Superthermal >36-eV ions observed in the near-tail region of Venus by the Pioneer Venus Orbiter neutral mass spectrometer

Journal Article · Mon Jul 01 04:00:00 EDT 1991 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/91JA00677· OSTI ID:5224554

Kasprzak, W T; Grebowsky, J M; Niemann, H B ^[1]; Brace, L H ^[2]

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1. NASA Goddard Space Flight Center, Greenbelt, MD (USA)
2. Univ. of Michigan, Ann Arbor (USA)

The Pioneer Venus orbiter neutral mass spectrometer (ONMS) has made measurements of >36-eV ions in the altitude range 1,300-3,700 km for solar zenith angles greater than 120 {degrees}. The superthermal ions form part of the near-tail region of Venus termed the ionotail. The ONMS superthermal ions represent only a small fraction (about 0.3%) of the plasma density in the ionotail region. The composition is mainly O{sup +}, but He{sup +}, N{sup +}, (CO{sup +} N{sub 2}{sup +}), NO{sup +}, and O{sub 2}{sup +} have been identified. CO{sub 2}{sup +} is very rarely observed in this region, and H{sup +} is not measured. The directions of the apparent O{sup +} ion flow in the ecliptic plane show predominantly tailward components with a similar number of nontailward components. Since the energy of the superthermal ions is sufficient for planetary escape, the >36-eV O{sup +} escape flux in the ionotail is estimated to be about 10{sup 5} cm{sup {minus}2} s{sup {minus}1}. Other species observed also have enough energy to escape. The O{sup +} flux data show a factor of 2.5 increase from solar minimum to solar maximum implying a photoionization source for these ions. Neither the origin of the >36-eV ions nor the acceleration mechanism is precisely known. The O{sup +} flux observations do not appear to be correlated with the direction of the cross-tail magnetic field as might be expected if the ions were due to the asymmetric pickup of newly ionized atmospheric neutrals above the ionopause. The composition of the superthermal ions in the ionotail suggests that their source is most likely the high-altitude nightside ionosphere where O{sup +} and not O{sub 2}{sup +} is the dominant ion. Transport of the superthermal O{sup +} across the terminator to the nightside has been observed, and measurements in the ionotail region at solar minimum near 2,000 km show that {sup +} is mainly superthermal.

• OSTI ID: 5224554
• Journal Information: Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 96:A7; ISSN JGREA; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

640107* -- Astrophysics & Cosmology-- Planetary Phenomena
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ACCELERATION
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CHARGED PARTICLES
CHARGED-PARTICLE TRANSPORT
COMPARATIVE EVALUATIONS
EVALUATION
HELIUM IONS
INTERACTIONS
ION DRIFT
IONIC COMPOSITION
IONIZATION
IONS
MARS PLANET
MASS SPECTROMETERS
MEASURING INSTRUMENTS
MOLECULAR IONS
NITRIC OXIDE
NITROGEN COMPOUNDS
NITROGEN IONS
NITROGEN OXIDES
ORIGIN
OXIDES
OXYGEN COMPOUNDS
OXYGEN IONS
PHOTOIONIZATION
PIONEER SPACE PROBES
PLANETARY IONOSPHERES
PLANETS
PLASMA DENSITY
RADIATION TRANSPORT
SOLAR ACTIVITY
SOLAR WIND
SPACE VEHICLES
SPECTROMETERS
VEHICLES
VENUS PLANET