Time: 15h on 1 December 2025
On-site location: In the Ole-Johan Dahl building, Kristen Nygaards sal (room 5370). It's on the 5th floor and you enter through elevator D, which is the northernmost in the building. See map sketch below.
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External evaluator: Tormod Kværna, NORSAR
Internal evaluator: Valérie Maupin, UiO
Supervisors: Sven Peter Näsholm, Marouchka Froment, Quentin Brissaud
Abstract:
Infrasound can travel large distances in atmospheric waveguides, carrying information about its source and propagation medium. On Venus, a hellish surface environment prevents the use of traditional ground-based sensors to record infrasound. As an alternative, atmospheric balloons equipped with acoustic sensors can be deployed in a more gentle environment, at around 60 km altitude, where they would drift with the wind. Previous studies on Earth and Venus have established that data from such balloon-borne sensors could be used to invert for both atmospheric and subsurface properties. However, no previous study has established that long-range propagation, through atmospheric waveguides, is possible on Venus. If such waveguides do exist, they would enable detection of explosive volcanism, meteor fragmentation, and even Venusian lightning—thereby facilitating and further justifying future balloon missions to Venus. In this thesis, we unveil the global morphology of infrasound propagation on Venus, to investigate the potential of detecting explosive volcanism by balloon-borne sensors in the atmosphere.
The results reveal waveguides in both zonal and meridional directions, created by superrotational winds (more than 50 times faster than the planet's rotation) in the middle atmosphere, as well as subsolar-antisolar winds in the upper atmosphere. These waveguides enable ground-to-ground and ground-to-balloon propagation, showing that the conditions for long-range propagation on Venus are more favourable than previously considered.
We further investigate the balloon detectability of explosive volcanism on Venus by showing 1) the peak source pressure needed for detection, 2) the number of detectable volcanoes, and 3) the maximum detectable range for a given source pressure. The results reveal the possibility of long-range detection, with some eruptions detectable more than 5000 km from the source. Furthermore, the probability of detecting explosive eruptions on Venus is estimated for a 60-day balloon mission, resulting in cumulative detection probabilities between 35% and 65% for a Signal-to-Noise ratio of 2, depending on the number of volcanoes included.
For validation, a similar framework is applied on Earth, modelling the propagation of broadband infrasound signals from the Hunga Tonga–Hunga Ha'apai eruption on 15 January 2022. The eruption was recorded both by balloons from the Stratéole-2 campaign and by ground-based arrays of infrasound sensors. Considering the spatial extent of a volcanic plume, we use elevated sources to predict the observed wave amplitudes for seven different stations (up to 8658 km in range), with a root mean square power spectrum ratio of 4.9 dB, suggesting that relative amplitudes were wrong on average by a factor 1.8.
Altogether, our discovery of long-range waveguides increases confidence in the feasibility of employing balloon-borne sensors to detect and study potential explosive volcanism on Venus.
