Formation of striations in ionospheric plasma clouds

Abstract

It is suggested that a low‐frequency ‘gradient drift’ instability may be important for the formation of striations in barium ion clouds released in the ionosphere above the E layer. The theory predicts that the trailing edge (with respect to the neutrals) of the plasma cloud will be unstable while the leading edge is stable, in qualitative agreement with observations. The growth time is relatively independent of wavelength and is of order d/Uo, where d is the typical density gradient length and Uo is the velocity of the cloud with respect to neutrals. Predicted growth times varying from seconds in the auroral zone at high latitude to minutes at midlatitudes are also in agreement with observation. Wavelengths of waves with zero phase velocity are found to be given approximately by λ = 2πd, which can be verified by optical observations.

Backscatter measurements of 11‐cm equatorial spread‐F irregularities

“…Although the striations in barium clouds are initiated by the gradient-drift instability (e.g., Linson and Workman, [1970]), which is a direct analog of the Rayleigh-Taylor instability that initiates equatorial spread F (e.g., Ossakow and Chaturvedi [1978]), both the striations and the background ionosphere (into which the barium cloud is injected) have higher electron densities than required to trigger high-frequency drift waves. This conclusion is consistent with Aside from the consideration of the source mechanism, the observation of li-cm FAI is also noteworthy because it represents detection of FAI with a spatial wavelength that is closer to the electron gyroradius and, possibly, also closer to the Debye length than any other measurement of ionospheric FAI.…”

Late time barium cloud striations and their possible relationship to equatorial spread F

“…The time domain analysis discussed above is not the best representation of the data, since the rocket velocity was great compared with the phase velocity of the waves associated with the E x B instability [Linson and Workman, 1970]. Also, the unstable waves have k. B = 0.…”

“…The linear theory was developed by Simon [1963] for laboratory plasmas and adapted by Linson and Workman [1970] for ionospheric applications to barium clouds and tested in a computer simulation by Zabusky et al [1973]. The linear process can be understood after a brief description of the initial development of a large barium cloud.…”

“…As discussed by Linson and Workman [1970], the growth rate 'y is a function of wavelength but is usually normalized to % = E'/BL s -•, where L is the density gradient in the cloud and E' is the electric field in the neutral frame (E + V,• x B). For typical values of E'/B equal to 100 m/s and L equal to 4 km, % = 2.5 X 10 -2 s. Since the earliest striation measurements made in this rocket series were at 42 min after barium release, time was available for more than 50 e folds (e '•t) of any initial perturbations.…”

Coherent HF radar backscatter characteristics associated with auroral forms identified by incoherent radar techniques: A comparison of CUTLASS and EISCAT observations

“…The ExB instability acts in the presence of a background electron density gradient with a component perpendicular to B and with an applied electric field E which has a component along the gradient. In the one-dimensional linear case the growth rate of this instability is given by y= Vo/L for ro << Vin [Linson and Workman, 1970] , where k is the irregularity wave vector. The electron density gradient can be in the horizontal or vertical directions; though as B is directed almost vertically at high latitudes, any vertical electron density gradient would have to be large for L to become significant.…”