Comparison of a thermospheric photochemical model with Student Nitric Oxide Explorer (SNOE) observations of nitric oxide

Abstract

A time‐dependent thermospheric model has been used to calculate the nitric oxide density in the lower thermosphere for a 935‐day period, 11 March 1998 to 30 September 2000. This model uses daily values of the observed solar soft X‐ray irradiance (2–7 nm) as an energy input parameter. The model does not include an energy input from auroral electron precipitation. The results of the model calculation of nitric oxide density at 110 km were compared with observations of nitric oxide density made with the Student Nitric Oxide Explorer (SNOE) for the 935‐day period. At the equator the model calculations and the observations agree very well with a linear correlation coefficient of 0.876. The correlation coefficient remains high for the altitude region 107–117 km, the region where solar soft X‐rays (2–7 nm) are the major source of nitric oxide production. The comparison of the model calculations with the observations as a function of latitude show that there is excess nitric oxide poleward of 30°N and S latitude particularly during the fall‐winter season. We believe that the source of this excess nitric oxide is the nitric oxide that is produced in the auroral region (65°–75°N and S geomagnetic latitude) by precipitating auroral electrons. We believe that aurorally produced nitric oxide is transported equatorward by horizontal winds. At midlatitudes the excess nitric oxide decays to about half of its initial value in one day. At times of large geomagnetic storms we believe that aurorally produced nitric oxide is transported all the way to the equator by horizontal winds. The excellent correlation of the model calculations and the SNOE observations of nitric oxide at 110 km between 30°S and 30°N support the hypothesis that solar soft X‐rays are the source of the variability of nitric oxide in the thermosphere at low latitudes.