Modifications in the nighttime low-latitude ionosphere after southward turning of the IMF

Abstract

Sharp decreases in Φ oF2 are found to occur frequently in the nighttime low-latitude ionosphere after southward turning of the IMF B z component, especially under isolated B z turnings, i.e. when the IMF has been northward for at least 6 h before its turning. These decreases occur simultaneously (within a 1-h time interval) with the B z turning. The effect is observed both when a substorm or a magnetic storm begins after B z has turned southward, and when a noticeable substorm does not 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 Φ oF2 and h mF2 for B z southward turning are analysed using a large amount of ground-based and topside sounding data. The decreases in Φ oF2 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 anomaly. 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 low-latitude ionosphere associated with B z southward turning.