Nitric oxide variations in the mesosphere and lower thermosphere during the November 1993 storm period

The variability of nitric oxide has not been fully quantified. Particle precipitation is known to enhance nitric oxide; however, a detailed understanding of this process is lacking, and its importance has not been quantified. In this paper, we present nitric oxide measurements between 80 and 200 km altitudes from the Halogen Occultation Experiment (HALOE) on UARS during the November 1993 space weather special study period, near solar minimum conditions. The nitric oxide mass mixing ratio routinely varies by 3 orders of magnitude above 80 km altitudes. The variation of NO with magnetic activity is discussed, using two measures of magnetic activity level. The first is the well-known Kp index, and the second is derived from the particle environment monitor (PEM) on UARS. The Kp index varied between 0 and 7 during the study period. A detailed comparison of NO enhancements with ionization rates derived from the PEM data indicates that the particle precipitation can account for much of the NO. In the context of the November 1993 storm period, this study is interesting because it emphasizes the potential importance of space weather for the entire atmosphere and not only regions above 100 km altitude.