Accurate representation of ensemble members of coupled Ionosphere‐Thermosphere (I‐T) is crucial to the ionosphere forecast in the ensemble Kalman filter (EnKF) data assimilation system. In this ...paper, besides the model drivers used in previous studies, including the solar 10.7 cm radio flux (F10.7), auroral hemispheric power (HP) and cross‐tail potential drop (CP), the eddy diffusion coefficient (ED), and nighttime vertical O+ flux at the top boundary are also perturbed to generate ensemble members. Based on our developed EnKF data assimilation system, the impact of perturbing different model external forcing parameters on ensemble generation and forecast capability of ionosphere and thermosphere has been investigated in detail through a series of sensitivity tests and data assimilation experiments. This system uses the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) as a background model. The findings are summarized as follows: (a) Associated with perturbing two additional model forcing parameters, better ensemble members of ionosphere and thermosphere states of the background model can be generated during both daytime and nighttime. (b) The improvement of the forecast capability of the ionosphere and thermosphere variables can be further enhanced. This study can provide a reference for ensemble generating strategy for the coupled I‐T EnKF data assimilation in the future.
Plain Language Summary
The ensemble member of EnKF data assimilation considering as much as possible variabilities existed in the coupled I‐T system should represent model uncertainties better. In the previous researches on EnKF ionosphere forecast, ensemble members are mainly generated by perturbing solar and geomagnetic activity index common model drivers (e.g., F10.7, KP). However, solely perturbing those indices is not sufficient to distribute the ionosphere and thermosphere states at all local times. In this study, in addition to perturbing those indices, the key model forcing parameters of the lower and upper boundary of the TIEGCM are also perturbed to generate the ensemble member of the coupled I‐T system. We found that an improved ensemble spread can be obtained at all local times in this way, which can further contribute to the forecast capability of the ionosphere and thermosphere states.
Key Points
The eddy diffusion and upper boundary are additionally perturbed to generate ensembles in EnKF I‐T assimilation system
Ensemble members can be better obtained at no matter daytime or nighttime
The forecasting capability of I‐T states can be further improved
The 15 January 2022 Tonga eruption seemed to have caused a strong depletion in the ionospheric electron density. However, the eruption occurred during a moderate geomagnetic storm, so that the ...depletion could be a local negative effect of the storm. In this work, for the first time, we analyze this depletion and discuss relative contributions of the eruption and the storm through measurements of GNSS‐derived vertical total electron content (VTEC), O/N2 ratio by TIMED/GUVI, ion density and temperature by ICON/IVM, and simulations by Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). We show that shortly after the eruption onset the VTEC in the vicinity of the volcano dropped by ∼80%–95% below the quiet‐time values. The depletion extended up to 4,000 km away from the volcano and lasted for ∼16 hr, that is, until local morning hours. Our results suggest that the depletion was initially caused by the eruption (60%–75% negative deviation) and was further reinforced by the storm, by at least 20%. Spatially, only ∼1,500 km could be attributed to the eruption. This study provides a good illustration of understanding the contributions of forcings from above and below to the ionosphere.
Key Points
The strong (i.e., ∼80%–95%) and long‐lasting (i.e., ∼16 hr) ionospheric depletion occurred after the 15 January 2022 Tonga eruption
The volcanic explosion is responsible for ∼60%–75% depletion and horizontal extension of ∼1,500 km
The depletion was reinforced by the moderate geomagnetic storm, and its extreme duration is due to local night hours
In this work, we constructed an ensemble Kalman filter (EnKF) ionosphere and thermosphere data assimilation system using the National Center for Atmospheric Research Thermosphere Ionosphere ...Electrodynamics General Circulation Model (NCAR‐TIEGCM) as the background model. We use a sparse matrix method to avoid significant matrix related calculation and storage. A series of observing system simulation experiments have been conducted to assess the performance of the system. The results show that the system optimizes ionosphere drivers efficiently by assimilating electron densities through their covariance. The short‐term forecast capability is enhanced significantly, and the effect of initial condition correction lasts for longer than 24 hr. To our knowledge, this is the first study to demonstrate that the EnKF‐based global ionosphere and thermosphere data assimilation can be conducted without using a supercomputer. This workstation‐based EnKF ionosphere and thermosphere data assimilation system benefits both scientific studies and near‐real‐time operation.
Plain Language Summary
In the ionosphere and thermosphere, the neutrals remember the past much longer than the ionized part does. In the recent decades, ensemble‐based data assimilation has been proven to be an efficient method to enhance the ionosphere forecast capability by assimilating electron densities through optimizing neutral state variables via their covariance. Due to the globality and large ensemble running, this kind of studies always relies on supercomputers. In this study, we used a sparse matrix method to do the matrix‐related calculation and storage in the EnKF and conducted EnKF ionosphere and thermosphere data assimilation on a workstation. We have done a series of observing system simulation experiment studies to evaluate the validity and reliability of the method. The results show that the EnKF can optimize the ionosphere drivers efficiently by assimilating electron density through their covariance. The short‐term forecast capability is enhanced significantly and extended to last longer than 24 hr. This is the first study to demonstrate that the EnKF‐based global ionosphere and thermosphere data assimilation system can be conducted without using supercomputers. It can benefit both scientific studies using the EnKF ionosphere and thermosphere‐related data assimilation system and the near‐real‐time operational purpose.
Key Points
A new ionosphere and thermosphere data assimilation system is developed with the ensemble Kalman filter
A sparse matrix method is used for the EnKF to militate the huge computation and storage problems
The system can be run on a workstation and can benefit both scientific studies and near‐real‐time operation
Ionospheric total electron content (TEC) is an important parameter in ionospheric researches and applications. However, the determination of the absolute value of TEC can be greatly influenced by the ...differential code biases (DCBs) estimation. Nowadays, there are more and more Global Navigation Satellite System (GNSS) signals available all over the world, which allow us to solve TEC and DCBs with the hypothesis of local spherical symmetry (LSS) imposed on the dual-frequency observations from only one individual station. For comparison, the results based on the global ionospheric map (GIM) act as a reference in this article. On the one hand, different combinations of Global Positioning System (GPS), GLONASS, and BeiDou Navigation Satellite System (BDS) are considered to illustrate the significance of multi-GNSS observations, with the mixed GPS, GLONASS, and BDS combination performing best when compared to the referenced results. On the other hand, different parameters in LSS condition are taken into account to investigate the suitable geometric constraint of LSS, with the differential longitude, latitude, and epoch suggested to be 3.0°, 0.6°, and 4 min, respectively. Moreover, a group of detailed comparisons from several different stations also show that the combined DCBs and ionospheric TEC derived from our method are compatible with those from the GIM-aided method, especially in the low-latitude area. In summary, with the advantage of the multi-GNSS signals from an individual station, our method can estimate the ionospheric TEC and DCBs independently, which could provide a potential tool in the future real-time applications.
Given that the ionosphere is strongly determined by the thermosphere and its state depends on thermospheric parameters, we propose a new method to extract exospheric temperature (Tex) from electron ...density (Ne) profiles based on the relationship between the variations in Tex and Ne profiles established through simulation. Ne profiles and corresponding Tex from the Millstone Hill incoherent scatter radar (ISR) observations are used to test the method. ISR Ne profiles are used for Tex retrieval and ISR Tex is used to make a comparison with Tex calculated by model and retrieved Tex. The results show that the retrieved Tex effectively captures diurnal, anomalous and short‐period variations. The relative deviation between the retrieved–observed Tex is approximately 2%, which is significantly improved compared with the Mass Spectrometer Incoherent Scatter model, especially under disturbed conditions. This result confirms that thermospheric temperature variation can be deduced from ionospheric profiles and our method can be considered a useful tool to obtain Tex from Ne profiles.
Plain Language Summary
Space weather can affect satellite communications and orbit determination and interfere with ground‐based power grids. Thermosphere/ionosphere forecasting based on simple current and past data offers a feasible solution to reduce the impact of space weather. However, measurements of the thermosphere are not an easy task and thermospheric data is deficient and discontinuous. In contrast, a large amount of continuous ionospheric observations have been accumulated. Furthermore, as a tightly coupled ionosphere/thermosphere system, the thermosphere and the ionosphere are interdependent on structure and variation. Thermospheric parameters such as neutral concentration, compositions, and temperature play a vital role in the production and loss of ionosphere plasma density and have a significant impact on electron density. Conversely, the structure and variation in the ionosphere should contain information about the surrounding thermosphere. Theoretically, thermospheric parameters can be obtained by solving the inverse problem of aeronomy. Since exospheric temperature (Tex) is an important parameter describing the upper thermosphere, we develop a method to retrieve Tex from electron density (Ne) profiles in this work. Subsequently, the Tex module in empirical models can be replaced by our retrieved Tex to obtain other thermospheric parameters. The good performance of the method is validated with incoherent scatter radar observations.
Key Points
We propose a new method to extract exospheric temperature (Tex) from electron density profiles
This method accurately reproduces diurnal, anomalous and short‐period variations in the thermosphere
The retrieved Tex agrees better with the incoherent scatter radar Tex than those from the NRLMSISE‐00 model, especially under magnetically disturbed conditions
To date, no natural media is available to derive the “ancient” ionosphere, so the current understanding of ionospheric long‐term trends mainly comes from analyzing ∼70 yr modern measurements ...supplemented by theoretical simulations. In this study, for the first time, we expanded the ionosphere simulation to the whole Holocene (9455 BCE to 2015 CE) using the Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model developed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, driven by a realistic geomagnetic field, CO2 level, and solar activity, all of which are a combination of ancient media derivations and modern observations. Through a series of control runs, we found that the ionospheric oscillation of the global mean profile is characterized by a nonlinear variation versus geomagnetic field effect, a decrease (increase) above (below) ∼200 km due to CO2 increase, and violent oscillations in phase with changes in solar activity, with corresponding contributions of approximately 20%, 20%, and 60%, respectively. The CO2 effect became nonnegligible and even more significant after ∼1800 CE. The solar activity effect is marked by a frequently occurring grand solar minimum. The ionospheric characteristics show linear variations versus the dipole moment of the geomagnetic field, the CO2 level, and the F10.7 index, with the growth rate having significant local time and latitude variations. Our prediction shows that a 400 ppm CO2 increase will cause a global mean decrease of 1.2 MHz in foF2, 34 km in hmF2, and 4 tecu in TEC, which will directly influence radio wave communication.
Plain Language Summary
The ionosphere (∼60–1,000 km above sea level), which is the ionized part of atmosphere, can influence the radio communication and satellite navigation significantly. Given the increasing human's space activity in near future, this influence will be more important. Our current understanding of the ionosphere is mainly obtained by analyzing measurements made by modern instruments dating back to the 1930s. To date, no natural media is available that can record “ancient” ionosphere, which restrict our prediction of future ionosphere in a longer time scale. In this study, for the first time, we try to reconstruct the paleoionosphere using our own theoretical model driven by realistic geomagnetic field, solar activity, and CO2 level as long as possible (9455 BCE to 2015 CE). Then we analyzed the ionospheric evolution due to the variations of different drivers. Based on the simulation, we further analyzed the variations of ionospheric characteristics (critical frequency, peak height, and total electron content), which are most frequently used in real application, versus different drivers. We then made a prediction of ionosphere during the coming century due to variations of different drivers. Our results are of significances for future ionosphere related applications due to potential CO2 increase and geomagnetic field variations.
Key Points
The ionosphere during the Holocene was first simulated using the Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model developed at the Institute of Geology and Geophysics, Chinese Academy of Sciences driven by realistic indices
The geomagnetic field, CO2, and solar activity contributed ∼20%, 20%, and 60% of ionospheric variability, respectively
A rough prediction of ionosphere was made for the following century under several scenarios
The COSMIC radio occultation observations are used to investigate the wavenumber‐4 (WN4) structure of the Sporadic E (Es) layer during 2007–2018. The WN4 pattern is observed in the Es occurrence rate ...over the middle‐ and low‐latitudes. This pattern is stronger at the low‐latitudes than at the middle‐latitudes. It shows the peak intensity in latitude bands between 5° and 15° dip latitudes in both hemispheres. The WN4 structure occurs predominantly during daytime. It appears above 110 km in the morning, then descends slowly down to about 105 km, and finally disappears after sunset. The WN4 structure moves eastward with a speed of approximately 90°/day. It is prominent in boreal autumn and summer, and disappears in boreal winter. The dominant role of the DE3 tide for the formation of the WN4 structure is supported by similar features between the Es occurrence rate and the DE3 tide of the zonal wind shear, such as the phase, eastward speed, seasonal variation, symmetrical distribution, and strong activity at the low‐latitudes.
Key Points
The wavenumber‐4 (WN4) structure is observed in the sporadic E layer over the middle‐ and low‐latitudes
The diurnal, seasonal, and altitudinal variations of the WN4 pattern are analyzed
The diurnal eastward non‐migrating tide with zonal wavenumber‐3 plays the dominant role in the formation of the WN4 structure
Ionospheric F2 region peak densities (NmF2) are expected to have a positive correlation with total electron content (TEC), and electron densities usually show an anticorrelation with electron ...temperatures near the ionospheric F2 peak. However, during the 17 March 2015 great storm, the observed TEC, NmF2, and electron temperatures of the storm‐enhanced density (SED) over Millstone Hill (42.6°N, 71.5°W, 72° dip angle) show a quiet different picture. Compared with the quiet time ionosphere, TEC, the F2 region electron density peak height (hmF2), and electron temperatures above ~220 km increased, but NmF2 decreased significantly within the SED. This SED occurred where there was a negative ionospheric storm effect near the F2 peak and below it, but a positive storm effect in the topside ionosphere. Thus, this SED event was a SED in TEC but not in NmF2. The very low ionospheric densities below the F2 peak resulted in a much reduced downward heat conduction for the electrons, trapping the heat in the topside in the presence of heat source above. This, in turn, increased the topside scale height so that even though electron densities at the F2 peak were depleted, TEC increased in the SED. The depletion in NmF2 was probably caused by an increase in the density of the molecular neutrals, resulting in enhanced recombination. In addition, the storm time topside ionospheric electron density profiles were much closer to diffusive equilibrium than the nonstorm time profiles, indicating less daytime plasma flow between the ionosphere and the plasmasphere.
Key Points
A SED in TEC but not in NmF2
Storm time topside ionospheric electron density profiles were in diffusive equilibrium
Increase in the topside but decrease in bottom side ionosphere
In this work, we examine the impact of increased anthropogenic emissions on equatorial plasma bubble (EPB) occurrence by modeling the growth rate of Rayleigh‐Taylor (R‐T) instability (γRT ${\gamma ...}_{RT}$), which is closely related to EPB generation. Using the global coupled ionosphere‐thermosphere‐electrodynamics model‐institute of geology and geophysics, Chinese Academy of Sciences model, γRT ${\gamma }_{RT}$ is calculated under three different CO2 emission levels. As CO2 increases, γRT ${\gamma }_{RT}$ significantly increases at low altitudes (<∼260 km) and decreases at high altitudes (>∼320 km). In the altitudes in between, γRT ${\gamma }_{RT}$ increases (decreases) before (after) midnight. Longitudinal variability of the γRT ${\gamma }_{RT}$ change is manifested apparently above ∼280 km, while it is insignificant for low altitudes. Term analysis revealed that changes in the gravity term and the electric‐field term are the main causes and that the neutral‐wind term is insignificant. The investigation indicates that increased anthropogenic emissions can change EPB occurrence and, in turn, the radio‐communication system and therefore influence modern technological systems, which is expected to be more serious in the future.
Plain Language Summary
Equatorial plasma bubbles (EPBs) are ionospheric plasma irregularities that negatively influence radio propagation and even disrupt communication and navigation systems. In this work, we attempt to elucidate whether increased anthropogenic emissions impact EPB occurrence. To quantitatively characterize the EPB occurrence, the Rayleigh‐Taylor (R‐T) instability growth rate (γRT ${\gamma }_{RT}$) defined by Sultan (1996), https://doi.org/10.1029/96ja00682 is used. based on a thermosphere‐ionosphere coupled model, global coupled ionosphere‐thermosphere‐electrodynamics model‐institute of geology and geophysics, chinese academy of sciences, γRT ${\gamma }_{RT}$ is calculated for three different CO2 emission levels. The results show that as CO2 increases, γRT ${\gamma }_{RT}$ significantly increases at low altitudes (<∼260 km) and decreases at high altitudes (>∼320 km). In the altitudes in between, γRT ${\gamma }_{RT}$ increases before midnight but decreases after midnight. Longitudinal variability of the γRT ${\gamma }_{RT}$ change is manifested apparently above ∼280 km but is insignificant at low altitudes. As a first step, this work revealed a possible link between long‐term anthropogenic climate change and short‐term space weather events that impact modern communication.
Key Points
Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model‐Institute of Geology and Geophysics, Chinese Academy of Sciences simulation revealed that the equatorial plasma bubble (EPB) occurrence would increase below 260 km and decrease above 320 km as CO2 emissions increase
The EPB occurrence change shows local‐time and longitudinal dependence that maximum generally occurred at American sector before midnight
CO2 impacts on the electron density, conductivity, and electric fields cause the EPB occurrence change, while dynamic impacts are limited
TIMED/Global Ultraviolet Imager (GUVI) limb measurements of far‐ultraviolet airglow emission have been used to investigate middle‐low latitude thermospheric composition and neutral temperature ...responses to the 20 and 21 November 2003 (day of year DOY 324 and 325) superstorm. Altitude profiles of O, N2 number densities and temperature, as well as O/N2 column density ratio (∑O/N2), on the storm days along the GUVI limb tracks are compared with those on DOY 323 (quiet time). The storm‐time composition and temperature responses were global and evolved continuously as the storm progressed. Specially, N2 and temperature increased almost globally at all altitudes during the storm and their perturbation structures were similar. The magnitudes of their enhancements both increased with altitude and latitude. The storm‐induced O perturbations decreased in the lower thermosphere but increased in the upper thermosphere. Transition heights of O perturbations from decrease to increase changed with latitude and time. During the storm main and recovery phases, the storm‐induced ∑O/N2 decreases were mostly related to the O depletion in the low‐middle thermosphere, whereas ∑O/N2 increases during the storm were primarily caused by N2 depletion. There was a remarkable hemispheric asymmetry in composition responses as they have different morphologies and lifetime, especially during the storm recovery phase.
Key Points
N2 density and neutral temperature increase almost globally at all altitudes during the storm with similar latitude structures
O density decreases in the lower thermosphere but increases in the upper thermosphere in the main and recovery phases of the storm
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