Ionospheric ions in the near Earth geomagnetic tail plasma lobes

By comparing data from the plasma experiment on ISEE‐2 and from the ion composition experiment on ISEE‐1, composition and plasma properties of tailward flowing ions in the magnetotail lobes are assessed, in the 50 eV/Z to 11 keV/Z energy range. This population consists mainly of singly charged oxygen ions, with H + ions contributing in general less than 10%, flowing roughly along magnetic field lines. Hence, it is concluded that these ions are of mainly ionospheric origin. They are detected only during magnetically disturbed periods. Estimates of the E z electric field componets in the lobes are given.