Magnetic Evidence for a Nonthermal He Population in the Venusian Exosphere

Helium-4 (He) in the Venusian atmosphere has previously been identified through remote spectroscopic observations as well as in-situ plasma and mass spectrometer measurements from earlier spacecraft missions (e.g., Mariner 10, Venera 11 and 12, and PVO).

In this study, we analyze magnetic field and plasma observations from the Venus Express (VEX) mission to investigate ion cyclotron waves (ICWs) generated by the pickup of exospheric He-ions by the solar wind. Through the detection and analysis of these wave signatures, we identify for the first time a nonthermal exospheric He population extending from approximately 1 Venus radius up to 11 Venus radii. Average kinetic temperatures of around 2700 K and an inferred exobase density of about 200 cm^-3, characterize the derived population.

Our results indicate that there are likely two energization mechanisms for this nonthermal helium component. The first mechanism is related to collisions between thermal He atoms and suprathermal oxygen atoms near or slightly above the exobase. This process produces a hotter nonthermal helium population, consistent with mechanisms previously proposed for both Venus and Mars. The second possible mechanism is the dissociation of HeH⁺ in the exobase region.

Using the helium escape flux derived in this study, approximately 4 × 10⁴ cm^-2 s^-1, we further investigate the long-term evolution of helium in the Venusian atmosphere. The comparatively low escape rate suggests that the present atmospheric He abundance could have accumulated since the last major resurfacing event, or potentially over even longer geological timescales