Joule heating and nitric oxide in the thermosphere, 2

During geomagnetic storms, gravity waves propagate from the polar regions toward the equator heating the thermosphere at 140 km and higher. These gravity waves are produced by Joule heating that occurs at latitudes of 60° and higher. The heating leads to an increase in the density of nitric oxide at 140 km in the thermosphere. On some occasions, the increased nitric oxide diffuses downward to the 110 km level causing the nitric oxide density at that level to increase substantially. Two and a half years (11 March 1998–30 September 2000) of Student Nitric Oxide Explorer (SNOE) observations of nitric oxide were examined to look for occurrences of increased nitric oxide produced by Joule heating initiated gravity waves and to determine how often downflow of nitric oxide occurs. The results of this study show that gravity wave heating occurs frequently, about 12–14% of the time at 40° latitude. For about 50% of these events, downflow of nitric oxide from 140 km to 110–120 km occurs the following day. About 2–3% of the time, gravity waves propagate all the way to the 20°N and S latitude band around the equator. On special occasions, downflow of nitric oxide occurs at the equator as the result of Joule heating occurring in the polar regions. This happened on five occasions during the two and a half year period in 1998–2000.