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The Millstone Hill incoherent scatter (IS) radar is used to measure spectra close to perpendicular to the Earth's magnetic field, and the data are fit to three different forward models to estimate ionospheric temperatures. IS spectra measured close to perpendicular to the magnetic field are heavily influenced by Coulomb collisions, and the temperature estimates are sensitive to the collision operator used in the forward model. The standard theoretical model for IS radar spectra treats Coulomb collisions as a velocity independent Brownian motion process. This gives estimates of Te/Ti < 1 when fitting the measured spectra for aspect angles up to 3.6°, which is a physically unrealistic result. The numerical forward model from Milla and Kudeki (2011, https://doi.org/10.1109/TGRS.2010.2057253) incorporates single‐particle simulations of velocity‐dependent Coulomb collisions into a linear framework, and when applied to the Millstone data, it predicts the same Te/Ti ratios as the Brownian theory. The new approach is a nonlinear particle‐in‐cell (PIC) code that includes velocity‐dependent Coulomb collisions which produce significantly more collisional and nonlinear Landau damping of the measured ion‐acoustic wave than the other forward models. When applied to the radar data, the increased damping in the PIC simulations will result in more physically realistic estimates of Te/Ti. This new approach has the greatest impact for the largest measured ionospheric densities and the lowest radar frequencies. The new approach should enable IS radars to obtain accurate measurements of plasma temperatures at times and locations where they currently cannot. Key Points: Millstone Hill measured incoherent scatter spectra at small aspect angles to test different models of determining ionospheric temperaturesCollisions narrowed measured spectra at aspect angles less than 4.6° for high densities, and at angles less than 1.25° for low densitiesA nonlinear particle‐in‐cell forward model is shown to produce larger, more realistic, Te/Ti estimates than current forward models [ABSTRACT FROM AUTHOR]

Incoherent scatter radars (ISR) estimate the electron and ion temperatures in the ionosphere by fitting measured spectra of ion‐acoustic waves to forward models. For radars looking at aspect angles within 5° off perpendicular to the Earth's magnetic field, the magnetic field constrains electron movement and Coulomb collisions add an additional source of damping that narrows the spectra. Fitting the collisionally narrowed spectra to collisionless forward models leads to errors or underestimates of the plasma temperatures. This paper presents the first fully kinetic particle‐in‐cell (PIC) simulations of ISR spectra with collisional damping by velocity‐dependent electron‐electron and electron‐ion collisions. For aspect angles between 0.5° and 2° off perpendicular, the damping effects of electron‐ion and electron‐electron collisions in the PIC simulations are the same and the resulting spectra are narrower than what current theories and models predict. For aspect angles larger than 3° away from perpendicular, the simulations with electron‐ion collisions match collisionless ISR theory well, but spectra with electron‐electron collisions are narrower than theory predicts at aspect angles as large as 5° away from perpendicular. At aspect angles less than 5° the PIC simulations produce narrower spectra than previous simulations using single‐particle displacement statistics that include both electron‐ion and electron‐electron collisions. The narrowing of spectra by electron‐electron collisions in the PIC code between 3° and 5° away from perpendicular is currently neglected when fitting measured spectra from the Jicamarca and Millstone Hill radars, leading to underestimates of electron temperatures by as much as 25% at small aspect angles. Key Points: Collisional and nonlinear effects are important for incoherent scatter theory when looking close to perpendicular to Earth's magnetic fieldSimulations show electron‐electron collisions cause temperature underestimates at angles within 5° of perpendicular to the magnetic fieldParticle‐in‐cell simulations show different collisional effects compared to simulations using single‐particle displacement statistics [ABSTRACT FROM AUTHOR]

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 daytime E region 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. Plain Language Summary: Since the 1950s, radars near the Earth's geomagnetic equator have frequently seen their signals reflected from electrically charged structures in the upper atmosphere. A tremendous amount of research has gone into understanding what creates charged structures on different size scales, as well as how they evolve individually, but we understand relatively little about how those differently sized structures interact. This paper presents results of numerical simulations that leverage modern supercomputing power to model a few sections of the electrically charged gas of the upper atmosphere. The simulations were designed to test the hypothesis that a wave 1 km long can create electric fields strong enough to produce waves of only a few meters which move up and down the larger wave's trough and crest. The result? Yes they can. Not only do the smaller waves' motions explain decades of radar reflections—they also shed light on some strange rocket observations from the 1980s and tie both components together to answer a longstanding question about how fast these waves travel. Key Points: Kilometer‐scale primary wave drives meter‐scale secondary waves via polarization electric fieldMeter‐scale waves transport plasma across the magnetic fieldAnomalous plasma transport reduces wave electric field, creating flat‐top waveform and reducing wave speed [ABSTRACT FROM AUTHOR]

Abstract: Incoherent scatter radars (ISR) rely on Thomson scattering of very high frequency or ultrahigh frequency radio waves off electrons in the ionosphere and measure the backscattered power spectra in order to estimate altitude profiles of plasma density, electron temperature, ion temperature, and ion drift speed. These spectra result from the collective behavior of coupled ion and electron dynamics, and, for most cases, existing theories predict these well. However, when the radar points nearly perpendicular to the Earth's magnetic field, the motion of the plasma across the field lines becomes complex and Coulomb collisions between electrons and ions become important in interpreting ISR measurements. This paper presents the first fully kinetic, self‐consistent, particle‐in‐cell simulations of ISR spectra with electron‐ion Coulomb collisions. We implement a grid‐based Coulomb collision algorithm in the Electrostatic Parallel Particle‐in‐Cell simulator and obtain ISR spectra from simulations both with and without collisions. For radar directions greater than 5° away from perpendicular to the magnetic field, both sets of simulations match collisionless ISR theory well. For angles between 3° and 5°, the collisional simulation is well described by a simplified Brownian motion collision process. At angles less than 3° away from perpendicular the Brownian motion model fails, and the collisional simulation qualitatively agrees with previous single particle simulations. For radar directions exactly perpendicular to the magnetic field the simulated collisional spectra match those from the Brownian motion collision theory, in agreement with previous single particle simulations. [ABSTRACT FROM AUTHOR]

Abstract: A new module, which includes both the turbulence electron heating and the electron‐neutral cooling rate correction associated with the Farley‐Buneman onstability (FBI) in the National Center for Atmospheric Research‐Thermosphere Ionosphere Electrodynamics Global Circulation Model, enables us, for the first time, to investigate the intimate coupling between polar turbulence electron heating and thermosphere disturbances in the context of a first‐principle, self‐consistent model. Our simulation results show that during geomagnetic storms, after taking the FBI effects into account, Joule heating is almost doubled in the E region at these locations of strong convection electric field. This increases (~6%) the phase speed of traveling atmosphere disturbances at around 400 km that are launched from both hemispheres. Additional heating sources also directly produce a divergence in the zonal wind with magnitudes of 20 m/s in the lower thermosphere. However, the FBI impact on the thermosphere at F region altitudes is not so straightforward. E region anomalous electron heating in association with the FBI can change the phase speeds and magnitudes of traveling atmosphere disturbances from high latitudes in both hemispheres and consequently modify global neutral wind circulation at F region altitudes. This, in turn, can affect the neutral temperature through adiabatic compressional heating and expansive cooling by about 40 K. This study demonstrates how the ionosphere‐thermosphere physical processes across different temporal and spatial scales are tightly coupled together throughout the whole upper atmosphere domain. [ABSTRACT FROM AUTHOR]

Since the 1960s, the Farley‐Buneman instability has played an important role in probing the E‐region ionosphere. The intervening years have seen significant progress in the linear theory of this instability, its relation to other instabilities, and some of its observational signatures. However, the saturation mechanism and nonlinear behavior remain open topics because of their role in controlling energy flow in the E‐region plasma. This paper explores the saturated state of the Farley‐Buneman instability in 2‐D and 3‐D kinetic simulations of the high‐latitude ionosphere, at three different simulated altitudes: 107, 110, and 113 km. These simulations show irregularity amplitude growth and saturation in all runs, but irregularity growth takes much longer in 2‐D than in 3‐D. Once the simulations reach saturation, wave power in the meter‐scale regime falls off as a power law below the wavelength of peak growth, but the power law index is larger in 3‐D than in 2‐D. At longer wavelengths, the 3‐D spectrum is much flatter than the 2‐D spectrum. This implies that purely 2‐D simulations of the Farley‐Buneman instability may overestimate irregularity amplitudes at decameter scales and may also underestimate the efficiency of ion Landau damping at the ion mean‐free‐path scale. From a physical perspective, the relatively flat spectra above the wavelength of peak growth in 3‐D simulations imply a wavelength‐independent saturation mechanism across a range of altitudes. Finally, both 2‐D and 3‐D simulations demonstrate the importance of accounting for zeroth‐order ion drift when estimating the flow angle of density irregularities. Plain Language Summary: At approximately 90–120 km in altitude, the Earth's atmosphere comprises a gas containing mostly neutral molecules, and a small number of positively charged ions and electrons. This sort of gas is called a weakly ionized plasma. Aeronomy researchers have known for decades that the weakly ionized plasma around 90–120 km in Earth's atmosphere can grow unstable and reflect radio waves, and they have used radio‐wave reflection as one technique to probe this region of geospace. However, the aeronomy community still does not understand the nature of the fully developed plasma instabilities in this region. This work addresses that problem with computer simulations. It finds that 2‐D simulations do not capture all the characteristics of 3‐D simulations, which should mimic reality more closely. It also finds that whatever mechanism creates the fully developed 3‐D instability should apply to wavelengths from a few meters to tens of meters. Finally, this work suggests that the direction along which the unstable waves travel should change in a predictable way as altitude increases. These conclusions will help future researchers estimate atmospheric parameters based on radar and rocket measurements. Key Points: Instability growth differs between 2‐D and 3‐D simulationsSaturated density in 3‐D spectra are flat above peak wavelength and fall off sharply belowThe variation of relative plasma drift with altitude largely controls the variation in flow angle [ABSTRACT FROM AUTHOR]