Signatures for magnetospheric substorms in the geomagnetic field of dayside equatorial region: Origin of the ionospheric component

Abstract

An event-based study is made of the conditions that lead to the ionospheric current component in the geomagnetic field signature for magnetospheric substorms in the dayside dip equatorial region. The substorm activity studied here is characterized by two onsets separated by ~40 min: The first one occurred during steady southward interplanetary magnetic field (IMF) and steadily increasing cross-polar cap potential and enhanced energy input into the magnetosphere, while the second one occurred in close association with a northward transition of IMF and a rapid reduction of the polar cap potential and energy input into the magnetosphere. Earlier studies using multispacecraft data as well as ground-based optical, radar, and magnetic recordings ascertained the first onset to be associated with a pseudo-breakup and the second onset with a ``true'' breakup. Magnetometer recordings from a meridional network of stations in the Indian and Pacific sectors revealed the response of daytime equatorial H-field to the pseudo-breakup to be weak and not readily identifiable. In contrast, a distinct negative bay-like disturbance with an unambiguous dip equator enhancement prevailed in the H-field starting with the onset of the expansive phase of the fully developed substorm. This behavior indicative of a significant contribution of ionospheric currents to the negative H-field bay is seen both in the Indian and Pacific sectors covering the 1200-1600 LT region. The present case study thus suggests that a sudden and prominent reduction in the cross-polar cap potential at the substorm expansive phase is responsible for the prevalence of the ionospheric current component in the geomagnetic disturbance in the dayside equatorial region. It is also found for the first time that the amplitude of the negative H-field disturbance in the afternoon sector exhibits a marked hemisphere asymmetry at midlatitudes: It is higher by a factor of about 3 in the summer hemisphere than in the winter hemisphere. The equatorial enhancement of the negative H-component disturbance is interpreted as the signature of the ``over-shielding'' effect, namely, direct penetration of the perturbation component of the large-scale electric field associated with the rapid reduction in convection at the onset of the substorm expansion phase. The marked summer-winter asymmetry of the H-field disturbance at midlatitudes is suggested as the outcome of the competing magnetic effects of field-aligned currents and polar origin DP2 currents.