Venus Magnetosheath Research Articles

A statistical study of ions and magnetic fields in the Venus magnetotail

Plasma and magnetic field data from 223 orbits in the first nine tail seasons of Pioneer Venus are examined to provide a statistical description of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail. This description is based solely on instrumental responses, with no a priori assumptions about existing regions or their plasma and field characteristics. Over 22,000 plasma spectra taken from 7 to 12 Rv downtail are categorized as to whether they contain shocked solar wind, pickup ions, or no discernible plasma. The magnetic field characteristics associated with each plasma spectrum category are examined, and the average two‐dimensional cross‐tail ion flow and magnetic field structures of the Venus tail are determined at a resolution of 0.25×0.25 Rv. Plasma flows everywhere tailward, slowing from 450 km/s in the sheath to less than 300 km/s at the tail axis. Weak outward deflections of 50 km/s or less are found within the tail. The magnetotail is found to be highly draped, with a field‐reversing current sheet not more than 0.25 Rv thick. At these distances the tail is filled with shocked solar wind but also contains pickup ions and a plasma component with fluxes that are not detectable by the Pioneer Venus orbiter plasma analyzer. The bulk flow and field configurations imply a number density of 1.2 cm−3 and a temperature of 9×106 °K for a current sheet composed of 90% protons and 10% O+. These conditions are at or below the limit of instrumental detection. Weak magnetic field asymmetries are associated with the plasma dropouts. The undiscernible plasma component in the tail is consistent with a planetary ion population with fluxes below the instrumental detection threshold. A high E/q plasma population previously interpreted as planetary pickup ions (O+) is found asymmetrically both within the tail and in the adjacent sheath. The undiscernible and pickup ion plasmas (plasmas of Venusian origin) are observed from 5 to 15% of the time within the tail region. This rate depends on solar EUV flux, indicating a photoionization source. Thus the Venus tail is filled with plasma that is primarily shocked solar wind at sometimes undetectable fluxes, which coexists with a photoion population that produces asymmetries in the bulk plasma and magnetic field properties.

The effect of the hot oxygen corona on the interaction of the solar wind with Venus

A numerical gas dynamic model, which includes the effects of mass loading of the shocked solar wind, was used to calculate the density and magnetic field variations in the magnetosheath of Venus. These calculations were carried out for conditions corresponding to a specific orbit of the Pioneer Venus Orbiter (PVO orbit 582). A comparison of the model predictions and the measured shock position, density and magnetic field values showed a reasonable agreement, indicating that a gas dynamic model that includes the effects of mass loading can be used to predict these parameters.

On the role of the quasi‐parallel bow shock in ion pickup: A lesson from Venus?

Previous observations at Venus show convincing evidence of planetary (O+) ion pickup by the large‐scale motional (−V × B) electric field in the magnetosheath when the interplanetary magnetic field is perpendicular to the solar wind flow. However, the presence of magnetic field fluctuations in the magnetosheath downstream from the quasi‐parallel bow shock should allow pickup to occur even when the upstream magnetic field B and plasma velocity V are practically coaligned. Single‐particle calculations are used to demonstrate that convecting magnetic field fluctuations similar to those observed in the Venus magnetosheath when the subsolar bow shock is quasi‐parallel can efficiently accelerate cold planetary ions by means of the electric field associated with their transverse components. This ion pickup process, which is characterized by a spatial dependence determined by the bow shock shape and the orientation of the upstream magnetic field, is likely also to occur at Mars and may be effective at comets.

Origin of large magnetic fluctuations in the magnetosheath of Venus

The origin of large‐amplitude hydromagnetic waves in the Venus magnetosheath down‐stream of the quasi‐parallel bow shock is investigated by means of numerical simulation. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O+ ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.

Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the − V X B acceleration process, stochastic pickup of ionospheric ions, and J X B force “scavenging” at the ionopause all occur at various times.

Magnetic field fluctuations in the Venus magnetosheath

Using a model for the convection pattern of the shocked solar wind flow around the Venus obstacle, Pioneer Venus observations of ultra‐low‐frequency (∼10‐40s period) magnetic field fluctuations in the magnetosheath have been traced along streamlines to the region of the quasi‐parallel bow shock. The periods and polarizations of the sinusoidal fluctuations are similar to those observed upstream of the quasi‐parallel bow shock, where streaming superthermal particles are believed to produce MHD waves by a beam‐plasma instability. The results suggest that both disturbances at the ionopause at Venus and the earth's magnetopause may be caused by convection of turbulent magnetic fields from the subsolar bow shock when the interplanetary field direction produces a quasi‐parallel shock there.

Charge‐exchange in the magnetosheaths of Venus and Mars: A comparison

The amount of solar wind absorption due to charge‐exchange in the Martian magnetosheath is evaluated and found to be about an order of magnitude less than that in the Venus magnetosheath. This difference might explain the observed difference in the scaled position and shape between the shocks at Venus and Mars. The lower solar wind absorption for Mars is attributable to the less dense hot oxygen corona of Mars compared to Venus.

Approximate calculation of the current distribution when a conducting fluid flows along a channel within a magnetic field

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the −VXB acceleration process, stochastic pickup of ionospheric ions, and JXB force “scavenging” at the ionopause all occur at various times.