Additional stratospheric NO x production by relativistic electron precipitation during the 2004 spring NO x descent event

We analyze in detail the February 2004 Global Ozone Monitoring by Occultation of Stars (GOMOS) NO 2 observations in the northern polar latitudes during the springtime descent of NO x from the mesosphere into the stratosphere. We combine GOMOS observations with SABER‐observed NO 5.3 μ m 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 NO x , supplementing the original amounts that were already descending. At altitudes of 50–70 km, GOMOS observations of NO 2 showed a delayed response to the geomagnetic storm, with NO 2 being generated 3 days after the start of the storm. The delayed response and duration of NO 2 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 NO 2 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 × 10 6 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 × 10 4 el cm −2 sr −1 s −1 for 2–6 MeV. Prior to the storm the descending NO 2 had average mixing ratio values of ∼150 ppbv. The geomagnetic storm–induced relativistic electron precipitation event doubled the amount of NO x descending into the stratosphere to ∼300 ppbv after the storm.