Latitudinal and longitudinal displacement of cusp ion precipitation controlled by IMF By and Bz

Abstract

Abstract Dependence of the location of the cusp precipitation on the orientation of interplanetary magnetic field (IMF) is investigated using data from the Akebono satellite taken at altitudes of several thousands of km. More than a hundred cusp precipitation events have been identified with the low-energy particle detector (LEP) onboard Akebono. The observed energy spectra of ions precipitating at the cusp are dispersed by the convection motion of field lines. We pay special attention to the location where precipitating ions have the highest energy in the energy dispersion curve as recorded along the satellite’s path; such location would represent the one nearest to the foot point of the dayside reconnection line (we here assume that the cusp ion injection is triggered by the magnetopause reconnection). We study this location as a proxy for the foot point of the reconnection line and call it „ion entry point” in this paper. Our analysis shows that the location of the „ion entry point” has a strong dependence on the sign and magnitude of IMF B y and B z. B y and B z have almost orthogonal effects on the location of the entry point, with the latitudinal displacement linearly related to B z, and the longitudinal displacement linearly related to B y. We find that the B z dependence of the ion entry point is represented by a single regression line for entire range of B z irrespective of its north/south polarity. We also find that the B y dependence is larger for positive B z than for negative B z. The distribution of ion entry points for positive IMF B y is almost a mirror image of that for negative IMF B y with respect to the noon-midnight meridian plane. We discuss the dependence of the „ion entry point” on the IMF from the viewpoint of where and how the magnetopause reconnection (merging) occurs for various orientations of IMF.