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 R_E < X ≤ −8 R_E. 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 W_res ≥ 10 keV and pitch angles α_res ∼ 100°–130° and α_res ∼ 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 W_res ∼ 10–20 keV could potentially be scattered into the loss cone by the low frequency whistler waves with f_w ∼ (0.05–0.2)f_ce (f_w is the wave frequency and f_ce 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.