SUDDEN IMPULSES AT LOW-LATITUDE STATIONS - STEADY-STATE RESPONSE FOR SOUTHWARD INTERPLANETARY MAGNETIC-FIELD

When the interplanetary field is northward and the solar wind dynamic pressure suddenly increases, the increase in the H component magnetic field on the surface of the Earth at low and mid-latitudes is governed principally by the currents on the magnetopause. Currents in the tail, which act in the opposite sense to the magnetopause currents, are also enhanced. These currents have a smaller effect than the magnetopause currents and have a greater effect at night than in the daytime hours. In this study we examine the response of the H component when the IMF is southward. We find that the dayside response to solar wind pressure increases is over 25 % smaller when the IMF is southward than when it is northward, presumably because of the region 1 current system associated with dayside reconnection. At night, the opposite situation ensues. Mid-latitude bays associated with triggered substorms appear to be responsible for an enhanced nighttime response. Thus the ground level response to sudden pressure changes in the solar wind is very sensitive to the direction of the interplanetary magnetic field.