Data from multiple orbiters give a clearer picture of how density and temperature interact and what that could mean for future satellite missions.

If it weren’t for the exosphere—Earth’s outermost layer of atmosphere—we might all be at the mercy of the Sun’s blinding rays and other objects hurling toward us from outer space. In the exosphere, the air becomes very thin, almost vacuum-like, with very few particles, as many atoms and molecules from Earth’s atmosphere begin to escape into space.

Another consequence of the near-vacuum conditions is that there is very little friction, which makes this region of the atmosphere ideal for satellites. However, regions of the exosphere can differ greatly in terms of temperature and density, which can affect the orbits of spacecraft placed in this region. Incoming radiation, particularly from the Sun, can also influence the density of gases in the exosphere. Therefore, understanding the variation in temperature and density of the exosphere can provide valuable information for scientists when they are considering where to place satellites.

In this new study, Weimer et al. analyze density data from two satellites, Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE), measured by accelerometers found on board. The measured densities can then be converted to temperatures using the Naval Research Laboratory’s Mass Spectrometer, Incoherent Scatter Radar Extended Model.

The researchers found that the densities in the exosphere—which, in turn, correlated to differences in temperature—varied on the basis of solar cycles and other geomagnetic fluctuations. The scientists then averaged out these temperatures in grids such that the cells would have equal areas using NASA Jet Propulsion Laboratory’s Hierarchal Equal Area Isolatitude Pixelization of a Sphere (HEALPix). Typically, grid cells constructed by latitude and longitude lines get smaller and smaller near the poles and do not have equivalent areas. HEALPix rectifies this issue, however, allowing researchers to more easily compare averages from different regions of the exosphere.

With the methods used here, the authors propose a new way to create more accurate models of the exosphere. Those models could help researchers understand variations in exospheric temperatures or determine where to place certain spacecraft. (Journal of Geophysical Research: Space Physics, doi:10.1002/2016JA022691, 2016)

—Wudan Yan, Freelance Writer