The Electron Absorption Signature of 1989N1

Abstract

A deep minimum in the energetic (∼ 1 MeV) electron intensity at Neptune is associated with absorption by the satellite 1989N1. The absorption signature is modeled by assuming that the electron losses are replenished by diffusion due to violation of the third adiabatic invariant. It is predominantly a steady state feature, or macrosignature, for which approximate solutions of the diffusion equation lead to an estimate of the radial diffusion coefficient, D_(LL) ≈ 2×10^(−8) s^(−1) at L =5. The model suggests a strong L dependence for D_(LL) (≈L^7),but an outward displacement of the macrosignature by ≈0.3 unit in L from the location predicted by the model, and differences in the observed and model energy spectra, may indicate that processes such as pitch angle and energy diffusion need to be included for an accurate assessment of the transport coefficients. An approximate solution is also found for the time-dependent, two-dimensional diffusion equation including particle drifts and satellite absorption. It shows that the transient microsignatures of the satellite are largely dispersed by drifts and filled in by diffusion. However, the deep intensity minimum appears to include microsignatures near each of the two satellite minimum orbital L shell locations, and these are expected from the diffusion model. At the time of the observations the satellite had recently encountered the inner location, and its microsignature was not yet significantly affected by diffusion. The observed microsignatures are closer together than the minimum orbital L shell values based on the magnetic field model and, although they are nearly consistent with absorption by 1989N1 alone, the possibility that one was formed by other orbital material cannot presently be ruled out.