Search for: All records

Models currently fail to reproduce observations of the coldest regions in the Sun’s atmosphere, though recent work suggests the thermal Farley–Buneman instability (TFBI) may play a critical role. This meter-scale, electrostatic, multifluid plasma instability causes turbulence and heating in the coldest regions of the solar chromosphere. This paper describes how TFBI turbulence and heating varies across multifluid 2D, kinetic 2D, and kinetic 3D simulations. It also presents the first 3D simulations of the TFBI. We find that multifluid and kinetic 2D simulations produce similar results overall, despite using vastly different approaches. Additionally, our kinetic 3D simulations produce a similar or somewhat larger amount of heating compared to 2D, as contributions from the parallel electric field account for only (13 ± 2.5)% of the total turbulent heating in 3D. 

We present FARR (Finite-difference time-domain ARRay), an open source, high-performance, finite-difference time-domain (FDTD) code. FARR is specifically designed for modeling radio wave propagation in collisional, magnetized plasmas like those found in the Earth’s ionosphere. The FDTD method directly solves Maxwell’s equations and captures all features of electromagnetic propagation, including the effects of polarization and finite-bandwidth wave packets. By solving for all vector field quantities, the code can work in regimes where geometric optics is not applicable. FARR is able to model the complex interaction of electromagnetic waves with multi-scale ionospheric irregularities, capturing the effects of scintillation caused by both refractive and diffractive processes. In this paper, we provide a thorough description of the design and features of FARR. We also highlight specific use cases for future work, including coupling to external models for ionospheric densities, quantifying HF/VHF scintillation, and simulating radar backscatter. The code is validated by comparing the simulated wave amplitudes in a slowly changing, magnetized plasma to the predicted amplitudes using the WKB approximation. This test shows good agreement between FARR and the cold plasma dispersion relations for O, X, R, and L modes, while also highlighting key differences from working in the time-domain. Finally, we conclude by comparing the propagation path of an HF pulse reflecting from the bottomside ionosphere. This path compares well to ray tracing simulations, and demonstrates the code’s ability to address realistic ionospheric propagation problems. 

Abstract Models fail to reproduce observations of the coldest parts of the Sun’s atmosphere, where interactions between multiple ionized and neutral species prevent an accurate MHD representation. This paper argues that a meter-scale electrostatic plasma instability develops in these regions and causes heating. We refer to this instability as the Thermal Farley–Buneman Instability (TFBI). Using parameters from a 2.5D radiative MHD Bifrost simulation, we show that the TFBI develops in many of the colder regions in the chromosphere. This paper also presents the first multifluid simulation of the TFBI and validates this new result by demonstrating close agreement with theory during the linear regime. The simulation eventually develops turbulence, and we characterize the resulting wave-driven heating, plasma transport, and turbulent motions. These results all contend that the effects of the TFBI contribute to the discrepancies between solar observations and radiative MHD models. 

Abstract. The Andenes specular meteor radar shows meteor trail diffusion rates increasing on average byabout 10 % at times and locations where a lidar observes noctilucentclouds (NLCs). This high-latitude effect has been attributed to the presenceof charged NLC after exploring possible contributions from thermal tides. Tomake this claim, the current study evaluates data from three stations athigh, middle, and low latitudes for the years 2012 to 2016 to show that NLCinfluence on the meteor trail diffusion is independent of thermal tides. Theobservations also show that the meteor trail diffusion enhancement during NLCcover exists only at high latitudes and near the peaks of NLC layers. Thispaper discusses a number of possible explanations for changes in the regionswith NLCs and leans towards the hypothesis that the relative abundance ofbackground electron density plays the leading role. A more accurate model ofthe meteor trail diffusion around NLC particles would help researchersdetermine mesospheric temperature and neutral density profiles from meteorradars at high latitudes. 

Abstract 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. 

Abstract 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.