Abstract
Geomagnetic disturbances from the period April to June 1910 are analyzed to detect the possible effects of H the comet alley on the solar wind-magnetosphere interaction. Data from six mid-latitude geomagnetic observatories are used to calculate the longitudinally symmetric (i. e. Dst) and asymmetric fields. An application of a linear prediction filter to separate the solar wind dynamic pressure effect on the disturbances from that of the ring current, suggests that there exists a compressional variation in the Dst on May 18 which is around or slightly earlier than the time of estimated cometary tail encounter. The normal-run magnetogram from Agincourt on the dayside and that from Lu-Kia-Pang on the nightside, also indicate rather strong (i. e. 30-40nT) compressional variation. The disturbances characteristic to the solar wind-magnetosphere interaction under the southward IMF condition and that of the ring current development are seen during the period. An exponential recovery of the Dst also supports the above interpretation. These results suggest that the Earth's magnetosphere had been affected by a dense cometary plasma tail with high dynamic pressure, though the solar wind-magnetosphere interaction typically observed under the southward IMF condition had been taking place during the encounter. It is also suggested that the width of the cometary plasma tail was considerably greater (>0.09AU) than that of the theoretically expected though the possibility of overestimation caused by cometary tail fluttering can not be excluded.
