Thermosphere

The thermosphere is the layer of Earth's atmosphere that stretches from roughly 85 kilometers to as high as 600 kilometers above the surface, and it is where most low-Earth-orbit satellites, including the International Space Station, actually fly. Despite its name, which comes from the Greek word for heat, the thermosphere would not feel hot to a person because the air there is so thin that individual molecules, though moving extremely fast and carrying enormous kinetic energy, rarely collide with anything to transfer that heat. The thermosphere's temperature and density are highly sensitive to space weather: solar ultraviolet and X-ray radiation, along with energy deposited during geomagnetic storms, can heat this layer dramatically, causing it to expand outward. This expansion increases atmospheric drag on satellites, which can meaningfully shorten their orbital lifetimes or force operators to perform extra maneuvers to maintain altitude — a real, well-documented effect during major geomagnetic storms. The thermosphere overlaps with the ionosphere, the region where atmospheric gases are ionized by solar radiation, and the two are closely linked in how they respond to solar activity.