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

Abstract

Abstract. Observations are presented from the Indian magnetometer network (dipole latitude range 1.2øS to 13.5øN) 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).

Statistical analysis of dayside equatorial ionospheric electric fields and electrojet currents produced by magnetospheric substorms during sawtooth events

“…In fact, Sastri et al [2001] reported a case in which a positive bay occurred in the dayside magnetometer H component after a substorm onset, and they termed it “rather puzzling.” If the IMF was southward and stable in their case, the substorm onset would cause an eastward electric field in the dayside ionosphere and result in a positive bay. Kikuchi et al [2000, 2003]found that the substorm‐associated counter‐electrojet is induced by the dominant region 2 field‐aligned currents when the region 1 field‐aligned currents decrease abruptly because of the northward turning of the IMF.…”

Variations of low‐latitude geomagnetic fields and Dst index caused by magnetospheric substorms

“…The slight decrease in the magnitude of the nightside westward electric field at Jicamarca is very likely to be associated with the gradual decrease of the IMF B z magnitude after 0305 UT. In previous studies, low‐latitude ionospheric response to substorm onsets was described as an eastward electric field perturbation on the nightside [ Fejer et al , 1979; Gonzales et al , 1979] and a decrease in the ground magnetometer H component on the dayside [ Kikuchi et al , 2000; Sastri et al , 2001]. However, the substorm onsets in the previous observations were related to IMF northward turnings.…”

“…A number of significant issues associated with low‐latitude ionospheric signatures of substorms are not well understood. As mentioned above, it was reported that a negative bay of the dayside magnetometer H component was coincident with the substorm onsets [ Kikuchi et al , 2000; Sastri et al , 2001]. However, it is not clear whether the negative bay was caused by a substorm onset or by a northward IMF turning that triggered the substorm onset.…”

“…The substorm onsets in their cases were coincident with northward reversals of the IMF. Kikuchi et al [2000] and Sastri et al [2001] found that the dayside magnetometer H component at middle and low latitudes showed a decrease (a negative bay) after substorm onsets. Sastri et al [2001] also reported on an event in which a positive bay occurred in the H component of low‐latitude magnetometers during the substorm expansion phase.…”

Quantification and hemispheric asymmetry of low‐latitude geomagnetic disturbances caused by solar wind pressure enhancements

“…When the IMF turns from northward to southward, the interplanetary electric field can penetrate into the equatorial ionosphere and cause an eastward electric field in the dayside ionosphere, resulting in disturbances in the ionospheric currents and magnetic field. Magnetospheric substorms also generate geomagnetic disturbances at middle and low latitudes [Kikuchi et al, 2000[Kikuchi et al, , 2003Sastri et al, 2001]. As discussed by Huang et al [2004], tail current disruption, magnetotail dipolarization, and electric field penetration processes associated with substorms can cause geomagnetic disturbances on both the dayside and nightside.…”