Supersonic and Subsonic Measurements of Mesospheric Ionization

Abstract

An Arcas rocket-parachute system was used at night to compare supersonic and subsonic ionization measurements below 75 km. A hemispherical nose-tip probe was used on ascent and a parachute-borne blunt probe on descent to measure polar conductivities, which were due entirely to positive and negative ions. The velocity of the supersonic probe was ~Mach 2.5 at 50 km and 1.75 at 70 km; the blunt probe was subsonic below 71 km. Between 65 and 75 km the ratio of negative to positive conductivities (and thus of mobilities) determined by the blunt probe was about 1.2, and it approached 1 below this altitude range. The ratio obtained by the nose-tip probe varied from 1.5 at 75 km to .6 at 65 km, thus indicating a rapid variation of the effects of the shock wave on the sampled ions. The absolute values of positive conductivity measured subsonically and supersonically were essentially identical from 60 to 75 kin, indicating that the sampled ions were unchanged by the shock. However, below 60 km the shock apparently 'broke up' the positive ions, as indicated by higher measured conductivities. The negative ion conductivities for the supersonic case indicated that the sampling was badly disturbed by the shock. The negative ions were apparently lighter (higher mobility) than those sampled subsonically above 72 km and below 50 km. However, they appeared heavier (lower mobility) at the intervening altitudes.

In-situ electron and ion measurements and observed gravity wave effects in the polar mesosphere during the MaCWAVE program

“…Because the rocket velocity was supersonic (particularly, at the lower altitudes), a significant shock front developed in front of the probe. As a result, assumptions of "frozen chemistry" are not valid (Hale et al, 1972). We can expect that the ions are hydrated, but fragmentation or sublimation is likely to occur within the shock front.…”