12th International EISCAT Workshop, Kiruna, Sweden, 29 August - 3 September 2005

The atmospheric effect of highly relativistic electron events at high latitudes

E. Turunen1, Th. Ulich1, C.-F. Enell1, A. Kero1, P. T. Verronen2, A. Seppälä2, F. Honary3

1Sodankylä Geophysical Observatory, Sodankylä, Finland,
2Finnish Meteorological Institute, Helsinki, Finland,
3Department of Communication Systems, Lancaster University, U.K.


Recent balloon observations of bremsstrahlung X-rays have shown that precipitation of electrons of relativistic energies up to several MeV occurs also at high latitudes. First such observation was made on August 20, 1996 at 1530 UT near Kiruna Sweden (L=5.7) detecting a burst of X-rays, which were best accounted for by atmospheric bremsstrahlung from mono-energetic 1.7 MeV precipitating electrons. Since the energy deposition at these energies occurs in the upper stratosphere and mesosphere, it is important to study the effect of such excess ionisation on atmospheric minor constituents.

In this paper we present theoretical estimates on local effects of highly relativistic electron precipitation events on background concentration of nitric oxide, as well as two experimental observations by the EISCAT VHF incoherent scatter radar and the IRIS imaging riometer. We use the Sodankylä Ion Chemistry model, which is a 1-dimensional time-dependent model of the lower ionosphere. It solves for concentrations of 36 positive ions, 27 negative ions, and 14 neutral species. The altitude range is from 20 to 150 km, with 1 km resolution. The model includes several hundred chemical reactions, the important photoionisation/dissociation processes, as well as vertical transport.