Solar eruptions cause geomagnetic storms in the near-Earth environment, creating spectacular aurorae visible to the human eye and invisible dynamic changes permeating all of geospace. Just equatorward of the aurora, radars and satellites often observe intense westward plasma flows called subauroral polarization streams (SAPS) in the dusk-to-midnight ionosphere. SAPS occur across a narrow latitudinal range and lead to intense frictional heating of the ionospheric plasma and atmospheric neutral gas. SAPS also generate small-scale plasma waves and density irregularities that interfere with radio communications. As opposed to the commonly observed duskside SAPS, intense eastward subauroral plasma flows in the morning sector were recently discovered to have occurred during a super storm on 20 November 2003. However, the origin of these flows termed “dawnside SAPS” could not be explained by the same mechanism that causes SAPS on the duskside and has remained a mystery. Through real-event global geospace simulations, here we demonstrate that dawnside SAPS can only occur during major storm conditions. During these times the magnetospheric plasma convection is so strong as to effectively transport ions to the dawnside, whereas they are typically deflected to the dusk by the energy-dependent drifts. Ring current pressure then builds up on the dawnside and drives field-aligned currents that connect to the subauroral ionosphere, where eastward SAPS are generated. The origin of dawnside SAPS explicated in this study advances our understanding of how the geospace system responds to strongly disturbed solar wind driving conditions that can have severe detrimental impacts on human society and infrastructure.

Radars detect plasma trails created by the billions of small meteors that impact the Earth’s atmosphere daily, returning data used to infer characteristics of the meteoroid population and upper atmosphere. Researchers use models to investigate the dynamic evolution of the trails, enabling them to better interpret radar results. This paper presents a fully kinetic, 3D code to explore the impacts of three trail characteristics: length, neutral wind speed, and ablation altitude. The simulations characterize the turbulence that develops as the trail evolves and these are compared to radar data. They also show that neutral winds drive the formation of waves and turbulence in trails, and that wave amplitudes increase with neutral wind speed. The finite trail simulations demonstrate that the bulk motion of the trail flows with the neutral wind. A detailed analysis of simulated trail spectra yield spectral widths, and evaluate signal strength as a function of aspect angle. Waves propagate primarily along the length of the trail in all cases, and most power is in modes perpendicular to $\mathit{\vec{B}}$. Persistent waves develop at wavelengths corresponding to the gradient scale length of the original trail. Our results show that the rate at which power drops with respect to aspect angle in meter-scale modes increases from $5.7$ dB/degree to $6.9$ dB/degree with a 15 km increase in altitude. The results will allow researchers to draw more detailed and accurate information from non-specular radar observations of meteors.

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° of 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 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 are the same and the resulting spectra are narrower than what current theories 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 all aspect angles the particle-in-cell simulations produce narrower spectra than previous simulations using single particle displacement statistics. The narrowing of spectra by electron-electron collisions 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 50% at these radars.

Meteoroids smaller than a microgram constantly bombard the Earth, depositing in the mesosphere and lower thermosphere. Meteoroid ablation, the explosive evaporation of meteoroids due to erosive impacts of atmospheric particles, consists of sputtering and sublimation. This paper presents the first atomic scale modeling of sputtering, the initial stage of ablation where hypersonic collisions between the meteoroid and atmospheric particles cause the direct ejection of atoms from the meteoroid surface. Because meteoroids gain thermal energy from these particle impacts, these interactions are important for sublimation as well. In this study, a molecular dynamics simulator calculates the energy distribution of the sputtered particles as a function of the species, velocity, and angle of the incoming atmospheric particles. The sputtering yield generally agrees with semi-empirical equations at normal incidence but disagrees with the generally accepted angular dependence. Λ, the fraction of energy from a single atmospheric particle impact incorporated into the meteoroid, was found to be less than 1 and dependent on the velocity, angle, atmospheric species, and meteoroid material. Applying this new Λ to an ablation model results in a slower meteoroid temperature increase and mass loss rate as a function of altitude. This alteration results in changes in the expected electron line densities and visual magnitudes of meteoroids. Notably, this analysis leads to the prediction that meteoroids will generally ablate 1 - 4 km lower than previously predicted. This affects analysis of radar and visual measurements, as well as determination of meteoroid mass.

Strong subauroral plasma flows were observed in the dawnside ionosphere during the 20 November 2003 super geomagnetic storm. They are identified as dawnside subauroral polarization streams (SAPS) in which plasma drift direction is eastward and opposite to the westward SAPS typically found in the dusk sector. Both dawnside and duskside SAPS are driven by the enhanced meridional electric field in the low latitude portion of Region-2 field-aligned currents (FACs) in the subauroral region where ionospheric conductance is relatively low. However, dawnside eastward SAPS were only observed in the main and recovery phases while duskside westward SAPS were found much earlier before the sudden storm commencement. Simulations with the Multiscale Atmosphere-Geospace Environment (MAGE) model demonstrate that the eastward SAPS are associated with dawnside ring current build-up. Unlike the duskside where ring current build-up and SAPS formation can occur under moderate driving conditions, strong magnetospheric convection is required for plasmasheet ions to overcome their energy-dependent drifts to effectively build up the dawnside ring current and upward Region-2 FACs. We further used test particle simulations to show the characteristic drift pattern of energetic protons under strong convection conditions and how they are related to the dawnside SAPS occurrence. This study demonstrates the connection between the level of solar wind driving condition and a rare ionospheric structure, eastward SAPS on the dawnside, which only occur under strong convection typically associated with intense or super storms. Dawnside SAPS are suggested as a unique feature of major geomagnetic storms.