Arthur D. Richmond

Ionospheric currents and their associated magnetic perturbations are difficult to predict due to the high variability of their driving mechanisms. In addition, the 3D structure of the currents is challenging to unravel due to the lack of direct observations within the current flow region, particularly in the ‐layer. In this study, we aim to address these difficulties by combining a physics‐based model of the 3D ionospheric currents with ground and satellite observations. The physics‐based model is used to construct a novel set of spatial basis functions, which we term spatial modes, designed to capture the dominant 3D spatial flow patterns of the ionospheric currents at the diurnal variation (DV) period and associated harmonics. We additionally develop a set of temporal modes using observations from the global geomagnetic observatory network. Principal components analysis (PCA) is used to construct both the spatial and temporal modes by analyzing time series of observations at spatially distinct locations in order to identify correlations and reduce dimensionality of the parameter space. These modes are then used to construct a data‐driven empirical model of the 3D ionospheric currents spanning nearly two decades. Our empirical model is built using a hybrid approach in both the frequency domain, to facilitate the calculation of electromagnetically induced fields, and the time domain, to enable the inclusion of satellite observations in our fitting problem. Our model includes the spatial patterns and time evolution of all major ionospheric current systems and is valid from January 2000 to August 2018.

A new version of the US National Science Foundation National Center for Atmospheric Research (NSF NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) has been developed and released. This paper describes the changes and improvements of the new version (3.0) since its last major release (2.0) in 2016. These include: (a) increasing the model resolution in both the horizontal and vertical dimensions, as well as in the ionospheric dynamo solver; (b) upward extension of the model upper boundary to enable more accurate simulations of the topside ionosphere and neutral density in the lower exosphere; (c) improved parameterization for thermal electron heating rate; (d) resolving transport of minor species N( ² D); (e) treating helium as a major species; (f) parameterization for additional physical processes, such as SAPS and electrojet turbulent heating; (g) including parallel ion drag in the neutral momentum equation; (h) nudging of prognostic fields near the lower boundary from external data; (i) modification to the NO reaction rate and auroral heating rate; (j) outputs of diagnostic analysis terms of the equations; (k) new functionalities enabling model simulations of certain recurrent phenomena, such as solar flares and eclipses. We present examples of the model validation during a moderate storm and compare simulation results by turning on/off new functionalities to demonstrate the related new model capabilities. Furthermore, the model was upgraded to comply with the new computer software environment at NSF NCAR for easy installation and run setup and with new visualization tools. Finally, the model limitations and future development plans are discussed.

A new version of the US National Science Foundation National Center for Atmospheric Research (NSF NCAR) thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) has been developed and released. This paper describes the changes and improvements of the new version 3.0 since its last major release (2.0) in 2016. These include: 1) increasing the model resolution in both the horizontal and vertical dimensions, as well as the ionospheric dynamo solver; 2) upward extension of the model upper boundary to enable more accurate simulations of the topside ionosphere and neutral density in the lower exosphere; 3) improved parameterization for thermal electron heating rate; 4) resolving transport of minor species N(2D); 5) treating helium as a major species; 6) parameterization for additional physical processes, such as SAPS and electrojet turbulent heating; 7) including parallel ion drag in the neutral momentum equation; 8) nudging of prognostic fields near the lower boundary from external data; 9) modification to the NO reaction rate and auroral heating rate; 10) outputs of diagnostic analysis terms of the equations; 11) new functionalities enabling model simulations of certain recurrent phenomena, such as solar flares and eclipse. We present examples of the model validation during a moderate storm and compare simulation results by turning on/off new functionalities to demonstrate the related new model capabilities. Furthermore, the model is upgraded to comply with the new computer software environment at NSF NCAR for easy installation and run setup and with new visualization tools. Finally, the model limitations and future development plans are discussed.

The wind dynamo in the ionosphere leads to differential motion of ions and electrons, which in turn sets up electric fields and currents. Observations show that daytime lower thermospheric horizontal winds have large vertical gradients. Numerical modeling conducted approximately 50 years ago demonstrated that the zonal wind shears in the ∼130–180 km altitude range can generate off‐equatorial relative minima (dips) in the daytime height‐integrated eastward current density, appearing as westward sidebands north and south of the equatorial electrojet (EEJ). This study observationally confirms this connection for the first time by combining Ionospheric CONnection explorer zonal wind profiles and Swarm latitudinal zonal currents. We demonstrate observationally that the magnitude of the EEJ sideband current is proportional to the strength of westward turning winds with altitude in the Pedersen conductivity dominated region. Additional numerical experiments explain the importance of wind shear in different altitude regions in generating the sideband current. This study contributes to the better understanding of the neutral wind effect on the local current generation.

An expression of Rayleigh‐Taylor (R‐T) instability growth rate based on the field‐line integrated theory is newly established. This expression can be directly utilized in ionosphere models with magnetic flux tube structure based on Modified Apex Coordinates. The R‐T instability growth rates are calculated using the thermospheric and ionospheric conditions based on the coupled Whole Atmosphere Model and Ionosphere Plasmasphere Electrodynamic model (WAM‐IPE). The parameters used in this calculation include the field‐line integrated conductivities and currents, which consider the Quasi‐Dipole Coordinates and the modifications to the equations of electrodynamics. Detailed description of the new formulas and comprehensive analyses of diurnal, longitudinal, and seasonal variations of the R‐T instability growth rate are carried out. The dependencies of growth rates on pre‐reversal enhancement (PRE) vertical drifts and solar activity are also examined. The results show that pronounced R‐T growth rates are captured between 18 and 22 local time (LT) when strong PRE occurs in the equatorial ionosphere. The simulated R‐T growth rate increases with increasing solar activity levels and demonstrates strong correlations with the angle between the sunset terminator and the geomagnetic field line. These results are consistent with plasma irregularity occurrence rates shown in various satellite observations, suggesting that the newly developed R‐T growth rate calculation can effectively capture the probability of irregularities by considering the changes along magnetic flux‐tubes in the ionosphere. Since the WAM‐IPE is running in operation at National Oceanic and Atmospheric Administration Space Weather Prediction Center, the new calculations can be potentially implemented in the near future to provide forecasted information of the R‐T growth rate.

We report the first lidar observations of regular occurrence of mid‐latitude thermosphere‐ionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. Detection of tenuous Na layers (∼0.1–1 cm⁻³ from 150 to 130 km) was enabled by high‐sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from ∼125 km after dusk and from ∼150 km before dawn. The downward‐progression phase speeds are ∼3 m/s above 120 km and ∼1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (∼105–110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na⁺ layers. Vertical drift velocity of TINa⁺ calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis.

Reduced daytime upward E×B drifts and weakened fountain effects in equatorial ionosphere have been frequently observed during the initial stage of solar flares. The cause of this phenomenon, however, remains unresolved. The latest state‐of‐art whole geospace model provides an unprecedented opportunity to explore the origin of this response. We show that both prompt penetration electric fields (PPEFs) and internal changes in the wind dynamo process are responsible for the reduced upward ion drifts. Solar‐flare‐induced PPEFs are caused by a reduced high‐latitude potential as a result of flare‐enhanced ionospheric conductances which are distinct from traditional PPEFs that respond to changes in solar wind conditions or magnetosphere dynamics. The neutral wind dynamo source is mainly a reduction in the background low‐latitude eastward electric field. This reduction occurs to maintain current continuity in response to the flare enhancement of low‐latitude Cowling conductance that is relatively greater than the enhancement of the dynamo current source.

The magnetosphere‐ionosphere (MI) coupling is crucial in modeling the thermosphere‐ionosphere (TI) response to geomagnetic activity. In general circulation models (GCMs) the MI coupling is typically realized by specifying the ion convection and auroral particle precipitation patterns from for example, empirical or assimilative models. Assimilative models, such as the Assimilative Mapping of Ionospheric Electrodynamics, have the advantage that the ion convection and auroral particle precipitation patterns are mutually consistent and based on available observations. However, assimilating a large set of diverse data requires expert knowledge and is time consuming. Empirical models, on the other hand, are convenient to use, but do not capture all the observed spatial and temporal variations. With the availability of Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data, there is an opportunity for employing field‐aligned currents (FAC) in GCMs to represent the MI coupling. In this study, we will introduce a new method which enables us to use observed FAC in GCMs and solve for the interhemispherically asymmetric electric potential distribution. We compare Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) simulations of a geomagnetic storm period using the new approach and two other often‐used methods for specifying MI coupling based on empirical and assimilative high latitude electric potentials. The comparison shows general similarities of the TI storm time response and improved temporal variability of the new method compared to using empirical models, but results also illustrate substantial differences due to our uncertain knowledge about the MI coupling process.

The NASA Global‐scale Observations of the Limb and Disk ultraviolet imaging spectrograph performs observations of upper atmosphere airglow from the sunlit disk and limb of the Earth, in order to infer quantities such as the composition and temperature of the thermosphere. To interpret the measurements, the observational and solar illumination geometry must be considered. We use forward models of upper atmosphere density and composition, photoelectron impact, airglow emissions, radiative transfer, and line‐of‐sight integration, to describe the expected observations, and here test those calculations against observations near the terminator, and near the limb. On the nightside of the terminator, broad regions of faint airglow are seen, particularly near the winter solstice. These are caused by photoelectrons generated in magnetically conjugate areas in the other hemisphere that are still illuminated, transported along field lines, and then precipitated back into the atmosphere. Model calculations demonstrate that this process is the source of the emission, and obtain good agreement with its morphology and intensity. In some regions, the observed emissions are not as bright as the model simulations. Some of the reduction in electron flux is explained by changes in magnetic field strength; in other cases, particularly at high magnetic latitude, the cause is unknown, but must occur along extended field lines that reach into the plasma sheet.