The dawn-dusk asymmetry of energetic electron in the Earth's magnetotail: Observation and transport models

摘要

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 R E), (6) the plasma source region is steady at X GSM ～ ?100 R E, (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 r g for electron/ion.