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

• Published in: Journal of Geophysical Research. A. Space Physics 2002-12, Vol.107 (A12), p.SIA 9-1-SIA 9-8
• Main Author: Sastri, J. Hanumath
• Format: Article
• Language: English
• Subjects: field-aligned currents; ionospheric current system; midlatitude and equatorial effects; penetration electric fields; ssc models; storm sudden commencement (ssc)
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/2002JA009453

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.