Daytime equatorial geomagnetic H field response to the growth phase and expansion phase onset of isolated substorms: Case studies and their implications

Abstract

Observations are presented from the Indian magnetometer network (dipole latitude range 1.2°S to 13.5°N) of short-lived (<1 hr) disturbances in the daytime equatorial geomagnetic H field associated with specific phases of isolated substorms. Three well-documented substorms are examined here, of which the expansion onset of each is closely associated with sudden transitions of interplanetary magnetic field (IMF) Bz/By after a >40-min interval of southward Bz. A positive baylike perturbation is found to prevail during the substorm growth phase, followed by a negative baylike disturbance starting precisely at the onset of expansion phase activity. The amplitude of the positive as well as negative bay-type disturbance showed a clear-cut enhancement at locations inside the equatorial electrojet belt when compared with stations away from the electrojet influence, indicative of a significant contribution of ionospheric currents to the bays. This pattern of response, which is found in two out of the three events studied, constitutes the first-time evidence for the occurrence of equatorial H field perturbations related to the growth phase as well as the expansion phase onset for individual substorms. The H field perturbations are suggested to be signatures of prompt penetration by electric fields associated with rapid changes in magnetospheric convection brought about by the swift transitions in the IMF Bz, before shielding by the ring current becomes effective. In the third event, where the growth phase is weak and prolonged, there is no perceptible simultaneous disturbance in the equatorial H field, while the expansion phase onset is associated with a positive bay of very small amplitude (≤4 nT).