[P-EM15] Heliosphere and Interplanetary Space

[PEM15-03] Importance of the whistler waves intensity for electron acceleration at Earth’s bow shock

*Miki Mauda¹, Takanobu Amano¹, Mitsuo Oka², Naritoshi Kitamura¹ (1.Department of Earth and Planetary Science, University of Tokyo, 2.Space Science Laboratory, University of California Berkeley, Berkeley, CA 94720-7450 USA)

Non-thermal high-energy particles are frequently observed in space. Collisionless shock waves are one of the primary sources of these non-thermal particles. In fact, particles accelerated in the vicinity of collisionless shocks have frequently been observed. However, how and when particle acceleration occurs at collisionless shocks is not understood very well. The first-order Fermi acceleration is one of the particle acceleration processes. In this mechanism, efficient particle acceleration requires resonant scattering by low-frequency MHD waves. However, low-energy electrons cannot resonate with the MHD waves and will not be accelerated. Therefore, some other processes are necessary to accelerate electrons up to the energy where the first-order Fermi acceleration becomes efficient. To solve this problem, Katou & Amano (2019) suggested a mechanism called stochastic shock drift acceleration. In this mechanism, the cyclotron resonance scattering with high-frequency waves is important. The most promising candidate for the scattering agent is whistler waves with frequencies ranging from 10% to 50% of the electron cyclotron frequency. On the other hand, Oka et al. (2006) showed statistically that the electron acceleration efficiency tends to be higher when the shock is super-critical with respect to the whistler critical Mach number. This result suggests that the whistler waves play a role in the non-thermal electron acceleration at Earth's bow shock. However, the consistency with theory has not yet been understood.
Katou & Amano (2019) and Amano et al. (2020) predicts that electron acceleration occurs only when the intensity of the whistler waves exceeds a certain threshold. The theoretical threshold is proportional to (M_A/cosθ_Bn)^-2 and is qualitatively consistent with Oka et al. (2006) if we assume that the wave intensity is constant. However, the whistler wave intensity will, in several, depends on shock parameters. In this study, we investigate statistically the relation between the whistler wave intensity and shock parameters at Earth's bow shocks and examine the validity of the theoretical threshold.
We use the data of shock crossing events observed by the Magnetospheric Multiscale (MMS) spacecraft in burst mode. First, we calculate the time variation of whistler wave intensity with a time resolution of 1s during the shock transition including upstream and downstream, by using Search Coil Magnetometers(SCM) data. We define the transition layer as the region where high-energy (~1keV) electrons are effectively confined and investigate the relationship between the whistler wave intensity in this region and various shock parameters. We find a positive correlation, in particular with M_A/cosθ_Bn. Based on the finding, we discuss the relation between the electron acceleration efficiency and the whistler wave.