first‐order cyclotron resonance with the observed chorus waves. We also found that the timescale for the change in the electron pitch angle distribution ranges from several seconds to a few tens of seconds. This timescale is much faster than that expected by quasi‐linear diffusion theory, suggesting that nonlinear wave‐particle interactions play important roles in the deformation of the electron pitch angle distributions.

Abstract Electrostatic Cyclotron Harmonic (ECH) waves have been considered a potential cause of pitch angle scattering of electrons in the energy range from a few hundred eV to tens of keV. Theoretical studies have suggested that scattering by ECH waves is enhanced at lower pitch angles near the loss cone. Due to the insufficient angular resolution of particle detectors, it has been a great challenge to reveal ECH‐driven scattering based on electron measurements. This study reports on variations in electron pitch angle distributions associated with ECH wave activity observed by the Arase satellite. The variation is characterized by a decrease in fluxes near the loss cone, and energy and pitch angle dependence of the flux decrease is consistent with the region of enhanced pitch angle scattering rates predicted by the quasi‐linear diffusion theory. This study provides direct evidence for energy‐pitch angle dependence of pitch angle scattering driven by ECH waves.