Influence of the solar wind on the Venusian hydrogen upper atmosphere: SPICAV limb observations and radiative transfer modeling

Although Venus is often referred to as Earth's twin, its surface environment differs remarkably, with surface temperatures reaching approximately 700 K and no liquid water. The observed D/H ratio in the atmosphere suggests that Venus once possessed a substantial water inventory, and its loss is attributed to hydrogen escape into space. Revealing hydrogen escape processes is therefore crucial for understanding the planet's long-term water loss and atmospheric evolution. The hydrogen exosphere consists of two populations: a cold component in radiative equilibrium with the atmosphere and a hot component produced by non-thermal processes such as charge exchange between cold hydrogen atoms and ionospheric ions^[1]. Because Venus, which lacks an intrinsic magnetic field, the solar wind directly interacts with the ionosphere, resulting in density and temperature variation of the ionosphere. Thus, the solar wind variation could affect the rates of charge exchange reactions in the ionosphere and change the density and temperature distributions of the hydrogen exosphere may be modulated by solar wind variability. However, the nature of this coupling remains incompletely understood.　

Hisaki spacecraft observed the disk-averaged brightness of Ly-α (121.6 nm) and Ly-β (102.5 nm) airglow of Venus ^{[2], [3], [4],[5]} increased by approximately 20% over 2–3 days after the arrival of corotating interaction regions (CIRs). Although this variation could be caused by changes in the hot hydrogen density distribution due to charge exchange, it was difficult to study the variation of the spatial distribution of the hydrogen exosphere given Hisaki's limited spatial resolution. In this study, for a complementary analysis, we use the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV spectrometer aboard Venus Express ^{[6], [7]}, which performed limb observations capable of resolving vertical brightness profiles. To investigate the solar wind influence on the Venusian hydrogen exosphere, we analyzed hydrogen Ly-α emission during a period when the CIRs were arrived at Venus on March, 2014, which is the same observation period of Hisaki. Solar wind velocity and density data from the Ion Mass Analyzer (IMA) ^[8]aboard Venus Express confirmed the arrival of high-speed solar wind associated with CIRs on March 15 and March 25, 2014.

We analyzed Ly-α vertical intensity profiles from three SPICAV orbits: March 16 (altitude range ~500–2000 km), March 20 (~600–10,000 km), and March 26 (~0–10,000 km). Observations of the interplanetary Ly-αemission on March 15 and March 25 were used to subtract this background from the Venusian coronal observations. Comparing orbits with sufficient high-altitude coverage, Ly-α brightness above 2000 km increased by approximately 20% between March 20 and March 26, bracketing the second CIR arrival on March 25. To separate contributions of the cold and hot hydrogen components to the brightness change, we applied a three-dimensional Monte Carlo radiative transfer model based on the framework developed by Chaufray et al. ^{[9], [10], [11]}. In this work, we parallelized and optimized the radiative transfer code, reducing the computational cost compared to previous implementations. This improvement makes it feasible to conduct a statistical analysis of the hydrogen corona using the ~8-year Venus Express dataset in future studies.

From the current best-fit model parameters, the following changes were found between March 20 and March 26, surrounding the second CIR arrival: the cold hydrogen density at 4000 km altitude decreased by approximately 30%, the cold component temperature decreased by approximately 17%, the hot hydrogen density increased by approximately 20%, and the hot component temperature remained unchanged. We will discuss how the density and temperature structure of the hydrogen exosphere may change in association with CIR arrivals, separately for the cold and hot components.

 

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