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Ground magnetic signatures of ULF and substorm activity during an interval of abnormally weak solar wind on May 11, 1999

On May 11, 1999 the solar wind density became extremely low, dropping slowly over several hours to below 1 particle/cc at a time when the wind speed was relatively low (∼360–380 km/s) and the north‐south component of the interplanetary magnetic field (IMF) was fluctuating within ±2 nT of zero. During this period, we tried to establish whether or not ground based magnetometers detected any unusual signatures over and above a weakened version of the normal signatures of activity. In this paper we report that, although the oval was contracted, the activity along the oval demonstrated no unusual characteristics. However, at high latitudes there was significant ULF activity in a latitudinally localized strip during the nighttime hours. We contend that this activity is associated with resonant oscillations in the magnetotail which couple to field lines around the high latitude edge of the plasma sheet.