What Can We Learn About the Ionosphere Using the EISCAT Heating Facility?

Apart from being used for plasma physics, the HF facility near Tromsø, Norway, can be used to perturb the ionosphere at various heights in different ways, thereby giving information about the ionosphere. The co-located incoherent scatter radars are probably the most powerful instrument for probing the ionosphere, but HF techniques can complement the radars and even have some advantages. The principal perturbation method is to increase the electron temperature in a controlled way, some examples of which are presented here. Artificial electron heating in the E and F regions is useful for testing aeronomical models. More recently it has been discovered that electron heating can dramatically affect polar mesospheric echoes observed by VHF and UHF radars. Particularly the overshoot effect promises to be a powerful diagnostic of the physics and chemistry related to the formation of these layers, which are thought to involve dust, ice particles and aerosols. Radio induced optical emissions provide a way of measuring the lifetimes of excited species at different heights in the ionosphere, thereby providing a way of measuring the neutral density which is one of the most important parameters determining the lifetime. The technique of creating artificial periodic irregularities set up in the standing wave pattern of the upgoing and ionospherically reflected HF wave provides valuable information all heights below reflection. One particular feature of this method is that it can detect the presence of layers around 50 km and measure vertical winds, and electron densities and temperatures at various heights.