Average magnetotail electron and proton pitch angle distributions from Cluster PEACE and CIS observations

Abstract

We present results from the first systematic survey of proton and electron pitch angle distributions in the magnetotail, based on Cluster CIS and PEACE data binned by proton plasma beta (beta(p)). The proton distributions conform to the canonical picture of magnetotail ions - a made up of Earthward streaming and bidirectional field-aligned particles, consistent with recent observations of time-varying beamlets, which gives way to a broadly isotropic central plasma sheet when beta(p) similar to 3. The electron distributions are significantly different from the canonical picture. A boundary layer made up of bidirectional field-aligned electrons is observed to values of beta(p) as high as 17. This quickly gives way to perpendicular-dominated electrons close to the neutral sheet. Hence, our results suggest that, on average, there is no extended, isotropic electron plasma sheet and that the proton plasma sheet is not routinely encountered until higher beta(p) than commonly assumed. Citation: Walsh, A. P., C. J. Owen, A. N. Fazakerley, C. Forsyth, and I. Dandouras (2011), Average magnetotail electron and proton pitch angle distributions from Cluster PEACE and CIS observations, Geophys. Res. Lett., 38, L06103, doi:10.1029/2011GL046770.