Probe experiment characterizing 30-MHz radio wave scatter in the high-latitude ionosphere

A probe experiment, consisting of radio links between a common 30-MHz trans mitter located at Murmansk, Russia, and two receivers used as the imaging r iometer (two-dimensional 64 multiple-beam antenna) located at Ny Alesund, S valbard, and Tjornes, Iceland, was carried out to characterize wave scatter in the high-latitude ionosphere. They are nearly aligned with and perpendi cular to the geomagnetic meridian, respectively. In experiments conducted i n March-April 1994, the 30-MHz Probe signals were identified at nighttime m ore frequently than during the day at both receiver stations during periods of increased geomagnetic activity near the path midpoints, indicating that a relationship between the propagation path and the location of the aurora l oval controls signal identification. For the nighttime propagation paths within or crossing through the auroral oval, duty cycles of the probe signa ls were roughly correlated with increases in geomagnetic activity. Their ar rival directions showed a spread with a dominant power on the low elevation and a normal distribution in azimuth. These results indicate that the prob e signals are characterized as nonmeteoric "auroral E" scatter caused by ir regular, large-scale profiles of electron density enhancements at the lower edge of the ionosphere. However, on 2 days of weak geomagnetic activity, s trong probe signals with bursty behavior were identified by an extremely hi gh duty cycle (similar to 98%) for the nighttime meridian path only, and th eir arrival directions showed an isotropic spread in aziniuth. Such nonmete oric probe signals are characterized as "coherent" scatter caused by small- scale (similar to 5 m) field-aligned irregularities in electron density in the E region ionosphere, related to "sporadic E" occurrence.