SCATHA measurements of electron decay times at 5 < L ≤ 8

Abstract

The electron decay timescale (τ) is well known to be associated with radiation belt loss processes. Knowledge of τ is important for understanding pitch angle and radial diffusion mechanisms. Previous studies reported decay timescales from the inner belt to geosynchronous orbits; however, relatively few statistical studies have been focused on the region beyond the traditional outer belt. In this paper, a systematic calculation of electron decay times at 5 < L ≤ 8 is performed using 11 years (1979–1989) of electron data from the Spacecraft Charging AT High Altitudes (SCATHA) satellite. The decay timescale is determined using daily median fluxes, providing resolution of τ > 1 day. τ is examined as a function of energy, pitch angle, L shell, Kp, and AE during magnetically disturbed periods when Dst ≤ −50 nT. Results show that τ increases with increasing electron energy at L < 6.6 for electron energies from 50 to 300 keV, but is independent of energy at L > 6.6. This suggests radial transport as the dominant effect at L > 6.6. Additionally, τ decreases with increasing L‐shell. This dependence has the strongest correlation and is seen in all energies and pitch angles. However,τ has no systematic dependence with pitch angle suggesting that pitch angle diffusion also plays a key role in the electron loss process. Based on our results, τ can be expressed as a function of energy and L, and coefficients are provided for a two‐variable fit. Surprisingly,τ is slightly longer for higher activity cases at L < 6.6, which is inconsistent with the current radial or pitch angle diffusion models. Global effective decay times on the timescale of days place an upper bound on the true loss timescale.