Association between energetic particle bursts and Birkeland currents in the geomagnetic tail

Abstract

IMP 8 observations in the geomagnetic tail of energetic particles at hundreds of keV and of the magnetic field are examined for a substorm period from 1700 to 1800 UT on March 3, 1976. For this interval, it is found that bursts of energetic protons detected at a downstream distance of ∼37 RE in the midnight tail region are accompanied by east‐west magnetic field perturbations indicative of Birkeland current sheets. The magnitude of the magnetic field perturbations ranges from ∼1 to 12 nT, corresponding to current densities of 1 to 9 mA/m when integrated over the current sheet thickness. These values are comparable to previous observations of Birkeland currents in the near‐earth tail (X > −20 RE) and with the auroral‐arc‐associated Birkeland current densities at low altitudes scaled out to the tail region. The favorable comparison suggests that the closure of small‐scale Birkeland current systems, at least in these instances, lies tailward of ∼40 RE. The observed Birkeland current system and the magnitude of current densities are found to be consistent with ion tearing instability in the tail. The inferred wavelength of the tearing mode along the tail axis is ∼2 to 17 RE. In the context of the tearing model, the presence of Birkeland currents and the occasional isotropic distribution of energetic electrons (Ee > 0.22 MeV) suggest that the energetic particle events in this study occur on closed magnetic field lines, even though strong tailward streaming dominates the particle anisotropy in these events.