# Influence of the solar wind on the Venusian hydrogen in upper atmosphere observed by Hisaki

Chizuru Nose, Kei Masunaga, Fuminori Tsuchiya, Shotaro Sakai, Yasumasa Kasaba, Ichiro Yoshikawa, Atsushi Yamazaki, Go Murakami, Tomoki Kimura, Hajime Kita, et al.

► To cite this version:

Chizuru Nose, Kei Masunaga, Fuminori Tsuchiya, Shotaro Sakai, Yasumasa Kasaba, et al. Influence of the solar wind on the Venusian hydrogen in upper atmosphere observed by Hisaki. EPSC-DPS Joint Meeting 2025, Sep 2025, Helsinki, Finland. 10.5194/epsc-dps2025-1791. hal-05279473

HAL Id: hal-05279473

https://hal.science/hal-05279473v1

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# Influence of the solar wind on the Venusian hydrogen in upper atmosphere observed by Hisaki

**Chizuru Nose**<fnref n="1" />, **Kei Masunaga**<fnref n="2" />, **Fuminori Tsuchiya**<fnref n="1" />, **Shotaro Sakai**<fnref n="3" />, **Yasumasa Kasaba**<fnref n="1" />, **Ichiro Yoshikawa**<fnref n="4" />, **Atsushi Yamazaki**<fnref n="5" />, **Go Murakami**<fnref n="5" />, **Tomoki Kimura**<fnref n="6" />, **Hajime Kita**<fnref n="7" />, **Jean-Yves Chaufray**<fnref n="8" />, and **Francois Leblance**<fnref n="8" />,<fnref n="9" />

## Introduction

One of the outstanding questions regarding Venus is whether the planet once retained a significant amount of water. Observations of hydrogen atoms provide critical insights into atmospheric escape processes. Previous studies using Venus Express/SPICAV indicate that the Venusian hydrogen atmosphere consists of two distinct components characterized by different scale heights: a hot component and a cold component [1]. The hot hydrogen component primarily arises from charge exchange reactions and momentum transfer between cold hydrogen atoms and ionospheric ions [2]. Conversely, the cold component originates from the dissociation of sulfuric acid in the lower atmosphere. It is well known that, due to the absence of an intrinsic magnetic field, Venusian atmosphere interacts directly with the solar wind. However, it remains unclear whether the Venusian hydrogen corona dynamically responds to variations in solar wind conditions.

## Observation

To address this question, we analyzed variations in global hydrogen column densities derived from the brightness of resonantly scattered Ly-α (121.6 nm) and Ly-β (102.6 nm) emissions observed by Hisaki [3-5], solar wind velocities and densities measured by ASPERA-4 on Venus Express [6], and solar UV irradiance at Ly-α and Ly-β wavelengths obtained from the Flare Irradiance Spectral Model (FISM) for Planets [7]. The analysis periods spanned March 9 to April 3, 2014 (Period1), and April 25 to May 23, 2014 (Period2). High-speed solar wind events were confirmed during Period1 but not during Period2.

## Result

We derived variations in hydrogen column density at altitudes above approximately 310 km and 90 km from the observed Ly-α and Ly-β airglow brightness. Figure 1 shows that after the arrival of high-speed solar wind originating from a corotating interaction region (CIR) in Period1, the hydrogen column density derived from Ly-α increased by approximately 18% within a few days and subsequently remained nearly constant for several weeks. In contrast, the hydrogen column density

<footnote n="1">¹Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Sendai, Japan
(nose.c@pparc.gp.tohoku.ac.jp)</footnote>

<footnote n="2">²Institute of Arts and Sciences, Yamagata University, Yamagata, Japan.</footnote>

<footnote n="3">³Faculty of Environment and Information Studies, Keio University, Fujisawa, Japan</footnote>

<footnote n="4">⁴Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, University of Tokyo, Tokyo, Japan</footnote>

<footnote n="5">⁵Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan</footnote>

<footnote n="6">⁶Faculty of Science Division I, Department of Physics, Tokyo University of Science, Tokyo, Japan</footnote>

<footnote n="7">⁷Department of Information and Communication Engineering, Tohoku Institute of Technology, Sendai, Japan</footnote>

<footnote n="8">⁸LATMOS/ CNRS, Paris, France</footnote>

<footnote n="9">⁹Sorbonne University, Paris, France</footnote>
derived from Ly-β remained relatively stable throughout the same period. Differences between Ly-α and Ly-β brightness suggest an increase in hydrogen atom abundance at higher altitudes during high-speed solar wind events. In Period 2, when no significant increase in both solar wind velocity and density was observed, there was no clear indication of the arrival of a corotating interaction region. During this period, the hydrogen column density remained nearly constant for both Ly-α and Ly-β.

Figure1 (a and b)Times series of column densities of Venusian hydrogen atoms derived from Ly-α and Ly-β observed by Hisaki respectively. The red line indicates the 1-day moving average. (c and d) Solar wind velocity and density respectively observed by Venus Express.

Discussion

A possible explanation for the observed ~18% variation in Ly-α emission is an increase in high altitude hot hydrogen abundance due to charge exchange reactions and momentum transfer between neutral hydrogen and ionospheric ions. By considering charge exchange between cold hydrogen and ionospheric ions as a production process, and charge exchange between hot hydrogen and the solar wind as a loss process, we estimated the reaction timescales and found consistency with the observed variation. Alternative explanations include an increase in low-altitude cold hydrogen abundance or a rise in hydrogen temperature. These findings provide important implications for understanding non-thermal hydrogen escape mechanisms, thus contributing significantly to our knowledge of the atmospheric evolution of Venus.

[1] Chaufray, J. Y., et al., Icarus, 217, 2, 767, 2012

[2] Hodges, R. R., and E. L. Breig, Journal of Geophysical Research: Space Physics, 96, 7697, 1991

[3] Yoshikawa, I., et al., Space Science Reviews, 184, 237, 2014

[4] Yoshioka, K., et al., Planetary and Space Science, 85, 250, 2013

[5] Yamazaki, A., et al., Space Science Reviews, 184, 259, 2014

[6] Barabash, S., et al., Planetary and Space Science, 55, 12, 1772, 2007

[7] Chamberlin, P. C., et al., Space Weather, 6., S05001, 2008