Foreshock structure of the slow-mode shocks in the Earth's magnetotail

The ion distribution function in the upstream region of slow-mode shocks observed in the Earth's magnetotail are investigated. We have found the existence of a region called the ''foreshock region'' upstream of slow-mode shocks that can be clearly distinguished from the main dissipation region of the slow-mode shocks. This foreshock region is characterized by counterstreaming ions: backstreaming ions that flow from the shock surface toward the upstream region along the magnetic field and the lobe cold ions that flow from the upstream region into the shock surface. Backstreaming ions may be hot plasma sheet ions escaping from the downstream region toward the upstream region. The incident cold ions are heated about 3% - 20% of the total ion heating throughout the slow-shock transition in this foreshock region. Wave-particle interaction with the electromagnetic ion cyclotron waves generated by the counterstreaming ions is a candidate of the cold ion heating mechanism in the foreshock region.