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[1] We have studied favorable conditions for energetic electron acceleration during magnetic reconnection in the Earth's magnetosphere using the Geotail data. We have found the strong energetic electron acceleration in some reconnection events. On the other hand, the other reconnection events show weak electron acceleration. To discuss what reconnection characteristics determine energetic electron acceleration efficiency, we have studied the reconnection characteristics for 10 events in which the Geotail satellite observed the vicinity of the diffusion region. We have classified the relationship between the reconnection characteristics and the electron acceleration efficiency into three types: (1) good correlation (absolute value of correlation coefficient ∣r∣ > 0.6); (2) ambiguous correlation (0.6 > ∣r∣ > 0.3); and (3) no correlation (0.3 > ∣r∣). We found that ion heating, electron heating, current sheet thickness, reconnection electric field, and converging normal electric field could be categorized into good correlation. Ion/electron temperature ratio, total amount of reconnected magnetic energy, and reconnection rate were classified in ambiguous correlation. We could not find any correlation between energetic electron acceleration efficiency and absolute value of outflow velocity, current density parallel to magnetic field (Hall current system), and satellite location in the Earth's magnetosphere. From our analysis we claimed that the electrons are efficiently accelerated in a thin current sheet during fast reconnection events.

[1] We have studied the dawn-dusk asymmetry of the energetic/thermal particle in the plasma sheet using the Geotail data. We found the species- and energy-dependent dawn-dusk asymmetries in the plasma sheet. The dawn-dusk asymmetries have some typical structures, and we used a finite width two-dimensional model of the magnetotail to explain their characteristics. The characteristics and assumptions of our model can be summarized as follows: (1) the magnetotail convection is steady (both of magnetic field and electric field is steady), (2) the directions of electric fields are duskward, (3) the directions of magnetic fields are northward, (4) the magnetic field gradient is constant for earthward (constant for other directions), (5) the width of the magnetotail is constant in the entire plasma sheet (40 RE), (6) the plasma source region is steady at XGSM ∼ −100 RE, (7) the magnetic moment μ is conserved during transport, and (8) the particle spatial diffusion takes place only for dawn-dusk direction. In this model, we found that the energetic electrons cannot be provided sufficiently at the dusk flank with the simple adiabatic process (no spatial diffusion). Furthermore we applied the Bohm-type diffusion to the model and found that the observed asymmetries are well reproduced in the case that the diffusion coefficient is equal to 0.006/0.03v⊥rg for electron/ion.

[1] Energetic electrons in an earthward reconnection outflow region have been observed by Cluster/Research with Adaptive Particle Imaging Detectors. We found a good correlation between the energetic electron enhancement and a normal magnetic field (Bz) enhancement within a 0.25-s time resolution. The large normal magnetic field is thought to be associated with magnetic reconnection because the negative/positive Bz reversal observed during the fast proton tailward/earthward flow reversal is a good indicator of magnetic reconnection. Using the four-spacecraft Cluster, we can clearly see that this large positive Bz structure propagates in the earthward direction. Furthermore, we find that the energy spectrum of the energetic electrons becomes harder toward the downstream region. A negative Bz enhancement is also observed. The intensity of energetic electron enhancement associated with the negative Bz enhancement is weaker than that associated with the positive one. To discuss the temporal and spatial profile of energetic electron acceleration in the magnetic reconnection region, we determined the spacecraft position in the temporally evolving magnetic structures of reconnection. Our observation clearly indicates second-step acceleration, in addition to X line acceleration, of energetic electrons in the downstream reconnection outflow region.

Accepted: 2005-04-01, 資料番号: SA1003460000