Decameter and meter wavelength radar studies of artificial plasma clouds in the lower ionosphere, 2. Unstable evolution in the lowerElayer and some implications, regarding sporadicE

Abstract

Radio frequency scattering experiments have indicated that irregular fluid motions of tens of meters spatial scale and tenths of seconds temporal scale occur in the E region. It has been suggested that there can also occur in this region a dissipative plasma instability (similar to what has been termed the gradient-drift or E×B instability in previous work) that displays these spatial and temporal scales and that may be of importance in the evolution of sporadic E. This paper describes an attempt to study the circumstances under which such an instability might appear by injecting alkali plasma clouds into the lower E region and observing them with decameter and meter wavelength, coherent-pulse-Doppler radar. After an initial blast-wave-driven expansion phase, an alkali plasma cloud undergoes a period of irregular behavior, in which a number of discrete echoes from seemingly coherent, large, short-lived reflectors appear superimposed on the generally diffuse plasma cloud radar echo. Their large apparent size and spectral discreteness indicate that they may arise from an ordered, possibly wavelike structure, which must involve the entire plasma cloud. Comparison of the observed growth rates and probable spatial scales of the instabilities in the alkali plasma clouds with order-of-magnitude calculations of these quantities based on linear instability analysis confirms that the postulated plasma instability can indeed explain the observed behavior. In one phase of plasma cloud evolution a reflecting layer similar to sporadic E is formed, which permits decameter wavelength radiation, incident obliquely on the cloud, to be reflected back to earth in much the same manner as sky-wave transmission is achieved via sporadic E. The similarity in behavior of an artificial sporadic E patch to the natural variety is evident. Also, a nonlinear, two-dimensional plasma instability analysis is capable of explaining the spectral appearance and temporal variation of the decameter radar data from the alkali plasma cloud. An association is suggested between these results and the mechanism by which temperate latitude sporadic E is formed.