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1. Toward An Integrated View of Ionospheric Plasma Instabilities: 5. Ion‐Thermal Instability for Arbitrary Ion Magnetization, Density Gradient, and Wave Propagation

   https://doi.org/10.1029/2020JA028349  

   Makarevich, Roman A. ( September 2020 , Journal of Geophysical Research: Space Physics)

   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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2. Toward an Integrated View of Ionospheric Plasma Instabilities: 6. Analytic Analysis of Thermal Effects

   https://doi.org/10.1029/2021JA029178  

   Makarevich, Roman A. ( September 2021 , Journal of Geophysical Research: Space Physics)

   Thermal effects in the ionospheric plasma instabilities including the Farley‐Buneman instability (FBI), the gradient‐drift instability (GDI), and the ion‐thermal instability (ITI) are analyzed, focusing on analytic analysis of the previously obtained general expression of the combined instability growth rate. It is shown that thermal effects lead to generally nonmonotonic behavior of the growth rate with the background electric fieldor, equivalently with the plasma drift speed, in contrast with the conventional quadratic (FBI) or linear (GDI) dependence. The threshold electric field for FBI is demonstrated to exist at all altitudes, although at higher altitudes and/or at longer wavelength the threshold fields become too high to be observed in the ionosphere. The GDI growth rate is shown to be significantly modified by the thermal effects at long wavelengths. In the absence of thermal effects, the growth rate is proportional to a cosine of the flow angle, the angle between the differential plasma drift and the wavevector. Thermal effects result in an additional phase shift that maximizes at an altitude around 120 km, using representative high‐latitude ionospheric parameters, and in an increase in the proportionality coefficient by up to a factor of 2. The study highlights the usefulness of an analytic treatment for revealing additional insights into the instability behavior in the broad range of ionospheric parameters that may remain hidden using only numerical treatment.

    

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3. Simulations of Secondary Farley‐Buneman Instability Driven by a Kilometer‐Scale Primary Wave: Anomalous Transport and Formation of Flat‐Topped Electric Fields

   https://doi.org/10.1029/2018JA026072  

   Young, Matthew A. ; Oppenheim, Meers M. ; Dimant, Yakov S. ( January 2019 , Journal of Geophysical Research: Space Physics)

   Since the 1950s, high frequency and very high frequency radars near the magnetic equator have frequently detected strong echoes caused ultimately by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI). In the 1980s, coordinated rocket and radar campaigns made the astonishing observation of flat‐topped electric fields coincident with both meter‐scale irregularities and the passage of kilometer‐scale waves. The GDI in the daytimeEregion produces kilometer‐scale primary waves with polarization electric fields large enough to drive meter‐scale secondary FBI waves. The meter‐scale waves propagate nearly vertically along the large‐scale troughs and crests and act as VHF tracers for the large‐scale dynamics. This work presents a set of hybrid numerical simulations of secondary FBIs, driven by a primary kilometer‐scale GDI‐like wave. Meter‐scale density irregularities develop in the crest and trough of the kilometer‐scale wave, where the total electric field exceeds the FBI threshold, and propagate at an angle near the direction of total Hall drift determined by the combined electric fields. The meter‐scale irregularities transport plasma across the magnetic field, producing flat‐topped electric fields similar to those observed in rocket data and reducing the large‐scale wave electric field to just above the FBI threshold value. The self‐consistent reduction in driving electric field helps explain why echoes from the FBI propagate near the plasma acoustic speed.

    

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4. Observation of Scintillation Enhancements and Large‐Scale Structures Within the Equatorial Ionization Anomaly During a Sudden Stratospheric Warming Event

   https://doi.org/10.1029/2022JA030985  

   Valladares, C. E. ; Chen, Y. ‐J. ; Hairston, M. ; Chau, J. L. ; Dhanya, R. ( February 2023 , Journal of Geophysical Research: Space Physics)

   Total electron content (TEC) and L‐band scintillations measured by several networks of GPS and GNSS receivers that operate in South and Central America and the Caribbean region are used to observe the morphology of the equatorial ionization anomaly (EIA), examine the evolution of plasma bubbles, and investigate the enhancement of L‐band scintillations that occurred on February 12 and 13, 2016. A few weak and short magnetic storms developed these days, and a minor sudden stratospheric warming (SSW) event was initiated a few days before. During these unusual conditions, TEC maps reported a split of the otherwise continuous crests of the EIA and the formation of a large‐scale (thousands of kilometers) almost‐circular structure. The western part of the southern crest faded, and a north‐south aligned segment developed near the center of the South American continent, joining the north and south crests of the EIA, forming an anomaly that resembled a closed loop on the eastern side of the continent. Concurrently with the anomaly events, several GPS stations reported increases in the L‐band scintillation index from 0.4 to values greater than 1. We analyzed TEC values from receivers between ±6° from the magnetic equator to identify and follow TEC depletions associated with plasma bubbles when they reach different stations. Although the magnetic activity was moderate (K_p = 3°), we believe that the anomaly redistribution and the scintillation enhancements are not related to a prompt penetration electric field but to enhancing the semidiurnal lunar tide propitiated by the onset of the minor SSW event. We found that depending on the lunar tide phase cycle, the neutral wind's meridional component can augment sub‐km scale irregularities and enhance L‐band scintillations through the wind gradient instability when U·n < 0 or the action of wind gradients (U) within the bubbles. Our observations imply that the SSW event enables prominent changes in the thermosphere wind system at F‐region altitudes.

    

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5. Nonlinear Three-Dimensional Simulations of the Gradient Drift and Secondary Kelvin–Helmholtz Instabilities in Ionospheric Plasma Clouds

   https://doi.org/10.3390/atmos14040676  

   Almarhabi, Lujain ; Skolar, Chirag ; Scales, Wayne ; Srinivasan, Bhuvana ( April 2023 , Atmosphere)

   A newly developed three-dimensional electrostatic fluid model solving continuity and current closure equations aims to study phenomena that generate ionospheric turbulence. The model is spatially discretized using a pseudo-spectral method with full Fourier basis functions and evolved in time using a four-stage, fourth-order Runge Kutta method. The 3D numerical model is used here to investigate the behavior and evolution of ionospheric plasma clouds. This problem has historically been used to study the processes governing the evolution of the irregularities in the F region of the ionosphere. It has been shown that these artificial clouds can become unstable and structure rapidly (i.e., cascade to smaller scales transverse to the ambient magnetic field). The primary mechanism which causes this structuring of ionospheric clouds is the E×B, or the gradient drift instability (GDI). The persistence and scale sizes of the resulting structures cannot be fully explained by a two-dimensional model. Therefore, we suggest here that the inclusion of three-dimensional effects is key to a successful interpretation of mid-latitude irregularities, as well as a prerequisite for a credible simulation of these processes. We investigate the results of 2D and 3D nonlinear simulations of the GDI and secondary Kelvin–Helmholtz instability (KHI) in plasma clouds for three different regimes: highly collisional (≈200 km), collisional (≈300 km), and inertial (≈450 km). The inclusion of inertial effects permits the growth of the secondary KHI. For the three different regimes, the overall evolution of structuring of plasma cloud occurs on longer timescales in 3D simulations. The inclusion of three-dimensional effects, in particular, the ambipolar potential in the current closure equation, introduces an azimuthal “twist“ about the axis of the cloud (i.e., the magnetic field B). This azimuthal “twist” is observed in the purely collisional regime, and it causes the perturbations to have a non-flute-like character (k‖≠0). However, for the 3D inertial simulations, the cloud rapidly diffuses to a state in which the sheared azimuthal flow is substantially reduced; subsequently, the cloud becomes unstable and structures, by retaining the flute-like character of the perturbations (k‖=0).

    

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