Rapid response of the thermosphere to variations in Joule heating

Total mass density measurements from the CHAMP satellite near 400 km altitude are used to define the response time of the thermosphere to high‐latitude heating sources. A series of three geomagnetic storms occurred during the period of 20–29 July 2004 each with unique characteristics of energy input, affording the opportunity to observe the response characteristics of the thermosphere. By studying these storms in such close vicinity to one another, the aliasing effects of satellite sampling and seasonal and solar cycle variations are mitigated. Differences between the daytime and nighttime density response at all latitudes are discussed in the context of the various mechanisms. Generally, response times are between 3 and 4 h at low latitudes while less than 2 h at midlatitudes to high latitudes. These time lags are significantly shorter than those conventionally used to drive many empirical models of the thermosphere. During the night, response times are less accurate because of the somewhat sporadic arrival of traveling atmospheric disturbances at different latitudes.