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Strong electric fields in the auroral and equatorial electrojets can distort the background ion distribution function away from the Maxwellian. We developed a collisional plasma kinetic model using the Boltzmann equation and a simple BGK collision operator to predict a relatively simple relationship between the intensity of the background electric field and the resulting ion distribution function. To test the model, we perform 3-D plasma particle-in-cell simulations and compared the results to the model. Both the simulation and the analytical model assume a constant ion-neutral collision rate. The simulations show less ion heating in the Pedersen direction than in the analytical model but nearly identical overall heating. The model overestimates heating in the Pedersen direction because the simple BGK operator models collisions as a kinetic friction only in the Pedersen direction. On the other hand, the fully kinetic particle-in-cell code captures the physics of ion scattering in 3-D and therefore heats ions more isotropically. Although the simple BGK analytical theory does not precisely model the non-Maxwellian ion distribution function, it does capture the overall momentum and energy flows and therefore can provide the basis of further kinetic analysis of E-region wave evolution during strongly driven conditions. [ABSTRACT FROM AUTHOR]

Empirical models of the solar chromosphere show intense electron heating immediately above its temperature minimum. Mechanisms such as resistive dissipation and shock waves appear insufficient to account for the persistence and uniformity of this heating as inferred from both UV lines and continuum measurements. This paper further develops the theory of the Farley-Buneman instability (FBI) which could contribute substantially to this heating. It expands upon the single-ion theory presented by Fontenla by developing a multiple-ion-species approach that better models the diverse, metal-dominated ion plasma of the solar chromosphere. This analysis generates a linear dispersion relationship that predicts the critical electron drift velocity needed to trigger the instability. Using careful estimates of collision frequencies and a one-dimensional, semi-empirical model of the chromosphere, this new theory predicts that the instability may be triggered by velocities as low as 4 km s-1, well below the neutral acoustic speed. In the Earth's ionosphere, the FBI occurs frequently in situations where the instability trigger speed significantly exceeds the neutral acoustic speed. From this, we expect neutral flows rising from the photosphere to have enough energy to easily create electric fields and electron Hall drifts with sufficient amplitude to make the FBI common in the chromosphere. If so, this process will provide a mechanism to convert neutral flow and turbulence energy into electron thermal energy in the quiet Sun. [ABSTRACT FROM AUTHOR]

150 kilometer echoes are strong, coherent echoes observed by equatorial radars looking close to perpendicular to Earth's magnetic field. Observations over a day show a distinct necklace pattern with echoes descending from 170 km at sunrise to 130 km at noon, before rising again and disappearing overnight. This paper shows that the upper hybrid instability will convert photoelectron energy into plasma wave energy through inverse Landau damping. Using parameters from a WACCM‐X simulation, the upper hybrid wave growth rates over a day show a nearly identical necklace pattern, with bands of positive growth rate following contours of the plasma frequency. Small gaps in altitude with no echoes are explained by thermal electrons Landau damping the instability where the upper hybrid frequency is a multiple of the gyrofrequency. This theory provides a mechanism that likely plays a crucial role in solving a long‐standing mystery on the origin of 150‐km echoes. Key Points: Kinetic theory is developed for photoelectron‐driven upper hybrid instabilityUsing WACCM‐X for background parameters, the instability growth rates follow the same morphology as 150‐km echoesLandau damping by thermal electrons creates observed gaps in altitude where the upper hybrid frequency is a multiple of the gyrofrequency Plain Language Summary: For decades, large radars have observed strong, unexplained echoes returning from altitudes of 130–170 km in the atmosphere. All radars work by reflecting radio waves off a target and measuring the returned signal. For atmospheric radars, the targets are free electrons within the plasma in the upper atmosphere. Since the free electrons are typically a disordered gas, the radio waves are reflected in random directions. This means some process is needed to create a coherent structure in the plasma for the radio waves to strongly reflect back in the direction of the radar. In this paper, we show that the region between 130–170 km in the upper atmosphere is likely to create and maintain a specific set of plasma waves that act as a coherent structure for radar measurements. We show that predictions of where the plasma waves are generated match well with the observed patterns of these "150‐km echoes." This is the first research to provide a specific explanation for what causes 150‐km echoes. In understanding this cause, we learned more about the Sun's influence on our upper atmosphere while expanding the capabilities of atmospheric radars. [ABSTRACT FROM AUTHOR]

The Center for Applied Electromagnetics (AppEl) at the University of Maryland had started development of a sub-THz gyrotron for detecting concealed radioactive materials. The concept is based on the use of a high-power gyrotron whose power being focused in a small spot with dimensions on the order of a wavelength exceeds the threshold level required for initiating a freely localized microwave breakdown in air. However, in the absence of radioactive materials, the ambient electron density is so small that there is a very small probability to find a free electron in this small volume to trigger the avalanche breakdown process. Therefore the fact that the breakdown was observed would indicate that there is a hidden radioactive material in the vicinity of a focused wave beam. We present the design data for a 200-300 kW, 670 GHz gyrotron operating with a pulsed solenoid and describe a single-shot pulsed solenoid producing 27-28 T magnetic fields. Also numerous issues in this specific application are discussed, viz. threshold conditions for initiating the breakdown, production of gamma rays by concealed radioactive materials and their role in producing low energy electrons outside a container, wave beam focusing in a small spot by a limited-size antenna, random walk of energetic electrons which may result in appearance of free electrons in a given volume during the RF pulse and comparison of diffusion time with the time required for competing processes, such as ionization and three-body attachment. [ABSTRACT FROM AUTHOR]