Proceedings with Extended Abstracts (single PDF file) - Radio ...

non-negativity of the image is prior informationwhich, when incorporated into the imageinversion along with any other informationavailable, permits finer resolution thanthe Nyquist sampling theorem would otherwiseallow. A compelling discussion of therationale for the MaxEnt along these lines hasbeen given by Jaynes (1982). Other reviewsof the MaxEnt principle have been given byJaynes (1985), Daniell (1991), and Skilling(1991). The computational algorithm we followin our radar work has been described byHysell and Woodman (1997). It requires theanalyst to supply all available radar interferometriccross-spectra along with estimates ofthe error covariances. Our problem differsfrom the one in radio astronomy mainly inthat radar range gating adds the third dimensionto the images. In addition, the time evolutionof the scattering medium in the radarcase can be evaluated by comparing imagesfrom successive data integrations.What follows are three examples of radarimages drawn from low-, middle-, and highlatitudeexperiments. They illustrate the efficacyof radar imaging in situations where anunambiguous or high-definition determinationof the target bearing is essential. At Jicamarca,high-resolution images are necessaryfor observing the fine structure in equatorialspread F flows. In mid-latitude experimentsconducted in the Caribbean, radar imagingallowed unambiguous common-volume radarexperiments to be performed in conjunctionwith operations at Arecibo. Radar imagery ofauroral zone irregularities can be compareddirectly with in situ measurements made byrockets whose tracks can be followed in theradar images.At Jicamarca, imaging has become an integralpart of research into equatorial spreadF. In postsunset F region imaging experiments,we observe the growth and evolutionof large-scale plasma depletions and radarplumes as thin, precursor scattering layersevolve into full-blown ESF events (Hysell,2000). An example of a fully developed radarplume is shown in Figure 1. The brightness ofeach pixel represents the signal-to-noise ratioon a log scale, the hue represents the firstFigure 1: Radar image of plasma irregularitiesin a radar plume during a fully developedESF event over Jicamarca.moment Doppler velocity (red shifted echoesare red, blue shifted are blue), and the saturationrepresents the spectral half width (purecolors are spectrally narrow, pastel colors arebroad.)Notice the serpentine, branched, fractalquality of the plume in the image. Thestructuring is mainly due to secondary, winddriven gradient drift instabilities that formonce the plume has developed. Animatedsequences of images like this one elucidatethe evolution of the underlying instability,the background flow surrounding the plume,and the dynamics of the plume once fullyevolved. The two-dimensional images arespatially and temporally unambiguous andare directly comparable to numerical simulations.Figure 2 presents an example of mid-99