GEOTAIL observation of ring‐shaped ion distribution functions in the plasma sheet‐lobe boundary

The ring‐shaped velocity distribution of ions with energies of a few keV of the gyrating motion has at times been detected in the boundary region between the plasma sheet and the lobe in the distant tail at X GSM ∼ −70 and −170Re. The normal direction of the ring‐shaped distribution is almost parallel to the magnetic field, and the distribution moves tailward. In many cases the fast flowing ions of the Plasma Sheet Boundary Layer (PSBL) are observed simultaneously. The density of the ring is comparable to the density of the cold ion stream observed in the lobe. We discuss the possibility that these ring ions are generated from cold proton streams by the thermoelectric field that exists at the plasma sheet‐lobe boundary.