Drop-off of the polar rain flux near the plasma sheet boundary

The Akebono satellite has often observed the drop-off of polar rain flux near the polar cap boundary, The energy of the cutoff frequently showed decreasing trend with decreasing latitude, In this paper, we propose a model in which the drop-off is explained by disruption of the earthward polar rain flux at the X line in the tail, This model also predicts the behavior of the polar rain in the distant tail: The polar rain flux, usually bidirectional in the tail lobe, should become unidirectional near the plasma sheet boundary, as it is directed tailward (earthward) on the Earth (tail) side of the X line. Such a unidirectional polar rain layer has been newly detected by the Geotail satellite at X(GSM) = -40R(E) to -200R(E) recently, Modeling this new feature of the polar rain is the main purpose of this study, The proposed model also predicts a relation between the direction of the polar rain flux in this layer and the direction of ion bulk flow in the adjacent plasma sheet, The Geotail data show a good agreement with this prediction, Being convinced that the model is reasonable, the position of the X line relative to the satellite has been derived, It is shown by the Geotail survey over a wide range in GSM X that the neutral line formation occurs most frequently at X(GSM) = -50R(E) to -150R(E).