Assessment of precision in ionospheric electron density profiles retrieved by GPS radio occultations

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six satellite radio occultation mission that was launched in April 2006. The close proximity of these satellites during some months after launch provides a unique opportunity to evaluate the precision of Global Positioning System (GPS) radio occultation (RO) retrievals of ionospheric electron density from nearly collocated and simultaneous observations. RO data from 30 consecutive days during July and August 2006 are divided into ten groups in terms of daytime or nighttime and latitude. In all cases, the best precision values (about 1%) are found at the F peak height and they slightly degrade upwards. For all daytime groups, it is seen that electron density profiles above about 120 km height exhibit a substantial improvement in precision. Nighttime groups are rather diverse: in particular, the precision becomes better than 10% above different levels between 120 and 200 km height. Our overall results show that up to 100–200 km (depending on each group), the uncertainty associated with the precision is in the order of the measured electron density values. Even worse, the retrieved values tend sometimes to be negative. Although we cannot rely directly on electron density values at these altitudes, the shape of the profiles could be indicative of some ionospheric features (e.g. waves and sporadic E layers). Above 200 km, the profiles of precision are qualitatively quite independent from daytime or latitude. From all the nearly collocated pairs studied, only 49 exhibited a difference between line of sight angles of both RO at the F peak height larger than 10°. After analyzing them we find no clear indications of a significant representativeness error in electron density profiles due to the spherical assumption above 120 km height. Differences in precision between setting and rising GPS RO may be attributed to the modification of the processing algorithms applied to rising cases during the initial period of the COSMIC mission.     

Behavior of the equivalent slab thickness over three European stations

The total electron content (TEC) derived from the global positioning system (GPS) and the F2-layer peak electron density obtained from Digisonde data have been used to study the diurnal, seasonal and solar activity variations of the ionospheric equivalent slab thickness (τ) over three European stations located at Pruhonice (50.0°N, 15.0°E), Ebro (40.8°N, 0.5°E) and El Arenosillo (37.1°N, 353.3°E). The diurnal variation of the τ is characterized by daytime values lower than nighttime ones for all seasons at low solar activity while daytime values larger than nighttime characterizes the diurnal variation for summer at high solar activity. A double peak is noticeable at dusk and at dawn, better expressed for winter at low solar activity. The seasonal variations of τ depend on local time and solar activity, the daytime values of τ increases from winter to summer whereas nighttime values of τ show the opposite. The effect of the solar activity on τ depends on local time and season, there being very sensitive for winter nighttime values of τ. The results of this study are compared with those presented by other authors.     

Response of low latitude D-region ionosphere to the total solar eclipse of 22 July 2009 deduced from ELF/VLF analysis

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog. lat., 25.27° N; Geog. long., 82.98° E; Geomag. lat. = 14° 55’ N) was investigated using ELF/VLF radio signal. Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE). Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence. The changes in D-region ionospheric VLF reflection heights (h) and electron density (n_e: 22.6–24.6 cm⁻³) during eclipse have been estimated from tweek analysis. The reflection height increased from ∼89 km from the first occurrence of tweek to about ∼93 km at the totality and then decreased to ∼88 km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5 km during the eclipse. The reflection heights at the time of totality during TSE are found to be less by 2–3 km as compared to the usual nighttime tweek reflection heights. This is due to partial nighttime condition created by TSE. A significant increase of 3 dB in the strength of the amplitude of VLF signal of 22.2 kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day. The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days.     

The quiet nighttime low-latitude ionosphere as observed by TIMED/GUVI

In this paper, we examine the nighttime ionosphere climatology structure in the low latitude region and discrepancies between Global Ultraviolet Imager (GUVI) observations and the IRI model predictions using (1) the magnetic zonal mean of electron number density as a function of altitude and magnetic latitude, (2) vertical electron density profiles at various levels of F10.7 index, (3) nighttime descent and magnitude decrease of the ionosphere, (4) point-to-point comparisons of F-peak height (hmF2) and density (NmF2), and (5) the magnetic longitudinal variations of hmF2 and NmF2. The data collected from the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) mission since its launch in December 2001 have provided great opportunities for many scientific investigations of the ionosphere. In this analysis, we investigate the climatology of the nighttime low-latitude ionosphere under low geomagnetic activity (kp ? 4) using the electron density profiles inferred from the airglow measurements obtained by the GUVI aboard the TIMED spacecraft and compared with the results obtained from IRI (International Reference Ionosphere) model-2001. The observed climatology is an essential tool for further understanding the electrodynamics in the low-latitude region and improving the model’s prediction capability. The time range of the GUVI data used in this study is from 2002 (day 053) to 2006 (day 304), and the IRI model predictions were produced at every GUVI location. The ionosphere observed is generally of greater density than what IRI predicts throughout the night for all four seasons for low and moderate solar activity while the model over-predicts the electron density near the F-region peak at high solar activity before midnight. Observations show that the height of the F-region peak has a steep descent from dusk to midnight and near midnight the height of layer is insensitive to solar conditions, significantly different than what is predicted by IRI. Longitudinal features shown in GUVI data are present in the low-latitude ionosphere after sunset and continue through to midnight after which the low-latitude ionosphere is largely zonally symmetric.     

Effects of solar activity variations on the low latitude topside nighttime ionosphere

We have studied the topside nighttime ionosphere of the low latitude region using data obtained from DMSP F15, ROCSAT-1, KOMPSAT-1, and GUVI on the TIMED satellite for the period of 2000–2004, during which solar activity decreased from its maximum. As these satellites operated at different altitudes, we were able to discriminate altitude dependence of several key ionospheric parameters on the level of solar activity. For example, with intensifying solar activity, electron density was seen to increase more rapidly at higher altitudes than at lower altitudes, implying that the corresponding scale height also increased. The density increased without saturation at all observed altitudes when plotted against solar EUV flux instead of F10.7. The results of the present study, as compared with those of previous studies for lower altitudes, indicate that topside vertical scale height increases with altitude and that, when solar activity increases, topside vertical scale height increases more rapidly at higher altitudes than at lower altitudes. Temperature also increased more rapidly at higher altitudes than at lower altitudes as solar activity increased. In addition, the height of the F2 peak was seen to increase with increasing solar activity, along with the oxygen ion fraction measured above the F2 peak. These results confirm that the topside ionosphere rises and expands with increasing solar activity.     

Equatorial total electron content (TEC) at low and high solar activity

Total electron content (TEC) derived from ionosonde data recorded at the station of Korhogo (Lat = 9.33°N, Long = 5.43°W, Dip = 0.67°S) are compared to the International Reference Ionosphere (IRI) model predicted TEC for high (1999) and low (1994) solar activity conditions. The results show that the model represents the diurnal variation of the TEC as well as a solar activity and seasonal dependence. This variation is closer to that of the ionosonde-inferred TEC at high solar activity. However, at low solar activity the IRI overestimates the ionosonde-inferred TEC. The relative deviation ΔTEC is more prominent in the equinoctial seasons during nighttime hours where it is as high as 70%. At daytime hours, the relative deviation is estimated to 0–30%.     

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390 km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13 UT (09:36–12:13 KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2 dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53 UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km⁻¹ from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7 km and 0.055 km⁻¹, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71 km. The reflection height is found to increase by ∼7 km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.     

Low latitude nighttime ionospheric vertical E × B drifts at African region

The nighttime vertical E × B drifts velocities of the F2-region were inferred from the hourly hmF2 values obtained from ionosonde data over an African equatorial station, Ilorin (8.50^oN, 4.68^oE; dip lat. 2.95^o) during period of low solar activity. For each season, the plasma drift Vz is characterized by an evening upward enhancement, then by a downward reversal at 1900 LT till around 0000 LT, except for June solstice. This was explained using the Rayleigh–Taylor (R-T) instability mechanism. The occasional drift differences in Vz obtained by inferred and direct measurement over Ilorin and Jicamarca, respectively are reflective of the importance of chemistry and divergent transport system due to both the E region electric and magnetic fields instead of simple motions. The pre-reversal enhancement (PRE) magnitude is higher during the equinoctial months than the solsticial months over Jicamarca, highest during December solstice and the equinoctial months over Ilorin, suggesting the dominance of higher E × B fountain during equinoxes at both stations. The lowest PRE magnitude was in June solstice. The appearance of post-noon peak in NmF2 around 1700 LT is highest during the equinoctial months and lowest during the solsticial period. A general sharp drop in NmF2 around 1800 LT is distinct immediately after sunset, lowest during June solstice and highest in March equinox. Our result suggests that between 0930 and 2100 LT, the general theory that vertical drifts obtained by digisonde measurements only match the E × B drift if the F layer is higher than 300 km is reliable, but does not hold for the nighttime period of 2200–0600 LT under condition of solar minima. Hence, the condition may not be sufficient for the representation of vertical plasma drift at nighttime during solar minima. This assertion may still be tentative, as more equatorial stations needed to be studied for better confirmation.     

Theoretical simulation of electron density and temperature distribution at Indian equatorial and low latitude ionosphere

The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented and results are compared with the electron density and electron temperature measured in situ by Indian SROSS C2 satellite at an altitude of ∼500 km within 31°S–34°N and 75 ± 10°E that covers the Indian sector during a period of low solar activity. Equatorial Ionization Anomaly (EIA) observed in electron density, morning and afternoon enhancements, equatorial trough in electron temperature have been simulated by the model within reasonable limits of accuracy besides reproducing other normal diurnal features of density and temperature.     

An empirical model of electron density in low latitude at 600 km obtained by Hinotori satellite

An empirical model of electron density (Ne) was constructed by using the data obtained with an impedance probe on board Japanese Hinotori satellite. The satellite was in circular orbit of the height of 600 km with the inclination of 31 degrees from February 1981 to June 1982. The constructed model gives Ne at any local time with the time resolution of 90 min and between −25 and 25 degrees in magnetic latitude with its resolution of 5 degrees in the range of F_10.7 from 150 to 250 under the condition of K_p < 4. Spline interpolations are applied to the functions of day of year, geomagnetic latitude and solar local time, and linear interpolation is applied to the function of F_10.7. Longitude dependence of Ne is not taken into account. Our density model can reproduce solar local time variation of electron density at 600 km altitude better than current International Reference Ionosphere (IRI2001) model which overestimates Ne in night time and underestimates Ne in day time. Our density model together with electron temperature model which has been constructed before will enable more understanding of upper ionospheric phenomenon in the equatorial region.     

Variation of ionospheric electron and ion temperatures during periods of minimum to maximum solar activity by the SROSS-C2 satellite over Indian low and equatorial latitudes

To study the variation of ionospheric electron and ion temperatures with solar activity the data of electron and ion temperatures were recorded with the help of Retarding Potential Analyzer payload aboard Indian SROSS-C2 satellite at an average altitude of ∼500 km. The main focuses of the paper is to see the diurnal, seasonal and latitudinal variations of electron and ion temperatures during periods of minimum to maximum solar activity. The ionospheric temperatures in the topside show strong variations with altitude, latitude, season and solar activity. In present study, the temperature variations with latitude, season and solar activity have been studied at an average altitude ∼500 km. The peak at sunrise has been observed during all seasons, in both electron and ion temperatures. Further, the ionospheric temperatures vary with latitude in day time. The latitudinal variation is more pronounced for low solar activity than for high solar activity.     

Comparative study of high-latitude,mid-latitude and low-latitude ionosphere on basis of local empirical models

The analysis of the regular features of the high-, mid- and low-latitude ionosphere characteristics has been carried out using local empirical models. The local empirical models were derived from the manual scaled ionogram data recorded by DPS-4 Digisondes located at Norilsk (69 N, 88E), Irkutsk (52 N, 104E) and Hainan (19 N, 109E) for a 6-year period from December, 2002 to December, 2008. The technique used to build the local empirical model is described. Primary focuses are diurnal, seasonal and solar cycle variations of the peak electron density and the peak height under low solar activity and their changes with increasing solar activity. The main objective of the paper is to reveal both common and specific features of high-, mid- and low-latitude ionosphere. Based on earlier comparisons with the International Reference Ionosphere model, we analyze how the common and specific features are reproduced by this model.     

Towards a new reference model of hmF2 for IRI

A numerical model of the peak height of the F2 layer, hmF2_top, is derived from the topside sounding database of 90,000 electron density profiles for a representative set of conditions provided by ISIS1, ISIS2, IK19 and Cosmos-1809 satellites for the period of 1969–1987. The model of regular hmF2 variations is produced in terms of local time, season, geomagnetic latitude, geodetic longitude and solar radio flux. No geomagnetic activity trends were discernible in the topside sounding data. The nighttime peak of hmF2_top evident for mid-latitudes disappears near the geomagnetic equator where a maximum of hmF2_top occurs at sunset hours when it can exceed 500 km at solar maximum. The hmF2 given by the IRI exceeds hmF2_top at the low solar activities. The hmF2_top, obtained by extrapolation of the first derivative of the topside profile to zero shows saturation similar to foF2 the greater the solar activity. The proposed model differs from hmF2 given by IRI based on M(3000)F2 to hmF2 conversion by empirical relationships in terms of foF2, foE and R12 with these quantities mapped globally by the ITU-R (former CCIR) from ground-based ionosonde data. The differences can be attributed to the different techniques of the peak height derivation, different epochs and different global distribution of the source data as well as the different mathematical functions involved in the maps and the model presentation.     

低纬100—200km间电子数密度的变化

利用光化平衡模式计算了低纬100—200km间白天电子数密度的变化。求得E-F₁谷区的谷深,谷宽、谷高的变化特征。获得如下结果:a.太阳活动明显影响电子数密度随高度及太阳天顶角的变化,发现太阳活动指数与电子数密度间不仅存在正相关,而且存在负相关;b.太阳活动明显影响E-F₁谷区的形态。在一定太阳活动条件下,对同一太阳赤纬和地理纬度,谷深、谷宽与太阳天顶角的关系难以用一简单函数来表示;c.太阳耀斑、地磁活动对该区电子密度有明显影响;d.在讨论100—200km间电子密度时不能忽略O+（2P）和NO的光电离率。      

Storm time behavior of topside scale height inferred from the ionosphere–plasmasphere model driven by the F2 layer peak and GPS-TEC observations

The International Reference Ionosphere model extended to the plasmasphere, IRI-Plas, presents global electron density profiles and total electron content, TECiri, up to the altitude of the GPS satellites (20,000 km). The model code is modified by input of GPS-derived total electron content, TECgps, so that the topside scale height, Hsc, is obtained minimizing in one step the difference between TECiri and TECgps observation. The topside basis scale height, Hsc, presents the distance in km above the peak height at which the peak plasma density, NmF2, decays by a factor of e (∼2.718). The ionosonde derived F2 layer peak density and height and GPS-derived TECgps data are used with IRI-Plas code during the main phase of more than 100 space weather storms for a period of 1999–2006. Data of seven stations are used for the analysis, and data from five other stations served as testing database. It is found that the topside basis scale height is growing (depressing) when the peak electron density (critical frequency foF2) and electron content are decreased (increased) compared to the median value, and vice versa. Relative variability of the scale height, rHsc, and the instantaneous Hsc are inferred analytically in a function of the instantaneous foF2, median fmF2 and median Hmsc avoiding a reference to geomagnetic indices. Results of validation suggest reliability of proposed algorithm for implementation in an operational mode.     

Coordinated in situ measurements of plasma irregularities and ground based scintillation observations at the crest of equatorial anomaly

First comparison of in situ density fluctuations measured by the DEMETER satellite with ground based GPS receiver measurements at the equatorial anomaly station Bhopal (geographic coordinates (23.2°N, 77.6°E); geomagnetic coordinates (14.29°N, 151.12°E)) for the low solar activity year 2005, are presented in this paper. Calculation of the diurnal maximum of the strength of the equatorial electrojet, which can serve as precursor to ionospheric scintillations in the anomaly region is also done. The Langmuir Probe experiment and Plasma Analyzer onboard DEMETER measure the electron and ion densities respectively. Irregularities in electron density distribution cause scintillations on transionospheric links and there exists a close relationship between an irregularity and scintillation. In 40% of the cases, DEMETER detects the irregularity structures (dNe/Ne ? 5% and dNi/Ni (O⁺) ? 5%) and GPS L band scintillations (S4 ? 0.2) are also observed around the same time, for the low solar activity period. It is found that maximum irregularity intensity is obtained in the geomagnetic latitude range of 10–20° for both electron density and ion density. As the GPS signals pass through this irregularity structure, scintillations are recorded by the GPS receiver installed at the equatorial anomaly station, Bhopal it is interesting to note that in situ density fluctuations observed on magnetic flux tubes that pass over Bhopal can be used as indicator of ionospheric scintillations at that site. Many cases of density fluctuations and associated scintillations have been observed during the descending low solar activity period. The percentage occurrence of density irregularities and scintillations shows good correspondence with diurnal maximum of the strength of electrojet, however this varies with different seasons with maximum correspondence in summer (up to 66%) followed by equinox (up to 50%) and winter (up to 46%). Also, there is a threshold value of EEJ strength to produce density irregularities ((dNe/Ne)_max ? 5%) and for moderate to strong scintillations (S4 ? 0.3) to occur. For winter this value is found to be ∼40 nT whereas for equinox and summer it is around 50 nT.     

Effect of vertical plasma transport on ionospheric F2-region parameters at equatorial latitude

The variability of the F2-layer even during magnetically quiet times are fairly complex owing to the effects of plasma transport. The vertical E × B drift velocities (estimated from simplified electron density continuity equation) were used to investigate the seasonal effects of the vertical ion drifts on the bottomside daytime ionospheric parameters over an equatorial latitude in West Africa, Ibadan, Nigeria (Geographic: 7.4°N, 3.9°E, dip angle: 6°S) using 1 year of ionsonde data during International Geophysical Year (IGY) of 1958, that correspond to a period of high solar activity for quiet conditions. The variation patterns between the changes of the vertical ion drifts and the ionospheric F2-layer parameters, especially; f_oF₂ and h_mF₂ are seen remarkable. On the other hand, we observed strong anti-correlation between vertical drift velocities and h′F in all the seasons. We found no clear trend between N_mF₂ and h_mF₂ variations. The yearly average value of upward daytime drift at 300 km altitude was a little less than the generally reported magnitude of 20 ms⁻¹ for equatorial F-region in published literature, and the largest upward velocity was roughly 32 ms⁻¹. Our results indicate that vertical plasma drifts; ionospheric F2-layer peak height, and the critical frequency of F2-layer appear to be somewhat interconnected.     

Comparison of diurnal,seasonal and latitudinal effect of MUF VR and NmF2 VR during some solar cycle epochs

The diurnal and seasonal changes of the variability (VR) of Maximum Useable Frequency (MUF) are compared with those of peak electron density (NmF2) at Ibadan (7.4°N, 3.9°E, 6°S dip) in the African sector. Also compared is the latitudinal effect on both characteristics by combining data from Singapore (1.3°N, 103.8°E, 17.6°S dip) in the East Asian sector and Slough (51.5°N, 359.4°E, 66.5°S dip) in the European sector. MUF VR is found to be about half of NmF2 VR at all the hours and seasons and during the solar cycle epochs considered for the three stations. While nighttime MUF VR is greater in June Solstice and September Equinox during both low and moderate solar activities and in September Equinox and December Solstice during high solar activity, nighttime NmF2 VR is greater in June Solstice and September Equinox during high solar activity and greater at the equinoxes during low and moderate solar activities. This signifies a shift in nighttime MUF peak VR from the middle six months during low and moderate solar activities to the last half of the year during high solar activity. Daytime VR of both characteristics are not observed to show any seasonal variation. MUF VR and that of NmF2 are found to increase and decrease alternately with the Zurich sunspot number (Rz) for Ibadan and Singapore. For Slough, the VR of both characteristics increases with Rz during the first half of the day. It then increases and decreases alternately with Rz during the remaining hours of the day. While nighttime MUF VR decreases with latitude, just like nighttime NmF2 VR, no latitudinal effect is found for daytime VR of both characteristics.     

The equatorial ionization anomaly at the topside F region of the ionosphere along 75°E

Electron density measured by the Indian satellite SROSS C2 at the altitude of ∼500 km in the 75°E longitude sector for the ascending half of the solar cycle 22 from 1995 to 1999 are used to study the position and density of the equatorial ionization anomaly (EIA). Results show that the latitudinal position and peak electron density of the EIA crest and crest to trough ratios of the anomaly during the 10:00–14:00 LT period vary with season and from one year to another. Both EIA crest position and density are found to be asymmetric about the magnetic equator and the asymmetry depends on season as well as the year of observation, i.e., solar activity. The latitudinal position of the crest of the EIA and the crest density bears good positive correlation with F_10.7 and the strength of the equatorial electrojet (EEJ).