RAPID RAY MOTIONS IN BARIUM PLASMA CLOUDS AND AURORAS

Barium plasma clouds released at high latitudes characteristically become striated with many field-aligned rays. The rays which often resemble auroral features usually drift as a whole with the E x B/B2 drift of the cloud and alter position only slowly (order of tens of seconds). On two evenings in 1968, in released from Andoya, Norway, anomalous field-aligned brightenings or emission enhancements of up to 3X were observed to move rapidly (10-20 km/s) through three different Ba+ clouds. Similar effects were observed in Ba+ clouds released from rockets launched from Poker Flat, Alaska: On March 21, 1973, in two Ba thermite releases and on March 22, 1980, in the Ba-shaped charge experiment ''Miss Peggy.'' On these occasions, auroras on or near the Ba+ L shell, also exhibited active rapid ray motions. This leads us to the assumption that the two phenomena are related and the expectation that an explanation of the rapid ray motions in the Ba+ clouds would lead to a better understanding of the physics of auroral ray motions and the auroral ionosphere. Seven possible mechanisms to produce the observed moving emission enhancements are discussed. Direct excitation by energetic electron bombardment is ruled out on the basis of energy fluxes required for realistic cross sections. Direct motion of an isolated Ba+ ray past the other rays by E x B/B2 motion seems very unlikely due to the observed variations in the enhancements and the large E field required (>500 mV/m). Compressional waves do not seem to be of sufficient amplitude or velocity. Formation of folds or curls in a sheet beam instability require moving radial fields of 500 mV/m but would explain the auroral features as well as the Ba+ by the same mechanism. Qualitatively, the redistribution of Ba ions as a flux tube encounters a higher electron density in the ionosphere may result in enhancements. Absorption or radiation of Doppler shifted Ba+ emissions by ions gyrating or moving at a few kilometers per second seems to be the most promising mechanism for producing the enhancements. The observations provide compelling evidence for the existence of transient electric fields of order 100 mV/m at altitudes as low as 200 km during active aurora with rapid ray motions. The affected regions have dimensions of order a few kilometers across B and move eastward at 10-20 km/s.