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 $$\left| {{{\left[ {E_{y} + \left( {V \times B} \right)_{y} } \right]} \mathord{\left/ {\vphantom {{\left[ {E_{y} + \left( {V \times B} \right)_{y} } \right]} {\left( {V \times B} \right)_{y} }}} \right. \kern-\nulldelimiterspace} {\left( {V \times B} \right)_{y} }}} \right|$$ exceeds 0.5, i.e., the mismatch of E y 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/R E while the accumulated flux transport within the DF intervals to be ~0.1–2.8 MWb/R E. Assuming a dawn-dusk dimension of 3 R E 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.
Highlights
Two recently very popular topics in magnetospheric research are fast transient plasma flows, called bursty bulk flows (BBFs) (e.g., Angelopoulos et al 1992; Nakamura et al 2001), and dipolarization fronts (DFs) (e.g., Nakamura et al 2002; Runov et al 2009; Schmid et al 2011)
This is done in this work by comparing the values of Ey component with the values of −(V × B)y, where Ey is the y-component of the electric field, V is the plasma bulk flow, and B is the magnetic field
The field line condition (FIC) that allows magnetic flux being transported by plasma motion does not hold for many DFs
Summary
Two recently very popular topics in magnetospheric research are fast transient plasma flows, called bursty bulk flows (BBFs) (e.g., Angelopoulos et al 1992; Nakamura et al 2001), and dipolarization fronts (DFs) (e.g., Nakamura et al 2002; Runov et al 2009; Schmid et al 2011). Statistical studies show that BBFs carry significant amount of mass, energy, and magnetic flux earthward (Angelopoulos et al 1994; Liu et al 2011). BBFs and DFs are associated phenomena, but their time scales are different. The time scale of BBFs is ~10 min, while that of DFs is ~1–2 min. In spite of the short durations for DFs, their magnetic flux transport is estimated to be just as significant as BBFs. In particular, Liu et al (2013, 2014) studied the magnetic flux transport by DFs, referring them as dipolarization flux bundles and proposing them as elementary elements for the substorm current wedge
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