We present the signature of filamentary field-aligned irregularities (FAI) in the artificially modified ionosphere using data from a rocket-borne floating 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. We find that this signature is consistent with the presence of a radially directed effective electric field E&vec;eff within each of the over 180 filaments. The direction of E&vec;eff is consistent with that of an ambipolar electric field associated with the rapid perpendicular diffusion of ions out of each filament, or with an apparent electric field due to an inward electron temperature gradient associated with the presence of hotter plasma inside each filament. Our model also gives an estimate 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 consistency of these results confirms that the previously reported density depletions are quasi-steady, cylindrically symmetric, spatial structures. A small shift in the apparent angle between the double-probe boom and the filament-rocket velocity places a lower limit on the Earth-frame drift velocity of the filaments, away from the heater beam. Since the filaments are observed near the westward edge of the heater beam, well away from the bulk of the heater Poynting flux, we expect that the filaments are observed in the process of decaying, that is, that ions are diffusing inward rather than outward. Thus we identify E&vec;eff as a temperature gradient and use it to estimate a minimum filament temperature enhancement of ~100 K. ¿ 1999 American Geophysical Union