Spectral Analysis and Step Response of Radio Frequency Scattering from a Heated Ionospheric Volume

Abstract

Spectral analysis of field‐aligned VHF backscatter from a heated ionospheric volume in the F layer over Platteville, Colorado, reveals a complex frequency‐space picture for both center line and plasma line scattering modes. The spectra are observed to fall roughly into one of three categories: (1) narrow band (in relation to the 100‐Hz unambiguous frequency interval), (2) noiselike (essentially uniform over at least 100 Hz), and, (3) composite structure, consisting of a narrow‐band peak and a noiselike wide‐band component. For the field‐aligned plasma line observations the presence of a mechanism different from that responsible for the Arecibo Ionospheric Observatory plasma line observations is strongly indicated. Signal bandwidths are found to be range dependent, with the narrowest spectral widths consistently being observed at the southern periphery of the heated volume. Measurements of ionospheric drift velocities along a nominally north‐south axis are also presented. Values are found to range from 0 to 30 m/s. The result of a measurement of cross‐section decay in response to step function heater power variation is found to admit a simple interpretation, i.e., that the Fourier component of the electron density fluctuations at half the radar wavelength smooths out by a diffusion process in planes perpendicular to B, with a time constant proportional to λ². This interpretation is consistent with other observations in which, after sudden turn‐off of heater excitation, the scattering is always found to decay slower at longer radar wavelengths.

SuperDARN observations of spectral enhancements excited during an ionospheric heating experiment

“…Although it has not yet been demonstrated that these 27 enhancements are stimulated by the HAARP pump wave (this will be done below), it is worth noting that the backscattered spectrum from above HAARP is surprisingly complex, and the more so since we are not aware that any of these enhancements have been previously reported (with the exceptions of those near zero Doppler shift (number 1 in Figure 1) and near the HFUHL (numbers 14–16) [ Minkoff et al , 1974; Carpenter , 1974; Minkoff and Kreppel , 1976; Hedberg et al , 1984; Hughes et al , 2003]).…”

“…The indexed vertical dotted lines mark enhancements with SNRs > 3s. Figure 1) and near the HFUHL (numbers 14-16) [Minkoff et al, 1974;Carpenter, 1974;Minkoff and Kreppel, 1976;Hedberg et al, 1984;Hughes et al, 2003]).…”

Ionospheric modification and parametric instabilities

“…According to this point of view, the observed spectra could depend partly on the intrinsic scattering spectra of the fieldaligned striations, partly on the Doppler shift due to a regular drift of the ambient ionosphere, and partly on the Doppler shift due to irregular drifts that are independent of the heating process. This might make the interpretation of the observed spectra [Minkoff and Kreppel, 1976] extremely difficult.…”

Observations of E region irregularities generated at auroral latitudes by a high‐power radio wave

“…Early measurements of AFAI's having short spatial scales (1-10 m) across geomagnetic field lines were made at Platteville, Colorado, in the early 1970's (e.g., Radio Science, volume 9, number 11, and Minkoff and Kreppel [1976]). The aspect sensitive scattering from these irregularities is detectable using HF and VHF radars of moderate size.…”