Double-probe measurements in field-aligned irregularities produced by intense electromagnetic radiation

We present the signature of filamentary field-aligned irregularities (FAI) in the artificially modified ionosphere using data from a rocket-borne floa ting double-probe gathered near the critical layer of the Arecibo HF heater beam. We model the double-probe signature as proportional to each filament 's density gradient, in the plane perpendicular to the geomagnetic field. W e find that this signature is consistent with the presence of a radially di rected effective electric field (E) over right arrow(eff) within each of th e over 180 filaments. The direction of (E) over right arrow(eff) is consist ent with that of an ambipolar electric field associated with the rapid perp endicular diffusion of ions out of each filament, or with an apparent elect ric field due to an inward electron temperature gradient associated with th e presence of hotter plasma inside each filament. Our model also gives an e stimate of the impact parameter at which each filament is encountered. The mean square impact parameter shows the correct relation to the mean square filament transit time, assuming cylindrical field-aligned filaments. The co nsistency of these results confirms that the previously reported density de pletions are quasi-steady, cylindrically symmetric, spatial structures. A s mall shift in the apparent angle between the double-probe boom and the fila ment-rocket velocity places a lower limit on the Earth-frame drift velocity of the filaments, away from the heater beam. Since the filaments are obser ved near the westward edge of the heater beam, well away from the bulk of t he heater Poynting flux, we expect that the filaments are observed in the p rocess of decaying, that is, that ions are diffusing inward rather than out ward. Thus we identify (E) over right arrow(eff) as a temperature gradient and use it to estimate a minimum filament temperature enhancement of simila r to 100 K.