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

Abstract

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 ø. 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 outflowing 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 W, He +, 0 +, and O ++ ions that were stationary in the Earth's corotating frame, i.e., ions observed in the spacecraft ram direction.

The polar wind: Recent observations

“…Consequently, the fraction of the polar wind ion population beyond the upper energy-limit of the thermal ion measurements is also expected to increase, and this may account for the reduction in the observed outflow rate at very high Kp index. Horita et al (1993) reported the frequent occurrence of ion depletion zones at altitudes down to 3000 km in the nightside polar cap above 701 invariant, in which the thermal-energy ion density was below the detection limit of the SMS instrument (10 À2 cm À3 ). These authors noted the lack of Kp dependence of ion depletion zone occurrence, and attributed the active-time occurrence to the increase in ion energization to beyond the upper energy limit of the thermal ion measurements (and the quiet-time occurrence to convection vortex in the nightside polar cap).…”

The O+ density trough at 5000 km altitude in the polar cap

“… Robinson [1991], using Sondrestrom radar observations, revealed deep minima in the F region electron densities in the dawn sector. At higher altitudes, Horita et al [1993] reported that the suprathermal ion mass spectrometer on the EXOS D (Akebono) satellite frequently observed ion depletion zones, where the ion densities were less than 10 −2 cm −3 . In their report, the ion depletion zones observed by the satellite were primarily located in the nightside region of the polar cap magnetosphere at invariant latitudes above 70° and at altitudes between 8000 and 10,000 km.…”

History of kinetic polar wind models and early observations

“…Another decade later, the effect of the photoelectrons driving the polar wind was inferred from the observed day/ night asymmetry in the outflow of thermal O + in the polar cap (e.g. Horita et al, 1993), and the availability of reliable measurements of thermal electron anisotropies (e.g. Yau et al, 1995) from the Akebono satellite.…”