• relativistic electron precipitation;
  • stratosphere;
  • odd nitrogen

[1] We analyze in detail the February 2004 Global Ozone Monitoring by Occultation of Stars (GOMOS) NO2 observations in the northern polar latitudes during the springtime descent of NOx from the mesosphere into the stratosphere. We combine GOMOS observations with SABER-observed NO 5.3 μm radiated power and an AARDDVARK-derived radio wave index (RWI) to describe the impact of the 11 February geomagnetic storm. Energetic electron precipitation generated some additional NOx, supplementing the original amounts that were already descending. At altitudes of 50–70 km, GOMOS observations of NO2 showed a delayed response to the geomagnetic storm, with NO2 being generated 3 days after the start of the storm. The delayed response and duration of NO2 production was found to be consistent with the increase in the flux of relativistic electrons measured by GOES at geostationary orbit and by POES through relativistic electron contamination of the >16 MeV proton channel. Using the Sodankylä Ion and Neutral Chemistry model (SIC), we found that a good fit to the observed NO2 mixing ratios at the peak of the geomagnetic storm effect was produced by a monoenergetic 1.25 MeV electron beam with a flux of ∼0.3 × 106 el cm−2 sr−1 s−1 keV−1 or with a “hard” electron spectra taken from Gaines et al. (1995) but with fluxes enhanced by a factor of 15, i.e., 8 × 104 el cm−2 sr−1 s−1 for 2–6 MeV. Prior to the storm the descending NO2 had average mixing ratio values of ∼150 ppbv. The geomagnetic storm–induced relativistic electron precipitation event doubled the amount of NOx descending into the stratosphere to ∼300 ppbv after the storm.