Ion flows in Venus' magnetotail

Abstract:

The plasma environment of Venus, including its magnetotail up to 3 Venus radii distance has been studied by the Venus Express spacecraft since 2006. We use the ASPERA/IMA instrument that measures protons and oxygen ions in the eV to keV range. Oxygen ions deriving from Venus’ atmosphere and flowing down the magnetotail have been reported and contribute to atmospheric escape (Barabash et al. 2007). There have also been ions observed that flow Venusward in the tail (Dubinin et al. 2014). Signatures of magnetic reconnection have been found (Zhang et al. 2012), which might contribute to these flows.

We have numerically calculated flow velocities and densities of protons and oxygen ions in the tail. Since the IMA instrument cannot cover all directions during one scan, we manually identified hundreds of cases of clear and mostly unbiased plasma flows. This approach avoids cases that lead to incorrect results.

We find that the Venusward fluxes of both protons and oxygen ions are on average smaller but on the same order of magnitude as the tailward escape fluxes. Venusward fluxes are commonly quasi-steady i.e. observed throughout most of a tail passage, which takes several ten minutes.

The instantaneous flow directions can differ by more than 100 degrees in the tail. Their net tailward or Venusward direction is opposite in about half of the cases. Comparison with magnetic field data shows that tailward and Venusward bulk flows are roughly field-aligned.

The calculation of plasma moments does not imply that the underlying distributions are thermal and described well with a Maxwellian distribution. About a third of the proton spectra show a suprathermal component. Such cases are more common for Venusward fluxes and can be observed over longer periods. In some cases the additional component is a clearly separated second peak but most of the time it fits well with a flat power law (exponents 0 to 3).