Ion acceleration processes in magnetic reconnection: Geotail observations in the magnetotail

Ion outflow formation processes in magnetic reconnection are investigated on the basis of 3‐D ion velocity distribution function observations for magnetic reconnection in the magnetotail with the spacecraft Geotail. Ion acceleration processes up to 40 keV can be divided into two types, and these two types consist of two‐step processes. The first type is found close to the X line in the central part of the ion‐electron decoupling region of magnetic reconnection where an intense crosstail current layer exists and electrons make high‐speed outflow jets. In this region, inflowing ions and ions making the meandering motion coexist. Ions are accelerated up to ~10 keV in the north–south direction during the inflowing process, and then they are accelerated farther by the reconnection electric field in the duskward direction. The second type is found far from the X line in the outer parts of the ion‐electron decoupling region of magnetic reconnection. In this region, the Hall current loops are formed and the inflowing Hall electrons exist near the separatrix layers. Ions are accelerated and make the dawnward motion inside the Hall current loop in the inflowing process, and then they are farther accelerated to form tailward and earthward outflows. In this region, significant amount of ions is supplied as inflows, and outflows containing ions in the wide energy range (5–40 keV) are formed. Hence, both the north–south (vertical) electric field, which is probably the Hall electric field, and the duskward reconnection electric field operate ion acceleration processes for magnetic reconnection in the magnetotail.

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