Studies of electron and ion temperatures at 500 km altitude during sunrise using Indian SROSS C2 satellite

Solar dependence of electron and ion temperatures (T_e and T_i) in the ionosphere is studied using RPA data onboard SROSS C2 at an altitude of ∼500 km and 77°E longitude during early morning hours (04:00–07:00 LT) for three solar activities: solar minimum, moderate and maximum during winter, summer and equinox months in 10°S–20°N geomagnetic latitude. In winter the morning overshoot phenomenon is observed around 06:00 LT (T_e enhances to ∼4000 K) during low-solar activity and to T_e ∼ 3800 K, during higher solar activity. In summer, it is observed around 05:30 LT, but the rate of T_e enhancement is higher during moderate solar activity (∼2700 K/hr) than the low-solar activity (∼1700 K/hr). During equinox, this phenomenon is delayed and is observed around 06:00 LT (∼4200 K) during all three activities.     

Validation of B0 and B1 in the IRI 2001 model at low solar activity for Ilorin an equatorial station

A new set of data obtained at low solar activity from Ilorin, Nigeria (geog. latitude 8.5°N, geog longitude, 4.6°E, dip 4.1°S) is used to validate the IRI 2001 model at low solar activity. The results show in general a good agreement between model and observed B0 at night but an over estimation during daytime. The overestimation is greatest during the morning period (0600LT–1000LT). The model prediction for B1 is fairly good at night and during the day. A dependence of B0 on solar zenith angle χ is observed during the daytime. A formulation of the form B0 = A[cos(χ)ⁿ] is therefore proposed. Values of the constants n and A were determined for the period of low solar activity for this station.     

The solar cycle variation of plasma parameters in equatorial and mid latitudinal areas during 2005–2010

Based on the ISL data detected by DEMETER satellite, the solar cycle variation in electron density (Ne) and electron temperature (Te) were studied separately in local daytime 10:30 and nighttime 22:30 during 2005–2010 in the 23rd/24th solar cycles. The semi-annual, annual periods and decreasing trend with the descending solar activity were clearly revealed in Ne. At middle and high latitudes, there exhibited phase shift and even reversed annual variation over Southern and Northern hemisphere, and the annual variation amplitudes were asymmetrical at both hemispheres in local daytime. In local nighttime, the annual variations of Ne at south and north hemispheres were symmetrical at same latitudes, but the annual variation amplitudes at different latitudes differed largely, showing obviously zonal features. As for Te, the phase shift in annual variations was not as apparent as Ne with the increase of latitudes at Southern and Northern hemisphere in local daytime. While in local nighttime the reversed annual variations of Te were shown at low latitudinal areas, not at high latitudes as those in Ne. The correlation study on Ne and Te illustrated that, in local daytime, Ne and Te showed strong negative correlation at equator and low latitudes, but during the solar minimum years the correlation between Ne and Te changed to be positive at 25–30° latitudes in March 2009. The correlation coefficient R between Ne and Te also showed semi-annual periodical variations during 2005–2010. While in local nighttime, Ne and Te exhibited relatively weak positive correlation with R being about 0.6 at low latitudes, however no correlation beyond latitudes of 25° was obtained.     

A study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station

The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.16°N, 72.78°E; Geomagnetic: 12.90°N, 147.35°E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S₄ index_, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70–96% scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S₄ ? 0.2) and enhancement of ROTI with (ROTI ? 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.     

Occurrence of equatorial plasma bubbles over Kolhapur

This paper reports the nightglow observations of OI 630.0 nm emissions, made by using all sky imager operating at low latitude station Kolhapur (16.8°N, 74.2°E and dip lat. 10.6°N) during high sunspot number years of 24th solar cycle. The images are analyzed to study the nocturnal, seasonal and solar activity dependence occurrence of plasma bubbles. We observed EPBs in images regularly during a limited period 19:30 to 02:30 LT and reach maximum probability of occurrence at 22:30 LT. The observation pattern of EPBs shows nearly no occurrence during the month of May and it maximizes during the period October–April. The equinox and solstice seasonal variations in the occurrence of plasma bubbles show nearly equal and large differences, respectively, between years of 2010–11 and 2011–12.     

Variability of foF2 in the African equatorial ionosphere

This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equatorial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35–55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time.     

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.     

Peculiarities of the nighttime winter foF2 increase over Irkutsk

An analysis of properties and peculiarities of the nighttime winter foF2 increases (NWI) in the East Siberia is made on data of ionospheric station Irkutsk in the periods 1958–1992 and 2002–2009 and the empirical model of the F2 layer critical frequency under the geomagnetic quiet conditions deduced from these data (model Q-F2). It is revealed, that the NWI is the stable regularity of the quiet ionosphere over Irkutsk. The amplitude of the NWI (the difference between maximum and minimum foF2 values at night hours) is the greatest in December–January and nearly the same at low and middle solar activity. It is a peculiarity of the quiet ionosphere in the East Siberia. Maximum in night foF2 under quiet geomagnetic conditions is observed mainly after midnight (02-04 LT) and is shifted to predawn hours as solar activity increases. At low solar activity the quiet ionosphere at ∼02–04 LT shows the following properties: (a) the fluctuations of foF2 and hmF2 are in the reverse correlation but this dependence is weak; (b) very strong fluctuations of foF2 (|δfoF2| > 30%) occur seldom (∼4% of events) and almost all of them are positive; an example of very strong fluctuations of foF2 up to 60% can be an extreme increase in the foF2 on 19.12.2008; (c) the very strong enhancements of foF2 in the NWI maximum can be observed at the low geomagnetic activity, they occur more often during substorms but very seldom during geomagnetic storms. Possible reasons of these properties of NWI are discussed.     

Seasonal variability of GPS-VTEC and model during low solar activity period (2006–2007) near the equatorial ionization anomaly crest location in Chinese zone

Variability of vertical TEC recorded at Fuzhou (26.1°N, 119.3°E, geomagnetic latitude 14.4°N), Xiamen (24.5°N, 118.1°E, geomagnetic latitude 13.2°N), Nanning (22.8°N, 108.3°E, geomagnetic latitude 11.4°N), China, during the low solar activity in 2006–2007 have been analyzed and discussed. Remarkable seasonal anomaly was found over three stations with the highest value during spring and the lowest value during summer. The relative standard deviation of VTEC is over 20% all the time, with steady and smooth variation during daytime while it has a large fluctuation during nighttime. The biggest correlation coefficient was found in the VTEC-sunspot pair with a value of over 0.5. It seems that solar activity has a better correlation ship than geomagnetic activity with the variation of VTEC and better correlations are found with more long-term data when comparing our previous study. The results of comparing observation with model prediction in three sites reveal again that the SPIM model overestimates the measured VTEC in the low latitude area.     

Response of the extratropical middle atmosphere to the September 2002 major stratospheric sudden warming

The effects of a major stratospheric sudden warming (SSW) at extratropical latitudes have been investigated with wind and temperature observations over a Brazilian station, Cachoeira Paulista (22.7°S, 45°W) during September–October 2002. In response to the warming at polar latitudes a corresponding cooling at tropical and extratropical latitudes is prominent in the stratosphere. A conspicuous signature of latitudinal propagation of a planetary wave of zonal wavenumbers 1 and 2 from polar to low latitude has been observed during the warming period. The polar vortex which split into two parts of different size is found to travel considerably low latitude. Significant air mass mixing between low and high latitudes is caused by planetary wave breaking. The meridional wind exhibits oscillations of period 2–4 days during the warming period in the stratosphere. No wave feature is evident in the mesosphere during the warming period, although a 12–14 day periodicity is observed after 2 weeks of the warming event, indicating close resemblance to the results of other simultaneous investigations carried out from high latitude Antarctic stations. Convective activity over the present extratropical station diminishes remarkably during the warming period. This behavior is possibly due to destabilization and shift of equatorial convective active regions towards the opposite hemisphere in response to changes in the mean meridional circulation in concert with the SSW.     

Polarization pattern of low and mid-frequency magnetic pulsations in the polar cap: A comprehensive analysis at Terra Nova Bay (Antarctica)

The polarization pattern of ULF pulsations (f ≈ 1–100 mHz) at Terra Nova Bay (Antarctica, CGM λ ∼ 80°) has been determined for the entire 2003, soon after the solar maximum. A comparison with the results of previous investigations, conducted at the same station close to the solar minimum (1994–96), allows to focus common elements and major differences among different frequency bands which persist through the entire solar cycle. Basically, between f ∼ 1.5 and 5 mHz, the day can be divided into four sectors with alternate polarizations. The local time and latitudinal dependence of the observed pattern can be tentatively interpreted in terms of a latitude of resonant field lines reaching λ ∼ 80° in the noon sector; on the other hand, resonance effects of lower latitude field lines can be clearly identified also far from the noon meridian when the station moves into the deep polar cap. Moreover, in the morning sector the resonance region would extend to lower latitudes than in the evening sector. The proposed profile of the resonant region can interpret also the results obtained at other cusp/auroral stations and appears consistent with that one inferred in the northern hemisphere at smaller latitudes. The resonance region progressively shifts toward lower latitude with increasing frequency; correspondingly, the four-sector pattern progressively disappears at TNB. Above f ∼ 20 mHz, the experimental observations might suggest an additional contribution from Sunward propagating waves, possibly via the magnetotail lobes.     

F3 layer feature under low and medium solar activity observed at a Chinese low latitude station Fuke

Ionogram observations from the ionosonde at Fuke (9.5°N geomagnetic latitude), a Chinese low latitude station, in 2010–2012 are analyzed to present the features of F3 layer under low and moderate solar activity conditions. Structure of the ionogram, displaying the F3 layer, was more distinct and clear during MSA than LSA periods especially during spring to summer. Start time of occurrence of the F3 layer is about at 0830–0900 LT and is approximately the same for LSA and MSA conditions. The average duration time of the F3 layer occurrence was 181 min per day under F_10.7 = 75 condition, 263 min in F_10.7 = 99 and 358 min in F_10.7 = 125, respectively. The differences of h′F2 and h′F3 exhibited obvious semiannual variation observed at Fuke from March 2010 to June 2012 and increased with increasing solar activity. The difference of foF2 and foF3 in the months February, March, September, October and November is less evident in the middle solar activity period 2011–2012 than the low solar activity 2010 and in the other period it shows a slight increase (0.5 MHz) or keeps constant. The results show that the solar activity dependence of the F3 layer occurrence at low latitude away from the magnetic equator is different from that at near the magnetic equator.     

The variability and IRI2007-predictability of hmF2 over South Africa

This paper presents an investigation into the variability and predictability of the maximum height of the ionospheric F2 layer, hmF2 over the South African region. Data from three South African stations, namely Madimbo (22.4°S, 26.5°E, dip angle: −61.47°), Grahamstown (33.3°S, 26.5°E, dip angle: −64.08°) and Louisvale (28.5°S, 21.2°E, dip angle: −65.44°) were used in this study. The results indicate that hmF2 shows a larger variability around midnight than during the daytime for all seasons. Monthly median hmF2 values were used in all cases and were compared with predictions from the IRI-2007 model, using the URSI (Union Radio-Scientifique Internationale) coefficient option. The analysis covers the diurnal and seasonal hourly hmF2 values for the selected months and time sectors e.g. January, July, April and October for 2003 and 2005. The time ranges between (03h00–23h00 UT; LT = UT + 2h) representing the local sunrise, midday, sunset and midnight hours. The time covers sunrise, midday, sunrise, and midnight hours (03–06h00 UT, 07–11h00 UT, sunrise 16–18h00 UT and 22–23h00 UT; LT = UT + 2h). The dependence of the results on solar activity levels was also investigated. The IRI-2007 predictions follow fairly well the diurnal and seasonal variation patterns of the observed hmF2 values at all the stations. However, the IRI-2007 model overestimates and underestimates the hmF2 value during different months for all the solar activity periods.     

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.     

Cross-hemisphere comparison of mid-latitude ionospheric variability during 1996–2009: Juliusruh vs. Hobart

Analysis of a long-time series of hourly median characteristics of the ionospheric plasma at two mid-latitude locations in the Northern and Southern hemisphere, Juliusruh (54.6N; 13.4E) and Hobart (42.9S; 147.3E), reveals patterns of their synchronous and independent variability. We studied timelines of GPS vTEC, ionogram-derived F2-layer peak electron density NmF2, ionospheric equivalent slab thickness τ, and their ratios at two locations during the complete 23rd solar cycle and its following period of the extremely low solar activity in 2008–2009. This study has also involved the comparative analysis of the observed data versus the model predictions by IRI-2012. During the high solar activity in 2000–2002, seasonal variations show a complicated cross-hemisphere behavior influenced by the winter and semi-annual anomalies, with the largest noon-time values of TEC and NmF2 observed around equinoxes. Strength of the winter anomaly in NmF2 was significantly greater at Juliusruh in comparison with Hobart. The winter anomaly in GPS vTEC values was much weaker than in NmF2 for the Northern hemisphere mid-latitudes and was entirely absent at the Southern hemisphere. Cross-hemisphere analysis of the equivalent slab thickness shows its clear seasonal dependence for all levels of solar activity: the day-time maximum τ_max is observed during local summer, whereas the day-time minimum τ_min is observed during local winter. The night-time values of τ were higher compared to the day-time values during the winter and equinox seasons. Comparative model-data study shows rather good IRI performance of the day-time NmF2 for mid-latitudes of both hemispheres and rather noticeable overestimations for the mid-night NmF2 values during high solar activity. Analysis of IRI vTEC demonstrates the model limitations, related with the absence of the plasmaspheric part, and actual demand in a reliable and standard ionosphere–plasmasphere model for analysis of GPS vTEC.     

An investigation of the solar cycle impact on the lower thermosphere O(S) nightglow emission as observed by WINDII/UARS

The yearly variation of the integrated emission rate of the O(¹S) nightglow in the lower thermosphere is studied and the solar cycle impact is examined from the observations of the Wind Imaging Interferometer (WINDII) operated on the Upper Atmosphere Research Satellite (UARS). More than 300,000 volume emission rate profiles of the O(¹S) nightglow observed by WINDII for 40°S–40°N latitudes during November 1991–August 1997 over half of a solar cycle are utilized. These profiles are vertically integrated for the altitude range of 80–100 km and the equivalent column integrated emission rates are then zonally averaged for bins with 10° latitude and 3 month intervals. It is found that for each latitude the O(¹S) nightglow emission rate appears to increase with increasing solar F10.7 cm flux, following a linear relationship. This characterizes the solar cycle impact on the O(¹S) nightglow, while the solar influence is modulated by a seasonal variation. Based on these variations, an empirical formula is derived for predicting the three-month averages of the O(¹S) nightglow integrated emission rate. The standard error of the estimated values from the formula is smaller than 30 Rayleigh.     

Variability study of ionospheric total electron content at crest of equatorial anomaly in China from 1997 to 2007

The variation of TEC data at Wuhan station (geographic coordinate: 30.5°N, 114.4°E; geomagnetic coordinate: 19.2°N, 183.8°E) at crest of equatorial anomaly in China from January 1997 to December 2007 were analyzed. Variability with solar activity, annual, semiannual, diurnal and seasonal variation were also analyzed. The MSIS00 model and ISR model were used to analyze the possible mechanisms of the variabilities found in the results. The TEC data in 1997 and 2001 deduced from another crest station Xiamen (geographic coordinate: 24.4°N, 118.1°E; geomagnetic coordinate: 13.2°N, 187.4°E) were used to contrast. Analysis results show that long-term variations of TEC at Xiamen station are mainly controlled by the variations of solar activities. Typical diurnal variation behaves as a minimum of the TEC in the pre-dawn hours around 05:00–06:00LT and a maximum on the afternoon hours around 13:00–15:00LT. Some features like the semiannual anomaly and winter anomaly in TEC have been reported. The anomaly may be the result of common action of the electric field over the magnetic equatorial and the [O/N₂] at the crest station.     

Comparison of GPS TEC measurements with IRI-2007 TEC prediction over the Kenyan region during the descending phase of solar cycle 23

This paper presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service receiver (formerly IGS) at Malindi (2.9°S, 40.1°E), Kenya for the periods 2004–2006 during the declining phase of solar cycle 23. The diurnal, monthly and seasonal variations of the TEC are compared with TEC from the latest International Reference Ionosphere model (IRI-2007). The GPS–TEC exhibits features such as an equatorial noon time dip, semi-annual variations, Equatorial Ionization Anomaly and day-to-day variability. The lowest GPS–TEC values are observed near the June solstice and September equinox whereas largest values are observed near the March equinox and December solstice. The mean GPS–TEC values show a minimum at 03:00 UT and a peak value at about 10:00 UT. These results are compared with the TEC derived from IRI-2007 using the NeQuick option for the topside electron density (IRI–TEC). Seasonal mean hourly averages show that IRI-2007 model TEC values are too high for all the seasons. The high prediction primarily occur during daytime hours till around midnight hours local time for all the seasons, with the highest percentage deviation in TEC of more 90% seen in September equinox and lowest percentage deviation in TEC of less than 20% seen in March equinox. Unlike the GPS–TEC, the IRI–TEC does not respond to geomagnetic storms and does overestimate TEC during the recovery phase of the storm. While the modeled and observed data do correlate so well, we note that IRI-2007 model is strongly overestimating the equatorial ion fountain effect during the descending phase of solar cycle, and this could be the reason for the very high TEC estimations.     

Ionospheric variability at taiwan low latitude station: Comparison between observations and IRI-2001 model

This paper presents the observed ionospheric F-region critical frequency, foF2, and peak height, hmF2, at northern crest of equatorial ionization anomaly (EIA) area station, namely Chung-Li (24.9°N, 121.1°E, dip 35°), and to be compared with International Reference Ionosphere model (IRI-2001) predictions for the period from 1994 to 1999, corresponding to half of the 23rd solar cycle. The diurnal and seasonal variation of foF2 and hmF2 are analyzed for different solar phases, respectively. The result shows the largest discrepancies were observed during nighttime for foF2 and hmF2, respectively. The value of foF2 both CCIR and URSI selected in the IRI model produced a good agreement during the daytime and underestimated during the noon time for high solar activities. The underestimation at noon time is mainly caused by the fountain effect from equator. Further, the peak height hmF2 shows a larger variability around the midnight than daytime in the equinox and winter seasons and reserved in summer, respectively. The study shows that the monthly median values of observed hmF2 is somewhat lower than those predicated by the IRI model, at night time in all the seasons except the period of 04:00–06:00 LT and reverse at daytime in summer. In general the IRI model predictions with respect to the observed in hmF2 is much better than foF2. The percentage deviation of the observed foF2 (hmF2) values with respect to the IRI model varies from 5% to 80% (0–25%) during nighttime and 2–17% (0–20%) at daytime, respectively. In general, the model generates good results, although some improvements are still necessary to implement in order to obtain better simulations for ionospheric low-latitudes region.     

Statistical characteristics of low latitude Pc4s: Influence of solar wind and IMF parameters

Pc4 signatures for the year 2013, extracted from geomagnetic north–south and east–west components of induction coil magnetometer (LEMI 30) from low latitude station Desalpar (DSP), operated by Institute of Seismological Research (ISR), India have been investigated vis-à-vis the prevalent interplanetary parameters (IMF) as well as the geomagnetic activity indices. A clear dominance of Pc4-5 (467 events) over Pc3 (17 events) is observed. Local time variation of Pc4 shows a peak in the noon sector in both X and Y components. Our investigations show that the dominant peak frequency is 10 mHz at low latitude region. Correlations with solar wind and IMF parameters illustrate highest occurrence of Pc4 for a solar wind speed of 300–400 km/s and average IMF B field of 3–6 nT. The amplitude of Pc4s at DSP shows an increase with increasing solar wind speed, plasma density, solar wind dynamic pressure and average B field which is also reflected in the trend of frequency variation of these pulsations. We report that IMF clock angle at low latitude does not have influence on Pc4 occurrence. Based on the characteristics of these events, detected in latitudinally distributed stations from low and mid-latitudes from northern and southern hemisphere, we infer that modes were compressional, which could be driven by K-H instability or solar wind dynamic pressure, as compressional modes can propagate to low latitude with little attenuation.