Plasma transition at the flanks of the Venus ionosheath: Evidence from the Venus Express data

Abstract

[1] Measurements conducted with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) instrument in the Venus Express spacecraft reveal the presence of a plasma transition within a boundary layer that extends along at the flanks of the Venus ionosheath and where the solar wind exhibits changes similar to those reported from previous missions (Mariner 5, Venera, and Pioneer Venus). At the plasma transition there is a sharp downstream decrease in the density of the solar wind electrons and a sudden increase in their temperature embedded within the boundary layer where more gradual changes in the speed, temperature, and density of the solar wind ions are observed. The ASPERA-4 data also show important fluxes of planetary ions measured downstream from the plasma transition and whose dominant velocity component is in the Sun-Venus direction. The speed of those ions is slower than the local solar wind speed and thus is different from that expected from the convective electric field acceleration in which both speed values should be comparable. The boundary layer is interpreted as representing a feature that results from the transport of solar wind momentum to the Venus upper ionosphere, and the ASPERA-4 data provide information on the kinetic properties of the eroded planetary ion population that is seen to stream mostly in the Sun-Venus direction. From the comparison of the ASPERA-4 measurements with those of the magnetic field obtained with the magnetometer of the Venus Express, it is found that in the near wake crossing of the plasma transition the magnetic field intensity decreases to lower values with downstream distance from the planet in agreement with measurements conducted with the Mariner 5 and the PVO. From the analysis of data for orbits with evidence of the plasma transition within the boundary layer, it is found that the momentum flux of planetary ions measured in the wake can be accounted for from the incident momentum flux of the solar wind protons implying an approximate balance as would result from the transport of solar wind momentum to the planetary particles.

O+outflow channels around Venus controlled by directions of the interplanetary magnetic field: Observations of high energy O+ions around the terminator

“… Pèrez‐de‐Tejada [2001] suggested that in this region the viscous force becomes dominant to cause a momentum exchange between the solar wind and the planetary plasma, and then accelerates the planetary plasma. Such regions were also found in VEX measurements [ Pèrez‐de‐Tejada et al , 2011]. In fact, when the high energy O+ flux is observed at 01:46 UT in Figure 1, there is a strong deficit of the kinetic energy of the solar wind protons.…”

An induced global magnetic field looping around the magnetotail of Venus

“…On the other hand, the downstream planetary ions (O+) flow mostly along the Sun-planet direction with a speed slower than the local solar wind speed, which is inconsistent with the convection electric field acceleration in which both speeds should be comparable and a transverse velocity component should be measured [Pérez-de Tejada et al, 2011]. Thus, there should be an additional acceleration mechanism along the Sun-planet lines; the wave-particle interaction has been suggested by Pérez-de Tejada et al [2009].…”

Short large‐amplitude magnetic structures (SLAMS) at Venus

“…There were three encounters with the magnetosheath (marked in pink on the time line) and associated bow shock crossings (pink circles). The clearest and best example is the final (right most in Figure 2) bow shock crossing at ∼02:53:45, after which the magnetosheath is clearly visible in Figure 2g by an increase in | B |, and an increase in flux of electrons over a broad range of energies, consistent with heating at the bow shock [ Pérez‐de‐Tejada et al , 2011]. The two other transitions into the sheath at ∼02:45:00 and ∼02:52:30 were brief, but are evident by the change the orientation of the magnetic field, an increase in | B |, and an increase in electron flux consistent with that of the final bow shock crossing.…”