A statistical study of ELF‐VLF plasma waves at the magnetopause

Abstract

A statistical study of the broadband ELF‐VLF plasma waves at the magnetopause has been performed using ISEE 1 plasma wave data. It is found that enhanced wave intensities are detected at 85% of all magnetopause crossings. Although wave amplitudes are highly variable from event to event and even within an event (particularly during high‐intensity events), the wave spectra averaged over many passes are remarkably similar at dawn, noon, and dusk local hours. Thus to first order, the average wave intensity and spectral shape are independent of local time. At select frequencies, the intensities are 10 −8 , 10 −13 , and 3 × 10 −17 V 2 /m 2 Hz at 10 1 , 10 3 , and 10 5 Hz and 3 × 10 −4 and 5 × 10 −8 nT 2 /Hz at 10 1 and 10 2 Hz, respectively. The average wave intensity is independent of latitude, magnetosheath field strength, and magnetopause position. The only parameter found to be correlated with wave intensity is the magnitude of the Z component of the magnetosheath magnetic field. The broadband wave intensities increase with increasing negative B z . These observational results put strong constraints on any proposed generation mechanism for the broadband magnetopause boundary layer waves. Reasonable mechanisms should be able to explain the lack of local time, latitude, and interplanetary parameter dependences. The dependence of IMF B z and possible correlation with magnetic reconnection should be a principal feature in the model. These observations support the conjecture that cross‐field diffusion of magnetosheath plasma by wave particle interaction with these waves (Tsurutani and Thorne, 1982) is the steady state source of the low‐latitude boundary layer, and that pitch angle scattering and the consequential particle precipitation into the ionosphere is the mechanism for the dayside aurora. For cyclotron resonant interaction with E >100 eV electrons during southward IMF B z events, a total precipitation rate of 1.0 to 1.2 ergs/cm 2 s is determined.