Introduction to ionospheric heating at Tromsø—I. Experimental overview

The high power radio facility, built jointly by the Max-Planck-Institut fur Aeronomie, Lindau and the University of Tromso, at Tromso in Northern Norway, has been operational since 1980. Since that time an extremely wide range of ionospheric modification phenomena, induced by the powerful radio beam or pump, has been studied by a multitude of diagnostic techniques. This review is concerned, in particular, with the role played by the anomalous absorption of the high power wave. Anomalous absorption arises when the high power electromagnetic waves excite thermal parametric plasma instabilities in the diffusion dominated ionospheric F-region. This leads to the growth of plasma density irregularities, which are highly elongated along the geomagnetic field and which very efficiently couple electromagnetic and electrostatic plasma waves. The conversion of the electromagnetic modifying wave to rapidly damped electrostatic waves constitutes the anomalous absorption process which gives rise to strong plasma heating and the self extinction of the pump. After briefly reviewing some relevant results from earlier experiments carried out in the USA, a number of modification experiments undertaken at Tromso, using both HF diagnostics and the European Incoherent Scatter radar (EISCAT), are described in detail. These experiments were designed to investigate anomalous absorption processes, large scale plasma heating, pump self action and cross-modulation effects. The relevant theories required to interpret these experimental results are also presented. In particular, the relative importance of collisional and anomalous heating at the high latitude site is examined. The importance of the Tromso high power facility to aeronomical studies is also briefly discussed.

The heating of the high lattitude ionosphere by high power radio waves

A high frequency electromagnetic pump wave transmitted into the ionospheric plasma from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave. The numerous spectral features of these stimulated electromagnetic emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and related theoretical, numerical, and simulation results are discussed. On long (thermal) time scales the SEE constitutes a self-organization of the ionospheric plasma which depends on the interaction of nonlinear processes in a hierarchy of time scales in response to the electromagnetic pumping. Particularly, the appearance of the rich SEE spectrum is associated with the slow self-structuring of the plasma density into a spectrum of magnetic field-aligned density striations. The dependence of the SEE on electron gyroharmonic effects and the presence of density striations suggests that the existence of a magnetic field in the plasma is important for plasma turbulence to dissipate into non-thermal electromagnetic radiation during the long time quasi-stationary state of the turbulence evolution.

Stimulated electromagnetic emissions by high-frequency electromagnetic pumping of the ionospheric plasma

Review of ionospheric modification experiments at Tromsø

EISCAT: early history and the first ten years of operation

The experiments performed up to mid 1984 with the heating facility at Ramfjordmoen near Tromso, Norway, are summarized. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF frequencies, excitation of E -region small-scale irregularities and of F -region small- and large-scale irregularities, anomalous absorption of HF wave on long and short time scales, excitation of incoherent backscatter plasma and ion lines, stimulated radio wave emission and F -region in situ measurements.

H. Derblan

Papers

Ionospheric modification experiments with the tromso heating facility