Abstract. The role of high-speed solar wind streams in driving relativistic electron acceleration within the Earth's magnetosphere during solar activity minimum conditions has been well documented. The rising phase of the new solar activity cycle (cycle 23) commenced in 1996, and there have 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 enhancements in the magnetospheric energetic particle population while others do not. This paper compares and contrasts relativistic electron signatures observed by the POLAR, SAMPEX, Highly Elliptical Orbit, and geostationary orbit spacecraft during two magnetic cloud events: May 27-29, 1996, and January 10-11, 1997. Sequences were observed in each case in which the interplanetary magnetic field was first strongly southward and then rotated northward. In both cases, there were large solar wind density enhancements toward the end of the cloud passage at 1 AU. Strong energetic electron acceleration was observed in the January event, but not in the May event. The relative geoeffectiveness for these two cases is assessed, and it is concluded that large induced electric fields (9B/9t) caused in situ acceleration of electrons throughout the outer radiation zone during the January 1997 event.
Abstract.A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (Bz--25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst--170 nT). SAMPEX data showed that relativistic electrons (E _> 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4• These electrons were not directly "injected" from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for-2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.
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