Day-to-day variability of equatorial anomaly in GPS-TEC during low solar activity period

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionization anomaly (EIA) crest region is investigated by using dual-frequency signals of the Global Positioning System (GPS) acquired from Rajkot (Geog. Lat. 22.29°N, Geog. Long. 70.74°E; Geom. Lat. 14.21°N, Geom. Long. 144.90°E), India. The day-to-day variability of EIA characteristics is examined during low solar activity period (F10.7∼83 sfu). It is found that the daily maximum TEC at EIA crest exhibits a day-to-day and strong semi-annual variability. The seasonal anomaly and equinoctial asymmetry in TEC at EIA is found non-existent and weaker, respectively. We found a moderate and positive correlation of daily magnitude of crest, Ic with daily F10.7 and EUV fluxes with a correlation coefficient of 0.43 and 0.33, respectively indicating an existence of a short-term relation between TEC at EIA and the solar radiation even during low solar activity period. The correlation of daily Ic with Dst index is also moderate (r = −0.35), whereas no correlation is found with the daily Kp index (r = 0.14) respectively. We found that the magnitude of EIA crest is moderately correlated with solar flux in all seasons except winter where it is weakly related (0.27). The magnitude of EIA crest is also found highly related with EEJ strength in spring (r = 0.69) and summer (r = 0.65) than autumn (0.5) and winter (r = 0.47), though EEJ is stronger in autumn than spring.     

Variation in the ionospheric propagating factor M(3000)F2 at Ouagadougou,Burkina Faso

We use hourly monthly median values of propagation factor M(3000)F2 data observed at Ouagadougou Ionospheric Observatory (geographic12.4°N, 1.5°W; 5.9^o dip), Burkina Faso (West Africa) during the years Januar1987–December1988 (average F10.7 < 130 × 10⁻²² W/m²/Hz, representative of low solar flux conditions) and for January 1989–December1990 (average F10.7 ? 130 × 10⁻²² W/m²/Hz, representative of high solar epoch) for magnetically quiet conditions to describe local time, seasonal and solar cycle variations of equatorial ionospheric propagation factor M(3000)F2 in the African region. We show that that seasonal trend between solar maximum and solar minimum curves display simple patterns for all seasons and exhibits reasonable disparity with root mean square error (RMSE) of about 0.31, 0.29 and 0.26 for December solstice, June solstice and equinox, respectively. Variability Σ defined by the percentage ratio of the absolute standard deviation to the mean indicates significant dissimilarity for the two solar flux levels. Solar maximum day (10–14 LT) and night (22–02 LT) values show considerable variations than the solar minimum day and night values. We compare our observations with those of the IRI 2007 to validate the prediction capacity of the empirical model. We find that the IRI model tends to underestimate and overestimate the observed values of M(3000)F2, in particular, during June solstice season. There are large discrepancies, mainly during high solar flux equinox and December solstice between dawn and local midnight. On the other hand, IRI provides a slightly better predictions for M(3000)F2 between 0900 and 1500 LT during equinox low and high solar activity and equinox high sunspot number. Our data are of great importance in the area of short-wave telecommunication and ionospheric modeling.     

Equatorial vertical E × B drift velocities inferred from ionosonde measurements over Ouagadougou and the IRI-2007 vertical ion drift model

F-region vertical plasma drift velocities were deduced from the hourly hmF2 values acquired from ionogram data over a near dip equatorial station Ouagadougou (12.4°N, 358.5°E, dip angle 5.9°N) in Africa. Our results are compared against the global empirical model of Scherliess and Fejer (1999) incorporated in the IRI model (IRI-2007) for 1600 to 0800 LT from 1 year of data during sunspot maximum year of 1989 (yearly average solar flux intensity, F10.7 = 192) corresponding to the peak phase of solar cycle 22, under magnetically quiet conditions. The drifts are entirely downward between 2000 and 0500 LT bin for both techniques and the root mean square error (RMSE) between the modeled and the ionosonde vertical plasma drifts during these periods is 3.80, 4.37, and 4.74 m/s for June solstice, December solstice and equinox, respectively. Ouagadougou average vertical drifts show evening prereversal enhancement (PRE) velocity peaks (V_ZP) of about 16, 14, and 17 m/s in June solstice, December solstice, and equinox, respectively, at 1900–2000 LT; whereas global empirical model average drifts indicate V_ZP of approximately 33 m/s (June solstice), 29 m/s (December solstice), and 50 m/s (equinox) at 1800 LT. We find very weak and positive correlation (+0.10376) between modeled V_ZP versus F10.7, while ionosonde V_ZP against F10.7 gives worst and opposite correlation (−0.05799). The results also show that modeled V_ZP–Ap indicates good and positive correlation (+0.64289), but ionosonde V_ZP–Ap exhibits poor and negative correlation (−0.22477).     

Solar activity dependence of total electron content derived from GPS observations over Mbarara

Vertical total electron content (VTEC) observed at Mbarara (geographic co-ordinates: 0.60°S, 30.74°E; geomagnetic coordinates: 10.22°S, 102.36°E), Uganda, for the period 2001–2009 have been used to study the diurnal, seasonal and solar activity variations. The daily values of the 10.7 cm radio flux (F_10.7) and sunspot number (R) were used to represent Solar Extreme Ultraviolet Variability (EUV). VTEC is generally higher during high solar activity period for all the seasons and increases from 0600 h LT and reaches its maximum value within 1400 h–1500 h LT. All analysed linear and quadratic fits demonstrate positive VTEC-F_10.7 and positive VTEC-R correlation, with all fits at 0000 h and 1400 h LT being significant with a confidence level of 95% when both linear and quadratic models are used. All the fits at 0600 h LT are insignificant with a confidence level of 95%. Generally, over Mbarara, quadratic fit shows that VTEC saturates during all seasons for F_10.7 more than 200 units and R more than 150 units. The result of this study can be used to improve the International Reference Ionosphere (IRI) prediction of TEC around the equatorial region of the African sector.     

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.     

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).     

Determination of the threshold value of F10.7 in the dependence of foF2 on solar activity

By introducing the two-segmented linear regression model instead of the well known quadratic fit, we were able to describe the solar activity dependence of the F2 critical frequency. Saturation features were observed and the corresponding F10.7 values at which this phenomenon occurs were obtained for different hours. The seasonal average values were found to be around 154 sfu, 138 sfu, 177 sfu and 150 sfu for March equinox, June solstice, September equinox and December solstice respectively. These affirmed that saturation phenomenon is more pronounced at the equinoxes than solstices. On the average, the threshold value of F10.7 was obtained to be 154.5 sfu for this station in the African sector of the equatorial region.     

Equatorial Ionospheric Anomaly (EIA) and comparison with IRI model during descending phase of solar activity (2005–2009)

The ionospheric variability at equatorial and low latitude region is known to be extreme as compared to mid latitude region. In this study the ionospheric total electron content (TEC), is derived by analyzing dual frequency Global Positioning System (GPS) data recorded at two stations separated by 325 km near the Indian equatorial anomaly region, Varanasi (Geog latitude 25°, 16^/ N, longitude 82°, 59^/ E, Geomagnetic latitude 16°, 08^/ N) and Kanpur (Geog latitude 26°, 18^/ N, longitude 80°, 12^/ E, Geomagnetic latitude 17°, 18^/ N). Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the descending phase of solar activity from 2005 to 2009. It is found that the maximum TEC (EIA) near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semi-annual variation is seen with two maxima occurring in both spring and autumn. Results also showed the presence of winter anomaly or seasonal anomaly in the EIA crest throughout the period 2005–2009 only except during the deep solar minimum year 2007–2008. The correlation analysis indicate that the variation of EIA crest is more affected by solar activity compared to geomagnetic activity with maximum dependence on the solar EUV flux, which is attributed to direct link of EUV flux on the formation of ionosphere and main agent of the ionization. The statistical mean occurrence of EIA crest in TEC during the year from 2005 to 2009 is found to around 12:54 LT hour and at 21.12° N geographic latitude. The crest of EIA shifts towards lower latitudes and the rate of shift of the crest latitude during this period is found to be 0.87° N/per year. The comparison between IRI models with observation during this period has been made and comparison is poor with increasing solar activity with maximum difference during the year 2005.     

Evidence for the lowering of the centroid of daytime thermospheric O(D) 630.0 nm emission over the magnetic equator: First results

It is shown in this paper for the first time that the intensity of the daytime thermospheric O(¹D) 630.0 nm airglow as measured by the ground-based dayglow photometer over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), a geomagnetic dip equatorial station, exhibit a direct correlation with the electron density at 180 km. This altitude is about ∼40 km lower than the believed centroid of the O(¹D) 630.0 nm dayglow emission i.e. 220 km. This observation is contrary to the understanding of the behavior of O(¹D) 630.0 nm dayglow over equatorial/low latitudes. Over these latitudes, the variations of the measured intensity of O(¹D) 630.0 nm dayglow are known to be associated with the changes in the electron density at altitudes around 220 km, the centroid of this emission. In this context, the present results indicating the lowering of the peak altitude of O(¹D) 630.0 nm emission from ∼220 to ∼180 km over the dip equator is new. Recent results on solar XUV flux indicate that this could be an important parameter that controls the O(¹D) 630.0 nm dayglow excitation rates through modulations in the neutral and ionic composition in lower thermosphere-ionosphere region. However, the lowering of the centroid of O(¹D) 630.0 nm emission, as shown in this study, has been ascribed primarily to the fountain effect associated with the equatorial ionization anomaly.     

Longitudinal distribution of thermospheric densities and ionization rates in the upper atmosphere of Mars at mid-latitude using MGS ACC data

The Accelerometer Experiment (ACC) onboard Mars Global Surveyor (MGS) measured 1600 density profiles in the upper atmosphere of Mars during aerobraking. These measurements reveal large-scale and small-scale structure in the thermosphere of Mars. Here, the measurements of mass density for 115 orbits (#P0670–P0789) from November 1 to 30, 1998, under spring equinox and medium solar activity conditions (average F_10.7 ∼ 137) during phase 2 of the aerobraking in the thermosphere of Mars at different altitudes and longitudes are presented for northern mid-latitude (17–42°N) in the dayside atmosphere using ACC onboard MGS. From these mass densities, the neutral densities of different gases are derived from their mixing ratios. Using these neutral densities, the longitudinal distribution of photoionization rates and photoelectron impact ionization rates are calculated at wavelength range 1–102.57 nm due to EUV and soft X-ray radiation under photochemical controlled region using Analytical Yield Spectrum approach (AYS). These conditions are appropriate for MGS Phase 2 aerobraking period from which the accelerometer data is used. Under the photochemical equilibrium condition, the electron density near the peak varies as the square root of the total peak ionization rate. Using this fact, an attempt is being made to estimate the mean primary and secondary peak electron density by averaging the longitudinal variations of total peak ionization rates in the northern mid-latitude (17–42°N) ionosphere of Mars, as there is no radio science measurement at this latitude region by MGS.     

Variation of ionospheric total electron content in Taiwan region of the equatorial anomaly from 1994 to 2003

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionospheric anomaly (EIA) region is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a chain of nine observational sites clustered around Taiwan (21.9–26.2°N, 118.4–112.6°E). In this study, we present results from a statistical study of seasonal and geomagnetic effects on the EIA during solar cycle 23: 1994–2003. It is found that TEC at equatorial anomaly crests yield their maximum values during the vernal and autumnal months and their minimum values during the summer (except 1998). Using monthly averaged I_c (magnitude of TEC at the northern anomaly crest), semi-annual variations is seen clearly with two maxima occurring in both spring and autumn. In addition, I_c is found to be greater in winter than in summer. Statistically monthly values of I_c were poorly correlated with the monthly Dst index (r = −0.22) but were well correlated with the solar emission F_10.7 index (r = 0.87) for the entire database for the period during 1994–2003. In contrast, monthly values of I_c were correlated better with Dst (r ? 0.72) than with F_10.7 (r ? 0.56) in every year during the low solar activity period (1994–1997). It suggests that the effect of solar activity on I_c is a longer term (years), whereas the effect of geomagnetic activity on I_c is a shorter term (months).     

Longitudinal differences in the equatorial spread F characteristics between Vietnam and Brazil

We present the results of a comparative study of the equatorial spread F (ESF) and the F layer critical parameter, the base height of the F layer bottomside (h′F) over the two equatorial sites, Ho Chi Minh City – HCM (dip latitude: 2.9°N) in Vietnam and Sao Luis – SL (dip latitude: ∼2°S) in Brazil. The study utilizes simultaneous data collected by a CADI at HCM and a digisonde at SL during the year 2002 with the monthly mean solar 10.7 cm flux (F10.7) varying from ∼120 to ∼185. This study focuses on the quiet time seasonal behavior of the F layer parameters in the two widely separated longitude sectors, and addresses the question as to what can we learn from such comparative studies with respect to the ambient ionospheric and thermospheric parameters that are believed to control the ESF generation and hence its longitudinal occurrence pattern. The observed differences/similarities in the diurnal and seasonal patterns of the F Layer height vis-à-vis the ESF occurrences are evaluated in terms of the known longitudinal differences in the F layer heights, thermospheric meridional winds and the geomagnetic peculiarities of the two sites.     

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.     

Equatorial F-region vertical ion drifts during quiet solar maximum

Median values of ionosonde h′F data acquired at Ibadan (Geographic:7.4°N, 3.9°E, Magnetic: dip 6°S, and magnetic declination, 3°W), Nigeria, West Africa, have been used to determine vertical ion drift (electric field) characteristics in the postsunset ionosphere in the African region during a time of high solar activity (average F_10.7 −208). The database spans from January and December 1958 during the era of International Geophysical Year (IGY) for geomagnetic quiet conditions. Bimonthly averaged diurnal variations patterns are very similar, but differ significantly in magnitude and in the evening reversal times. Also, monthly variations of F-region vertical ion drift reversal times inferred from the time of h′F maximum indicates early reversal during equinoxes and December solstice months except for the month of April. Late reversal is observed during the June solstice months. The equatorial evening prereversal enhancement in vertical ion drift (V_zp) occurs largely near 1900 LT with typical values 20–45 m/s. Comparison of Ibadan ionosonde V_zp with the values of prereversal peak velocity reported for Jicamarca (South America), Kodaikanal (India), and Scherliess and Fejer global model show considerable disparity. The changes of postsunset peak in virtual height of F-layer (h′F_P) with prereversal velocity peak V_zp are anti-correlated. Investigation of solar effects on monthly values of V_zp and h′F_P revealed that these parameters are independent of monthly averaged solar flux intensity during quiet-time sunspot maximum conditions.     

Study of maximum electron density NmF2 at Karachi and Islamabad during solar minimum (1996) and solar maximum (2000) and its comparison with IRI

The monthly hourly medians of maximum electron density, N_mF₂, at two Pakistani ionospheric stations, Karachi and Islamabad, have been determined for solar minimum (1996) and solar maximum (2000) and compared with IRI predictions using the URSI coefficients. At night and pre-noon period the N_mF₂ values at both stations are almost equal during the 2 years. However, at post-noon the values at Karachi are considerably larger than those at Islamabad due to the equatorial or geomagnetic anomaly. Karachi (geomag. coord. 16.44°N, 139.08°E) lies near the region of the equatorial anomaly (+20 and −20 geomagnetic latitude), so most of the N_mF₂ values at Karachi are larger than those at Islamabad (geomag. coord. 24.46°N, 145.67°E). The maximum monthly values of N_mF₂ show a semi-annual variation at Karachi and Islamabad both during 1996 and 2000 as predicted by IRI.     

Photoelectron flux variations observed from the FAST satellite

This paper examines high resolution (ΔE/E = 0.15) photoelectron energy spectra from 10 eV to 1 keV, created by solar irradiances between 1.2 and 120 nm. The observations were made from the FAST satellite at ∼3000 km, equatorward of the auroral oval for the July–August, 2002 solar rotation. These data are compared with the solar irradiance observed by the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and fluxes calculated using the Field Line Interhemispheric Plasma (FLIP) code. The 41 eV photoelectron flux, which corresponds to solar EUV fluxes near 20 nm, shows a clear solar rotation variation in very good agreement with the EUV flux measurements. This offers the possibility that the 41 eV photoelectron flux could be used as a check on measured solar EUV fluxes near 20 nm. Because of unexpected noise, the solar rotation signal is not evident in the integral photoelectron flux between 156 and 1000 eV corresponding to EUV wavelengths between 0.1 and 7 nm measured by the SEE instrument. Examination of daily averaged photoelectron fluxes at energies between 25 and 500 eV show significant changes in the photoelectron spectra in response X and M class flares. The intensity of photoelectrons produced in this energy region is primarily due to two very narrow EUV wavelength regions at 2.3 and 3 nm driving Auger photoionization in O at 500 eV and N₂ at ∼360 eV. Comparison of calculated and daily averaged electron fluxes shows that the HEUVAC model solar spectrum used in the FLIP code does not reproduce the observed variations in photoelectron intensity. In principle, the 21 discrete photoelectron energy channels could be used to improve the reliability of the solar EUV fluxes at 2.3 and 3 nm inferred from broad band observations. In practice, orbital biases in the way the data were accumulated and/or noise signals arising from natural and anthropogenic longitudinally restricted sources of ionization complicate the application of this technique.     

Comparison between African equatorial station ground-based inferred vertical E × B drift,Jicamarca direct measured drift,and IRI model

The Incoherent Scatter Radar measurement over Jicamarca, together with the IRI model-2007 measurements were compared with ground-based digisonde inferred E × B drift over Ilorin in the African region during year of solar minima (F10.7 = 81). Seasonally, Ilorin pre-reversal enhancement (PRE) had peak drift velocities of 7.2, 3.7 and 7.9 m/s for March equinox, September equinox and December solstice respectively, while Jicamarca drifts indicated 13.0, 10.5 and 5.2 m/s; as well as the IRI model with 14.3, 8.4 and 0.7 m/s in similar order. PRE value was insignificant during June solstice. The PRE magnitude of the IRI-model during the equinoxes is twice the value obtained at Ilorin. The daytime E × B drift peaked over Ilorin 1–2 h earlier than both the modeled and Jicamarca observations. This could be due to the difference in sunset time at the conjugate points corresponding to the altitude of the observation. During the evening time PRE, the respective correlation coefficients (R) for Vz–F10.7 relation over Jicamarca, Ilorin and the modeled observations are −0.5559, 0.4796 and −0.4979. Similarly, the Vz–Ap relation exhibit excellent anti-correlation coefficient (R = −0.8637) for the IRI-model, −0.4827 over Jicamarca and 0.3479 for Ilorin. Annual mean drift velocities over Jicamarca, Ilorin and IRI model measurements respectively are 10, 5.6 and 10 m/s for the peak PRE observation; 15, 16 and 21 m/s for the daytime pre-sunrise peak values; and −21, −9 and −16 m/s for the nighttime downward reversals. The root-mean square (RMS) deviation between IRI-model and the Ilorin drift between 2000 and 0500 h is 4.37, 2.03, 3.71 and 2.42 m/s for March equinox, June solstice, September equinox and December solstice respectively. For Jicamarca–Ilorin drift relation, RMS deviation is 5.48, 2.30, 3.47 and 1.27 m/s in the same order respectively. Annual hmF2 inferred drift over Ilorin during daytime is higher by a factor of ≈2 and 3 at Jicamarca and IRI model measurements respectively; and by a factor of ≈5 for both during the night-time period. The limitations in using hmF2 to infer drifts are discussed.     

The variation of equatorial spread-F occurrences observed by ionosondes at Thailand longitude sector

The equatorial spread-F (ESF) is a phenomenon of ionopheric irregularities which are mainly generated by the generalized Rayleigh–Taylor (R–T) instability mechanism in conjunction with the other physical mechanisms, originated at the bottom side of the F-layer in the equatorial region after sunset. It degrades the quality of signals that propagate through these irregularities, especially in the navigation satellite system, which requires the high integrity signals. In this work, we analyze the ESF statistics obtained from the FM/CW ionosonde stations over Thailand longitude sector. One is at Chumphon (10.72°N, 99.37°E, dip latitude 3.0°), located near the geomagnetic equator, and the other station is located at Chiangmai (18.76°N, 98.93°E, dip latitude 12.7°). Both stations are as part of the South-East Asia Low Latitude Ionospheric Network (SEALION) project. The ionograms are obtained at every 15 min from September 2004 to August 2005, which has the monthly mean of solar 10.7 cm flux (F10.7) from ∼80 to ∼110. In addition, we compare the diurnal patterns between the ESF occurrences and the variation of virtual height of the F-layer bottom side (h’F) of these two stations. The results show that the ESF occurrences at Chumphon stations are higher than Chiangmai station in all seasons. The high ESF occurrences of both stations mostly occur in equinoctial months corresponded with the rapid rising of the monthly mean h’F in the post-sunset. However, some inconsistent results are still observed, implying the role of other factors such as gravity waves and planetary waves to ESF occurrences.     

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.