Abstract. Spatial structures in enhanced ionization of the ionosphere are observed by absorption of cosmic radio waves. These structures are resolved by using theoretically derived imaging riometer antenna array directivities. These directivities are calculated from beam phasing of 64 crossed dipole elements of the 38.2-MHz antenna array at SANAE IV, Antarctica. In order to ensure that these derived directivities are represemative of the actual viewing directions of the 64-beams, a radio transmitter was flown by helicopter across the antenna array. In this paper variations in the receiver signal strengths, recorded when flying across beam-viewing directions, are compared with the spatial and angular-dependem profiles of expected receiver output responses, derived theoretically from the directivities of the antenna array. A Global Positioning System (GPS) device on board the helicopter was used for positional recording. The derived and recorded profiles did coincide occasionally, but at other instances relative displacements and differences in magnitude of responses were observed. These displacements and differences could be attributed to degradation in position fixes imposed deliberately by selective availability on the GPS system. Excellent coincidence for a number of beam crossings proved that the viewing directions are accurate in all the beam directions, since the multi-dimensional Butler matrix produces 64 simultaneous beams.

[1] The expansion phase of a substorm on the evening of 1 May 1997 was studied from recordings of all-sky optical aurora and a 64-beam imaging riometer at the Antarctic base SANAE IV (L = 4.1). Digitized all-sky, low-level white light images of auroral optical emissions were mapped onto the angular directivity functions of the 64-beam imaging riometer. During the period of investigation, several optical arcs appeared and disappeared, while a persistent absorption structure appeared in the south (poleward). This and new structures changed only slowly in time. The differences in morphologies of optical emissions and absorption regions, together with temporal differences, imply that there are two categories of energetic auroral electrons: the softer electrons (<10 keV) causing optical emissions in the E and F regions of the ionosphere and the harder (>10 keV) electrons ionizing down into the D region for cosmic radio noise absorption (CRNA). Optical emissions tend to lead the appearance of cosmic radio noise absorptions by 0-60 s in a specific region of ionospheric space. Furthermore, in the all-sky optical data, westward traveling surges (WTS) were observed coincident with a CRNA WTS and leading the CRNA WTS by $30-50 s. In the CRNA data the WTS was ribbon-like, traveling westward at a speed of 2.3 km s À1 . These observations suggest that the softer electrons, causing E and F region optical emissions, may be injected onto drift paths closer to Earth than the more energetic electrons, ionizing down into the D region for cosmic radio noise absorption. The harder electrons could have been accelerated by, for instance, an increasing dawn-to-dusk electric field for later precipitation.

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Experimental m easuremen ts usi ng scale model techniqu es ha ve been carried o u t to determine t he effectiveness of a ground system of lon g-wire radials to obtain low a ng les of dep ar ture of t ra nsmission . Since transmission was to be in one direction only, t he gro und wires were laid out to form a g round-plane sector approximately 18° wide cen te red i n t he direction of trans mi ssion. The antenna was a base-driven vertical monopole. Measurem en ts were made of t he relative respon se in decibels for t he mo nopole used as a receivin g anten na at a freq uency of 400 m egacycles per seco nd . The target tr a nsmi tter a nte nna was always lo wted at a di stance of 200 wave lengths. At t his separation the ground pla ne sector was in t he neal' fi eld of t he target t ransmitting an tenna a nd appropriate co rrections must be m ade.The received signa l strength improvement due to the presence of t he g round secto r was approximately 14 decibels. The m eas ured lobe positions of the first a nd seco nd beam m aximum s and t he fir st null a rc in good ag ree men t with t heo ry.

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