Correlation between proton anisotropy and magnetic field direction in the distant Geotail

Abstract

We report the results of a statistical analysis of the anisotropy of suprathermal protons and of the polarity of the magnetic field during the time period April 10‐16, 1983, when the ISEE‐3 spacecraft was at lunar distances (˜61‐78 radii) in the geomagnetic tail of the earth, well within the nominal magnetopause. Using 128 sec time averages of the angular distributions of 33 keV protons, we correlate the first order anisotropy with the latitude angle, θ, and the z‐component, Bz, of the magnetic field. We find that the frequency distribution of the anisotropy direction, in particular for large anisotropies, is strongly peaked in the earthward and tailward direction, indicative of fast earthward and tailward flows. For large anisotropies and within 5 RE (earth radii) of the nominal neutral sheet position we find a strong correlation of earthward streaming suprathermal protons with northward polarity of the magnetic field whereas large tailward anisotropies are generally correlated with southward magnetic field polarity. The simplest interpretation of this correlation is in terms of a neutral line (or reconnection) model, with the neutral line being tailward or earthward of the spacecraft, respectively.