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Abstract Behavior of unstable plasma waves generated by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI) is analyzed in the transitional valley region near 120 km in altitude. The analysis is based on the expression for the FBI/GDI growth rateγthat has been recently generalized to include ion inertia effects for arbitrary altitude and wavelength, within the limits imposed by the fluid and local approaches. It is found that the ion inertia leads to a different instability behavior when the convection component is between the two critical values determined by the ion acoustic speedCsand the ratioribetween the ion collision and gyrofrequency. The most interesting case occurs near 120 km, just below whereri=1. From analysis of electron density gradientsG=∇n/nthat result in marginal instability conditionγ=0 (i.e., critical gradientsG0), there exists a critical scale whereG0=0 and below which all waves are unstable to FBI. Above this scale,G0>0 and gradients need to be sufficiently strongG>G0for the plasma to become unstable through GDI. There also exists a maximum in dependence, which refers to the least unstable scale and gradient. For convection outside of the specified range, no critical or least unstable scale exists, which is a typical situation outside of the transitional valley region. Overall, this analysis shows that the FBI convection thresholds and the GDI critical gradients are modified by the ion inertia and that the effects are most pronounced in the transitional valley region near 120 km.
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Toward An Integrated View of Ionospheric Plasma Instabilities: 5. Ion‐Thermal Instability for Arbitrary Ion Magnetization, Density Gradient, and Wave Propagation
Abstract A unified fluid theory of ionospheric electrostatic instabilities is presented that includes thermal effects due to nonisothermal processes for arbitrary ion magnetization, background density gradient, and wave propagation. The theory considers arbitrary altitude within the limits imposed by the fluid and collisional models and integrates the ion‐thermal instability (ITI) with the Farley‐Buneman and gradient‐drift plasma instabilities (FBI and GDI). A general dispersion relation is obtained and solved numerically for the complex wave frequencyωby using either an iterative or a polynomial (quadric) form inω. An analytic explicit expression for the instability growth rate is also derived under the local and slow growth approximations. The previously considered limiting cases of the FBI/ITI at long wavelengths and the FBI/GDI for isothermal plasma are successfully recovered. In the high‐latitudeE‐region near 110 km in altitude, thermal effects are found to be destabilizing at long wavelengths near m and stabilizing at shorter wavelengths near 10 m. In theF‐region, the effects are destabilizing at m but much weaker that those of GDI for moderate gradients. At shorter wavelengths, they become comparable so that a significant fraction of propagation directions at m have positive growth rates, in contrast with the isothermal FBI/GDI case, where stronger gradients are needed to destabilize the plasma at these short wavelengths. The overall conclusion is that the thermal effects modify the growth rate terms traditionally associated with FBI and GDI rather than being purely additive.
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- Award ID(s):
- 2028441
- PAR ID:
- 10450013
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 125
- Issue:
- 9
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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