A statistical study of magnetic dipolarization for sawtooth events and isolated substorms at geosynchronous orbit with GOES data

Abstract

We investigate whether the dipolarization process during sawtooth events is global or not through an examination of the local time distribution of the magnetic tilt angle (i.e. the angle between the B vector and the equatorial plane in dipole VDH coordinates) at geostationary orbit. From the statistical analysis of 207 individual teeth and 212 isolated substorms, we find that individual teeth follow a dipolarization pattern similar to that observed for isolated substorms. The dipolarization for individual teeth initiates in the sector between 22:00 magnetic local time (MLT) and 00:00 MLT. Then the dipolarization expands both eastward and westward. However, relative to isolated substorms, we find that the nightside magnetosphere is much more stretched prior to the onset, and the change of the tilt angle during the dipolarization process is much larger during individual teeth. The magnetic dipolarization is seen over a wider local time sector during individual teeth than during isolated substorms. However, the magnetic dipolarization is rarely observed near local noon during individual teeth. This suggests that the magnetic dipolarization process during individual teeth is still confined primarily to the nightside.