Equatorial Electron Density Research Articles

Quantifying Uncertainties in the Quiet‐Time Ionosphere‐Thermosphere Using WAM‐IPE

AbstractThis study presents a data‐driven approach to quantify uncertainties in the ionosphere‐thermosphere (IT) system due to varying solar wind parameters (drivers) during quiet conditions (Kp < 4) and fixed solar radiation and lower atmospheric conditions representative of 16 March 2013. Ensemble simulations of the coupled Whole Atmosphere Model with Ionosphere Plasmasphere Electrodynamics (WAM‐IPE) driven by synthetic solar wind drivers generated through a multi‐channel variational autoencoder (MCVAE) model are obtained. Applying the polynomial chaos expansion (PCE) technique, it is possible to estimate the means and variances of the QoIs as well as the sensitivities of the QoIs with regard to the drivers. Our results highlight unique features of the IT system's uncertainty: (a) the uncertainty of the IT system is larger during nighttime; (b) the spatial distributions of the uncertainty for electron density and zonal drift at fixed local times present 4 peaks in the evening sector, which are associated with the low‐density regions of longitude structure of electron density; (c) the uncertainty of the equatorial electron density is highly correlated with the uncertainty of the zonal drift, especially in the evening sector, while it is weakly correlated with the vertical drift. A variance‐based global sensitivity analysis suggests that the IMF Bz plays a dominant role in the uncertainty of electron density. A further discussion shows that the uncertainty of the IT system is determined by the magnitudes and universal time variations of solar wind drivers. Its temporal and spatial distribution can be modulated by the average state of the IT system.

Modeling the Development of Plasmasphere Ducts and Irregularities With SAMI3/WACCM‐X

AbstractWe show that atmospheric gravity waves can generate plasma ducts and irregularities in the plasmasphere using the coupled SAMI3/WACCM‐X model. We find the equatorial electron density is irregular as a function of longitude which is consistent with CRRES measurements (Clilverd et al., 2007, https://doi.org/10.1029/2007ja012416). We also find that plasma ducts can be generated for L‐shells in the range 1.5–3.0 with lifetimes of ∼ 0.5 hr; this is in line with observations of ducted VLF wave propagation with lifetimes of 0.5–2.0 hr (Clilverd et al., 2008, https://doi.org/10.1029/2007ja012602; Singh et al., 1998, https://doi.org/10.1016/s1364-6826(98)00001-7).

Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption

In this work, the temporal variations in the ionospheric F region current (FRC) during the 2022 volcanic eruption in Tonga are reported for the first time. The FRC increased and underwent several polarity reversals following the eruption, as observed by the dual Swarm satellites. By combining neutral wind observations derived from the Ionospheric Connection Explorer (ICON) satellite, we attempt to reveal the potential physical mechanisms responsible for these variations. The ICON and Swarm satellites were located at almost the same distance from the epicenter of the volcanic wave, thus providing a good opportunity for analyzing coordinated observations. The F-layer neutral wind data reflect eastward and westward fluctuations that are basically consistent with the polarity reversals identified in the FRC. The E-layer neutral wind also exhibits polarity reversals that affect the equatorial ionosphere electron density anomaly. These results show that the atmospheric fluctuations caused by the analyzed volcanic eruption affected the wind pattern distribution in the E-F region, thus further affecting the spatial distribution of the FRC and electron density in the F layer.

LnIII (Ln = La, Gd, and Dy) Benzimidazolium Tricarboxylate Coordination Polymers with Hydrogen Bonding Modulated Magnetic Relaxation

The novel LnIII {Ln = Dy (1-Dy), Gd (2-Gd), and La (3-La)} benzimidazolium tricarboxylate coordination polymers have been synthesized, and their magnetic properties have been investigated. Single-crystal X-ray analysis revealed that 1-Dy, 2-Gd, and 3-La are one-dimensional coordination polymers with general molecular formulas of {[Ln(L)2(H2O)4]·(6Br)}∞ [L = 3,3′,3′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(1-(carboxymethyl)-benzimidazolium)], which crystallized in the monoclinic, P21/c space group. The solid-state packing of these coordination polymers shows the spiral propagation in a one-dimensional direction. The direct current (dc) magnetic data (susceptibility and magnetization) were collected for 1-Dy and 2-Gd. The alternating current (ac) magnetic measurement for 1-Dy at zero field shows the characteristic signature of single-molecule magnets (SMMs) at low temperatures but without clear maxima. Further, to rationalize the experimentally observed magnetic behavior and to understand the factors affecting the dynamic magnetic behavior of 1-Dy, we performed detailed completed active space self-consistent field (CASSCF) based calculations on 1-Dy. Our detailed theoretical analysis suggests that the hydrogen bonding interaction between the coordinated water molecule and the Br– counteranion increases the equatorial electron density, eradicating the slow relaxation in 1-Dy.

Planetary Period Oscillations of Saturn’s Dayside Equatorial Ionospheric Electron Density Observed on Cassini’s Proximal Passes

AbstractWe examine electron density data obtained in Saturn’s dayside equatorial ionosphere during Cassini’s proximal periapsis passes, specifically data derived from wave spectra, for evidence of modulation with the planetary period oscillations (PPOs) observed ubiquitously throughout Saturn’s magnetosphere. Clear evidence is found on northern hemisphere inbound passes where northern system modulations occur in the ionospheric diffusive layer peaking at a factor of ∼7 at ∼3,200 km altitude, extending at lesser factors to ∼7,000 km altitude in the transport region. Simultaneous southern system modulations by factors ∼1.5–2.5 are also found above ∼3,400 km. Factor of ∼4–10 northern system modulations are also detected in the diffusive layer and transport regions to ∼6,000 km in the outbound pass data, though southern system modulations were not clearly discerned within the data scatter. Modulations were not detected in the photochemical equilibrium layer at altitudes ∼1,600–2,200 km. Where clear modulations were observed, their phases in the diffusive layer were close to those of the principal PPO meridians (determined previously from near‐apoapsis nightside magnetic data) for both systems, moving to somewhat earlier PPO phases by a few tens of degrees in the transport region. Comparison with magnetic oscillation phases determined directly from proximal pass data shows that electron density maxima occur ∼30°–50° earlier than azimuthal field maxima for both systems, ∼140°–160° earlier than colatitudinal field maxima for the northern system, nearer to antiphase than to leading quadrature, and ∼50°–70° later than colatitudinal field maxima for the southern system, somewhat less than lagging quadrature.

Coordinated Ground‐Based and Space‐Borne Observations of Ionospheric Response to the Annular Solar Eclipse on 26 December 2019

AbstractThis paper studies the ionosphere's response to the annular solar eclipse on 26 December 2019, utilizing the following ground‐based and space‐borne measurements: Global Navigation Satellite System (GNSS) total electron content (TEC) data, spectral radiance data from the Sentinel‐5P satellite, in situ electron density and/or temperature measurements from DMSP and Swarm satellites, and local magnetometer data. Analysis concentrated on ionospheric effects over low‐latitude regions with respect to obscuration, local time, latitude, and altitude. The main results are as follows: (1) a local TEC reduction of ∼4–6 TECU (30–50%) was identified along the annular eclipse path, with larger depletion and longer recovery periods in the morning eclipse compared to midday. (2) The equatorial electrojet current was significantly weakened when the eclipse trajectory crossed the magnetic equator in the morning (India) sector, which contributed to large and prolonged TEC depletion therein. (3) At midday, equatorial ionization anomaly exhibited enhancements of 20–40% as well as poleward shifting of 3–4°, likely triggered by modified neutral wind and electrodynamics patterns. (4) The behavior of equatorial ionospheric electron density showed considerable altitudinal differences in the topside, exhibiting ∼30% reduction around 500 km and ∼30% enhancement with 300–500 K Te reduction around 850 km, before the arrival of maximum eclipse. This may have been caused by the enhanced eastward electric field and equatorward neutral wind, and other possible factors are also discussed.

Excitation Mechanism of Ionospheric 6‐Day Oscillation During the 2019 September Sudden Stratospheric Warming Event

AbstractA sudden stratospheric warming (SSW) event in the Antarctic region occurred in September 2019. During the SSW event, the quasi 6‐day wave (Q6DW) was enhanced in the mesosphere, and strong 6‐day oscillations are observed in ionospheric parameters, such as the equatorial electrojet (EEJ) and electron density. The 6‐day variation in the EEJ has a westward moving structure with the zonal wave number 1, indicating the influence of the Q6DW. In this study, we investigate the excitation mechanism of the 6‐day variations in the EEJ and electron density using numerical simulations. The main results are as follows. The 6‐day variations in the ionosphere are not generated by the Q6DW but generated by the waves with periods from 10 to 14 hr. The amplitude of the 10–14 hr waves is modulated with a period of 6 days, due to the nonlinear interaction between the Q6DW and migrating semidiurnal tide. This leads to the 6‐day variations in the EEJ and electron density through the E region dynamo process. At a fixed local time, the secondary waves generated by the Q6DW‐tidal interaction produce westward moving ionospheric 6‐day variations with zonal wave number 1, which cannot be distinguished from the ionospheric variations by the Q6DW itself. The interference of secondary waves leads to a longitudinal asymmetry in the magnitude of the ionospheric 6‐day oscillation.

The Role of the Phase of QBO in Modulating the Influence of the SSW Effect on the Equatorial Ionosphere

AbstractThis study investigates the role of stratospheric quasi‐biennial oscillation (QBO) in modulating the response of equatorial/low‐latitude ionosphere over the Indian sector to the major sudden stratospheric warmings (SSWs) occurred during the period 2003 to 2013. The analysis based on the equatorial electrojet‐induced surface magnetic field, total electron content, and mesospheric wind data reveals that the equatorial ionosphere responds in distinctly different ways to the SSWs occurred during different QBO phases. The peaking time of the equatorial electrojet and occurrence time of counter electrojet displays a shift toward morning/evening sector during the westward/eastward phase of the QBO as inferred from the zonal mean zonal wind at 10‐hPa level (~30 km). The analysis reveals that the enhancement and depletions seen in total electron content over both the equatorial and low‐latitude ionosphere display a shift toward morning/evening during the westward/eastward phase of the QBO. The upper mesospheric tides (diurnal and semidiurnal) estimated using the meteor radar measured winds over the dip equator also exhibit similar shift in their phases. The occurrence of periodic counter electrojets during the SSW period is found to be coinciding with enhancement in polar stratospheric temperature and in close association with the increased amplitude of the semidiurnal tide. These observations clearly vindicate that the phase of the QBO plays a crucial role in structuring the equatorial electrodynamics and electron density distribution over the low‐latitudes during the SSW events.

Merging of Storm Time Midlatitude Traveling Ionospheric Disturbances and Equatorial Plasma Bubbles

AbstractPostsunset midlatitude traveling ionospheric disturbances (TIDs) and equatorial plasma bubbles (EPBs) were simultaneously observed over American sector during the geomagnetic storm on 8 September 2017. The characteristics of TIDs are analyzed by using a combination of the Millstone Hill incoherent scatter radar data and 2‐D detrended total electron content (TEC) from ground‐based Global Navigation Satellite System receivers. The main results associated with EPBs are as follows: (1) stream‐like structures of TEC depletion occurred simultaneously at geomagnetically conjugate points, (2) poleward extension of the TEC irregularities/depletions along the magnetic field lines, (3) severe equatorial and midlatitude electron density (Ne) bite outs observed by Defense Meteorological Satellite Program and Swarm satellites, and (4) enhancements of ionosphere F layer virtual height and vertical drifts observed by equatorial ionosondes near the EPBs initiation region. The stream‐like TEC depletions reached 46° magnetic latitudes that map to an apex altitude of 6,800 km over the magnetic equator using International Geomagnetic Reference Field. The formation of this extended density depletion structure is suggested to be due to the merging between the altitudinal/latitudinal extension of EPBs driven by strong prompt penetration electric field and midlatitude TIDs. Moreover, the poleward portion of the depletion/irregularity drifted westward and reached the equatorward boundary of the ionospheric main trough. This westward drift occurred at the same time as the sudden expansion of the convection pattern and could be attributed to the strong returning westward flow near the subauroral polarization stream region. Other possible mechanisms for the westward tilt are also discussed.

Simulations of Van Allen Probes Plasmaspheric Electron Density Observations

AbstractWe simulate equatorial plasmaspheric electron densities using a physics‐based model (Cold PLasma, CPL; used in the ring current‐atmosphere interactions model) of the source and loss processes of refilling and erosion driven by empirical inputs. The performance of CPL is evaluated against in situ measurements by the Van Allen Probes (Radiation Belt Storm Probes) for two events: the 31 May to 5 June and 15 to 20 January 2013 geomagnetic storms observed in the premidnight and postmidnight magnetic local time (MLT) sectors, respectively. Overall, CPL reproduces the radial extent of the plasmasphere to within a mean absolute difference of L. The model electric field responsible for E × B convection and the parameterization of geomagnetic conditions (under the Kp‐index and solar wind properties) implemented by CPL did not account for localized enhancements in the duskward electric field during increased activity. Rather, it was found to be largely dependent on the measure of the quiet time background. This property indicates that the agreement between these simulations and observations does not account for the complete set of physical processes during extreme (strong or weak) geomagnetic conditions impacting the plasmasphere. Nevertheless, at the presented resolution of the model CPL does provide good agreement in reproducing Radiation Belt Storm Probes observations of plasmaspheric density and plasmapause location.

Erosion and refilling of the plasmasphere during a geomagnetic storm modeled by a neural network

AbstractWe present a history‐dependent model of the equatorial plasma density of the inner magnetosphere using a feedforward neural network with two hidden layers. As the model inputs, we take locations and time series of SYM‐H, AL, and F10.7 indices. By considering not only the instantaneous values but also the past values of geomagnetic and solar indices, the model is history dependent on levels of geomagnetic and solar activity. The modeled electron density is continuous both spatially and temporally so that the evolution of the density can be studied (such as plasmaspheric refilling). The model is trained using the electron density inferred from the spacecraft potential from three THEMIS probes. The equatorial electron density is shown to be accurately reconstructed with a correlation coefficient of r ~ 0.953 between data and model target. Since the model is history dependent, it succeeds in reconstructing various density features and dynamic behaviors, such as the quiet time plasmasphere, erosion and recovery of the plasmasphere, as well as the plume formation during a storm on 4 February 2011. Our model may provide unprecedented insight into the behavior of the equatorial density at any time and location; as an example we show the inferred refilling rate from our model and compare it to previous estimates.

Spatial distribution of the demodulated low-frequency field in the low-latitude perturbed ionosphere

Features of propagation of low-frequency electromagnetic waves in the low-latitude ionosphere are considered in the case when such waves are generated by means of demodulation of the radiation of a high-power short-wave ground transmitter in the lower ionosphere. Calculations are performed in the ray approximation based on a simplified model (developed specially for the case under study) of the anomaly of the low-latitude equatorial electron density. It is demonstrated that latitude gradients of the equatorial anomaly may result in both additional focusing and defocusing of the beam of demodulated low-frequency waves.

Evidence and effects of the sunrise enhancement of the equatorial vertical plasma drift in the F region ionosphere

AbstractRecent studies based on the satellite observations demonstrated that the equatorial vertical plasma drift can also enhance near sunrise in a way similar to the prereversal enhancement. However, it is not clear whether the signature of this sunrise enhancement appears in observations with other sounding techniques. In this work, we explore the Jicamarca (12°S, 283.2°E) incoherent scatter radar measurements to present the evidence of sunrise enhancement in vertical plasma drift on 12 May and 10 June 2004, which are under magnetically quiet and solar minimum conditions. The effects of the sunrise enhancement on the ionosphere are, for the first time, investigated by analyzing the ionograms recorded by the Digisonde Portable Sounder at Jicamarca and conducting the Theoretical Ionospheric Model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences. The observations showed that, during the sunrise enhancement, the F2 layer peak height is lifted remarkably, and the F2 layer peak density and bottomside electron density tend to decrease compared to the days without sunrise enhancements. The simulations indicated that the sunrise enhancement drift can lift the equatorial ionosphere to higher heights and distort the equatorial electron density profiles. What is more, the simulations display an F3 layer in the equatorial F region during the sunrise enhancement, and a new F2 layer develops at lower altitudes under the jointed control of the usual photochemical and dynamical processes.

Upper atmospheric remote sensing using ELF – VLF lightning generated tweek and whistler sferics

Tweek and whistler atmospherics (sferics) recorded at a low latitude station, Suva (18.2oS, 178.3oE, geomag. lat. 22.2oS, L = 1.17) Fiji, in the South Pacific region, during September 2003–July 2004 and March–December 2006 were analyzed to probe the nighttime D-region ionosphere and plasmasphere. From the first harmonic analysis of the selected 1063 tweeks, the path integrated ionospheric reflection height was estimated mostly between 75–97 km with median and standard deviation of 83.6 and 4.1 km, respectively. The path integrated mean electron density was estimated to vary from 24.4–31.5 cm–3 over the reflection heights. The whistler recorded on 20 September 2006, a magnetically quite day with maximum three hourly Kp value of 2o, most likely propagated in the ducted mode to this station. The whistler spectrum was used to calculate the equatorial magnetic field and the equatorial electron density at L = 1.3 which are in agreement with low latitude whistler observations.

Evidence for a seasonally dependent ring plasma in the region between Saturn's A Ring and Enceladus' orbit

AbstractEquatorial electron density measurements from the Cassini Radio and Plasma Wave Science experiment are derived from the upper hybrid resonance frequency from Saturn Orbit Insertion (SOI) on 1 July 2004 through 21 May 2013. These densities are used to determine the characteristics of the plasma in the inner magnetosphere of Saturn between the outer edge of the A Ring and the orbit of Enceladus. Electron densities obtained when Cassini first arrived at Saturn on 1 July 2004 showed a plasma distribution decreasing radially outward from Saturn in the direction of Enceladus, the expected distribution of a centrifugally driven plasma expanding radially outward from a source in the main rings. We examine equatorial electron densities in the region between 2.4 and 4.0 Rs and show that the density measurements in this region exhibit a strong seasonal dependence resulting from photon‐induced decomposition of icy particles on the ring surfaces, a decomposition process which is controlled by the solar incidence angle. This seasonal dependence will have plasma density implications for Cassini when the spacecraft returns to the region just beyond the A Ring in 2016.

SWARM observations of equatorial electron densities and topside GPS track losses

AbstractThe SWARM satellites have both upward looking GPS receivers and Langmuir probes. The receivers repeatedly lost track of the L1 band signal in January–February 2014 at postsunset hours, when SWARM was at nearly 500 km altitude. This indicates that the signal was disturbed by ionospheric irregularities at this height and above. The track losses occurred right at density gradients associated with equatorial plasma bubbles and predominantly where the measured background density was highest. The signal showed strong phase scintillations rather than in amplitude, indicating that SWARM might be in the near field of an ionospheric phase screen. Density biteouts, depletions between steep gradients, were up to almost 3 orders of magnitude deep in the background of a more shallow trough centered at the magnetic equator. Comparison between satellites shows that the biteout structure strongly varied in longitude over ∼100 km and has in north‐south steep walls.

Theoretical study of the ionospheric plasma cave in the equatorial ionization anomaly region

AbstractThis paper investigates the physical mechanism of an unusual equatorial electron density structure, plasma cave, located underneath the equatorial ionization anomaly by using theoretical simulations. The simulation results provide important new understanding of the dynamics of the equatorial ionosphere. It has been suggested previously that unusual drifts might be responsible for the observed plasma cave structure, but model simulations in this paper suggest that the more likely cause is latitudinal meridional neutral wind variations. The neutral winds are featured by two divergent wind regions at off‐equator latitudes and a convergent wind region around the magnetic equator, resulting in plasma divergences and convergence, respectively, to form the plasma caves structure. The tidal‐decomposition analysis further suggests that the cave related meridional neutral winds and the intensity of plasma cave are highly associated with the migrating terdiurnal tidal component of the neutral winds.

Seismo‐ionospheric coupling appearing as equatorial electron density enhancements observed via DEMETER electron density measurements

AbstractWe report the processes and results of statistical analysis on the ionospheric electron density data measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite over a period of 6 years (2005–2010), in order to investigate the correlation between seismic activity and equatorial plasma density variations. To simplify the analysis, three equatorial regions with frequent earthquakes were selected and then one‐dimensional time series analysis between the daily seismic activity indices and the equatorial ionization anomaly (EIA) intensity indices, which represent relative equatorial electron density increase, were performed for each region. The statistically significant values of the lagged cross‐correlation function, particularly in the region with minimal effects of longitudinal asymmetry, indicate that some of the very large earthquakes with M > 5.0 in the low‐latitude region can accompany observable precursory and concurrent EIA enhancements, even though the seismic activity is not the most significant driver of the equatorial ionospheric evolution. The physical mechanisms of the seismo‐ionospheric coupling is consistent with our observation, and the possibility of earthquake prediction using the EIA intensity variation is discussed.

A study on ionospheric scintillation near the EIA crest in relation to equatorial electrodynamics

AbstractEquatorial electrojet (EEJ) data, which are considered as a proxy index of equatorial electric field, are analyzed in conjunction with equatorial ionosonde, total electron content (TEC) and scintillation data near the equatorial ionization anomaly (EIA) crest for the equinoctial months of high solar activity years (2011–2012) to identify any precursor index of postsunset evolution of equatorial electron density irregularities and subsequent occurrence of scintillation near the northern EIA crest. Only geomagnetically quiet and normal electrojet days are considered. The diurnal profiles of EEJ on the scintillation days exhibit a secondary enhancement in the afternoon to presunset hours following diurnal peaks. A series of electrodynamical processes conducive for generation of irregularities emerge following secondary enhancement of EEJ. Latitudinal profile of TEC exhibits resurgence in EIA structure around the postsunset period. Diurnal TEC profile near the EIA crest resembles postsunset secondary enhancement on the days with afternoon enhancement in EEJ. Occurrence of equatorial spread F and postsunset scintillation near the EIA crest seems to follow the secondary enhancement events in EEJ. Both the magnitude and duration of enhanced EEJ are found to be important for postsunset intensification of EIA structure and subsequent occurrence of equatorial irregularities. A critical value combining the two may be considered an important precursor for postsunset occurrence of scintillation near the EIA crest. The results are validated using archived data for the years 1989–1990 and explained in terms of modulation effects of enhanced equatorial fountain.

Assessment of correct fixing rate for precise point positioning ambiguity resolution on a global scale

Ambiguity resolution (AR) for a single receiver has been a popular topic in Global Positioning System (GPS) recently. Ambiguity-resolution methods for precise point positioning (PPP) have been well documented in recent years, demonstrating that it can improve the accuracy of PPP. However, users are often concerned about the reliability of ambiguity-fixed PPP solution in practical applications. If ambiguities are fixed to wrong integers, large errors would be introduced into position estimates. In this paper, we aim to assess the correct fixing rate (CFR), i.e., number of ambiguities correctly fixing to the total number of ambiguities correctly and incorrectly fixing, for PPP user ambiguity resolution on a global scale. A practical procedure is presented to evaluate the CFR of PPP user ambiguity resolution. GPS data of the first 3 days in each month of 2010 from about 390 IGS stations are used for experiments. Firstly, we use GPS data collected from about 320 IGS stations to estimate global single-differenced (SD) wide-lane and narrow-lane satellite uncalibrated phase delays (UPDs). The quality of UPDs is evaluated. We found that wide-lane UPD estimates have a rather small standard deviation (Std) between 0.003 and 0.004 cycles while most of Std of narrow-lane estimates are from 0.01 to 0.02 cycles. Secondly, many experiments have been conducted to investigate the CFR of integer ambiguity resolution we can achieve under different conditions, including reference station density, observation session length and the ionospheric activity. The results show that the CFR of PPP can exceed 98.0 % with only 1 h of observations for most user stations. No obvious correlation between the CFR and the reference station density is found. Therefore, nearly homogeneous CFR can be achieved in PPP AR for global users. At user end, higher CFR could be achieved with longer observations. The average CFR for 30-min, 1-h, 2-h and 4-h observation is 92.3, 98.2, 99.5 and 99.7 %, respectively. In order to get acceptable CFR, 1 h is a recommended minimum observation time. Furthermore, the CFR of PPP can be affected by diurnal variation and geomagnetic latitude variation in the ionosphere. During one day at the hours when rapid ionospheric variations occur or in low geomagnetic latitude regions where equatorial electron density irregularities are produced relatively frequently, a significant degradation of the CFR is demonstrated.