The thermospheric column O/N2 ratio (ΣO/N2) exhibits complex spatial and temporal variations and is a key parameter in diagnosing the state of the thermosphere and ionosphere. The solar cycle, ...seasonal, latitudinal and longitudinal variations of ΣO/N2 have been studied in the past few decades. However, the local time variation of ΣO/N2 is rarely investigated. At solstice, the most important feature of ΣO/N2 is the well‐known summer‐winter difference caused by the global meridional circulation. Based on TIMED/GUVI observations from 2002 to 2022, it was found that the daytime pattern of ΣO/N2 exhibits significant hemispheric differences superimposed on the more prominent seasonal distribution. ΣO/N2 decreases soon after sunrise in the winter hemisphere, while it occurs much later in the summer hemisphere. This hemispheric difference in the local time variation of ΣO/N2 is generally reproduced by the empirical model MSIS and the physics‐based model TIEGCM. And the TIEGCM simulation results suggest that it is mainly attributed to the meridional advection. In the summer hemisphere, upward winds which decrease ΣO/N2 are weak in the early morning and compete with the enhancement of ΣO/N2 caused by the meridional advection, resulting in a later turning time around 11:00 LT. In the winter hemisphere, the reduction of ΣO/N2 caused by daytime upward winds is superimposed on the reduction of ΣO/N2 induced by the meridional advection, resulting in an earlier turning time around 7:00 LT. Additionally, ΣO/N2 peaks much later in regions near the magnetic pole than those far from the magnetic pole in the summer hemisphere.
Key Points
During the day, the mid‐latitude ΣO/N2 peaks in the late morning in summer but does so in the early morning in winter
The hemispheric difference in local‐time pattern of ΣO/N2 is mainly caused by the opposite effects of meridional advection on ΣO/N2 change in two hemispheres
ΣO/N2 peaks later in regions near the magnetic pole than those far from the magnetic pole at mid‐high latitudes in the summer hemisphere
The influence of zonal electric fields as well as E × B plasma drifts on the ionosphere has been widely investigated, but the latitudinal variations of zonal electric fields have not been well ...understood. In this study, we investigate the driving mechanisms responsible for the latitudinal variations of zonal electric fields under geomagnetically quiet conditions using the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM). A series of case‐controlled TIEGCM simulations were conducted to explore the effects of neutral wind and ionospheric conductivity on the latitudinal variations of zonal electric fields. The major results are given as follows: (1) The eastward electric field at noon increases with latitude, which is mainly associated with latitudinal changes of poleward neutral winds. (2) The prominent latitudinal variations of zonal electric fields at sunrise and sunset are attributed to the strong longitudinal gradients of zonal winds, that is, the curl‐free mechanism. (3) The changes of the zonal electric fields at a given location are affected by the dynamo effect globally, which also produces the latitudinal structure of zonal electric fields.
Key Points
The eastward electric fields at noon increase with latitude, which is mainly related with the latitudinal variation of meridional winds
The latitudinal variations of the zonal electric fields at sunset and sunrise are associated with the longitudinal gradients of zonal winds
The latitudinal variations of zonal electric fields at a given location can be influenced by the global nature of dynamo process
Recently a Three‐dimensional EquatoriaL spread F (TELF) model using high‐resolving power advection schemes have been developed at the University of Science and Technology of China. With the inclusion ...of the field‐aligned plasma flow and realistic ionospheric conductivity configuration, the TELF model reproduced typical features of the observed equatorial spread F (ESF) phenomena, including plasma depletions along flux tubes, geomagnetic conjugate irregularities, and ESF morphology as seen in ground optical and satellite observations. Through control simulations, the effects of field‐aligned plasma flow and ionospheric conductivity on ESF are investigated. It is found that field‐aligned plasma flow could inhibit the growth of bubbles by modulating the ionospheric conductivity distribution along field lines, and the plasma bubbles from the TELF model behave more physically in the topside F‐region than those from the 2D model due to the realistic ionospheric conductivity specification. Additionally, the simulations demonstrate that the amplitude and altitude of the initial perturbations can also affect the growth rate of ESF by modulating the polarization electric fields.
Key Points
A new three‐dimensional equatorial spread F model named TELF with high‐order numerical schemes was developed
Field‐aligned plasma flow could inhibit the growth of equatorial spread F by redistributing plasma density and ionospheric conductivities along flux tubes
Compared with the 2D model, TELF is more appropriate for simulating topside bubbles due to the coupling nature of the ionosphere
The ratio of number density of atomic oxygen (O) to that of molecular nitrogen (N2) in the thermosphere (O/N2) on the constant pressure surface, which has complex temporal and spatial ...characteristics, is widely regarded as an important parameter connecting the terrestrial thermosphere and daytime ionosphere. Previous studies demonstrated that the thermospheric O/N2 increases with increasing solar activity, and the changes in O/N2 with solar activity show significant difference between winter and summer hemispheres. However, the root causes, which are responsible for the solar activity variation of O/N2, are not fully understood. In this study, the contributions of various physical and chemical processes on the response of O/N2 to the solar radiation change were quantitatively investigated through a series of controlled simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model. The simulation results suggested that the chemical processes lead to the increase of thermospheric O/N2 over the globe with increasing solar activity. The increase of O/N2 with solar activity is dominated by the enrichment of O abundance and the loss of N2 abundance in the lower and upper thermosphere, respectively. Moreover, the simulation results suggested that the stronger hemispheric asymmetry is attributed to the stronger thermospheric circulation, which changes the vertical advection of O/N2 through both direct and indirect effects.
Key Points
The chemical processes which change O and N2 abundances are mainly responsible for the increase of thermospheric O/N2 with solar activity
The solar activity dependence of O/N2 is attributed to the enrichment of O and loss of N2 in the lower and upper thermosphere, respectively
The variation of hemispheric asymmetry of O/N2 at solstice with solar activity is associated with the changes in thermospheric circulation
Previous studies showed that the density overcooling in the thermosphere during the recovery phases of the October 2003 storms was not reproduced by the National Center for Atmospheric Research ...Thermosphere‐Ionosphere Electrodynamics General Circulation Model. In this study, a series of controlled numerical experiments were carried out to explore the processes responsible for the neutral density overcooling. It was found that the simulation with the temperature‐dependent reaction rate of N(2D) + O2 from Duff et al. (2003, https://doi.org/10.1029/2002GL016720) can better capture the overcooling during the recovery phases of the October 2003 storms. Our study also demonstrated that the thermosphere recovery strongly depends on the altitudinal distribution of NO emission per mass rather than the NO cooling flux alone. During the storm recovery phases, the temperature starts to show overcooling in 110–180 km where the NO increases primarily, which has great contribution to the density overcooling.
Key Points
The TIEGCM with the Duff et al. (2003) reaction rate of N(2D) + O2 generally reproduced the observed thermospheric overcooling
The thermosphere recovery is strongly dependent on the altitudinal distribution of NO emission per mass rather than the NO cooling flux alone
The overcooling in neutral temperature initially occurs at 110–180 km where NO densities are primarily enhanced
The ionospheric irregularities known as equatorial spread F (ESF) have drawn great attention in decades due to their disruption of communications and navigation systems. ESF is generally attributed ...to Rayleigh–Taylor instability, but there are still many remaining issues about its formation and evolution. Numerical simulations have been widely used to study such a phenomenon due to the complex nonlinear evolution of ESF. A two‐dimensional ESF model was developed in this work and a series of experiments were conducted to study the effects of grid resolution and numerical diffusion on idealized ESF simulation. It was found that advection schemes with the total variation diminishing property are preferred for preventing spurious oscillations which occur near the steep density gradients in the ESF. On the other hand, schemes with low diffusion are also desirable in order to model complicated dynamic structures of the ESF. Moreover, numerical experiments suggest that some of the secondary instabilities in idealized ESF simulations, although with morphological similarities, are possibly initiated by numerical seeding, which may differ from the observed evolution of secondary ESF.
Key Points
Numerical considerations are critical in simulating the equatorial spread F
For the advection scheme with higher‐order reconstruction, a flux limiter with the TVD property is used to suppress numerical oscillations
The solution associated with secondary instability is influenced by the quality of the numerics
Abstract
Accurately estimating the mass density of the thermosphere can assist satellite operators in planning missions and optimizing satellite orbits, reducing the risk of collisions. The newly ...developed GOFT (Global Observation‐based Forecast for the Thermosphere) model specifies thermospheric mass density by optimizing uncertainty parameters in a physics‐based model based on model errors. This study extends the model to specify thermospheric mass density based on Two‐Line Elements (TLEs) from numerous space objects. The resulting mass density is validated against accelerometer‐derived mass density from different satellites. The comparison results revealed that assimilating TLEs into the GOFT model significantly improved the accuracy of the thermospheric mass density. In addition, the GOFT model also improved the accuracy of the electron density in the ionosphere, indicating that the assimilation capability of GOFT allows simultaneous specification of coupled ionosphere and thermosphere in future applications.
Plain Language Summary
The thermosphere (∼90–600 km) is the layer in the Earth's atmosphere directly above the mesosphere. Though the density in the thermosphere is at least a billion times lower than at sea level, aerodynamic drag continues to be the greatest uncertainty in determining the orbits of satellites at low altitudes. Thus, better specification of thermospheric mass density has become more and more important and imperative. One of the great challenges in space weather prediction is the paucity of observational data that are used to drive current physics‐based models. Herein, we have developed a method for specifying thermospheric mass density based on Two‐Line Elements (TLEs) from tens of space objects by optimizing uncertain parameters in a physics‐based model. This method improved the accuracy of thermospheric mass density by assimilation of TLEs, and the coupling assimilation of ionosphere and thermosphere is possible based on the method.
Key Points
Thermospheric mass density was specified through parameter optimization of a physics‐based model based on Two‐Line Elements
The model improved the accuracy of thermospheric mass density at various latitudes, local times, and altitudes
The model can be extended to assimilating neutral and electron densities in the coupled thermosphere and ionosphere simultaneously
In this work, we carry out a comprehensive modeling study, using the Thermosphere–Ionosphere–Electrodynamics General Circulation Model, to explore the physical processes by which the ...longitude‐dependent geomagnetic field drives the longitudinal variations of the sunrise enhancement of the zonal electric fields at the dip equator near the June solstice. Numerical experiments and diagnostic analyses of the electrodynamics equation show that the longitudinal differences of the equatorial zonal electric fields near sunrise are primarily associated with the longitudinal variations in the zonal wind dynamo, with those from the meridional wind dynamo contributing secondarily. Furthermore, the longitudinal differences of the wind dynamo near sunrise are mainly related to the longitudinal variations of U×B and conductance, which are caused primarily by the direct influence of the longitudinal structures of magnetic field declination and strength. Meanwhile, the longitudinal variations of neutral winds, which also result in moderate U×B longitudinal variations, play a secondary role in the longitudinal variations of the neutral wind dynamo, while plasma density, which has minor longitudinal differences near sunrise, contributes slightly by modifying the conductance. Overall, the sunrise enhancement in June is more significant at the longitudes where the magnetic field strength and distortion are larger or the magnetic field declination is smaller in the Northern Hemisphere.
Key Points
The causes for the longitudinal variations of equatorial zonal electric fields near sunrise are explored
These longitudinal variations are primarily associated with the longitudinal differences of zonal wind dynamo
Dynamo differences are mainly related to longitudinal variations of U×B and conductance caused by magnetic field declination and strength
When solar wind and interplanetary magnetic field (IMF) disturb, thermospheric winds change accordingly. Among the momentum forces driving high‐latitude thermospheric winds, ion drag is supposed to ...greatly affect wind variations through ion‐neutral coupling when abrupt and strong changes in ion drifts occur. However, due to the great inertia of thermospheric winds it needs a certain period of time for the wind changes to be prominent both in speed and direction. How long the neutral winds take to change from one steady state to another through the ion‐neutral coupling process is currently still a controversial issue. In this paper, we examine the high latitudinal ion‐neutral coupling time scale based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations, which can determine whether wind variations are dominantly driven by ion drag by analyzing the relative contribution of each momentum force. It is found that the spatial variation of ion‐neutral coupling time scale is primarily determined by local electron density, but also varies with neutral density and ion‐neutral collision frequency. Simulations during periods of medium solar activity at ∼250 km altitude show that the ion drag‐dominated region is generally located at the dayside convection inverse boundary and the coupling time scale (e‐folding time) is ∼1 hr when IMF By is the dominant component of the IMF and changes direction. Meanwhile, the southward component of IMF Bz enlarges the ion drag‐dominated region. When IMF Bz is southward with a large magnitude, ion drag‐dominated region is primarily located in the nightside auroral oval with ∼2 hr coupling time scale.
Key Points
The spatial variation of ion‐neutral coupling time scale depends primarily on local electron density
The ion drag dominant region is located at the dayside convection cell boundary (nightside auroral oval) when interplanetary magnetic field (IMF) By (negative Bz) disturbs
When wind variation is greatly affected by IMF By, the concurrent negative shift of IMF Bz obviously enlarges the ion drag dominant region
Thermospheric nitric oxide (NO) has a strong response to the energy injection, and it serves as the “natural thermostat.” Previous studies have tended to concentrate on NO radiative flux and its ...effectiveness on the global energy budget rather than the altitudinal dependence. In this study, measurements from TIMED/SABER (the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on TIMED satellite) were used to investigate the variation of the NO radiance cooling in thermosphere as a function of altitude, latitude, and local time. Using the methods of principal component analysis and spherical harmonic regression, we have developed an empirical model to capture the most significant variation in NO radiance cooling for the altitude range 100–280 km. The model reproduced the variations of NO radiation at different latitudes and altitudes. Moreover, the integrated model NO radiative flux is in good agreement with satellite measurements with a correlation coefficient up to 0.96. This empirical model can be used to investigate the NO cooling evolution and serve as a validation tool for future physics‐based simulations.
Key Points
Sounding of the Atmosphere using Broadband Emission Radiometry data are used to develop an empirical model of thermospheric nitric oxide (NO) radiance cooling
The global pattern of NO cooling, as a function of height, latitude, and local time, is well captured
The modeled NO cooling flux is in agreement with satellite measurements with the correlation coefficient up to 0.96
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