Evolution of meteor trails

First author draf

Photoelectron-induced waves: A likely source of 150 km radar echoes and enhanced electron modes

VHF radars near the geomagnetic equator receive coherent reflections from plasma density irregularities between 130 and 160 km in altitude during the daytime. Though researchers first discovered these 150 km echoes over 50 years ago and use them to monitor vertical plasma drifts, the underlying mechanism that creates them remains a mystery. This paper uses large‐scale kinetic simulations to show that photoelectrons can drive electron waves, which then enhance ion density irregularities that radars could observe as 150 km echoes. This model explains why 150 km echoes exist only during the day and why they appear at their lowest altitudes near noon. It predicts the spectral structure observed by Chau (2004) and suggests observations that can further evaluate this mechanism. It also shows the types and strength of electron modes that photoelectron‐wave interactions generate in a magnetized plasma.The authors would like to thank Juha Vierinen, David Hysell, Jorge Chau, and Roger Varney for their helpful discussions and suggestions. This material is based upon work supported by NASA under grant NNX14AI13G. This work used the XSEDE and TACC computational facilities, supported by National Science Foundation grant ACI-1053575. Simulation-produced data are archived at TACC and available upon request. (NNX14AI13G - NASA; ACI-1053575 - National Science Foundation

A newly discovered source of turbulence and, possibly, heating in the chromosphere

Othe

Anomalous electron heating effects on the E region ionosphere in TIEGCM

We have recently implemented a new module that includes both the anomalous electron heating and the electron‐neutral cooling rate correction associated with the Farley‐Buneman Instability (FBI) in the thermosphere‐ionosphere electrodynamics global circulation model (TIEGCM). This implementation provides, for the first time, a modeling capability to describe macroscopic effects of the FBI on the ionosphere and thermosphere in the context of a first‐principle, self‐consistent model. The added heating sources primarily operate between 100 and 130 km altitude, and their magnitudes often exceed auroral precipitation heating in the TIEGCM. The induced changes in E region electron temperature in the auroral oval and polar cap by the FBI are remarkable with a maximum Te approaching 2200 K. This is about 4 times larger than the TIEGCM run without FBI heating. This investigation demonstrates how researchers can add the important effects of the FBI to magnetosphere‐ionosphere‐thermosphere models and simulators.NNX14Al13G - NASA GCR; NASA LWS; NNX14AE06G; NNX15AB83G; NNX12AJ54G - NASA HGI; ACI-1053575 - National Science Foundatio

Effects of ion magnetization on the Farley-Buneman instability in the solar chromosphere

Intense heating in the quiet-Sun chromosphere raises the temperature from 4000 to 6500 K but, despite decades of study, the underlying mechanism remains a mystery. This study continues to explore the possibility that the Farley–Buneman instability contributes to chromospheric heating. This instability occurs in weakly ionized collisional plasmas in which electrons are magnetized, but ions are not. A mixture of metal ions generate the plasma density in the coolest parts of the chromosphere; while some ions are weakly magnetized, others are demagnetized by neutral collisions. This paper incorporates the effects of multiple, arbitrarily magnetized species of ions to the theory of the Farley–Buneman instability and examines the ramifications on instability in the chromosphere. The inclusion of magnetized ions introduces new restrictions on the regions in which the instability can occur in the chromosphere—in fact, it confines the instability to the regions in which heating is observed. For a magnetic field of 30 G, the minimum ambient electric field capable of driving the instability is 13.5 V/m at the temperature minimum.This work was supported by NSF-AGS Postdoctoral Research Fellowship Award No. 1433536 and NSF/DOE grant No. PHY-1500439. The authors also acknowledge a recent contribution from William Longley. (1433536 - NSF-AGS Postdoctoral Research Fellowship Award; PHY-1500439 - NSF/DOE grant)First author draftPublished versio

Plasma instabilities in meteor trails:linear theory

Ablation of micrometeoroids between 70 and 130 km altitude in the atmosphere creates plasma columns with densities exceeding the ambient ionospheric electron density by many orders of magnitude. Density gradients at the edges of these trails can create ambipolar electric fields with amplitudes in excess of 100 mV/m. These fields combine with diamagnetic drifts to drive electrons at speeds exceeding 2 km/s. The fields and gradients also initiate Farley-Buneman and gradient-drift instabilities. These create field-aligned plasma density irregularities which evolve into turbulent structures detectable by radars with a large power-aperture product, such as those found at Jicamarca, Arecibo, and Kwajalein. This paper presents a theory of meteor trail instabilities using both fluid and kinetic methods. In particular, it discusses the origin of the driving electric field, the resulting electron drifts, and the linear plasma instabilities of meteor trails. It shows that though the ambipolar electric field changes amplitude and even direction as a function of altitude, the electrons always drift in the positive ∇n × B direction, where n is the density and B the geomagnetic field. The linear stability analysis predicts that instabilities develop within a limited range of altitudes with the following observational consequences: (1) nonspecular meteor trail echoes will be field-aligned; (2) nonspecular echoes will return from a limited range of altitudes compared with the range over which the head echo reflection indicates the presence of plasma columns; and (3) anomalous cross-field diffusion will occur only within this limited altitude range with consequences for calculating diffusion rates and temperatures with both specular and nonspecular radars

Hybrid simulations of coupled Farley-Buneman/gradient drift instabilities in the equatorial E region ionosphere

Plasma irregularities in the equatorial E region ionosphere are classified as Type I or Type II, based on coherent radar spectra. Type I irregularities are attributed to the Farley‐Buneman instability and Type II to the gradient drift instability that cascades to meter‐scale irregularities detected by radars. This work presents the first kinetic simulations of coupled Farley‐Buneman and gradient drift turbulence in the equatorial E region ionosphere for a range of zeroth‐order vertical electric fields, using a new approach to solving the electrostatic potential equation. The simulation models a collisional quasi‐neutral plasma with a warm, inertialess electron fluid and a distribution of NO+ ions. A 512 m wave with a maximum/minimum of ±0.25 of the background density perturbs the plasma. The density wave creates an electrostatic field that adds to the zeroth‐order vertical and ambipolar fields, and drives Farley‐Buneman turbulence even when these fields are below the instability threshold. Wave power spectra show that Type II irregularities develop in all simulation runs and that Type I irregularities with wavelengths of a few meters develop in the trough of the background wave in addition to Type II irregularities as the zeroth‐order electric field magnitude increases. Linear fluid theory predicts the growth of Type II irregularities reasonably well, but it does not fully capture the simultaneous growth of Type I irregularities in the region of peak total electric field. The growth of localized Type I irregularities represents a parametric instability in which the electric field of the large‐scale background wave drives pure Farley‐Buneman turbulence. These results help explain observations of meter‐scale irregularities advected by kilometer‐scale waves.This work was supported by NSF grants AGS-1007789 and PHY-1500439, and NASA grants NNX11A096G and NNX14AI13G. This work used the XSEDE and TACC computational facilities, supported by NSF grant ACI-1053575, for simulation runs. Simulation-produced data are archived at TACC and are available upon request. This work also used the Massachusetts Green High Performance Computing Center for simulation data analysis. The authors thank one reviewer for insightful comments and critiques. (AGS-1007789 - NSF; PHY-1500439 - NSF; ACI-1053575 - NSF; NNX11A096G - NASA; NNX14AI13G - NASA

Mesospheric anomalous diffusion during noctilucent clouds

The Andenes specular meteor radar shows meteor-trail diffusion rates increasing on average by ~ 20% at times and locations where a lidar observes noctilucent clouds (NLCs). This high-latitude effect has been attributed to the presence of charged NLC but this study shows that such behaviors result predominantly from thermal tides. To make this claim, the current study evaluates data from three stations, at high-, mid-, and low-latitudes, for the years 2012 to 2016, comparing diffusion to show that thermal tides correlate strongly with the presence of NLCs. This data also shows that the connection between meteor-trail diffusion and thermal tide occurs at all altitudes in the mesosphere, while the NLC influence exists only at high-latitudes and at around peak of NLC layer. This paper discusses a number of possible explanations for changes in the regions with NLCs and leans towards the hypothesis that relative abundance of background electron density plays the leading role. A more accurate model of the meteor trail diffusion around NLC particles would help researchers determine mesospheric temperature and neutral density profiles from meteor radars.Public versio

Formation of plasma around a small meteoroid: electrostatic simulations

Obtaining meteoroid mass from head echo radar cross section depends on the assumed plasma density distribution around the meteoroid. An analytical model presented in Dimant and Oppenheim (2017a, https://doi.org/10.1002/2017JA023960; 2017b, https://doi.org/10.1002/2017JA023963) and simulation results presented in Sugar et al. (2018, https://doi.org/10.1002/2018JA025265) suggest the plasma density distribution is significantly different than the spherically symmetric Gaussian distribution used to calculate meteoroid masses in many previous studies. However, these analytical and simulation results ignored the effects of electric and magnetic fields and assumed quasi‐neutrality. This paper presents results from the first particle‐in‐cell simulations of head echo plasma that include electric and magnetic fields. The simulations show that the fields change the ion density distribution by less than ∼2% in the meteor head echo region, but the electron density distribution changes by up to tens of percent depending on the location, electron energies, and magnetic field orientation with respect to the meteoroid path.First author draf