A substorm-associated drift echo of energetic protons observed by Geotail: Radial density gradient structure

Abstract

We found a drifting population with a fairly long (∼several tens of minutes) dispersion at a geocentric distance >9 RE, indicating that energetic particles can lie on a closed drift path around the Earth much farther than the geosynchronous distance. In this event, Geotail was situated in the plasma sheet in the post-midnight sector at a distance of ∼10 RE and observed a drift echo of energetic protons with energies of several tens to hundreds of keV. This drifting population showed an azimuthal sector anisotropy in particle distribution as it passed by the spacecraft. The drift echo showed a lack of duskward-directed particle fluxes in its leading portion, became isotropic in the middle of the event, and finally showed enhanced duskward-directed fluxes in its trailing portion. Since the magnetic field was dominated by the Bz component during this event, this sequence of anisotropy can be considered in terms of a density gradient structure of the drifting population with a scale of the order of proton gyroradius (∼0.5 RE) in the equatorial plane. Because the duskward-directed flux at the spacecraft can be attributed to protons with their guiding centers sunward of the observation point, it is suggested that the drifting proton echo has a spatial structure such that protons are localized tailward of the spacecraft at the leading portion, while a sunward density gradient exists around the trailing portion. Thus the present study shows that a drift echo is not an uniform population but has a complicated structure with a sharp density gradient in the radial direction, which may reflect some characteristics of its source site as well as its drift path.