When "the fog comes on little cat feet," we want to see what it's hiding. The millimeterwave regime of the electromagnetic spectrum can show us-if we have the necessary vision. bed}. :is opposed 1o a perfect reflector, which has ,:' = 0 The emissivity of an object (v_hich is polauization-dcpendent) is a function of the dielectric properties of its co_lstitUClllS, its surf4cc roughness, and the angle of observation (,% samplcc_l-the mea_,uredemissixitiesol dixers materials at various frequencies is ei,,en in the table on the next page.) The radiation intensity of a 300-Kelvit+ blackbody t+all_ e×ponentialt> by about eight orders of magnitude from a peak \alue in the 1R to the millimeter-_,ave regime at around 94 GHz (Figure 1). This large dccrease m mtensit._, is partially corn pensated for bv the lower photon energy, th:tt occurs at millimeter-,aave fre quencies. H_,aexcr this '-,iluatioll is dramaticall\ reversed in fog and other inclenlent _ve_tther \_hcn one lakes into accotlilt the _,ignal attelltlation bv atlnospheiic constituents. Ilele the ,qrength of the propagated signal peaks in the millimeter _._,:l_.c region, as the figtire sho_,s. Fhu-,,ecnml reas(m, haldv, 41c Itlllitalions, is due tO the low inillilneler-wave pnv, cl Ilu,, hul i,, not the probleln it once ,aas. Se',eral recent technological advances haxe enabled the exploitation of millimeter waves. Receivers with mixcl ll(ml ends ti_,hlg Scholtky-b;_uricr diodes have demonstrated double ",idchand i_oise figure', :_I (_,to l(} dB over the _4 to 300 GHz regime,

A study of the modifications produced by high‐power, high‐frequency (HF) waves in the F region of a quiet (low solar cycle) polar ionosphere is presented. In the polar geometry, maximum ohmic heating occurs producing significant changes to the zero‐order electron temperature and density profiles. The temporal and spatial aspects of these changes are calculated, and their effects on relevant ionospheric parameters, such as the dc conductivity tensor, are found. An interesting result is the generation of a density depression at the reflection point of the HF waves. The dependence of ohmic heating on HF pump parameters and peak background density is considered.

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An advanced ray‐tracing numerical method is coupled to an ionospheric plasma transport code to model the self‐consistent propagation of an obliquely incident high‐frequency (HF) beam in the ionosphere. An HF radar propagation scenario is presented as an application. During the daytime, a density increase in the lower ionosphere is predicted causing the defocusing of the incident HF beam.