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Diffusive Electron Acceleration at Interplanetary CME Shocks: Comparison between events on 21 Feb 1994 and 15 July 2000
*Toshio Terasawa[1]
,Nobue Shimada [1],Yasuhiro Takei [1]
Shigeto Kawada [1],Mitsuo Oka [1],Mitsue Den [2]
Toshifumi Mukai [3],Toshifumi Saito [3]
Deptment of Earth and Planetary Science, University of Tokyo[1]
Communication Research Laboratory[2]
Institute of Space and Astronautical Science[3]
While the diffusive shock acceleration (DSA) process of electrons
has significant astrophysical importance, reports of in situ
observations of such process accompanying with interplanetary
CME shocks at 1 AU have been limited to several big events, such
as those on 21 Feb 1994 and 15 July 2000 [Shimada et al., ASS,
1999; Terasawa et al., ICRC, 2001]. In this presentation, we
will present the results of comparative studies of these important
events based on the GEOTAIL measurements.
While the diffusive shock acceleration (DSA) process of electrons has significant astrophysical importance, reports of in situ observations of such process accompanying with interplanetary CME shocks at 1 AU have been limited to several big events, such as those on 21 Feb 1994 and 15 July 2000 [Shimada et al., ASS, 1999; Terasawa et al., ICRC, 2001]. In this presentation, we will present the results of comparative studies of these important events based on the GEOTAIL measurements. Common features of these events are, (1) high average propagation speeds from the sun to 1AU ($
sim$1300 km/s and $
sim$1500 km/s), (2) high local propagation speeds at 1 AU ($
sim$920 km/s and $
sim$1100 km/s), (3) exponential upstream time profiles of nonthermal electrons (up to 40 keV), and (4) nearly power-law energy spectrum. Despite these similarities, one noticeable difference among them was the relative flux increases of accelerated electrons: In the energy range of several keV to 20 keV nearly two-order of magnitude flux increases were obser ved at the former shock, while the corresponding increase at the latter shock was only a factor of $
sim$3. We are now trying to identify the origin of this difference: One possibility is the different shock angles ($
sim$68 deg for the former, and $
sim$48 deg for the latter). Further search for the difference in scattering agents of these electrons is also under way (For the former shock, intensification of whistler waves of several Hz was identified.)