IAGA 2005 Scientific Assembly, Toulouse, France, 18-29 July 2005

NO₂ and ozone changes in the Northern Hemisphere middle atmosphere due to the October-November 2003 solar proton events

P. T. Verronen¹, A. Seppälä¹, E. Kyrölä¹, S. Hassinen¹, L. Backman¹, A. Hauchecorne², J. L. Bertaux², D. Fussen³, C. J. Rodger⁴, M. A. Clilverd⁵, C.-F. Enell⁶, E. Turunen⁶, Th. Ulich⁶

¹Finnish Meteorological Institute, Earth Observation, Helsinki, Finland,
²Service d'Aeronomie du CNRS, Verrieres-le-Buisson Cedex, France,
³Institut d'Aeronomie Spatiale de Belgique, Brussels, Belgium,
⁴Physics Dept, University of Otago, Dunedin, New Zealand,
⁵British Antarctic Survey, Cambridge, U.K.,
⁶Sodankylä Geophysical Observatory, Sodankylä, Finland,

Abstract

The GOMOS instrument on board the European Space Agency's Envisat satellite has been observing the middle atmosphere since March 1, 2002. GOMOS measures altitude profiles of several minor gas constituents, e.g. ozone and NO2, with a good global coverage including the poles. The advantage of the stellar occultation technique used by GOMOS is that measurements can be made in both day and night conditions with no strict restrictions on the local time. In the work presented here, observations of O3 and NO2 by GOMOS have been used to monitor both short-term (days, Oct 26-Nov 6) and long-term (months, from late October to the end of year) effect of the October-November 2003 series of solar proton events on the NH polar middle atmosphere. The expected effect of energetic proton precipitation includes production of odd hydrogen (HO_x) and odd nitrogen (NO_x) species through particle impact ionisation/dissociation and ion chemical reactions, and subsequent depletion of ozone in catalytic reactions with HO_x and NO_x.

In addition to GOMOS measurements, the short-term effects of proton precipitation were studied also with a 1-D ion and neutral chemistry model (SIC) using the proton flux measurements from the GOES-11 satellite as input. SIC predicts order-of-magnitude changes in HO_x and NO_x concentrations as well as ozone depletion by 20-95% at 40-85 km. The effect on ozone displays a diurnal dependence such that the largest depletions occur during sunrise and sunset. The modelled and measured NO2 profiles agree well at altitudes 35-60 km, particularly during times of large concentrations observed after the solar proton event onset. A comparison of the time series of ozone depletion shows a good agreement between SIC and GOMOS.

The long-term results from GOMOS show NO₂ enhancement of several hundred per cent and tens of per cent ozone depletion between 36 and 60 km, with the concentrations of NO₂ and ozone displaying a strong negative correlation. Depletion of ozone continues even after the proton fluxes have returned to quiet-time levels and, with time, the altitude of maximum depletion decreases as particle-produced NO2 descends from the upper stratosphere to lower altitudes inside the polar vortex. The maximum effect on ozone is seen at the end of November, at which time there is a ≈60% reduction at 42 km. Afterwards, a partial recovery of both NO₂ and ozone occurs towards the end of the year.