New Thermospheric Temperature Profiles from PROBA2 LYRA Solar Occultations

Abstract:

The Earth's thermosphere (>~120 km) contains both the atmosphere's warmest and coldest region, and spans both collisional 'fluid-like' and collisionless 'balistic' regimes. The thermosphere is heated predominantly by solar extreme ultraviolet (EUV) radiation, and is sensitive to geomagnetic and atmospheric wave forcing, all of which cause thermospheric variability in both space and time. This domain, rich in structure and variability, also spans the region of Low Earth Orbit and thermospheric variability directly impacts satellite drag, creating a critical space weather need for nowcasting the state of the thermosphere. However, the thermosphere has been historically difficult to measure, leading some to dub the neutral atmosphere between 120 and 300 km the 'Thermospheric Gap'. Recently, it has been shown, using the LYRA instrument onboard the ESA PROBA2 satellite, that solar EUV occultations with modern instrumentation are highly effective at measuring thermospheric density. This study expands these results to include thermospheric temperature between 150 and 350 km.

LYRA measures the summed N₂ and O neutral densities above ~150 km with its Zr foil filter photometer channel. Eclipse seasons occur annually from mid-October to mid-February, during which LYRA has measured occultations since 2010. Since LYRA occultations have been only proven effective with the single Zr channel, it lacks a second measurement to directly measure the major species (O and N₂) abundance. In this study, we show that by using a-priori knowledge of the expected thermospheric temperature profile shape and judicious application of assumptions related to the diffusive seperation of the major species, accurate temperature profiles can be determined from the LYRA data. We present comparisons of LYRA temperature measurements between 150 and 300 km with those predicted by the NRL-MSISE-00 model, showing the LYRA predictions are in good agreement with those from MSIS. In addition to providing an extensive new dataset of thermospheric temperatures, these results have important implications for space weather nowcasting and forecasting because of the small size, simplicity and low cost of EUV photometers, making EUV solar occultations an appealing option for future constellations of satellites monitoring thermospheric density.