Timescale for radiation belt electron acceleration by whistler mode chorus waves

TLDR

Electron acceleration by whistler‑mode chorus waves, most efficient near L = 4.5 outside the plasmapause, is required to explain the ∼MeV radiation‑belt flux increases during disturbed periods and is expected to produce energy‑dependent, flat‑topped or butterfly‑shaped pitch‑angle distributions. The study presents CRRES observations of the spatial distribution of chorus emissions during active conditions. Using CRRES wave data, the authors calculate pitch‑angle and energy diffusion rates across three magnetic local time sectors to derive an acceleration timescale. Chorus emissions in the prenoon sector most efficiently accelerate electrons at latitudes above 15° for equatorial pitch angles 20°–60°, and as electrons drift they are further accelerated on the nightside, yielding a ~1‑day timescale to increase 1 MeV flux by an order of magnitude, supporting wave‑acceleration theory.

Abstract

Electron acceleration inside the Earth's magnetosphere is required to explain increases in the ∼MeV radiation belt electron flux during magnetically disturbed periods. Recent studies show that electron acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for acceleration. We show that chorus emissions in the prenoon sector accelerate electrons most efficiently at latitudes above 15° for equatorial pitch angles between 20° and 60°. As electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave acceleration the electrons undergo many drift orbits and the resulting pitch angle distributions are energy‐dependent. Chorus scattering should produce pitch angle distributions that are either flat‐topped or butterfly‐shaped. The results provide strong support for the wave acceleration theory.