Ion and electron distribution functions in the distant magnetotail: modeling of Geotail observations

Ion and electron distribution functions measured by the Geotail spacecraft in the distant magnetotail are investigated considering single‐particle dynamics in sharp field reversals. The observed H + distribution functions exhibit two components, namely, a low‐energy core and a high‐energy population propagating tailward. In agreement with previous studies, we show that ion dynamics in a two‐dimensional (2‐D) reconnection field geometry naturally leads to such distributions and that the neutral line location can be analytically inferred from the velocity space structure. As for electron distribution functions, Geotail observations reveal characteristic “flattop” profiles with enhanced flux in the parallel direction at low energies and in the perpendicular direction at high energies. We interpret these distributions as the result of the electron chaotic behavior in the reconnection region. This chaotic regime, which occurs when the particle Larmor radius is comparable to the magnetic field line curvature radius, can lead to prominent pitch angle scattering. Near the neutral line, we show that at a given energy, this regime yields flux depletion in the parallel direction and enhancement in the perpendicular one. The flattop profile arises from this behavior affecting only the (relatively) high‐energy part of the electron distribution. Conversely, the threshold energy above which electrons exhibit such a behavior contains information on the field reversal characteristics. The electron distribution functions obtained numerically are in agreement with the observations, which support this interpretation framework and allow us to estimate the thickness of the neutral sheet.