On the diurnal variability in F2 –region plasma density above EISCAT Svalbard Radar

Moen, J.1, Oiu, X. C.1, Carlson, H. C.2, Fujii, R.3 and McCrea, I. W.4

1 University of Oslo, Oslo, Norway
2 Utah State University, Logan, USA
3 STEL, Nagoya University, Nagoya, Japan
4 Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK

Two separate months of continuous operation of the EISCAT Svalbard Radar (ESR) in the common program mode, looking up along the magnetic field line, have been analysed with respect to variability in the F2 region peak density. In order to characterize the diurnal variation, 2 minute resolution data were binned in 30 minute averages, and then averaged over all days of observations within each month; February 2001 and October 2002. The analysis resulted in a characteristic diurnal variation in the F2 region peak density, with one maximum located almost exactly at 12 MLT and one around 23 MLT. The ESR is sitting inside the polar cap most of the time for moderate and high activity. The peak near noon is consistent with transport of solar EUV ionized plasma into the polar cap in the cusp inflow region. The peak near magnetic midnight indicates that on average, the path of exit of high density plasma from the polar cap is shifted slightly towards pre-midnight, i.e. entirely consistent with a recent statistical study on the occurrence of airglow patches by Moen et al. (submitted to Geophys. Res. Lett, 2007). In February 2001 (October 2002), about 50% (17 %) of the 2 minute data dumps had F2 region peaks larger than 1012m-3, referred to as extreme densities. This high density plasma represents patches or source material for polar cap patches. The occurrence of extreme densities in the 10-11 MLT sector occur predominantly for IMF BY positive, and the occurrence of extreme densities in the 13-14 MLT sector occur predominantly for IMF BY negative. This is entirely consistent with the IMF control of zonal motion near noon. However, this represents the strongest evidence to date for no IMF BY preference in patch formation. Comparison with the international reference ionosphere model (IRI) demonstrates that the model value F2 peak is representative of solar EUV production of plasma, but not of long distance transport of F2 region plasma. Time continuous measurements by the ESR during IPY will constitute an important basis for empirical modelling of the polar cap ionosphere.