Dipolarization fronts and magnetic flux transport

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×ByV×By\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left| {{{\left[ {E_{y} + \left( {V \times B} \right)_{y} } \right]} \mathord{\left/ {\vphantom {{\left[ {E_{y} + \left( {V \times B} \right)_{y} } \right]} {\left( {V \times B} \right)_{y} }}} \right. \kern-\nulldelimiterspace} {\left( {V \times B} \right)_{y} }}} \right|$$\end{document} 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.