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The rise in relativistic electron flux observed by numerous spacecraft within the magnetosphere on January 10, 1997 has been simulated with a guiding center test particle code using output field data from a 3D global MHD simulation of the event driven by upstream solar wind parameters measured by the WIND spacecraft. The rise in flux is greatest in the 1.6 - 3.2 MeV energy range of the four GPS detector channels simulated, and greatest around the radial location of GPS (L = 4.2) in the equatorial plane of the simulations. The January 1997 event was characterized by an extended period of steady southward IMF which produced a sequence of substorms leading to a peak in average Kp=6 just prior to and during the rise in relativistic electron flux seen by GPS spacecraft. Simultaneous groundbased measurements in Alaska and Canada, which were situated postmidnight in local time, indicated enhanced ULF wave power coincident with the rise in relativistic electron flux measured inside geosynchronous orbit [Baker et al., 1998].
The acceleration mechanism proposed here is distinct from that attributed to rapid compressions of the magnetopause on the electron drift timescale, such as occurred for the March 24, 1991 CME-driven SSC [Li et al., 1993; Hudson et al., 1996; 1997]. In the latter case, it was shown that electrons drift eastward synchronously with a magnetosonic pulse launched within the magnetosphere by the dayside SSC shock compression. It is the azimuthal electric field component which transports electrons (and protons) radially inward in this case, increasing their energy as the first adiabatic invariant is conserved. The pulse spreads around the flanks of the magnetosphere at a characteristic magnetosonic speed determined by the plasma density and magnetic field, with maximum effect on those electrons drifting eastward at a comparable velocity. Lower energy electrons are not efficiently transported inward by such a pulse, which is mainly bipolar after reflection from the ionosphere (see Hudson et al., , Figure 3). Electrons in drift coherence with the magnetosonic pulse as it spreads around the flanks of the magnetosphere become drift-phase bunched in less than one drift period, as observed for the March 1991 event by energetic particle detectors on the CRRES satellite [Blake et al., 1992]. By contrast, the ULF-wave drift-coherent mechanism proposed here requires multiple drift orbits to increase electron energies from the hundred keV to MeV range, and drift echo features are not expected. Nonetheless, because it is a coherent process, it is much more efficient and rapid than acceleration by standard radial diffusion, based on incoherent electric or magnetic fluctuations.
In the broader context of so-called "killer electron" flux increase, the January 1997 event is noteworthy for the lack of 1 - 2 day delay typically observed for outer zone electron flux buildup associated with high speed solar wind stream interactions [Blake et al., 1997], as well as for moderate solar wind velocity (< 500 km/s), until after passage of the magnetic cloud on January 11 [Burlaga et al., 1998]. While the correlation between solar wind speed and outer zone electron fluxes is well documented [Paulikas and Blake, 1979], the additional factor of steady southward IMF Bz [Blake et al., 1997] clearly played a role in the January 1997 case. It appears that such steady southward IMF Bz, well known to facilitate substorms, may affect the relativistic electron population in two ways. First, it provides an inhanced seed population in the hundred keV energy range due to substorm injections. Second, the ULF wave activity in the 10 minute period range is enhanced. Further investigation of what we have interpreted as torroidal oscillations, based on analysis of the MHD simulation data, will be pursued elsewhere.
In summary, we have simulated an impulsive rise in relativistic electron flux on January 10, 1997, correlated with an observed increase in ULF wave activity on the nightside in a frequency range comensurate with electron drift periods. Solar wind parameters from the WIND spacecraft were used to drive the longest 3D global MHD simulation to date, and the output fields were used to push guiding center relativistic electron trajectories in the equatorial plane. Power enhancement in the radial electric field component of the simulations coincident with the electron drift period suggests drift-resonant acceleration over multiple electron drift periods. As electrons move inward, gaining energy from this radial electric field while conserving the first invariant, the flux peak moves inward to L=4, during a time interval both of enhanced ULF wave activity as seen by ground instrumentation on January 10, and the rise in relativistic electron fluxes seen by GPS [Reeves et al., 1998a]. A continuous and time-dependent electron injection scheme is under development, which will increase flux levels above those seen in Plates 1 and 2, and should improve direct comparisons with spacecraft measurements over the present results, which simply apply the time evolving MHD fields to an intial AE8MIN electron model.
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