Energetics of the Thermosphere Revealed through EUV Occulations

Dr. Ed Thiemann

Abstract: The thermosphere, the tenuous boundary between the atmosphere and space, is heated by solar extreme ultraviolet (EUV) radiation from above, by magnetospheric currents during geomagnetic storms, and by ubiquitous gravity waves propagating from below; yet the relative importance of these energy inputs remains poorly constrained. In this colloquium, I present recent results on the structure, variability and energetics of the thermosphere found by the method of solar occultation, where atmospheric properties are inferred using sunlight as it passes through an atmosphere. Because EUV light is strongly absorbed in the thermosphere, EUV occultations provide a self-calibrating, high signal-to-noise window into the ‘thermospheric gap’, a region from approximately 130 to 300 km with few vertically resolved measurements. I begin at Mars, where new findings show that thermospheric density near 180 km is driven by coupling with the lower atmosphere and by the planet’s large orbital eccentricity rather than by the 11-year solar cycle, with EUV heating becoming dominant only above approximately 230 km. I then turn to Earth and follow each of its thermospheric energy inputs. Occultations show storm-time density enhancements at 200 km that models systematically underpredict, pointing to poorly constrained Joule heating. They also reveal a non-linear dependence of exospheric temperature on EUV irradiance, which can inform heating efficiency parameterizations that currently compensates for a high EUV bias in models. I then present the upcoming Occultation Wave Limb Sounder (OWLS) mission, of which I am Principal Investigator (PI), planned for launch in late 2027 to quantify the thermal impact of gravity waves, where model predictions range from approximately 5 K to 100 K. Students have been major contributors to these results, and their participation is highlighted. I conclude with a future in which occultation sensors shrink to the size of a deck of cards, enabling hosted constellations for operational monitoring of the thermosphere.

To participate in this event virtually via Zoom, go to https://uiowa.zoom.us/j/92703451055?pwd=juyDNWSDvbCo9DpIYaMenMvDPeJYbM.1&from=addon.