Characteristics of Magnetic Variations Caused by Low-Latitude Aurorae Observed around 210.DEG. Magnetic Meridian.

Abstract

Optical and magnetic observations at the Moshiri Observatory (L = 1.6) of the Solar-Terrestrial Environment Laboratory, Nagoya University, show that invisible low-latitude aurorae sometimes appear in concert with ≥50-nT H component positive magnetic excursions and large-amplitude Pi pulsations during the main phase of magnetic storms, for example, on February 9, 26, 27, and 29 and May 10, 1992, even with a minimum Dst index of only ∼-130 nT. Magnetograms from the L = 1.03-2.13 stations of the 210° magnetic meridian show that the positive excursions in ΔH begin almost simultaneously in the northern and southern hemispheres during the low-latitude aurorae. However, there is large northern-southern asymmetry in the D component magnetic variations during the low-latitude amoral events. The D component magnetic perturbations localized around L = 2 in the southern hemisphere are nearly time derivative of the H component; that is, a bipolar structure. The H and D component magnetic variations have apparent phase delays. The equivalent ionospheric current pattern moves from the midnight side toward the evening and morning sides during the premidnight and postmidnight sectors, respectively. The apparent longitudinal velocity of the equivalent ionospheric current pattern is about 7 km/s at around 40° magnetic latitude. Equivalent ionospheric current vectors deduced from the magnetic variations on the ground in the southern hemisphere show a vortex structure with clockwise rotation. These magnetic variations suggest that an ionospheric Hall current vortex, in which a field-aligned current of 1.4 x 10-8 A/m2 flows upward, must be formed locally near the low-latitude aurorae.