Abstract
Abstract. In this paper we study the effect of artificial HF heating on cosmic radio noise absorption in the D-region ionosphere. The effect has earlier been studied theoretically in idealised cases and without experimental verification. Here we present a 3-dimensional modelling of the effect, taking into account the directivity patterns of the vertical beam of the EISCAT Heater at Tromsø, Norway, and the intersecting beam of the IRIS imaging riometer at Kilpisjärvi, Finland. The heater-induced enhancement of cosmic radio noise absorption at the IRIS frequency (38.2 MHz) is estimated to be between 0.02 dB and 0.05 dB in the most representative model cases. However, a statistical study of IRIS data from a selected set of heating experiments carried out during the years 1994–2004 shows that the median effect is between 0.002 dB and 0.004 dB, i.e. an order of magnitude less than theoretically predicted. This indicates that the actual HF heating effect at D-region altitudes is substantially overestimated by the present theory.
Highlights
Radio wave absorption is known to take place in the ionospheric D-region through collisions of electrons, which are accelerated by the electric field of the radio wave, with neutral atoms and molecules
The intensity of a radio wave, emitted by the heater antenna field, along a ray path r is determined by Eq (1), where the unabsorbed intensity is given by transmitter power P and antenna gain G in the ray direction: PG
Some minor-scale differences are found between seasons: summer atmosphere yields higher effects, except in the high electron density case with antenna field 2
Summary
Radio wave absorption is known to take place in the ionospheric D-region through collisions of electrons, which are accelerated by the electric field of the radio wave, with neutral atoms and molecules. In contrast to radar measurements, passive monitoring of any HF radio waves penetrating through the heated volume can be used as a diagnostic technique This is possible because the increased electron temperature raises the electronneutral collision frequency, and decreases the transparency of the plasma. This additional absorption can be supposed to take place and disappear practically immediately, in timescales of milliseconds, when the heater is turned on and off (Rietveld et al, 1986). The effect on cosmic radio noise intensity seen in the riometer beam overlapping the heated volume is expected to be a step function of a certain magnitude added to the natural variations of the transparency. This study is dedicated to finding the D-region heating effect and its magnitude, both by modelling and by statistical analysis of IRIS beam 9 data as contrasted to the other IRIS beams not overlapping the heated volume
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