High-latitude dayside energetic precipitation and IMF Bz rotations

Abstract

[1] X-ray images from space and optical and riometer data from the ground are used to examine two discrete high-latitude dayside arcs. The dayside auroral oval is usually associated with soft precipitation of ≤ 1 keV electrons but the observed X-ray features and riometer data indicate more energetic precipitation as the X-rays are produced by electrons with energies >2.5 keVand cosmic radio absorption is associated with ∼10 keV electron precipitation. Both the X-ray images from space as well as the ground based optical and riometer data show two energetic precipitation events appearing at high latitudes and subsequently moving equatorward at ∼0.5 km/s. The average energy of the electrons is estimated to be 4-8 keV, and the energy deposition is 6-10 mW/m2. Owing to the high latitudes it is unlikely that these electrons were energized on the nightside and adiabatically drifted to the dayside. Instead, we think that the high-latitude events are controlled by the solar wind. The precipitation appears at ∼77-79o magnetic latitude and coincides with a northward turning of the interplanetary field. Owing to the uncertainties in determining the exact location of open-closed boundary we interpret the events to be caused by either KH instabilities at the inner edge of the LLBL or by lobe reconnection. In the latter case the arcs are produced by electrons accelerated by parallel electric fields resulting from converging horizontal electric fields associated with convection enhancements due to lobe reconnection at the front surface of the magnetotail. These flux tubes map to the poleward edge of the cusp. The equatorward movement is attributed to erosion of field lines as the interplanetary field turns southward with associated dayside reconnection on flux tubes with footpoints at lower latitudes. Copyright 2003 by the American Geophysical Union.