SIGNATURES OF POLAR-CAP PATCHES IN-GROUND IONOSONDE DATA

The interpretation of localized density structures in the high-latitud e ionosphere as observed by ground ionosondes is a task that is compli cated by the complexity of those structures and by the variety of proc esses that ionosonde waves undergo. To aid interpretation, we have per formed a first-order analysis of the shapes of traces seen in ionosond e displays. This has been done using an idealized model of a particula r structure of current interest, a polar ionospheric patch. The patch is modeled as a spheroidal enhancement of density superposed on a hori zontally stratified F layer. Ray tracing is used to find O- and X-mode rays that reflect back to a ground ionosonde after total reflection. Theoretical curves are calculated for swept-frequency (h'f), fixed-fre quency (h't), and Doppler-frequency (f(D)t) operational modes. The com puted h'f curves have U shapes, the h't curves are quasi-hyperbolic, a nd the Doppler curves have reverse-S shapes. By moving the patch throu gh a series of horizontal separations from the ionosonde, the sequence of curves is calculated for a patch passing by an ionosonde. The theo retical traces have been compared with data from the Canadian advanced digital ionosondes (CADIs) at Resolute Bay, Eureka, and Alert. The CA DI h'f traces are observed usually to have incomplete U shapes, missin g their lower-frequency arms. In the light of the ray tracing the CADI results are consistent with the hypothesis that HF-scattering irregul arities are found on both the leading and trailing edges of patches bu t are stronger on the trailing edge. Similar results have been reporte d for scintillation-causing irregularities. This points to the dominan ce of the gradient-drift instability. The hyperbolic h't curves are as ymmetric, having slower downsweeps than upsweeps. This can be caused b y horizontal asymmetries in the patch density distribution. Often, pol ar patches seen by CADI are not single, isolated enhancements. Rather, they appear as composites of substructures, each of which mimics cert ain aspects of the single patch described by the present theoretical m odel.