Evidence that crater flux transfer events are initial stages of typical flux transfer events

Bipolar magnetic perturbations along the normal to the local magnetopause associated with field magnitude enhancements are signatures of typical flux transfer events (T-FTEs) and are interpreted as evidence of encounters with magnetic flux ropes with strong core fields. If the field magnitude dips at the center of the signature, we identify the event as a crater FTE (C-FTE). In the multiple-spacecraft data of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) between 1 May and 31 October 2007, we have identified 622 FTEs of which only 23 manifested C-FTE signatures. We analyze a C-FTE (30 July 2007) that evolved into a T-FTE and compare its properties with those of a T-FTE (May 20, 2007). For all 23 C-FTEs and 35 confirmed T-FTEs, we compare solar wind conditions and internal plasma and field properties. The similarity of solar wind properties for events in the two classes suggests that differences in their structures are not related to the solar wind conditions. Systematic differences in internal peak fields (BC-FTE < B-Magnetosphere < BT-FTE) and averaged number densities (NT-FTE < 0.5 x N-Magnetosheath < NC-FTE) between the two groups are consistent with the evolution of C-FTEs into T-FTEs. We propose that parallel flows inside C-FTEs deplete the internal ion densities and reduce the thermal pressures as the central field magnitude increases to maintain pressure balance.