Isobar Altitude Variations in the Upper Mesosphere Observed With IUVS-MAVEN in Response to Martian Dust Storms

We report limb measurements of the oxygen dayglow emission at 297.2 nm performed during four Martian dust storms. The emission peak provides a good remote sensing tool to probe changes of the altitude of the 39 mPa pressure level for the first time during dust storms. We illustrate the time variation of these changes and compare them with the infrared opacity in the lower atmosphere. We find that the 39 mPa level rises in response to the increase in dust opacity. It reaches a plateau, and additional dust load does not significantly increase its altitude. Numerical simulations with the LMD global circulation model shows a similar response, except for the event observed during MY33 regional storm when the model fails to reproduce the observed variations. Observations collected during the onset of the global dust storm in June 2018 show that the upper atmosphere rapidly responds within two Martian days to the increased amount of tropospheric dust. Plain Language Summary Earlier studies have shown that the oxygen dayglow emission at 297.2 nm is a good tracer of the response of the upper atmosphere to atmospheric perturbations such as dust storms. In this work, we use MAVEN-IUVS observations during four different dust storm episodes in order to follow their influence on the atmospheric structure. We combine these measurements with concurrent observations of the dust load in the lower atmosphere monitored by infrared absorption. We find that the atmospheric layers move up as the atmospheric dust load increases. However, we show that there is a saturation effect limiting the rise of the layers at a given dust content. High time resolution observations performed during the first phase of the 2018 global dust storm indicate that the upper atmosphere responds quite fast following the onset of the storm.