Consequences of geomagnetic activity on energization and loss of radiation belt electrons by oblique chorus waves

Abstract

Statistics of amplitudes and obliquity of lower band chorus whistler mode waves have been obtained from Cluster measurements in Earth's outer radiation belt and fitted as functions of L, latitude, magnetic local time, and three geomagnetic activity ranges for Dst∈[+10,−80] nT. Very oblique chorus waves have generally a much smaller average intensity than quasi‐parallel waves, especially on the nightside. Nevertheless, analytical estimates and full numerical calculations of quasi‐linear diffusion rates show that dayside very oblique waves (θ>60°) dominate pitch angle scattering of energetic electrons during moderately disturbed periods. As geomagnetic activity increases, leading to higher wave amplitudes, electron lifetimes are only slightly reduced, due to a decrease of the wave obliquity probably related to Landau damping by stronger incoming fluxes from the plasma sheet. As a result, electron energization by chorus waves for Dst>−80 nT generally occurs in a loss‐dominated regime in which energization increases at lower L. However, at L≥6 the most disturbed periods (Dst<−40 nT) produce a stronger energization independent of losses. Double‐belt structures may therefore arise when Dst<−40 nT, with two peaks of energization located just outside the plasmapause and at L∼6. The variability of lower band chorus wave obliquity with geomagnetic activity could actually account for some part of the observed variability of energization and loss in the outer belt. It is also suggested that quasi‐linear pitch angle diffusion by very oblique waves together with energy diffusion by parallel waves might contribute to the steep wave growth observed in the day sector between the equator and 25°.