Characterization of Alfvenic fluctuations in the magnetopause boundary layer

Abstract

The European Space Agency GEOS 2 spacecraft happened to cross the magnetopause several times, at various local times. Intense electric and magnetic fluctuations, in the ultralow‐frequency (ULF) range (0‐10 Hz) have been detected during each such crossing, with a peak at the magnetopause and still large amplitudes in the adjacent magnetosheath and magnetopause boundary layer. By applying spectral analysis and correlations to the electric and magnetic fluctuations, and a minimum variance analysis to the magnetic fluctuations, we investigate the nature of these fluctuations which appear as short‐lasting bursts in the spacecraft frame. Having reviewed possible interpretations, we show that the observed electric and magnetic signatures are consistent with small‐scale ( L ≈ ion Larmor radius) Alfvenic field‐aligned structures passing by the spacecraft at high speed. It is suggested that these structures correspond to nonlinear Alfvenic structures.