Substorms associated with azimuthal turnings of the interplanetary magnetic field

Whether the magnetospheric substorms can be triggered by the interplanetary magnetic field (IMF) variations is an important issue in the substorm rese arch. In this work we investigate observationally the relationship between substorm activities and IMF By variations, i.e., azimuthal turnings. We hav e searched for the EMT's azimuthal turning events for a period of one year using the data from multispacecraft monitoring the solar wind, WIND, IMP 8 and Geotail. Based on specific selection criteria, we have found 11 such ev ents that exhibit pure azimuthal turnings while the IMF B-z remains quasi-s teady. These events are found to be mostly in reasonable temporal associati ons with the substorm activities which were identified by multipoint measur ements using the geomagnetic bays, auroral images, geosynchronous energetic particle injections and magnetic dipolarizations. We find an average respo nse time of similar to8-9 min between the substorm onsets and the dayside m agnetopause contact times of the IMF By turnings. The results suggest that in addition to the more popular trigger by northward turnings of the IMF, i ts azimuthal (both positive and negative) turnings be also regarded as anot her possible external trigger of substorms. (C) 2001 Published by Elsevier Science Ltd.