Occurrence rate of earthward-propagating dipolarization fronts

Abstract

The occurrence rate of earthward-propagating dipolarization fronts (DFs) is investigated in this paper based on the 9 years (2001-2009) of Cluster 1 data. For the first time, we select the DF events by fitting the characteristic increase in Bz using a hyperbolic tangent function. 303 earthward-propagating DFs are found; they have on average a duration of 4 s and a Bz increase of 8 nT. DFs have the maximum occurrence at Z GSM 0 and r ≈ 15 RE with one event occurring every 3.9 hours, where r is the distance to the center of the Earth in the XY GSM plane. The maximum occurrence rate at ZGSM 0 can be explained by the steep and large increase of Bz near the central current sheet, which is consistent with previous simulations. Along the r direction, the occurrence rate increases gradually from r 20 to r 15 R E but decreases rapidly from r 15 to r 10 RE. This may be due to the increasing pileup of the magnetic flux from r 20 to r ≈ 15 R E and the strong background magnetic field at r