The solar wind interaction with Venus: Pioneer Venus observations of bow shock location and structure

Abstract

Pioneer Venus observations are used in conducting a study of the location and structure of the Venus bow shock. The trace of the shock in the solar wind aberrated terminator plane is nearly circular at an altitude of 1.38 RV independent of interplanetary magnetic field orientation with an extrapolated subsolar height of 0.38 RV. Gas dynamic relations and scaling of the terrestrial analogue are used to determine the effective impenetrable obstacle altitude from the mean shock surface with the conclusion that it lies beneath the observed height of the ionopause. The short‐term variability in shock position is similar to that found at the earth, while over the long‐term bow shock, altitude varies by up to ∼35% in phase with the solar cycle owing to causes other than changing solar wind Mach number. In contrast to ionopause position, which is shown to be well determined by external pressure measurements, bow shock altitude is found to be only weakly dependent upon ionopause height and solar wind dynamic pressure. These results are interpreted in terms of interactions with exospheric neutrals and/or lack of complete deflection of the incident solar wind by currents induced in the ionosphere modifying the flow about Venus from that associated with a tangential discontinuity obstacle of nearly constant radius. The downstream bow shock is smaller in diameter than that of terrestrial case despite the larger Mach cone angle at 0.72 AU most probably due to the smaller relative size of the Venus magnetotail. A brief survey of shock structure with Pioneer Venus instrumentation shows general agreement as to the time and location of the shock crossings with a transition layer thickness of the order of the ion inertial length scale. The observed variation in bow shock structure and the foreshock with upstream parameters was similar to that seen at the earth.

Mirror mode waves in Venus's magnetosheath: solar minimum vs. solar maximum

“…This is also the reason for the increase in ion cyclotron waves upstream of Venus's bow shock as shown by Delva et al (2015). The increased ionization also causes the bow shock and ionosphere to move outward (Alexander and Russell, 1985;Shan et al, 2015), albeit Slavin et al (1980) argue that charge exchange at low altitudes near the ionopause is causing the shock to move closer at solar minimum. The MM wave effect is to balance magnetic pressure B 2 /2µ 0 and plasma pressure n i k B T ⊥,i , and the instability is driven by the temperature anisotropy of the ions (see Eq.…”

The solar wind interaction with Venus

“…With this extrapolation, it was considered that the shock shape could be used to investigate the extent of solar wind absorption by Venus' atmosphere. Both Slavin et aL (1980Slavin et aL ( , 1983 and more recently Russell etal. (1985) made efforts to model the shock shape determined from magnetometer data using Spreiter and Stahara's (1980a) gas dynamic treatment and thereby deduce the altitude of the obstacle nose.…”

Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

“…In the absence of a significant crustal magnetic field, there are no seasonal or diurnal effects. Slavin et al (1980) and Tatrallyay et al (1983) showed that the bow shock at Venus flares more than might be expected based on gasdynamic models, suggesting that mass loading plays an important role. Consistent with this hypothesis, Alexander and Russell (1985) showed that the terminator bow shock moves outward during solar maximum when exospheric neutral densities should be enhanced.…”