Occurrence rate of earthward-propagating dipolarization fronts

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 B_z using a hyperbolic tangent function. 303 earthward-propagating DFs are found; they have on average a duration of 4 s and a B_z increase of 8 nT. DFs have the maximum occurrence at Z _GSM 0 and r ≈ 15 R_E 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 Z_GSM 0 can be explained by the steep and large increase of B_z 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 R_E. 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 <∼13 R _E, where the magnetic field changes from the tail-like to dipolar shape. The maximum occurrence rate of DFs (one event per 3.9 hours) is comparable to that of substorms, indicating a relation between the two.