MODIFICATIONS IN THE NIGHTTIME LOW-LATITUDE IONOSPHERE AFTER SOUTHWARD TURNING OF THE IMF

Sharp decreases in f(o)F2 are found to occur Frequently in the nightti me low-latitude ionosphere after southward turning of the IMF B-z comp onent, especially under isolated B-z turnings, i.e. when the IMF has b een northward for at least 6 h before its turning. These decreases occ ur simultaneously (within a 1-h time interval) with the B-z turning. T he effect is observed both when a substorm or a magnetic storm begins after B-z has turned southward, and when a noticeable substorm does no t occur. The effect is more pronounced after midnight and a maximum at 03 LT. Short-term (with scale times of about 1 h) variations of f(o)F 2 and h(m)F2 for B-z southward turning are analysed using a large amou nt of ground-based and topside sounding data. The decreases in f(o)F2 are shown to occur at first over the magnetic equator and then, during the second hour after the turning, at the crests of the equatorial an omaly. The ionosphere returns to its undisturbed state, on average, in 4-5 h (if other disturbing agents do not arise). These decreases are suggested to be caused by modifications in the electric field in the l ow-latitude ionosphere associated with B-z southward turning.