Generation and propagation of cyclotron maser emissions in the finite auroral kilometric radiation source cavity

Abstract

The properties of the electron‐cyclotron maser radiation produced in the auroral kilometric radiation (AKR) source cavity are investigated by means of two‐dimensional particle‐in‐cell simulations and observations made by the Fast Auroral SnapshoT (FAST) Explorer. The simulations assume a population of primary auroral electrons with a downgoing shell distribution. Driven simulations in a meridional plane with a finite magnetic field gradient demonstrate that the maser radiation builds up very rapidly with decreasing altitude and that bursts of radiation with timescales of the order of 0.5 ms are produced at only 5–6 km below the injection level. Initial value simulations in a longitudinal plane show that the radiation component striking the cavity boundary at normal incidence is damped away via particle absorption and conversion to the Z mode. In contrast, the longitudinally propagating component is able to be amplified along an extended path length. The FAST observations of the electric field in the source region reveal that the polarization in the plane perpendicular to the ambient magnetic field varies between being approximately isotropic to having a substantial (as much as a factor of 100) enhancement of the “along‐track” polarization. No evidence is found that the “across‐track” polarization is ever dominant, and thus the AKR emissions do not form a standing wave structure between the cavity boundaries.

High spectral and temporal resolution observations of Saturn kilometric radiation

“…[16] As pointed out by Pritchett et al [2002], there is no consensus theory for the more highly studied terrestrial kilometric radiation fine structure. Given this situation and no in situ observations in Saturn's source region at present, it is premature to expect a complete theoretical understanding of Saturn's fine structure.…”

“…Calvert [1982] suggested that a mechanism involving feedback, similar to a laser, could explain the narrowband spectrum. Pritchett et al [2002] argue that the feedback model is an unlikely explanation for the fine structure. Pottelette et al [2001] suggest that electron or ion phase space holes propagating in the auroral current regions could act as tiny radiation sources within the source region and their small size and motions could account for the narrowband drifting features in AKR.…”

“…Based in large part on theory developed for AKR at Earth [cf. Wu and Lee, 1979;Pritchett et al, 2002], it is assumed that Saturn's kilometric emissions are generated via the cyclotron maser instability near the electron cyclotron frequency f ce at the source [Galopeau et al, 1989].…”

Multipoint observations of plasma phenomena made in space by Cluster

“…Freely propagating electromagnetic radiation can be produced near f ce by the (relativistic) cyclotron maser instability (CMI) (Wu and Lee 1979;Fung and Viñas 1994;Pritchett et al 2002). Because there was no way to establish connections between separate observations near the source and observations at remote locations, it was never possible to associate the remote AKR observations with specific source regions.…”

“…Prior to the launch of Cluster, observations and theoretical modeling had shown that AKR was most likely generated by electrons moving along active auroral magnetic field lines and through electron density cavities (e.g., Delory et al 1998;Pritchett et al 2002). Freely propagating electromagnetic radiation can be produced near f ce by the (relativistic) cyclotron maser instability (CMI) (Wu and Lee 1979;Fung and Viñas 1994;Pritchett et al 2002).…”