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Evidence for Precipitation of Energetic Particles by Ionospheric «Heating» Transmissions

A paper with this title appeared in the J. Geophys. Res. 80, #31, p4383-4386, 1975. It is available as a pdf document here. The paper reports observations made in 1970 by the first prototype dynasonde in the form of a time-lapse movie, an electronic copy of which you see on the right. Although the dynasonde had then been in use for Heating diagnostics for about a year, the movie was produced for quite another purpose (to illustrate and advertise the new dynasonde's capabilities), and the film received rather little attention until 1974; during a review, the present phenomenon was discovered. In the experiment of 2 October 1970, the heater was tuned to 9.9 MHz, and cycled 15 minutes ON and 15 minutes OFF. The ON periods are tinted red in the film; 3 ionograms (of 4s duration) were obtained each minute. As displayed by a browser at 10 frames per second, the 762 ionograms are shown in 76 seconds, sped up by a factor of 200 over real time.

The circumstances of occurrence of «sudden Sporadic E layers», coincident with the Platteville Heater ON/OFF cycles, are discussed in the paper, together with several possible mechanisms. What was not previously available to a wider audience are the impressions of (imperfect) synchronization and variability of Es occurrance among the 8 or 9 Heater cycles of the sequence. Generally, the Es seems to be delayed from the Heater ON time, sometimes by many minutes.

Not mentioned in the paper, but evident in the movie, are downward-traveling perturbations of the «normal» ionogram traces. These also seem to be caused by the heating episodes, but natural «traveling disturbances» (AGWs) cannot be ruled out, at least for some events.

It is worth mentioning that attempts to reproduce these phenomena at the MPI Heating facility at EISCAT, during the author's work there 1983-1986, were unsuccessful (but see [82]). Since the present paper and film present evidence that the Es occurred only when the heating frequency was near the F-region peak plasma frequency, absence of the effects in Norway might be a consequence of the restricted number of heating frequencies available there.