Mid-latitude winds in the mesosphere: a superposed epoch analysis over thegeomagnetic storm times

This study aims at looking for the characteristic patterns of mesospheric w ind over the geomagnetic storm times. For this purpose, the geomagnetic sto rms preceded by a sudden commencement (SSC) have been selected from January 1995 to April 1999. By using the onset of SSC as the timing mark, a super- posed epoch analysis has been performed on the available neutral wind data measured with medium frequency (MF) radars at Yamagawa (31.2 degreesN, 130. 6 degreesE) and Wakkanai (45.4 degreesN, 141.7 degreesE). In doing so, the length of time chosen for the superposed analysis is from 7 days before the SSC onset to 21 days after the onset; subsets of wind data are superimpose d for summer and winter months, respectively. Then with harmonic analysis o n the superposed winds the mean winds in both summer and winter months have been obtained. Concerning mean wind characteristics, some interesting deta ils are the reversal heights of the summer zonal winds, which is 79-80 km a t Yamagawa and 84 km at Wakkanai. Strong wavy structures with 2-4 days peri od are observed at both Yamagawa and Wakkanai in both summer and winter. As for storm effects, significant enhancement of eastward wind is found 5 day s after SSC onset at both Yamagawa and Wakkanai in winter. Moreover, the no rthward wind turns southward at Wakkanai 2 days after the onset of SSC, and the southward wind lasts for several days thereafter. In summer months, th e post-storm enhancement tends to be small and mainly in the eastward wind at both Yamagawa and Wakkanai. (C) 2001 Elsevier Science Ltd. All rights re served.