Characteristics of Resonant Electrons Interacting With Whistler Waves in the Nearest Dipolarizing Magnetotail.

The Magnetospheric Multiscale spacecraft observations in the burst mode allow the determination of the characteristics of resonant electrons interacting with quasi‐parallel whistler waves during prolonged dipolarizations in the near Earth magnetotail at −22 RE < X ≤ −8 RE. We have detected 163 whistler bursts observed during 48 dipolarization events. The bursts were registered within ∼13 min following the dipolarization onset when the burst mode observations were available. In the majority of events, electrons with energies Wres ≥ 10 keV and pitch angles αres ∼ 100°–130° and αres ∼ 50°–80°made the maximum positive contribution to the growth rate of whistler waves propagating quasi‐parallel and antiparallel to the ambient magnetic field, respectively. Our analysis shows that electrons with Wres ∼ 10–20 keV could potentially be scattered into the loss cone by the low frequency whistler waves with fw ∼ (0.05–0.2)fce (fw is the wave frequency and fce is the electron gyrofrequency). The electrons that are scattered into the loss cone can contribute to electron precipitation within ∼13 min following the dipolarization onset. We suggest that whistler waves that are excited due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV) may, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. This may lead to the transformation of higher‐energy part of electron distribution from Maxwellian to the power law shape. Plain Language Summary: Magnetic dipolarization is an important process in the magnetotail dynamics which manifests in transformation of initially stretched tail‐like configuration into the dipole‐like configuration. The dipolarization is followed by various processes of energy dissipation and transformation including plasma heating and acceleration as well as generation of different wave modes. In collisionless plasma, wave‐particle interactions play a key role in energy exchange between different populations of plasma particles. Also particle interactions with waves may cause particle scattering into the loss cone and their precipitation in the auroral region. The processes of wave‐particle interactions usually occur at very short time scales, and their investigation "in situ" requires observations with high time resolution. In this study, we use the Magnetospheric Multiscale spacecraft observations in burst mode and determine the characteristics of resonant electrons interacting with quasi‐parallel whistler waves at time scales of the order of a few seconds. We have found that quasi‐parallel whistler waves can be generated in the course of dipolarization due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV). Once have been generated, these waves, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. We also found that some fraction of suprathermal electrons can be potentially scattered into the loss cone by the low‐frequency whistler waves. Such electrons can contribute to electron precipitation within ∼13 min following the dipolarization onset. Key Points: Perpendicular anisotropy of electrons with energies more than 10 keV is responsible for whistler waves generation during dipolarizationsWhistler waves play an important role in energy exchange between different parts of electron spectrum in energy range more than 2 keVElectrons with energies 10–20 keV can be scattered into the loss cone due to their interaction with the low‐frequency whistler waves [ABSTRACT FROM AUTHOR]