Jm. Grebowsky et al., ION MEASUREMENTS DURING PIONEER VENUS REENTRY - IMPLICATIONS FOR SOLAR-CYCLE VARIATION OF ION COMPOSITION AND DYNAMICS, Geophysical research letters, 20(23), 1993, pp. 2735-2738
During the final, low solar activity phase of the Pioneer Venus missio
n, the Orbiter Ion Mass Spectrometer measurements found all ion specie
s, in the midnight-dusk sector, reduced in concentration relative to t
hat observed at solar maximum. Molecular ion species comprised a great
er part of the total ion concentration as O+ and H+ had the greatest d
epletions. The nightside ionospheric states were strikingly similar to
the isolated solar maximum ''disappearing'' ionospheres. Both are ver
y dynamic states characterized by a rapidly drifting plasma and 30-100
eV superthermal O+ ions.