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.