Space Weather Effects on Thermosphere Drag

The pathway for insuring continuous and secure operations of Low Earth Orbit (LEO) spacecraft technology is to improve the estimate of the thermosphere state and predict its response to space weather events. Since 2001 there has been significant progress in improving estimates and advancing models of atmospheric mass density in the lower atmosphere (200-500 km), but further work is needed in this altitude range. Various forms of energy fluxes from space weather events (i.e., solar EUV flux, solar flares, coronal mass ejections, corotating interaction regions, etc.) significantly perturb thermosphere properties of temperature and composition that lead to mass density changes. These space weather inputs and subsequent processes need to be adequately represented to describe mass density changes and are required for the assessment of physics-based, data assimilated, and empirical models under different space weather conditions.

A key deficiency that requires much more investigation is estimating atmospheric drag in the upper register of the LEO regime (altitudes between 500 and 1000 km). In fact, atmospheric drag estimates are more coveted by operators in this domain than at lower altitudes because of the larger number of operational space assets and debris. For space weather research, this altitude domain includes the challenging transition from a gas continuum to particle dynamics, as the thermosphere gives way to the exosphere, and is highly sensitive to space weather and the underlying atmospheric layers. Furthermore, predictions of state in the upper altitude range are dependent on how well the lower altitude range is represented and how the composition of light species, like helium and hydrogen, is treated. Thus, improvements in estimating atmospheric drag in the lower register will be important for expanding the models into the higher register, making the 500-1000km altitude range particularly challenging and in need of improvements in all aspects of thermosphere models.