Anomalous Trapping of Low Pitch Angle Electrons by Coherent Whistler Mode Waves. Issue 7 (25th July 2019)

• Record Type: Journal Article
• Authors: Kitahara, M.; Katoh, Y.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 5568
• Page End: 5583
• Publication Date: 2019-07-25
• Subjects: whistler mode chorus emissions -- electron acceleration -- wave‐particle interaction; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026493
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Chorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v ‖ × B w Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution. Key Points: Coherent whistler mode waves cause anomalous trapping of electrons at low pitch angles Test particle simulations demonstrate the theoretically predicted motion of electrons The relation between wave intensity and electron flux near the loss cone is not proportional