Generation of artificial magnetic pulsations in the Pc1 frequency range byperiodic heating of the Earth's ionosphere: indications of ionospheric Alfven resonator effects

A series of six experiments with the EISCAT HF heater device assisted by th e EISCAT (European-Incoherent-Scatter) radar were carried out with the purp ose of producing artificial magnetic pulsations in the 0.1-3 Hz frequency r ange. In only 3 of the 30 h of experiment time under a variety of ionospher ic conditions was an artificial magnetic signal detected by ground-based ma gnetometers. A numerical model was used to explain the sporadic nature of t he artificial signal in terms of ionospheric parameters. For several experi ments the EISCAT radar provided an in situ electric field and/or electron d ensity values; otherwise standard neutral atmosphere and ionosphere models were used. The PGI model was only partially successful. It could produce th e right order of magnitude for the artificial signal when it was observed a nd it could demonstrate the different efficiencies when using either the O- or the X-mode of the HF wave, but it could not explain why the artificial signal was observed at a particular time and not at others. This is only pa rtially due to the uncertainty in one or more input parameters. When the ar tificial signal was observed its spectrum usually exhibited spectral resona nce structures of the Ionospheric Alfven Resonator (IAR), indicating that a "DC approach" is insufficient and that the generation of oscillating held- aligned currents, and thereby shear Alfven waves, has to be taken into acco unt. We believe, however, that even with the introduction of the IAR into t he model it will not be possible to resolve the sporadic character of the h ealing-induced artificial magnetic signal entirely. A more realistic way of D-region modeling will without doubt also be an important factor in resolv ing the puzzle. (C) 2000 Elsevier Science Ltd. All rights reserved.