AGU Fall Meeting 2005, San Francisco, California, USA, 5-9 December 2005

The Effects of the January 2005 Solar Events on the Middle Atmosphere

Annika Seppälä¹, Pekka T. Verronen¹, Mark A. Clilverd², Craig J. Rodger³, Erkki Kyrölä¹, Esa Turunen⁴, Thomas Ulich⁴

¹Finnish Meteorological Institute, Earth Observation, P.O.Box 503, FI-00101 Helsinki, Finland,
²Physical Sciences Division, British Antarctic Survey, Madingley Road, Cambridge, U.K.,
³Physics Department, University of Otago, P.O.Box 56, 9001 Dunedin, New Zealand,
⁴Sodankylä Geophysical Observatory, Tähteläntie 62, 99600 Sodankylä, Finland.

Abstract

The Solar Proton Event that began on January 16th, 2005, took an unusual turn on January 20th when the giant sunspot 720 produced a powerful X-class solar flare. This flare triggered an extraordinary solar proton storm: the flux of the solar protons with the highest energies (>100MeV) was of the same order as those in the well known October 1989 Solar Proton Event (SPE), whilst the lower energy fluxes remained at moderate levels, making the January event the hardest and most energetic proton event of Cycle 23 so far.

Solar protons with very high energies are able to cause ionization in the polar atmosphere down to the middle–lower stratosphere (proton energies associated with the Jan 20th event were high enough to cause a so called Ground Level Event). The subsequent ion chemistry leads to increased production of odd nitrogen (NO_x) and odd hydrogen (HO_x) in the polar middle atmosphere. Both NO_x and HO_x take part in catalytic ozone consuming reaction cycles.

We have used the Sodankylä Ion Chemistry model (SIC) and Very Low Frequency (VLF) subionospheric propagation observations and modelling to study the conditions in the northern polar atmosphere during the January events. The SIC model has been used to predict the response of the NO_x, and HO_x constituents to the precipitating high proton fluxes, and the subsequent effect of NO_x, and HO_x on ozone. The model also calculates the time-varying ionospheric D-region electron densities. These have been used as an input to a VLF subionospheric propagation model for comparison with experimental observations during the event. Furthermore we have used the nighttime observations of mesospheric and stratospheric ozone made by the GOMOS instrument on board the Envisat satellite to monitor the ozone depletion in the middle atmosphere due to the January SPEs.

We will present results from both modelling and observations of the middle atmosphere during the January 2005 solar proton storm series, contrasting the effects and significance of the different levels of proton spectral hardness.