Masahiro Kitahara

Method for direct detection of pitch angle scattering of energetic electrons caused by whistler‐mode chorus emissions

The Wave-Particle Interaction Analyzer (WPIA), a new instrument proposed by Fukuhara et al. (2009), measures the relative phase angle between the wave magnetic field vector and the velocity vector of each particle and calculates the energy exchange from waves to particles. In this study, we expand its applicability by proposing a method of using the WPIA to directly detect pitch angle scattering of resonant particles by plasma waves by calculating the g values. The g value is defined as the accumulation value of the Lorentz force acting on each particle and indicates the lost momentum of waves. We apply the proposed method to the results of a one-dimensional electron hybrid simulation reproducing the generation of whistler mode chorus emissions around the magnetic equator. Using the wave and particle data obtained at fixed observation points assumed in the simulation system, we conduct a pseudo-observation of the simulation result using the WPIA and analyze the g values. Our analysis yielded significant values indicating the strong pitch angle scattering for electrons in the kinetic energy and pitch angle ranges satisfying the cyclotron resonance condition with the reproduced chorus emissions. The results of this study demonstrate that the proposed method enables us to directly and quantitatively identify the location at which pitch angle scattering occurs in the simulation system and that the method can be applied to the results of space-based observations by the forthcoming Exploration of energization and Radiation in Geospace (ERG) satellite.

Direct measurements of two-way wave-particle energy transfer in a collisionless space plasma

Two-step energy transfer in space plasma Plasmas are ionized gases that contain negative electrons, positive ions, and electromagnetic fields. These constituents can oscillate in position over time, carrying energy as plasma waves. In principle, such waves could transfer energy between two different ion populations. Kitamura et al. analyzed data from the Magnetospheric Multiscale mission, a group of four spacecraft that are flying in tight formation through Earths magnetosphere. They discovered an event in which energy was transferred from hydrogen ions to plasma waves and then from the waves to helium ions. This energy transfer process is likely to occur in many other plasma environments. Science, this issue p. 1000 Energy transfer between H+ ions, plasma waves, and He+ ions is observed in a space plasma. Particle acceleration by plasma waves and spontaneous wave generation are fundamental energy and momentum exchange processes in collisionless plasmas. Such wave-particle interactions occur ubiquitously in space. We present ultrafast measurements in Earth’s magnetosphere by the Magnetospheric Multiscale spacecraft that enabled quantitative evaluation of energy transfer in interactions associated with electromagnetic ion cyclotron waves. The observed ion distributions are not symmetric around the magnetic field direction but are in phase with the plasma wave fields. The wave-ion phase relations demonstrate that a cyclotron resonance transferred energy from hot protons to waves, which in turn nonresonantly accelerated cold He+ to energies up to ~2 kilo–electron volts. These observations provide direct quantitative evidence for collisionless energy transfer in plasmas between distinct particle populations via wave-particle interactions.

Method for direct detection of pitch angle scattering caused by plasma waves

We propose a new method to directly detect pitch angle scattering of energetic particles caused by plasma waves in space plasma. The Wave-Particle Interaction Analyzer (WPIA), a new instrument proposed by [1], measures the energy exchange from waves to particles. In this study, we expand its applicability to detect pitch angle scattering of resonant particles by plasma waves by calculating g values. g value is defined as the accumulation value of the Lorentz force acting on each particle and indicates the lost momentum of waves. We apply the proposed method to the results of a one-dimensional electron hybrid simulation reproducing the generation of whistler-mode chorus emissions around the magnetic equator [2, 3]. Using the wave and particle data obtained at fixed observation points assumed in the simulation system, we conduct a pseudo-observation of the simulation result using the WPIA and analyze the g values. Our analysis yielded significant values indicating the strong pitch angle scattering for electrons in the kinetic energy and pitch angle ranges satisfying the cyclotron resonance condition with the reproduced chorus emissions. The results of this study demonstrate that the proposed method enables us to directly and quantitatively identify the location at which pitch angle scattering occurs in the simulation system and that the method can be applied to the results of space-based observations by the Exploration of energization and Radiation in Geospace (ERG) satellite.