Dipolarization fronts and magnetic flux transport

Abstract

Recent emphasis on dipolarization fronts (DFs) has led to the impression that DFs play a significant role in bringing magnetic flux to the inner magnetosphere during substorms. In this work, we investigate the amount of magnetic flux transport associated with DFs by examining the frozen-in field line condition (FIC) for previously reported DF events. A study of 18 DF cases shows that the FIC does not hold for 17 cases when the ratio of |[Ey+(V×B)y]/(V×B)y| exceeds 0.5, i.e., the mismatch of Ey and −(V × B)y exceeds 50 %; this criterion is applied only when the electric field magnitude exceeds 0.5 mV/m to eliminate times of low-level electric fluctuations. Furthermore, the peak magnetic flux transport rate for DFs in which FIC holds is found to be in the range of ~8–42 kWb/s/RE while the accumulated flux transport within the DF intervals to be ~0.1–2.8 MWb/RE. Assuming a dawn-dusk dimension of 3 RE for a DF, the accumulated magnetic flux transport is ~0.3–8 MWb, which amounts to ~0.1–2.2 % of what is needed to account for magnetic flux increase in the near-earth dipolarization during substorms. This result casts doubt on the idea that DFs play a significant role in substorm dipolarization.