Abstract
Abstract. The Es layer instability has been suggested as a participant in the creation of frontal structures observed in both the Es and F layers of the nighttime midlatitude ionosphere, in spite of the fact that the spatial scales of the frontal structures are very different in the two layers. The linear growth rate of the instability has a maxima in the vicinity of the wavelength observed for the Es layer structures (short wavelengths). However, the maxima is non-distinct, and simulations have shown that the instability is extremely nonlinear. Therefore, to understand the wavelength dependence of the Es layer instability it is necessary to factor in nonlinear behavior. Simulations have shown that the instability is active at the wavelengths observed in the F layer, and revealed that the Es layer behavior at these long wavelengths is so nonlinear that the common, highly localized Es layer observation techniques would likely miss the signature, which is highly visible in the F layer. However, there is currently no explanation for why long wavelengths so clearly dominate short (or intermediate) wavelengths in the F layer observations, and this is a weakness in arguments that the Es layer instability participates in the creation of F-region frontal structures. Herein we remove this weakness by showing that longer wavelengths grow to larger amplitudes before eventual nonlinear saturation, and couple more effectively to the F-region.
Highlights
Cosgrove and Tsunoda (2002) showed that the equilibrium configuration of a midlatitude sporadic-E (Es) layer at a wind shear node is unstable, at night, to plane wave per-turbations in altitude or field-line-integrated (FLI) density
Cosgrove turbations in altitude or field-line-integrated (FLI) density. This instability is a possible participant in frontal structuring events in Es layers, and in the F layer, which have been observed over the years by ionosonde, by coherent scatter radar, by all-sky images of 630.0 nm emissions, and by GPS time delay mapping (Tsunoda and Cosgrove, 2001; Tsunoda et al, 2004)
The instability is potentially involved in the source of large polarization electric fields in Es layers, and in the F layer, which have been indicated by incoherent scatter radar (Behnke, 1979), by coherent scatter radar (e.g. Schlegel and Haldoupis, 1994; Tsunoda et al, 1994; Fukao et al, 1991), and measured in situ during the two SEEK rocket campaigns (Fukao et al, 1998; Pfaff et al, 1998; Yamamoto et al, 2005; Pfaff et al, 2005)
Summary
Cosgrove and Tsunoda (2002) showed that the equilibrium configuration of a midlatitude sporadic-E (Es) layer at a wind shear node is unstable, at night, to plane wave per-. QP echoes have been found to form frontal structures with a preferred orientation matching that of the Es layer instability (Yamamoto et al, 1994, 1997; Hysell et al, 2004; Larsen et al, 2007; Saito et al, 2007). This orientation, observed in the Northern Hemisphere, is conjugate along magnetic field lines to that observed in the Southern Hemisphere by Goodwin and Summers (1970).
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