Perturbing the high-latitude upper ionosphere (F region) with powerful HF radio waves: A 25-year collaboration with EISCAT

We present a brief review of experimental investigations concerning the perturbation of the high-latitude upper ionosphere (F region) by powerful HF radio waves (pump waves), carried out by the team from the Arctic and Antarctic Research Institute (AARI) during a 25-year collaboration with the European Incoherent Scatter (EISCAT) Scientific Association. Three types of HF pumping experiments were carried out by the Russian team at EISCAT. The first type of experiments was related to the modification of the ionosphere-magnetosphere coupling by HF pumping into the nightside ionosphere auroral E and F regions. The second type of experiments was concerned with investigations of geophysical phenomena by using the EISCAT HF heating facility and multi-instrument diagnostics. Most of the experiments were closely related to radio science and nonlinear plasma experiments (a third type of experiment). The main attention was paid to the investigation of artificial ionospheric turbulence in the upper ionosphere (F region) induced by the extraordinary (X-mode) HF pump waves. A great number of X-mode experiments, carried out by the Russian team at EISCAT, clearly demonstrated for the first time that an X-polarized HF pump wave was able to produce various HF-induced phenomena in the high-latitude ionospheric F region, which could be even much stronger than the O-mode effects. We found for the first time that the powerful X-mode HF radio waves, injected into the high-latitude ionospheric F2 layer towards the magnetic zenith, were capable of generating artificial field-aligned irregularities, radio-induced optical emissions in the red (630 nm) and green (557.7 nm) lines, strong electron acceleration leading to Ne enhancements, ion-acoustic and Langmuir electrostatic waves, and spectral components in the narrowband stimulated electromagnetic emission observed at a large distance from the HF heater. The features and behavior of the X-mode effects were analyzed and compared with the O-mode effects.