Wind measurements and dynamics in Venus’ upper atmosphere measured by high-resolution terahertz spectroscopy of atomic oxygen

Atomic oxygen is important for the photochemistry in the mesosphere and thermosphere of Venus and can be used as tracer for atmospheric dynamics. The altitude range where it predominantly occurs is between 90 km and 130 km with a peak at 100 km – 110 km. Atomic oxygen is mainly generated through photolysis of CO₂ on the dayside. From there it is transported to the nightside by the subsolar to antisolar circulation. It accumulates near the antisolar point and recombines to molecular oxygen [1, 2 ,3, 4]. The region between 90 km and 120 km altitude is the transition region from superrotation to subsolar-antisolar flow and is not yet well understood. This is the altitude range probed in this work.

We have detected atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm) with the upGREAT (German Receiver for Astronomy at Terahertz Frequencies) heterodyne spectrometer on board SOFIA (Stratospheric Observatory for Infrared Astronomy) [5]. This is a direct detection in contrast to most past and current detection methods, which are indirect and rely on photochemical models to obtain atomic oxygen concentrations [1, 2]. We have used this transition to determine Doppler shifts and the corresponding wind velocities. Due to the high spectral resolution of the upGREAT heterodyne spectrometer of 0.2 MHz it is possible to measure the speed at which the atomic oxygen is moving towards the observer [6]. The observations were made on Nov. 10, Nov. 11 and Nov. 13 2021. The 2.5-m diameter telescope of SOFIA was pointing at Venus. The telescope provides a diffraction-limited beam with 6.3 arcsec, which is about five times smaller than the apparent diameter of Venus (29 arcsec). The phase of Venus was 42%.

The 4.7-THz channel of upGREAT has seven pixels in a hexagonal pattern separated by 13.6 arcsec. While most of the pixels were on Venus, three pixels were pointing at its limb. At these positions the component of the wind vector which points towards the observer is sufficiently large to be measured with upGREAT. As a reference we take the transition frequency measured by a pixel which points towards the center of the disk of Venus where the wind vector component towards the observer is negligible. The measurements at 45° and 15° north don’t show a wind speed component which is significantly different from zero (13±38 m/s and 2±31 m/s, respectively) while the measurement close to the south pole exhibits a wind speed component of 120±75 m/s. These values are in agreement with the global circulation model (GCM) of Navarro et al. [6].

For those spectra which are not close to the Venus limb, the wind component towards the observer is too small to be measured by upGREAT. However, the variation of column density and temperature of atomic oxygen may serve as an indicator for the dynamics. When comparing the column density determined by upGREAT with the wind velocity provided by the GCM of Navarro et al. [7] it stands out that on the night side the column density peaks at the time between 19 and 20 hour local time where the gradient of the wind speed is strongest (Fig. 1). This might be an indication of an adiabatic flow of an air parcel in the atmosphere of Venus which leads to an increased density when the wind speed drops sharply.

Fig. 1: Column density of atomic oxygen measured by upGREAT (dots, data from [5]) and zonal wind speeds in the region between 90 km and 130 km (from [6]). The dashed line marks the terminator at the surface of Venus. The grey lines are streamlines showing the circulation (from [7]).

References

[1] A. S. Brecht et al., Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations. Icarus 217, 759–766 (2012).

[2] L. Soret et al., Atomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping. Icarus 217, 849–855 (2012).

[3] J.-C. Gérard, Aeronomy of the Venus upper atmosphere. in: Space Sci Rev. Venus III edited by B. Bézard et al., Springer Dordrecht (2017).

[4] G. Gilli, et al., Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements. Icarus 366, 114432 (2021).

[5] H.-W. Hübers et al., Direct detection of atomic oxygen on the dayside and nightside of Venus, Nature Communications, 14:6812 (2023).

[6] C. Risacher et al., The upGREAT dual frequency heterodyne arrays for SOFIA, J. Astron. Instrum. 7, 1840014 (2018).

[7] T. Navarro et al., Venus´ upper atmosphere revealed by a GCM: I. Structure and variability of the circulation, Icarus 366, 114400 (2021).