On consecutive bursts of low-latitude Pi2 pulsations

Abstract

Consecutive bursts of low-latitude Pi2 pulsations are examined with magnetic field data obtained by the Sino Magnetic Array at Low Latitudes (SMALL) in 1999. To ascertain the relationship with the substorm onset, 33 events consisting of two consecutive Pi2 bursts are selected from the observations at the Wuhan and Beijing stations, with reference to H-component magnetic bays in high-latitude magnetograms. The dominant frequencies of these consecutive Pi2 bursts are mostly in the range 10– 20 mHz . The frequencies of the two Pi2 bursts are less correlated with each other at the lower latitude station, Wuhan, than at Beijing. The dominant frequency increases as the Kp index increases. Observations with ACE and Wind satellite show that the first Pi2 burst occurs significantly after the southward turning of the interplanetary magnetic field (IMF) and the second Pi2 often occurs shortly after a northward turning of the IMF. Thus, the first Pi2 burst is not a signature of the beginning of the flux transfer to the tail but perhaps signals the initial onset of reconnection in the tail. The interplanetary data can be used to estimate the amount of reconnected magnetic flux transported to the tail. The delay time of two consecutive bursts of low-latitude Pi2 is correlated with this estimated flux pileup. If the second burst of Pi2 signals the onset of reconnection of the open field lines in the tail lobes, then this observation implies that the rate of reconnection of the closed field lines in the plasma sheet is more rapid the greater is the rate of reconnection at the nose. Thus, we would expect plasmoids to be created more quickly for strong southward IMF than for weakly southward IMF.