Ion depletion zones in the polar wind: EXOS D suprathermal ion mass spectrometer observations in the polar cap

In the high‐altitude polar cap, the suprathermal ion mass spectrometer (SMS) on the EXOS D (Akebono) satellite frequently “observed” ion depletion zones (IDZ) in which the thermal‐energy ion flux was below the detection limit of SMS, corresponding to thermal‐energy ion densities less than 10−2 cm−3. These IDZ are located primarily in the nightside region of the magnetosphere at invariant latitudes above 70° and at altitudes preferentially near apogee and between 8000 and 10,000 km (EXOS D apogee) but extending down to 3000 km. In contrast, outside the IDZ, the SMS regularly observed 3/16/2009 2:01:59 PMoutflowing H+, He+, O+, and O++ polar wind ions with energies typically less than 10 eV in the polar cap. Also, at sufficiently low altitudes below the IDZ the SMS instrument always observed H+, He+, O+, and O++ ions that were stationary in the Earth's corotating frame, i.e., ions observed in the spacecraft ram direction.