Nitric oxide and the winter anomaly in absorption - Rocket results

Results of an extended series of rocket measurements at South Uist (57.4 deg N, 7.4 deg W) aimed primarily at investigation of the winter anomaly in radio-wave absorption are described. Initial flights, in which D and lower-E region electron densities were measured, show that three types of winter day can be recognized, with electron density becoming progressively more enhanced in the region 75 km-90 km as radio-wave absorption increases. A later group of rocket flights investigated the relationship with ion densities and minor constituents on a day of very high absorption. The electron concentrations, compared to those for a low absorption day, are significantly increased from 70 km-95 km. The NO(+) population, is very clearly correlated to the electron concentrations at all heights from 70 km upwards and is the dominant ion. The transition from diatomic molecular ions to water cluster ions dominating occurred at about 70 km. A major enhancement of NO concentrations at mesosphere and lower thermosphere heights on winter anomaly days is suggested.