Cluster observation of continuous reconnection at dayside magnetopause in the vicinity of cusp

Abstract

In this paper, we present a case study of continuous reconnection at the dayside magnetopause observed by the Cluster spacecraft. On 1 April 2003, the four Cluster spacecraft experienced multiple encounters with the Earth's dayside magnetopause under a fairly stable southwestward interplanetary magnetic field (IMF). Accelerated plasma flows, whose magnitude and direction are consistent with the predictions of the reconnection theory (the Walén relation), were observed at and around the magnetopause current layer for a prolonged interval of ~3 h at two types of magnetopause crossings, one with small magnetic shears and the other one with large magnetic shears. Reversals in the Y component of ion bulk flow between the magnetosheath and magnetopause current layer and acceleration of magnetosheath electrons were also observed. Kinetic signatures using electron and ion velocity distributions corroborate the interpretation of continuous magnetic reconnection. This event provides strong in-situ evidence that magnetic reconnection at the dayside magnetopause can be continuous for many hours. However, the reconnection process appeared to be very dynamic rather than steady, despite the steady nature of the IMF. Detailed analysis using multi-spacecraft magnetic field and plasma measurements shows that the dynamics and structure of the magnetopause current layer/boundary can be very complex. For example, highly variable magnetic and electric fields were observed in the magnetopause current layer. Minimum variance analysis shows that the magnetopause normal deviates from the model normal. Surface waves resulting from the reconnection process may be involved in the oscillation of the magnetopause.