Annual Asymmetry Research Articles

The annual asymmetry in the F2 layer during deep solar minimum (2008–2009): December anomaly

AbstractAnnual January/July midlatitude daytime asymmetry in monthly median NmF2 and model thermospheric parameters has been considered during deep solar minimum, (2008–2009), when solar and geomagnetic activities were at the lowest level, to analyze the background effect due to the Sun‐Earth minimum distance, perihelion, in the vicinity of the December solstice. Averaged over 10 midlatitude station pairs, the NmF2 asymmetry was found to be ≈1.23, while the average asymmetry for the annual component in NmF2 variations is ≈1.17. Annual asymmetry in monthly median neutral composition and temperature predicted by Mass Spectrometer Incoherent Scatter 86 (MSIS86) and MSISE00 thermospheric models along with the 7% increase in solar EUV flux in the vicinity of the December solstice is sufficient to explain the observed annual asymmetry in NmF2. A hierarchy of aeronomic parameters responsible for the observed asymmetry in NmF2 has been established: the main contributor is atomic oxygen—about 50% of the total effect, [N2] contributes around 35% strongly compensating the [O] contribution, and solar EUV and Tn provide <10% each. The zonal mean annual asymmetry in MSIS86 atomic oxygen column density was shown to be 1.18 at low and middle latitudes, and this is close to the estimated asymmetry for the annual component in NmF2 variations. The earlier proposed mechanism of the December anomaly is considered as a plausible one to explain the 1.18 January/July asymmetry in the atomic oxygen variations and consequently the NmF2 annual daytime asymmetry at middle latitudes under the deep solar minimum.

Characteristics and mechanisms of the annual asymmetry of thermospheric mass density

In this paper, globally-averaged, thermospheric total mass density, derived from the orbits of ∼5000 objects at 250, 400, and 550 km that were tracked from 1967 to 2006, has been used to quantitatively study the annual asymmetry of thermospheric mass density and its mechanism(s). The results show that thermospheric mass density had a significant annual asymmetry, which changed from year to year. The annual asymmetry at the three altitudes varied synchronously and its absolute value increased with altitudes. The results suggest that there is an annual asymmetry in solar EUV radiation that is caused by the difference in the Sun-Earth distance between the two solstices and the random variation of solar activity within a year. This change in radiation results in an annual change in the thermospheric temperature and thus the scale height of the neutral gas, and is the main cause of the annual asymmetry of thermospheric mass density. The annual asymmetry of mass density increases with altitude because of the accumulating effect of the changes in neutral temperature and scale height in the vertical direction.

North-south components of the annual asymmetry in the ionosphere

A retrospective study of the asymmetry in the ionosphere during the solstices is made using the different geospace parameters in the North and South magnetic hemispheres. Data of total electron content (TEC) and global electron content (GEC) produced from global ionospheric maps, GIM-TEC for 1999–2013, the ionospheric electron content (IEC) measured by TOPEX-Jason 1 and 2 satellites for 2001–2012, the F2 layer critical frequency and peak height measured on board ISIS 1, ISIS 2, and IK19 satellites during 1969–1982, and the earthquakes M5+ occurrences for 1999–2013 are analyzed. Annual asymmetry is observed with GEC and IEC for the years of observation with asymmetry index, AI, showing January > July excess from 0.02 to 0.25. The coincident pattern of January-to-July asymmetry ratio of TEC and IEC colocated along the magnetic longitude sector of 270° ± 5°E in the Pacific Ocean is obtained varying with local time and magnetic latitude. The sea/land differences in the F2 layer peak electron density, NmF2, and the peak height, hmF2, gathered with topside sounding data exhibit tilted ionosphere along the seashores with denser electron population at greater peak heights over the sea. The topside peak electron density NmF2, TEC, IEC, and the hemisphere part of GEC are dominant in the South hemisphere which resembles the pattern for seismic activity with dominant earthquake occurrence in the South magnetic hemisphere. Though the study is made for the hemispheric and annual asymmetry during solstices in the ionosphere, the conclusions seem valid for other aspects of seismic-ionospheric associations with tectonic plate boundaries representing zones of enhanced risk for space weather.

Annual asymmetry in thermospheric density: Observations and simulations

AbstractIn this paper, the Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) observations during 2002–2010 are utilized to study the variation of the annual asymmetry in thermospheric density at 400 km under low solar activity condition (F10.7 = 80) based on the method of empirical orthogonal functions (EOFs). The derived asymmetry index (AI) in thermospheric density from the EOF analysis shows a strong latitudinal variation at night but varies a little with latitudes in daytime. Moreover, it exhibits a terdiurnal tidal signature at low to middle latitudes. The global mean value of the AI is 0.191, indicating that a 47% difference in thermosphere between the December and June solstices in the global average. In addition, the NCAR Thermosphere‐Ionosphere Electrodynamics Global Circulation Model (TIEGCM) is used to explore the possible mechanisms responsible for the observed annual asymmetry in thermospheric density. It is found that the standard simulations give a lower AI and also a weaker day‐to‐night difference. The simulated AI shows a semidiurnal pattern in the equatorial and low‐latitude regions in contrast with the terdiurnal tide signature seen in the observed AI. The daily mean AI obtained from the simulation is 0.125, corresponding to a 29% December‐to‐June difference in thermospheric density at 400 km. Further sensitivity simulations demonstrated that the effect of the varying Sun‐Earth distance between the December and June solstices is the main process responsible for the annual asymmetry in thermospheric density, while the magnetic field configuration and tides from the lower atmosphere contribute to the temporal and spatial variations of the AI. Specifically, the simulations show that the Sun‐Earth distance effect explains 93% of the difference in thermospheric density between December and June, which is mainly associated with the corresponding changes in neutral temperature. However, our calculation from the density observations reveals that the varying Sun‐Earth distance effect only accounts for ~67% of the December‐to‐June difference in thermosphere density, indicating that the TIEGCM might significantly underestimate the forcing originating from the lower atmosphere.

GPS observations at quasi‐conjugate points during solar minimum

This paper aims at investigating the climatology of ionospheric TEC at quasi‐conjugate points located at different latitude ranges during low solar activity in 2007 with focus on the annual/hemispheric and semiannual asymmetry. Data from four pairing GPS stations are used for investigating the symmetry/or asymmetry between the conjugate hemispheres. The stations are selected based on data availability and manipulation possibility, and based on location of the conjugate point that was obtained by IGRF/DGRF model parameters. Our observations were in good agreement with plenty of studies conducted in this area. The results show evident annual asymmetry, hemispheric differences, and also weak semiannual asymmetry in GPS‐TEC at all GPS stations. The annual asymmetry is occurring at all latitudes from equatorial to polar cap regions. The asymmetry was quantified based on differencing of the annual mean‐TEC between the conjugate hemispheres (i.e., TECN°‐TECS°) which show differences in the range between 10 and 30%. The annual asymmetry is mainly driven by several factors, mainly: the geomagnetic field configuration, the Sun‐Earth distance, and lower atmosphere tidal forcing. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination and is a manifestation of the seasonal variations. The semiannual anomaly (also equinoctial asymmetry) was seen at SBA/RESO and also SUWN/KARR with more activity seen in March equinox compared with September. The occurrence of semiannual anomaly suggests close couplings of the ionosphere in both hemispheres.

Ozone distribution in the middle latitude mesosphere as derived from microwave measurements at Lindau (51.66°N, 10.13°E)

[1] Mesospheric ozone measurements between 50 and 80 km altitude were carried out in middle latitude at Lindau (51.66°N, 10.13°E), Germany, by means of the microwave technique between 1993 and 2006, with an interruption from 1997 to 1998. We utilize data obtained between 1998 and 2006. The measurements show a tendency of typical ozone features that occurs in each year. The most marked patterns are a late summer maximum of ozone in the middle and lower mesosphere which is shifted into autumn and winter in the stratopause region, and a distinct nighttime maximum around 72 km during the winter season, whereas no annual maximum occurs there during daytime. A clear annual asymmetry of the nighttime ozone distribution exists in this domain, marked by a decline of the mean ozone values in January/February and an increase to a subsidiary annual maximum a few kilometers higher in March/April. This asymmetry at the height of the well-known middle mesospheric maximum of ozone (MMM, often called the tertiary ozone maximum) results from the asymmetric occurrence rate of sudden stratospheric warmings (SSWs) occurring more frequently after winter solstice than before. Additionally, the asymmetric annual variation of water vapor with lowest values just around spring equinox influences the annual variation of ozone. A strong influence on the nighttime ozone concentration is the zonal wind. The night-to-day ratios (NDRs) in the middle to upper mesosphere display a distinct winter anomaly marked by values more than twice as high as in summer. The NDR is modulated by pronounced oscillations with a planetary time scale. The maximum effect occurs at 65–70 km, clearly below the height of the MMM.

A global comparative study on the ionospheric measurements between COSMIC radio occultation technique and IRI model

[1] We compare ionospheric parameters including total electron content (TEC), peak density, and height of the F2 layer (NmF2 and hmF2) between FORMOSAT-3/COSMIC (F3/C) GPS radio occultation (RO) technique retrieved and International Reference Ionosphere (IRI-2001) model predicted during different seasons in a low solar activity (LSA) year 2007. The comparison of topside TEC (tTEC, obtained by integrating electron density from hmF2 to 800 km) between IRI and F3/C shows that the IRI overestimates tTEC during both equinox seasons at around ±15° magnetic latitudes during daytime, especially in March equinox while underestimation of tTEC predicted by IRI is a dominant feature for both solstices. Further, a common finding is that the IRI overestimates tTEC from evening to prenoon hours irrespective of season at around ±20° magnetic latitudes, which is most likely due to the underestimation of hmF2 (by around 30–40 km), a key parameter on which the build of electron density profile depends in the model and inaccurate representation of the real profile by the topside electron density profiler model in IRI. The global distributions and seasonal variations of NmF2 show clear semiannual and annual asymmetry features during daytime and such features also reflected in IRI predicted peak densities with few exceptions. This high degree of agreement between tTEC comparison and the characteristic features of NmF2 global distributions indicates that the majority of contribution for tTEC has come from the F2 region. Further, it is also presented the global distributions of topside vertical scale heights (VSH) computed using electron density profiles retrieved from F3/C and predicted by IRI model during different seasons in year 2007. An important finding is that the topside VSH of F3/C profile data is meticulously following the geomagnetic equator during daytime irrespective of season and tends to increase toward higher latitudes. An appreciable latitudinal difference is found in the season averaged scale heights that derived with F3/C data during daytime while completely opposite results are found for IRI predicted scale heights. The discrepancies of topside VSH between the F3/C derived and predicted by IRI indicating that the shape of the topside electron density profile in the IRI model should desperately be revised accordingly such that it more closely resembles the real situation.

Climatology of the mean total electron content derived from GPS global ionospheric maps

We analyzed 11 years' (1998–2008) worth of the total electron content (TEC) data derived at the Jet Propulsion Laboratory (JPL) from Global Positioning System (GPS) observations to investigate the overall climatological features of the ionosphere in a new way. The global ionospheric maps (GIM) of JPL TEC are averaged globally and over low‐, middle‐, and high‐latitude ranges in the southern (northern) hemisphere and both hemispheres to identify their capability of capturing the overall features of the ionosphere. These mean TEC data show strong annual/semiannual, solar cycle, and 27‐day variations. The mean TEC presents stronger solar activity sensitivity at lower‐latitude bands. Moreover, the saturation effect exists in these mean TEC versus solar index F10.7, more pronounced at low latitudes, while the mean TEC increases faster with higher solar EUV fluxes, being evident at high latitudes. The annual asymmetry (differences in June and December solstices) can be detected in the mean TEC averaged globally and at low latitudes under all solar epochs as well as at middle and high latitudes under most solar activities. The hemispheric asymmetry of the TEC in conjugate hemispheres follows the control of solar declination. Both the hemispheric differences and annual asymmetry are more marked with increasing solar activity. The annual components of the mean TEC are stronger in the southern hemisphere, and the semiannual components are of similar phases and comparable amplitudes in conjugate hemispheres, which suggest close couplings of the ionosphere in both hemispheres. Further, the mean TEC averaged in one hemisphere can reliably be used as ionospheric indices to monitor the solar activity variabilities and to capture the overall climatological features of the ionosphere over specified regions, while it should be cautioned that the mitigation of the dominant annual components with opposite phases in conjugate hemispheres leaves a significant semiannual component in the mean TEC averaged in both hemispheres, especially under low solar activity.

Ionospheric annual asymmetry observed by the COSMIC radio occultation measurements and simulated by the TIEGCM

Average F2‐layer electron densities at December solstice are higher than those at June solstice. This phenomenon, which is often called the F2‐layer annual asymmetry, has been observed for several decades, but its causes are still not fully understood. This study investigates global variations of this annual asymmetry observed from one year of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric radio occultation (IRO) measurements. The IRO observations show that there is a strong NmF2 annual asymmetry that has significant longitudinal and local time variations. A strong peak of the asymmetry occurs at about noon and another one at midnight, both located at around 25° geomagnetic latitude. Numerical simulations using the Thermosphere‐Ionosphere Electrodynamics Global Circulation Model (TIEGCM) are in very good agreement with these observations. The modeled NmF2 annual asymmetry has a similar magnitude, and similar semidiurnal and longitudinal variations as those in the observations. TIEGCM simulations show that changes in solar extreme ultraviolet (EUV) radiation between the December and June solstices and the displacement of the geomagnetic axis from the geographic axis are the two primary processes that cause the annual asymmetry and its associated longitudinal and local time variations. The tides propagating from lower altitudes also contribute to this asymmetry, but to a smaller extent.

Yearly variations of global plasma densities in the topside ionosphere at middle and low latitudes

In this paper, the 10‐year (1996–2005) measurements of total ion density (Ni) from the Defense Meteorological Satellite Program (DMSP) spacecraft at 0930 and 2130 LT have been analyzed to investigate the yearly variations of global plasma densities in the topside ionosphere at magnetic latitudes from 60°S to 60°N. Results indicate that there are strong yearly variations in the DMSP Ni at 840 km. The annual components of longitude‐averaged Ni dominate at most latitudes with maxima around the June solstices in the Northern Hemisphere and the December solstice in the Southern Hemisphere. In contrast, seasonal anomaly (maxima Ni around the December solstice) exists in the northern equatorial zone. Moreover, the differences in Ni at the two solstices are not symmetrical about the magnetic equator, being generally higher in the Southern Hemisphere than in the Northern Hemisphere. Conjugate‐averaged Ni is substantially greater at the December solstice than at the June solstice. This annual asymmetry is modulated by solar activity effect and has latitudinal and longitudinal structures. The longitude effects of the annual asymmetry depend on local time, being stronger in the evening sector than in the morning sector. The solstice differences and annual asymmetry are more marked with increasing solar activity. The annual asymmetry appears not only in the rising phase of the solar cycle but also in the declining phase. Thus the solar condition differences between the two solstices do not account for the Ni asymmetry. The concentration of neutral oxygen [O], provided from the NRLMSIS model, shows a similar pattern of annual and hemispheric asymmetries. Moreover, effects of the HWM model neutral winds are also constituent with the change patterns of Ni. Therefore, considering the principal processes in the topside ionosphere, the changes of [O] and the rates of thermospheric winds should contribute to the annual asymmetry in Ni at 840‐km altitude.

Why is there more ionosphere in January than in July? The annual asymmetry in the F2-layer

Abstract. Adding together the northern and southern hemisphere values for pairs of stations, the combined peak electron density NmF2 is greater in December-January than in June–July. The same applies to the total height-integrated electron content. This "F2-layer annual asymmetry" between northern and southern solstices is typically 30%, and thus greatly exceeds the 7% asymmetry in ion production due to the annual variation of Sun-Earth distance. Though it was noticed in ionospheric data almost seventy years ago, the asymmetry is still unexplained. Using ionosonde data and also values derived from the International Reference Ionosphere, we show that the asymmetry exists at noon and at midnight, at all latitudes from equatorial to sub-auroral, and tends to be greater at solar minimum than solar maximum. We find a similar asymmetry in neutral composition in the MSIS model of the thermosphere. Numerical computations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP) model give a much smaller annual asymmetry in electron density and neutral composition than is observed. Including mesospheric tides in the model makes little difference. After considering possible explanations, which do not account for the asymmetry, we are left with the conclusion that dynamical influences of the lower atmosphere (below about 30 km), not included in our computations, are the most likely cause of the asymmetry.

Statistical characteristics of the total ion density in the topside ionosphere during the period 1996-2004 using empirical orthogonal function (EOF) analysis

Abstract. We have applied the empirical orthogonal function (EOF) analysis to examine the climatology of the total ion density Ni at 840 km during the period 1996-2004, obtained from the Defense Meteorological Satellite Program (DMSP) spacecraft. The data set for each of the local time (09:30 LT and 21:30 LT) is decomposed into a time mean plus the sum of EOF bases Ei of space, multiplied by time-varying EOF coefficients Ai. Physical explanations are made on the first three EOFs, which together can capture more than 95% of the total variance of the original data set. Results show that the dominant mode that controls the Ni variability is the solar EUV flux, which is consistent with the results of Rich et al. (2003). The second EOF, associated with the solar declination, presents an annual (summer to winter) asymmetry that is caused by the transequatorial winds. The semiannual variation that appears in the third EOF for the evening sector is interpreted as both the effects of the equatorial electric fields and the wind patterns. Both the annual and semiannual variations are modulated by the solar flux, which has a close relationship with the O+ composition. The quick convergence of the EOF expansion makes it very convenient to construct an empirical model for the original data set. The modeled results show that the accuracy of the prediction depends mainly on the first principal component which has a close relationship with the solar EUV flux.

The global ionospheric asymmetry in total electron content

We use the NASA-JPL global ionospheric maps of total electron content (TEC), obtained from observations of GPS satellite signals, to construct average noon TEC maps for one-month periods centered on the June and December solstices of 2002. By combining data from equivalent points in the northern and southern hemispheres, we find that the combined north+south TEC in December substantially exceeds that in June, almost everywhere. The global average of an ‘Asymmetry Index’ (AI) used to characterize such patterns, AI=(Dec−June)/(Dec+June), results in AI=0.15 for 2002, far greater than the value of 0.035 that corresponds to the annual variation of the solar irradiance due to Sun-Earth distance. Since the F2-layer makes the major contribution to TEC, we regard this as a manifestation of the F2-layer annual asymmetry, discovered some decades ago but never explained. The neutral atomic oxygen/molecular nitrogen concentration ratio, derived from the GUVI experiment on the TIMED satellite, shows a similar but smaller asymmetry (AI=0.06), providing some evidence that the ionospheric asymmetry is related to changes in thermospheric composition, with a still smaller AI=0.03 in the NRL MSIS thermospheric model.

Pengaruh Luas Ungkapan Sukarela dan Asimetri Informasi Terhadap Cost of Equity Capital pada Perusahaan Publik di Indonesia

This study examines whether voluntary corporate disclosure level published in annual report and asymmetry information has an effect on cost of equity capital. The purpose of the study is to investigate whether the variances in corporate disclosure level and asymmetry information effect cost of equity capital. The sample consists of 61 companies listed on the Jakarta Stock Exchange in 1999 and 2000. The hypotheses tested was conducted by implementing pooled data method, for period of 1999- 2000. The results support the hypotheses that voluntary corporate disclosure, information asymmetry and beta have effect on cost of equity capital. More specifically, the results show that size significantly moderates the relationship between information asymmetry and cost of equity capital, but size does not significantly moderate the relationship between voluntary corporate disclosure level and cost of equity capital.