Annual Asymmetry Research Articles

Dependence of annual asymmetry in <i>NmF2</i> on local time

Based on the global empirical model of the F2 layer critical frequency median (Satellite and Digisonde Data Model of the F2 layer, SDMF2), an analysis was made of the properties of diurnal variations in the annual asymmetry in the concentration of the F2 layer maximum NmF2 at different values of the solar activity index F. The AI index, which characterizes the relative difference in NmF2 averaged over all longitudes and latitudes between January and July at a given local time, was used as a parameter of this asymmetry. It was found that the diurnal variations of the AI index are dominated by a semidiurnal mode with maxima in the daytime and at night. The daytime maximum of the AI index is almost independent of the level of solar activity. The nighttime AI maximum decreases with increasing solar activity. For low solar activity, the daytime and nighttime AI maxima almost coincide in amplitude when AI = 16—17%. The difference in the solar radio flux between January and July due to the ellipticity of the Earth’s orbit relative to the Sun makes a significant contribution to the AI index at all hours of the day. On average, it is 3—4% and can reach 5% with low solar activity at night. The difference in the AI index for low and high activity according to the IRI model (with URSI and, especially, CCIR coefficients) is overestimated relative to the SDMF2 model at almost all hours of the day, apparently due to the limited number of experimental data when obtaining the CCIR and URSI coefficients especially over the oceans

Statistical Characteristics of Total Electron Content Intensifications on Global Ionospheric Maps

AbstractGlobal ionospheric total electron content (TEC) maps exhibit TEC intensifications and depletions of various sizes and shapes. Characterizing key features on TEC maps and understanding their dynamic coupling with external drivers can significantly benefit space weather forecasting. However, comprehensive analysis of ionospheric structuring over decades of TEC maps is currently lacking due to large data volume. We develop feature extraction software based on image processing techniques to extract TEC intensification regions, that is, contiguous regions with sufficiently elevated TEC values than surrounding areas, from global TEC maps. Applying the software to the Jet Propulsion Laboratory Global Ionospheric Map data, we generate a TEC intensification data set for years 2003–2022 and carry out a statistical study on the number and strength of TEC intensifications. We find that the majority of the TEC maps (about 86%) are characterized with one or two intensification(s), while the rest of the TEC maps have three or more intensifications. Both the number and strength of TEC intensifications exhibit semi‐annual variation that peaks near equinoxes and dips near solstices, as well as an annual asymmetry with larger values around December solstice compared to June solstice. The number and strength of intensifications increase with enhanced solar extreme‐violet irradiance. The strength of intensifications also increases with elevated geomagnetic activity, but the number of intensifications does not. In addition, the number of intensifications is not correlated with the strength of intensifications.

Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Based on the medians of the F2-layer maximum electron concentration NmF2 for two ionosphericstations, Boulder and Hobart, in 1963–2013, an analysis of the dependence of the annual asymmetry localindex R at noon on solar activity has been performed, where the R index is the January/July ratio of the summaryNmF2 concentration for this pair of stations. Solar activity indices averaged over 81 days are used: Fobs isthe solar radio emission flux at a wavelength of 10.7 cm measured by ground-based radio telescopes and Fadjis the value of Fobs reduced to a fixed distance from the Sun of one astronomical unit. It is found that theregression equations that manifest the NmF2 medians dependence on Fobs make it possible to obtain theannual asymmetry index R for a fixed value of Fobs with allowance for a substitution of Fobs by cFobs in theseregression equations, the c coefficient is equal to 1.03 and 0.97 for January and July, respectively. The c = 1case corresponds to neglecting the annual asymmetry in the Fobs index due to the ellipticity of the Earth’sorbit. For the c = 1 version, the R index increases with a solar activity increase from 1.2 under low activity upto almost 1.4 under high activity. An additional allowance for the annual asymmetry in Fobs leads to anincrease in the R index by approximately 0.1 almost independently of the solar activity level. Apparently, thisconclusion is being drawn for the first time. The Fadj index also makes it possible to obtain a correct estimateof the R index, because the annual asymmetry in the solar radiation flux is indirectly taken into account viathe experimental values of NmF2.

Dependence of NmF2 Local Annual Asymmetry Index on Local Time and Solar Activity

Dependence of NmF2 Local Annual Asymmetry Index on Local Time and Solar Activity

Global Gridded Ionospheric Electron Density Derivation During 2006–2016 by Assimilating COSMIC TEC and Its Validation

AbstractThe long‐term accurate specification of the Earth's ionospheric states is crucial to scientific research and applications in the space weather community. In the current work, a global monthly mean three‐dimensional (3‐D) ionospheric electron density product has been obtained during one solar cycle from 2006 to 2016. Specifically, an accurate 3‐D product is reconstructed monthly by assimilating the Constellation Observing System for Meteorology, Ionosphere and Climate slant total electron content (TEC) into an empirical background model via the Kalman filter data assimilation algorithm. The outputs of the results have spatial resolutions of 2° in latitude, 5° in longitude, and 20 km in height, and temporal resolution of 1 hr in universal time. The accuracy and reliability of the results are systematically validated by the critical frequency at the F2 layer from global ionosonde stations, the in situ electron density from the CHAllenging Minisatellite Payload Planar Langmuir Probe, the TEC from the Massachusetts Institute of Technology, and Gravity Recovery and Climate Experiment. We found that the products agree well with the independent observations. Some well‐known ionospheric climatological patterns, including the solar and seasonal variation, annual asymmetry, the Weddell Sea Anomaly, and the longitudinal wave structure, can be well illustrated. The advantages of the products are its 3‐D and gridded electron density and continuous one solar cycle time series (2006–2016). It is useful to study the spatial‐temporal variations in ionospheric states from seasons to decades, which can also be used as the background parameters for atmospheric and ionospheric‐related scientific research and applications.

High‐Resolution and Accurate Low‐Latitude Gridded Electron Density Generation and Evaluation

AbstractThe accurate specification of three‐dimensional (3‐D) ionosphere states is crucial to positioning, navigation, and communication systems, especially in low‐latitude and equatorial regions. In this work, the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC‐2) radio occultation total electron content was assimilated around a whole year from October 2020 to September 2021 via the Kalman filter (KF) data assimilation system. This system includes an empirical ionosphere background model and the KF data assimilation algorithm. Accurate 3‐D ionosphere data assimilation results were produced hourly with spatial resolutions of 2° latitude, 5° longitude, and 20 km height in low‐latitude and equatorial regions. First, the results were validated by Massachusetts Institute of Technology vertical total electron content and global ionosonde data. The ionosphere vertical total electron content and the critical frequency at the F2 layer agree well with the independent observations. Then, a general ionosphere climatological pattern is found in the reconstructed 3D ionosphere results, including seasonal variation, annual asymmetry, and December anomaly. In addition, the results can well capture the daily spatial and temporal ionospheric variations in the day‐to‐day variability, the longitudinal wave 4 structures, and the interhemispheric asymmetry transition of the equatorial ionization anomaly crests. The current results demonstrate that assimilating COSMIC‐2 TEC observations can further improve the 3D ionosphere specification in low‐latitude and equatorial regions and can also contribute to physical research on ionosphere variability characteristics.

The Signature of Geomagnetic Field External Drivers in Virtual Observatory 30‐day Means Derived From Swarm Data

AbstractThe separation of contributions from different sources in the magnetic field signal measured at satellite altitude is an open challenge. An approach to this problem, using Principal Component Analysis, is here applied to geomagnetic external field series at Virtual Observatories (VOs). These series are computed from an enlarged data set of Swarm data covering all local times and all geomagnetic activity levels between January 2014 and December 2019. For each 30‐day time window, the Equivalent Source Dipole technique is used to reduce all measurements inside a cylinder to one single “observation” at its axis and 500 km altitude. Our results reveal a first principal mode with dipolar geometry and time variation following very closely the RC‐index of geomagnetic activity. They display a resolved second principal mode with annual periodicity and of approximately zonal quadrupolar radial pattern, reminiscent of results in a previous study using VO series from a filtered satellite data set and with lower time resolution. We resort to the recent comprehensive model CM6 to identify a possible source for this second mode. We propose that the dipolar mode is the expression of the magnetospheric ring current dynamics, at 30‐day time resolution, and the quadrupolar mode is the expression of the annual asymmetry between local summer and winter Sq current vortices. Two fainter modes could be related to the equinoctial amplification of Sq vortices and the ionospheric dynamo modulation by nonmigrating tides. We show that a more uniform local time sampling could contribute to better resolve ionospheric structures.

Dependence of the Annual Asymmetry in NmF2 on Geomagnetic Latitude and Solar Activity

Dependence of the Annual Asymmetry in NmF2 on Geomagnetic Latitude and Solar Activity

Dependence of the Local Index of Annual Asymmetry for NmF2 on Solar Activity

Dependence of the Local Index of Annual Asymmetry for NmF2 on Solar Activity

FoF2 Seasonal Asymmetry Diurnal Variation Study during Very Quiet Geomagnetic Activity at Dakar Station

This paper aims to study the foF2 seasonal asymmetry diurnal variation at Dakar station from 1976 to 1995. We show that equinoctial asymmetry is less pronounced and somewhere is absent throughout 21 and 22 solar cycles. The absence of equinoctial asymmetry may be due to Russell-McPherron mechanism and the vertical drift E × B . The solstice anomaly or annual anomaly is always observed throughout both 21 and 22 solar cycles as measured at Dakar ionosonde. The maximum negative value of σfoF2, fairly equal to -65%, is observed during the decreasing phase at solstice time; this value appeared usually at 0200 LT except during the maximum phase during which it is observed at 2300 LT. The maximum positive value, fairly equal to +94%, is observed at 0600 LT during solar minimum at solstice time. This annual asymmetry may be due to neutral composition asymmetric variation and solar radiation annual asymmetry with the solstice time. The semiannual asymmetry is also observed during all solar cycle phases. The maximum positive value (+73%) is observed at 2300 LT during solar maximum, and its maximum negative (-12%) is observed during the increasing phase. We established, as the case of annual asymmetry, that this asymmetry could not be explained by the asymmetry in vertical velocity E × B phenomenon but by the axial mechanism, the “thermospheric spoon” mechanism, and the seasonally varying eddy mixing phenomenon.

On the Annual Asymmetry of High‐Latitude Sporadic F

AbstractThe polar F region ionosphere frequently exhibits sporadic variability (e.g., Meek, 1949, https://doi.org/10.1029/JZ054i004p00339; Hill, 1963, https://doi.org/10.1175/1520‐0469(1963)020<0492:SEOLII>2.0.CO;2). Recent satellite data analysis (Noja et al., 2013, https://doi.org/10.1002/rds.20033; Chartier et al., 2018, https://doi.org/10.1002/2017JA024811) showed that the high‐latitude F region ionosphere exhibits sporadic enhancements more frequently in January than in July in both the northern and southern hemispheres. The same pattern has been seen in statistics of the degradation and total loss of GPS service onboard low‐Earth orbit satellites (Xiong et al. 2018, https://doi.org/10.5194/angeo‐36‐679‐2018). Here, we confirm the existence of this annual pattern using ground GPS‐based images of TEC from the MIDAS algorithm. Images covering January and July 2014 confirm that the high‐latitude (>70 MLAT) F region exhibits a substantially larger range of values in January than in July in both the northern and southern hemispheres. The range of TEC values observed in the polar caps is 38–57 TECU (north‐south) in January versus 25–37 TECU in July. First‐principle modeling using SAMI3 reproduces this pattern, and indicates that it is caused by an asymmetry in plasma levels (30% higher in January than in July across both polar caps), as well as 17% longer O+ plasma lifetimes in northern hemisphere winter, compared to southern hemisphere winter.

On the Sources of the Ionospheric Variability in the South American Magnetic Anomaly During Solar Minimum

AbstractWe investigate for the first time the variability of the F2 layer critical frequency (foF2), its peak height (hmF2), the thickness parameter B0, and the E region critical frequency (foE) over Santa Maria (29.7° S, 53.7° W, dip angle = −37°), a station located in the central region of the South American Magnetic Anomaly. The selected ionospheric parameters were obtained from ionograms recorded by a recent Digisonde Portable Sounder 4‐D. The time period covers 309 days from 1 September 2017 to 30 August 2018. The diurnal analyses revealed a large day‐to‐day ionospheric variability, with some peculiarities as a strong semiannual pattern superimposed to expected ionospheric behavior. Furthermore, the results show significant differences between the averaged foF2 in December and June solstices, revealing a possible presence of the annual asymmetry. The coefficient of variation (CV) is used as a quantitative description of the variability of each parameter versus time and season. Considering low solar flux and geomagnetically quiet days only, we note that CV is smaller during the daytime and larger during nighttime for all parameters. The least variable ionospheric parameter in our study is foE, while the most variable one is B0. Regarding the F2 layer parameters, we observe that foF2 is much more variable than hmF2. We attribute the observed CV to the neutral atmosphere source over Santa Maria. The ionospheric variability is in general enhanced during geomagnetically disturbed periods. The estimated CV is higher over Santa Maria than Wuhan, China (30.5° N, 114.4° E, dip angle = 46°), a station with no influence of the South American Magnetic Anomaly.

Annual Occurrence Rates of Ionospheric Polar Cap Patches Observed Using Swarm

AbstractDense, fast‐moving regions of ionization called polar cap patches are known to occur in the high‐latitude F region ionosphere. Patches are widely believed to be caused by convection of dense, sunlit plasma into a dark and therefore low‐density polar cap ionosphere. This leads to the belief that patches are a winter phenomenon. Surprisingly, a long‐term analysis of 3 years of ionospheric measurements from the Swarm satellites shows that large density enhancements occur far more frequently in local summer than local winter in the Southern Hemisphere (SH). The reverse is true in the Northern Hemisphere (NH). Previously reported patch detections in the SH are reexamined. Detection algorithms using only a relative doubling test count very small density fluctuations in SH winter due to extremely low ambient densities found there, while much larger enhancements occurring in SH summer are missed due to especially high ambient densities. The same problem does not afflict results in the NH, where ambient densities are more stable year‐round due to the ionospheric annual asymmetry. Given this new analysis, the definition of a patch as a doubling of the ambient density is not suitable for the SH. We propose a test for patches linked to long‐term averaged solar flux activity, characterized by the 81 day centered mean F10.7 index. Importantly, the current patch formation theory is at least incomplete in that it does not predict the observed lack of patches in SH winter, or the many large enhancements seen in SH summer.

FoF2 Seasonal Asymmetry Time Variation at Korhogo Station from 1992 to 2002

foF2 seasonal asymmetry is investigated at Korhogo station from 1992 to 2002. We show that equinoctial asymmetry is less pronounced and somwhere is absent trough out solar cycle phase. In general, the absence of equinoctial asymmetry may be due to the fact that in equinox and for each solar cycle phase, the asymmetry is due to Russell-McPherron mechanism. The solstice anomaly or annual anomaly is always observed throughout solar cycle phase. The minimum value of ΔfoF2 is inferior than −60% seen during all solar cycle phase at 0700 LT. This annual asymmetry may be due to interplanetary corpuscular radiation.

Simulations of the ionospheric annual asymmetry: Sun‐Earth distance effect

AbstractIt has been suggested that the difference of the Sun‐Earth distance between the December and June solstices has a great impact on the ionospheric annual asymmetry. In this study, the physical mechanisms of the Sun‐Earth distance effects on the ionospheric annual asymmetry are investigated using Thermosphere‐Ionosphere Electrodynamics General Circulation Model simulations. The main findings are the following: (1) The Sun‐Earth distance affects the ionospheric annual asymmetry mainly through photochemical processes. (2) During the daytime, this photochemical process results from the combined effect of ionization rate of atomic oxygen and the recombination with neutral species. The solar irradiation variation between December and June directly leads to about 6% December‐June electron density difference via ionization of atomic oxygen, whereas the recombination with neutral composition contributes to 12%–15% December‐June electron density difference. (3) In the plasma fountain‐prominent region (between 20° and 40° magnetic latitude), ambipolar diffusion can also be modulated by the Sun‐Earth distance effect and contribute to the ionospheric annual asymmetry. (4) During the nighttime, the Sun‐Earth distance effect impacts the annual asymmetry by changing thermospheric composition and ionospheric diffusion.

Ionospheric winter anomaly and annual anomaly observed from Formosat-3/COSMIC Radio Occultation observations during the ascending phase of solar cycle 24

Ionospheric winter and annual anomalies have been investigated during the ascending phase of solar cycle 24 using high-resolution global 3D – data of the FORMOSAT – 3/COSMIC (Formosa satellite – 3/Constellation Observing System for Meterology, Ionosphere and Climate) radio occultation observations. Our detailed analysis shows that the occurrence of winter anomaly at low-latitudes is confined only to the early morning to afternoon hours, whereas, the winter anomaly at mid-latitudes is almost absent at all local times during the ascending phase of solar cycle 24. Further, in the topside ionosphere (altitudes of 400km and above), the winter anomaly is completely absent at all local times. In contrast, the ionospheric annual anomaly is consistently observed at all local times and altitudes during this ascending phase of solar cycle 24. The annual anomaly exhibits strong enhancements over southern EIA crest latitudes during day time and around Weddle Sea Anomaly (WSA) region during night times. The global mean annual asymmetry index is also computed to understand the altitudinal variation. The global mean AI maximizes around 300–500km altitudes during the low solar active periods (2008–10), whereas it extends up to 600km during moderate to high (2011) solar activity period. These findings from our study provide new insights to the current understanding of the annual anomaly.

Climatology of plasmaspheric total electron content obtained from Jason 1 satellite

AbstractWe used more than 40 million total electron content (TEC) measurements obtained from the GPS TurboRogue Space Receiver receiver on board the Jason 1 satellite in order to investigate the global morphology of the plasmaspheric TEC (pTEC) including the variations with local time, latitude, longitude, season, solar cycle, and geomagnetic activity. The pTEC corresponds to the total electron content between Jason 1 (1336 km) and GPS (20,200 km) satellite altitudes. The pTEC data were collected during the 7 year period from January 2002 to December 2008. It was found that pTEC increases by about 10–30% from low to high solar flux conditions with the largest variations occurring at low latitudes for equinox. During low solar flux condition, pTEC is largely independent of geomagnetic activity. However, it slightly decreases with increasing geomagnetic activity at low latitudes during high solar flux. The seasonal variations such as the annual and semiannual anomalies in the ionosphere also exist in the low‐latitude plasmasphere. In particular, the American sector (around 300°E) shows strong annual asymmetry in the plasmaspheric density, being larger in December than in June solstice.

Can atomic oxygen production explain the ionospheric annual asymmetry?

AbstractThe causes of the ionospheric annual asymmetry, which refers to a larger averaged electron density at geomagnetic conjugate latitudes in December than in June, remain an unresolved problem that still generates considerable interest. The ionospheric annual asymmetry in the peak electron density of the F2 layer (NmF2) is typically 20–40%, which cannot be explained by the 7% annual asymmetry in photoionization caused by the shorter Sun‐Earth distance in December. Mikhailov and Perrone (2011, 2015) suggested that the annual asymmetry in atomic oxygen production due to O2 dissociation is sufficient to explain the ionospheric annual asymmetry at middle latitudes. In our study, a series of the Global Mean Model (GMM) simulations have been conducted to test this hypothesis. Although O2 dissociation and eddy diffusion processes are included in the GMM, the simulated annual asymmetry of NmF2 is only 13%. Furthermore, the annual asymmetry increase in neutral composition in our GMM simulations can only explain about one fifth of the ionospheric annual asymmetry. Therefore, the atomic oxygen production mechanism is unlikely to be a major contributor to the ionospheric annual asymmetry.

Effects of the equatorial ionosphere anomaly on the interhemispheric circulation in the thermosphere

AbstractWe investigate the interhemispheric circulation at the solstices, in order to understand why O/N2 is larger in the northern hemisphere winter than in the southern hemisphere winter. Our studies reveal that the equatorial ionosphere anomaly (EIA) significantly impacts the summer‐to‐winter wind through plasma‐neutral collisional heating, which changes the summer‐to‐winter pressure gradient, and ion drag. Consequently, the wind is suppressed in the summer hemisphere as it encounters the EIA but accelerates after it passes the EIA in the winter hemisphere. The wind then converges due to an opposing pressure gradient driven by Joule heating in auroral regions and produces large O/N2 at subauroral latitudes. This EIA effect is stronger near the December solstice than near the June solstice because the ionospheric annual asymmetry creates greater meridional wind convergence near the December solstice, which in turn produces larger O/N2 in the northern hemisphere winter than in the southern hemisphere winter.