CORONAL MASS EJECTIONS, MAGNETIC CLOUDS, AND RELATIVISTIC MAGNETOSPHERIC ELECTRON EVENTS - ISTP

The role of high-speed solar wind streams in driving relativistic elec tron acceleration within the Earth's magnetosphere during solar activi ty minimum conditions has been well documented. The rising phase of th e new solar activity,cycle (cycle 23) commenced in 1996, and there hav e recently been a number of coronal mass ejections (CMEs) and related ''magnetic clouds'' at 1 AU. As these CME/cloud systems interact with the Earth's magnetosphere, some events produce substantial enhancement s in the magnetospheric energetic particle population while others do not. This paper compares and contrasts relativistic electron signature s observed by the POLAR, SAMPEX, Highly Elliptical Orbit, and geostati onary orbit spacecraft during two magnetic cloud events: May 27-29, 19 96, and January 10-11, 1997. Sequences were observed in each case in w hich the interplanetary magnetic field was first strongly southward an d then rotated northward. In both cases, there were large solar wind d ensity enhancements toward the end of the cloud passage at 1 AU. Stron g energetic electron acceleration was observed in the January event, b ut not in the May event. The relative geoeffectiveness for these two c ases is assessed, and it is concluded that large induced electric fiel ds (partial derivative B/partial derivative t) caused in situ accelera tion of electrons throughout the outer radiation zone during the Janua ry 1997 event.