5:15 PM - 6:45 PM
[PPS04-P01] Solar wind influence of Venusian hydrogen airglow in thermosphere by Hisaki
★Invited Papers
Keywords:Venus, thermosphere, Hisaki
Understanding the variability in Venus’s upper atmosphere is important for comprehending the atmospheric evolution and climate change on Venus. The upper thermosphere is exposed to solar UV radiation and solar wind, while the lower thermosphere is influenced by dynamics of the lower atmosphere via atmospheric waves. However, dynamics of the thermosphere have not been fully understood yet. Oxygen and hydrogen airglows in Venusian thermosphere have been observed in EUV wavelength range. Variability in the oxygen airglow has been analyzed in previous by the Hisaki satellite, revealing 4-day periodic variations that are generated via atmospheric waves (Masunaga et al., 2015,2017, Nara et al., 2020). Detailed analysis of the hydrogen airglow has not been conducted yet.
We aim to understand that how Venusian thermosphere responds to the lower atmosphere dynamics and solar activity. We analyze periodic variation of the hydrogen airglow, along with the solar wind and solar ultraviolet radiation at Venus. The data-set include spectroscopic observation from Hisaki, solar wind velocity, density and dynamic pressure from ASPERA-4 installed on Venus Express, solar ultraviolet radiation irradiance at Ly-α(121.6nm) and Ly-β(102.6nm) wavelength from FISM-P (Flare Irradiance Spectral Model for Planets). The analysis periods were March 7th to April 3rd,2014 (Period1) and April 25th to May 23rd, 2014, (Period2) during which the arrival of high-speed solar wind was confirmed in Period 1 but not in Period 2.
We conducted time series analyses of airglow brightness, solar ultraviolet radiation, and solar wind flux for both Ly-αand Ly-β. The results revealed a positive correlation between the solar ultraviolet radiation and the Venusian brightness, indicating that the airglow is primary emitted by resonant scattering with solar ultraviolet radiation. By subtracting the predicted airglow component of resonant scattering obtained from the regression line determined by the least squares method, we found periodic variations of ±10-20% in the residual airglow brightness. Using the Lomb-Scargle periodogram method, a 9 to10-day periodicity in Ly-α, and a 7-day in Ly-βwere observed in Period 1, and a 14-day in Ly-αwas observed in the 99% confidence interval. The 4-day periodic variations were not identified.
We calculated the optical thickness in the Venusian upper atmosphere from the altitude distribution of hydrogen density by Venus Express observations (Chaufray et al., 2012) and estimated a typical altitude of the Ly-α about 310 km, higher than the oxygen emission (about 130km). This suggests that atmospheric wave effects do not extend to the upper thermospheric variation at 310 km.
Comparing the residual brightness and the time series of solar wind flux, during Period1, when high-speed solar wind arrived, the airglow brightness variations were minimized and then returned to the original brightness. In contrast, in Period2, this phenomenon was not observed. Comparing the time series of the residual brightness with altitude changes of the induced magnetosphere boundary (IMB) observed by Venus Express, it was found that the residual component had negative values when the IMB altitude decreased and had positive values when it increased.
From these analyses, it is inferred that solar wind contributes to the variations in hydrogen airglow. The compression and expansion of the ionosphere accompanying the arrival of solar wind led to changes in plasma density, resulting in controlling charge exchange reactions between thermospheric hydrogen and ionospheric plasma. This, in turn, is believed to cause fluctuations in the abundance of hydrogen atoms. Since charge exchange reactions produce non-thermal hydrogen, affecting the loss of neutral hydrogen, these results are important for understanding the process of Venusian atmospheric loss.
We aim to understand that how Venusian thermosphere responds to the lower atmosphere dynamics and solar activity. We analyze periodic variation of the hydrogen airglow, along with the solar wind and solar ultraviolet radiation at Venus. The data-set include spectroscopic observation from Hisaki, solar wind velocity, density and dynamic pressure from ASPERA-4 installed on Venus Express, solar ultraviolet radiation irradiance at Ly-α(121.6nm) and Ly-β(102.6nm) wavelength from FISM-P (Flare Irradiance Spectral Model for Planets). The analysis periods were March 7th to April 3rd,2014 (Period1) and April 25th to May 23rd, 2014, (Period2) during which the arrival of high-speed solar wind was confirmed in Period 1 but not in Period 2.
We conducted time series analyses of airglow brightness, solar ultraviolet radiation, and solar wind flux for both Ly-αand Ly-β. The results revealed a positive correlation between the solar ultraviolet radiation and the Venusian brightness, indicating that the airglow is primary emitted by resonant scattering with solar ultraviolet radiation. By subtracting the predicted airglow component of resonant scattering obtained from the regression line determined by the least squares method, we found periodic variations of ±10-20% in the residual airglow brightness. Using the Lomb-Scargle periodogram method, a 9 to10-day periodicity in Ly-α, and a 7-day in Ly-βwere observed in Period 1, and a 14-day in Ly-αwas observed in the 99% confidence interval. The 4-day periodic variations were not identified.
We calculated the optical thickness in the Venusian upper atmosphere from the altitude distribution of hydrogen density by Venus Express observations (Chaufray et al., 2012) and estimated a typical altitude of the Ly-α about 310 km, higher than the oxygen emission (about 130km). This suggests that atmospheric wave effects do not extend to the upper thermospheric variation at 310 km.
Comparing the residual brightness and the time series of solar wind flux, during Period1, when high-speed solar wind arrived, the airglow brightness variations were minimized and then returned to the original brightness. In contrast, in Period2, this phenomenon was not observed. Comparing the time series of the residual brightness with altitude changes of the induced magnetosphere boundary (IMB) observed by Venus Express, it was found that the residual component had negative values when the IMB altitude decreased and had positive values when it increased.
From these analyses, it is inferred that solar wind contributes to the variations in hydrogen airglow. The compression and expansion of the ionosphere accompanying the arrival of solar wind led to changes in plasma density, resulting in controlling charge exchange reactions between thermospheric hydrogen and ionospheric plasma. This, in turn, is believed to cause fluctuations in the abundance of hydrogen atoms. Since charge exchange reactions produce non-thermal hydrogen, affecting the loss of neutral hydrogen, these results are important for understanding the process of Venusian atmospheric loss.