Upper-atmospheric expansion

Definition
Upper-atmospheric expansion is

"an increase in the temperature and density of the Earth's upper atmosphere. This change is driven by solar activity and can have a direct impact on LEO spacecraft that are susceptible to the effects of atmospheric drag. Increased drag can pull satellites closer to Earth, changing their orbit, decreasing the lifespan of space assets, and making satellite tracking difficult. Understanding this phenomenon and its relationship to atmospheric drag is essential to maintain safe and effective operations of space-based assets through collision avoidance, accurate satellite tracking, object custody, and reentry prediction."