Generation of kilometer scale irregularities during the midnight collapse at Arecibo

Abstract

Coordinated observations of the incoherent scatter radar at Arecibo, Puerto Rico, and total electron content/scintillation measurements made by using the 137‐MHz transmissions from the geostationary satellite SMS 1 from the nearby station of Ramey are studied to determine the background ionospheric conditions necessary for the generation of kilometer scale irregularities in the midlatitude ionosphere. It is found that the well‐known midnight descent or collapse of the F region known to occur in this part of the world is sometimes associated with increases in the bottomside electron content, as well as large scintillation events (∼10 dB). From a measurement of vector ion velocities and electron densities during such events, it is determined that the scintillations occur in a region of eastward and northward electric fields coupled with a northward directed density gradient caused by the northward propagation of the midnight collapse. The role of possible plasma instability mechanisms, specifically the E × B gradient drift instability and the Perkins instability, is discussed in the formation of the kilometer scale irregularities giving rise to VHF scintillations.

Mid-latitude ionospheric fluctuation spectra due to secondary instabilities

“…Indeed, few examples of the mid-latitude case have been published in any form. Basu et al (1981) conjectured that sharp plasma gradients perpendicular to B should be unstable to the E Â B instability, as did Kelley (1989). But the origin of such gradients was not clear until recently, when airglow observations revealed their presence (Garcia et al, 2000a, b).…”

An important role of electric field reversals for the initiation of gigahertz scintillations at midlatitude during geomagnetic storms.

“…Namely, winds blow equatorward, electric fields are directed westward and the Flayer is lifted instead of the collapse. As in the theory of BAsu et al (1981), the equatorward propagating lift of the F layer will create equatorward density gradients. These processes take place on the poleward side of the crest of the equatorial anomaly, where background equatorward density gradients are intensified alone will not explain gigahertz scintillations during the disturbance events.…”

“…enhancement of midlatitude scintillations which were observed over Arecibo associated with the midnight collapse of the Flayer. According to BAsu et al (1981), the midnight collapse of the F layer is associated with northward neutral winds and northward directed density gradients which are generated through the northward propagation of the collapse. In addition, eastward electric fields are generated by the polarization effects.…”

Perkins instability revisited

“…It is clear that the work to be presented in this paper is solely motivated by the fact that the "Perkins" instability may very well help us understand some of the physical observations as mentioned in the literature [see, e.g., Miller et al, 1997 and references therein]. It has been suggested by Basu et al [1981] that the need for a zeroth-order horizontal density gradient is essential for the generation of kilometer-scale irregularities. Basu et al [1981] …”

“…It has been suggested by Basu et al [1981] that the need for a zeroth-order horizontal density gradient is essential for the generation of kilometer-scale irregularities. Basu et al [1981] …”