Penetration electric fields at the nightside dip equator associated with the main impulse of the storm sudden commencement of 8 July 1991

Abstract

The geomagnetic storm sudden commencement (ssc) of 8 July 1991 was reported [Wilson et al., 2001] to be characterized by a reduction (enhancement) of X component at midlatitudes in the noon (midnight) sector in the 1-hour period after its start at 1636 UT. This distinctive feature is seen even after accounting for the effects of the Chapman-Ferraro current by subtracting the step-like increase of X/H component at low-latitude stations on the same magnetic meridian from the midlatitude data. We present evidence to show that over the same 1-hour period after the start of the ssc on 8 July 1991 an eastward electric field disturbance (peak value ~1.2 mV/m) grew up and decayed at the premidnight dip equator. The eastward electric field evidenced at the nightside dip equator is interpreted as the signature of the penetration of the dawn-to-dusk electric field associated with an enhancement of region 1 field-aligned currents (FACs) driven by the solar wind. The negative disturbance in the H/X component at midlatitudes in the noon sector is explained as the magnetic effect of FACs that carry the large-scale electric fields from the magnetosphere to the polar ionosphere as well as the disturbance of polar origin-type 2 ionospheric currents excited by the large-scale electric field.