Retrieval of Atmospheric Tides from Solar Occultations: How Satellite Orbit Impacts Visibility

Vertically propagating atmospheric tides are important drivers of variability in Earth’s upper atmosphere as well as a key player in the region’s dynamics. As tides dissipate in Earth’s thermosphere, they deposit momentum and energy originating from lower altitudes into the background flow. Here, we present work that assesses how atmospheric tides can be observed and their parameters extracted from solar occultation measurements. Solar occultations extract atmospheric density from vertical profiles of extinction of the solar signal produced as the instrument line-of-sight to the Sun passes through the Earth’s atmosphere during transition into and out of eclipse. This creates a self-calibrating dataset capable of measuring within the thermospheric gap region. These datasets already exist and are ripe for analysis. Here, we seek to assess how the observation geometry and orbit selection impact the visibility of atmospheric tides in solar occultation datasets. The geometry of the solar occultation technique limits observations to the dawn and dusk terminators. The orbit of the measuring spacecraft dictates where, in terms of latitude, longitude, and altitude, observations occur. These facts complicate the extraction of tidal parameters due to the limited local time coverage and by the drift of the observation point in latitude and local time. We simulate observations from a tide-resolving global circulation model from a variety of orbits to understand how the solar occultation technique and orbit selection influence our ability to extract tidal parameters. This will inform analysis of current datasets as well as future missions. The OWLS mission, consisting of a solar occultation instrument suite, will investigate gravity waves and the degree of cooling they exert on the thermosphere. For this mission, it is desirable to remove variability due to tides to isolate the atmospheric response to gravity waves. The study presented here helps to inform how tides will appear in the OWLS observation and how best to extract their properties so they may be removed.