39th Annual Conference of the Finnish Physical Society, Espoo, Finland, 17-19 March 2005

Effects of artificial RF heating on riometer absorption in the ionospheric D region

A. Kero1, C.-F. Enell1, P. T. Verronen2, A. Seppälä2, Th. Ulich1, T. Raita1, E. Turunen1

1Sodankylä Geophysical Observatory, Sodankylä, Finland,
2Finnish Meteorological Institute, Helsinki, Finland.


The lowest part of the ionosphere, the D region, plays a key role in the coupling between the neutral atmosphere, which is dominated by particle collisions, and the ionosphere, which is determined by the plasma effects of free charges. In the D region the features from both worlds overlap in a complicated way, making it a challenging layer from the point of view of theoretical modelling. Moreover, the D region is also very difficult to access experimentally. Located at 50-100 km, it is practically too low for satellites and too high for balloons, and the ionisation is generally too weak to be studied efficiently with radars. Our current understanding of the D region is therefore to a large extent based on theoretical modelling. We therefore seek to explore new ways of experimental verification of current models.

Riometers (relative ionospheric opacity meters) are standard instruments in ionospheric monitoring and have been in operation globally for decades. Riometers are radio receivers which detect cosmic background HF radio noise through the ionosphere. There are also active instruments for ionospheric studies, which enhance electron temperatures by transmitting high power HF waves. This method is called artificial ionospheric heating and allows us to modify ionospheric conditions in a controlled manner.

Both these techniques are based on the radio wave absorption caused by collisions between neutral particles and electrons accelerated by the radio wave. In the D region, the electron density is already significant and the electron-neutral collision frequency is still high. Thus the D region provides optimal conditions for this absorption process to take place.

The combination of artificial heating and riometers therefore provides an appropriate tool for remote sensing of the D region. The Sodankylä Ion Chemistry (SIC) model has been developed for detailed modelling of the D region and is used, among many other applications, for interpretation of riometer data. In this poster we present a theoretical study of heating experiment effects on riometer absorption. Experiments are scheduled to be carried out with the EISCAT Heating facility in Tromsø, Norway, in near future.