Solar ﬂare; equatorial electrojet.
The effect of solar flare, sudden commencement of magnetic storm and of the disturbances ring current on the equatorial electrojet in the Eastern Brazil region, where the ground magnetic declination is as large as 20∘W is studied based on geomagnetic data with one minute resolution from Bacabal during November–December 1990. It is shown that the mean diurnal vector of the horizontal field was aligned along 2∘E of north at Huancayo and 30∘W of north at Bacabal during the month of December 1990. Number of solar flares that occurred on 30 December 1990 indicated the direction of solar flare related $\Delta H$ vector to be aligned along 5∘E of north at Huancayo and 28∘W of north at Bacabal. This is expected as the solar flare effects are due to the enhanced conductivity in the ionosphere. The SC at 2230 UT on 26 November 1990 produced a positive impulse in $\Delta X$ and negative impulse in $\Delta Y$ at Bacabal with $\Delta H$ vector aligned along 27∘W of north. At Huancayo the $\Delta H$ vector associated with SC is aligned along 8∘E of north, few degrees east to the alignment of the diurnal vector of H. The magnetic storm that followed the SC had a minimum Dst index of –150 nT. The corresponding storm time disturbance in $\Delta X$ at Huancayo as well as at Bacabal were about –250 nT but $\Delta Y$ at Bacabal was about +70 nT and very small at Huancayo, that give the alignment of the H vector due to ring current about 16∘W of north at Bacabal and almost along N–S at Huancayo. Thus alignment of the $\Delta H$ vector due to ring current at Bacabal is 14∘E of the mean direction of $\Delta H$ vector during December 1990. This is consistent with the direction of ring current dependent on the dipole declination at the ring current altitude which is about 5∘W of north over Bacabal and the deviation of declination due to the ring current during disturbed period given by the angle $(\psi-D)$.