Low Latitude Indian Station Research Articles

A Comparative Analysis of the Effect of Orbital Geometry and Signal Frequency on the Ionospheric Scintillations over a Low Latitude Indian Station: First Results from the 25th Solar Cycle

The equatorial post-sunset ionospheric irregularities induce rapid fluctuations in the phase and amplitude of global navigation satellite system (GNSS) signals which may lead to the loss of lock and can potentially degrade the position accuracy. This study presents a new analysis of L-band scintillation from a low latitude station at Guntur (Geographic 16.44°N, 80.62°E, dip 22.18°), India, for the period of 18 months from August 2021 to January 2023. The observations are categorized either in the medium Earth-orbiting (MEO) or geosynchronous orbiting (GSO) satellites (GSO is considered as a set of the geostationary and inclined geosynchronous satellites) for L1, L2, and L5 signals. The results show a higher occurrence of moderate (0.5 < S4 ≤ 0.8) and strong (S4 > 0.8) scintillations on different signals from the MEO compared to the GSO satellites. Statistically, the average of peak S4 values provides a higher confidence in the severity of scintillations on a given night, which is found to be in-line with the scintillation occurrences. The percentage occurrence of scintillation-affected satellites is found to be higher on L1 compared to other signals, wherein a contrasting higher percentage of affected satellites over GSO than MEO is observed. While a clear demarcation between the L2/L5 signals and L1 is found over the MEO, in the case of GSO, the CCDF over L5 is found to match mostly with the L1 signal. This could possibly originate from the space diversity gain effect known to impact the closely spaced geostationary satellite links. Another major difference of higher slopes and less scatter of S4 values corresponding to L1 versus L2/L5 from the GSO satellite is found compared to mostly non-linear highly scattered relations from the MEO. The distribution of the percentage of scintillation-affected satellites on L1 shows a close match between MEO and GSO in a total number of minutes up to ~60%. However, such a number of minutes corresponding to higher than 60% is found to be larger for GSO. Thus, the results indicate the possibility of homogeneous spatial patterns in a scintillation distribution over a low latitude site, which could originate from the closely spaced GSO links and highlight the role of the number of available satellites with the geometry of the links, being the deciding factors. This helps the ionospheric community to develop inter-GNSS (MEO and GSO) operability models for achieving highly accurate positioning solutions during adverse ionospheric weather conditions.

Evaluating the single-frequency static precise point positioning accuracies from multi-constellation GNSS observations at an Indian low-latitude station

Abstract Multiple constellations and their combinations in the global navigation satellite systems (GNSS) provide a great opportunity for single-station precise point positioning (PPP) models. The PPP models are of more importance for GNSS users as they are cost-effective with reasonable accuracy. There are abundant models in the market that use different data processing techniques based on the location and constellations used. In this study, we used the precise point positioning (PPP) software Net_Diff to verify the positioning accuracy at a low latitude Indian location using individual global satellite constellations (GPS, GLONASS, Galileo, BeiDou) and their combinations with GPS fixed. The ionospheric correction models such as GIM and Klobuchar are applied in the post-processing to determine the positioning accuracy. The Kalman filter method is applied to model the input data along with including the noise to derive the position solution. The results revealed that the GPS showed constant residual error for both quiet and disturbed days. In terms of single constellation Galileo produced less residuals in WGS 84 and ECEF coordinate systems whereas in the multi-constellation combination GPS, Galileo and GLONASS resulted in residuals of lesser magnitudes compared to the other combinations considered in this study. The RMS and STD of the residuals confirmed the high precision for Galileo and low precision for BeiDou constellations at the location.

ROTI and scintillation index correlation under quiet and disturbed periods over an Indian low latitude station, Waltair

The variation of correlation between ROTI and S4 index is examined by using GPS L1 and L5 signals over an Indian low latitude station, Waltair (17.73° N, 83.37° E) during the period from 2015 to 2017. The effect of geomagnetic storms on the correlation between ROTI and S4 index is the main focus of the present study. The influence of satellite elevation angle and signal frequency on the correlation between ROTI and S4 index is also investigated during different seasons and geomagnetic conditions. The correlation is found to vary between 0.28 and 0.63 for elevation angles above 30⁰ and between 0.56 and 0.87 for the elevation angles above 50⁰. The correlation coefficient values are decreasing with solar activity from 2015 to 2017 during quiet and disturbed geomagnetic periods. From the analysis on the seasonal correlation between ROTI and S4 index, it is found that solar activity dependent seasonal correlation is observed for both GPS L1 and L5 signals over lower elevation angle 30⁰. The correlation between ROTI and S4 index is increasing over higher elevation angles and for lower signal frequency.

Observation of ionospheric Alfven resonator with double spectral resonance structures at low latitude station, Shillong (dipole L=1.08)

Spectral Resonance Structures (SRS) of Ionospheric Alfven Resonator (IAR) are investigated by analysing the magnetic field data of a high sampling frequency induction coil magnetometer, installed at a low latitude Indian station, Shillong(25.56^circ N, 91.86^circ E, dipole L=1.08 ). One of the distinguishing features of IAR resonance structure observed at this latitude is the excitation of two spectral resonance structures simultaneously, one with a small frequency separation and the other with a large frequency separation of subsequent harmonics. The seasonal variation shows that the occurrence of double SRS is more favoured during winter, where, out of the total IAR events observed during winter, about 47% show clear signatures of double SRS and no double SRS are observed during summer. We have examined the altitudinal variation of refractive index using IRI 2016 model to understand the role of local ionospheric conditions in the formation of double spectral resonance structures. Our study reveals that the excitation of double SRS is related to the E-region to F-region variability of refractive index of the ionosphere. We also report that the double spectral resonance structures at Shillong are always observed only in the {B}_{x} component of the magnetic field variation.Graphical

The Role of Storm‐Time Electrodynamics in the Dawn and Dusk Sectors Across Equatorial and Low‐Latitude Ionosphere During December 19–21, 2015

AbstractWe present the distinct responses of the nighttime ionosphere due to an intense and long lasting geomagnetic storm of December 19–21, 2015 using a chain of ionosondes from magnetic equator to anomaly crest region over India. The observations exhibited unique features throughout the night including dawn and dusk sectors. The results showed twin episodes of pre‐reversal enhancement (PRE) in dusk sector associated with oscillatory vertical plasma drift due to shielding electric field that produced briefly post‐sunset equatorial spread F (ESF) irregularities. Accordingly, relative dominance of interplanetary magnetic field (IMF) and auroral electrojet (AE) related electric field fluctuations in vertical and zonal drifts at different times are investigated throughout the night. Also the application of wavelet/wavelet coherence analysis on interplanetary and geomagnetic conditions and ionospheric data showed distinct relationships. The results showed that vertical drift is in phase with interplanetary electric fields (IEFy) with period of ∼30 min at ∼18:30 IST (13:00 UT) that drives first PRE whereas ∼60 min period in AE which is in phase with vertical drifts drives second PRE at ∼20:30 IST (15:00 UT). However, other short term periods in vertical drifts are in phase with IEFy and AE at other times suggesting that both are contributing to the drift variations. The simultaneous upliftment of F layer at all stations producing fresh ESF irregularities in the dawn sector is attributed to the unique electrodynamical response of the Indian low latitude stations to the eastward disturbance dynamo electric fields (DDEFs). These results are also in concomitant with satellite observations and thermospheric ionospheric electrodynamics general circulation model (TIE‐GCM) drifts.

Earth’s Atmospheric Dynamics and Temperature and Pressure Profiles Over a Low-Latitude Indian Station Using Different Remote Sensing Techniques – A Case Study

Background/objectives: Even after plenty of inclusive studies, the earth’s lower atmospheric dynamics and a few important atmospheric parameters (temperature and pressure, etc.) have not yet been completely understood, particularly over equatorial and low-latitude regions. Methods/analysis: An attempt is made here by effectively utilising various ground- and satellite-based remote sensing instrument (Doppler sodar) data over a typical low-latitude Indian station, i.e. Gadanki (13.45°N, 79.16°E), to understand different atmospheric dynamics, and the critical behaviors of temperature and pressure profiles as measured by radiosonde and COSMIC RO techniques are presented. Findings: Zonal, meridional and vertical winds measured using the Doppler sodar showed distinct diurnal and nocturnal variations; most importantly, the evolution of night-time boundary layer was observed. A one-to-one correspondence was seen between radiosonde measured and COSMIC RO measured temperature and pressure profiles. Applications/improvements: This study recommends that it is very much essential to use different available datasets from various remote sensing instruments to completely understand the earth’s atmospheric dynamics since various dynamical interactions play a role in dictating them directly and indirectly near the earth’s surface.Keywords: Low-latitude Station, Doppler Sodar, Radiosonde, COSMIC RO Technique, Atmospheric Dynamics, Nocturnal Boundary Layer

Seasonal, annual, and interannual variability in MLT quasi-two-day waves over the low-latitude region Kolhapur (16.8°N; 74.2°E)

This study presents the quasi-two-day wave (Q2DW) characteristics of the mesosphere and lower thermosphere (MLT) region obtained by taking hourly mean values of horizontal wind velocities for 4 years (August 2013–July 2017) through continuous measurements using a medium-frequency (MF) radar (operating frequency – 1.98 MHz) located at the low-latitude Indian station Kolhapur (16.8°N; 74.2°E). The MF radar located at Kolhapur was upgraded in 2013, and these results of Q2DW have been reported for the first time after upgrading. The present study investigated variability in seasonal, annual, interannual, and solar indices of Q2DWs traveling in zonal (EW) and meridional (NS) components in the MLT region. The Q2DW activity is observed to be stronger during austral summer (January–February) (EW = ∼5 m/s and NS = ∼8–10 m/s) than during boreal summer (June–July) (EW = ∼5 m/s and NS = ∼6–8 m/s). The Q2DW amplitudes are larger in the meridional component than in the zonal one. A strong semiannual oscillation (SAO) has been observed in Q2DWs, with peak during January–February and June–July. In addition, small enhancement is seen in meridional Q2DW in October (∼5–6 m/s). It is observed that the entire spectrum (40–60 h) measured between 86 and 94 km contributes to the SAO amplitudes during January–February and June–July, whereas the waves measured between 42 h and 52 h contribute to enhancement in October similar to that reported elsewhere. In general, the Q2DW amplitude shows large interannual variability. The easterlies developed in the global circulation model in Northern hemisphere during May intensify up to around summer solstice. Q2DW activity peaks during westerly shear zone and intensifies with time at a lower thermospheric altitude (above 90 km). Small positive correlations (r = 0.2 for sunspot number and r = 0.1 for 10.7 cm solar flux) have been observed between Q2DW amplitudes and solar activity.

Comments on the percentage of occurrence methodology used in “a study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station” by H J Tanna, S P Karia and K N Pathak

Following Tanna et al. (2013), we computed the percentage of occurrence of S4 index for the period of 2012–2015 using the data of the dual frequency GPS receiver at the Tripura University, Agartala station (23.76°N, 91.26°E) situated at the northern crest of the equatorial ionization anomaly (EIA) region of the Indian Subcontinent. We have observed discrepancy in the results contradicting the actual scintillation occurrence. The distinctly noticeable discrepancy is that the maximum occurrence month is shifted to April 2013 instead of March 2014. The problem arises due to the denominator term used in the percentage of occurrence ratio i.e. the total number of days of observed scintillation activity during the complete period under consideration. But the conventional percentage of occurrence methodology uses the number of days of observation (the total number of days for which data is available) during each month in the denominator. It correctly assigns the maximum occurrence to March 2014 instead of April 2013 and the obtained monthly statistics follow the solar activity during this period.

Effect of geomagnetic storms on VHF scintillations observed at low latitude

A geomagnetic storm affects the dynamics and composition of the ionosphere and also offers an excellent opportunity to study the plasma dynamics. In the present study, we have used the VHF scintillations data recorded at low latitude Indian station Varanasi ($$\hbox {Geomag. latitude}\,=\,14^{^{\circ }}55^{\prime }$$N, long. = $$154^{^{\circ }}\hbox {E}$$) which is radiated at 250 MHz from geostationary satellite UFO-02 during the period 2011–2012 to investigate the effects of geomagnetic storms on VHF scintillation. Various geomagnetic and solar indices such as Dst index, Kp index, IMF Bz and solar wind velocity (Vx) are used to describe the geomagnetic field variation observed during geomagnetic storm periods. These indices are very helpful to find out the proper investigation and possible interrelation between geomagnetic storms and observed VHF scintillation. The pre-midnight scintillation is sometimes observed when the main phase of geomagnetic storm corresponds to the pre-midnight period. It is observed that for geomagnetic storms for which the recovery phase starts post-midnight, the probability of occurrence of irregularities is enhanced during this time and extends to early morning hours.

Ionospheric response to major storm of 17th March 2015 using multi-instrument data over low latitude station Kolhapur (16.8°N, 74.2°E, 10.6°dip. Lat.)

The optical observations of ionospheric and mesospheric OI 630.0 nm, OI 557.7 nm along with OH emission carried out from low latitude Indian station, Kolhapur (16.8°N, 74.2°E) using CCD based all-sky camera system. The features of night airglow variations observed during the period of a strong geomagnetic storm, which commenced on March 17, 2015, at ∼04:30 UT (10:00 IST (Indian Standard Time = UT + 5.5 h)). Dst of ∼−222 nT was seen in this storm suggest that this is among the strongest. The OI 630.0 nm images on 16, 17 and 18 March show the development of Equatorial Plasma Bubbles (EPBs) and bright intensity regions in OI 630.0 nm emission. Generally, EPBs move from west to east direction but it moved in reverse direction on the strong magnetically disturbed night. The EPBs drift velocity was less by ∼100 m/s than the velocity measured on magnetically quite night 16–17 March 2015. The bright intensity regions are also observed in OI 557.7 nm airglow, but there is no intensity enhancement seen in OH emission. It is also observed that the OI 630.0 nm intensity variation well matches with the GPS VTEC variation for PRN-2. The reversal in EPBs drift velocity and the nightglow intensity variations due to the strong magnetic storm are discussed.

Daytime VHF amplitude scintillations recorded at an Indian low-latitude station, Waltair (17.7°N, 83.3°E) during 1997–2003

In this research, it is presented the daytime amplitude scintillations recorded at VHF frequency (244MHz) at an Indian low-latitude station, Waltair (17.7°N, 83.3°E) during seven continuous years (1997–2003). Contrary to the nighttime scintillation seasonal trends, the occurrence of daytime scintillations maximizes during summer followed by winter and the equinox seasons. The fade depths, scintillation indices and the patch durations of daytime scintillations are meager when compared with their nighttime counterparts. A co-located digital high frequency (HF) ionosonde radar confirms the presence of sporadic (Es) layers when daytime scintillations are observed. The presence of daytime scintillations is evident when the critical frequency of the Es-layer (foEs) is ≥4MHz and Es-layers are characterized by a highly diffuse range spread Es echoes as can be seen on ionograms. It is surmised that the gradient drift instability (GDI) seems to be the possible mechanism for the generation of these daytime scintillations. It is quite likely that the spread Es-F-layer coupling is done through polarization electric fields (Ep) that develop inside the destabilized patches of sporadic E layers, which are mapped up to the F region along the field lines as to initiate the daytime scintillations through the GDI mechanism. Further, the presence of additional stratification of ionosphere F-layer, popularly known as the F3-layer, is observed on ionograms once the Es-layers and daytime scintillations are ceased.

GPS-TEC Variation during Low to High Solar Activity Period (2010-2014) under the Northern Crest of Indian Equatorial Ionization Anomaly Region

The measurements of ionospheric TEC (total electron content) are conducted at a low latitude Indian station Surat (21.16°N, 72.78°E Geog.), which lies under the northern crest of the equatorial anomaly in Indian region. The data obtained are for a period of five years from low to high solar activity (2010- 2014) using GPS (Global Positioning System) receiver. In this study, we report the diurnal and seasonal variation of GPS-TEC, dependence of GPS-TEC with solar activity, geomagnetic condition and EEJ strength. From the seasonal analysis, it is found that greater values of the GPS-TEC are observed during equinox season followed by winter and summer. The appearance (in the year 2011 and 2014) and disappearance (in the year 2010 and 2012) of “winter anomaly” have been observed at the station. From the correlation of GPS-TEC with different solar indices, i.e. solar EUV flux, F10.7 cm solar radio flux and Zurich sunspot number (SSN), it is concluded that the solar index EUV flux is a better controller of GPS-TEC, compared to F10.7 cm and SSN. Further, it is observed that there is no effect of rising solar activity on correlation. Moreover, the percentage variability of GPS-TEC and the standard deviation of GPS-TEC obtained for quiet and disturbed days show that dependence of GPS-TEC on geomagnetic condition is seasonal. Also, there is a positive correlation observed between GPS-TEC and EEJ strength.

Response of OH airglow emissions to mesospheric gravity waves and comparisons with full-wave model simulation at a low-latitude Indian station

Abstract. Quasi-monochromatic gravity-wave-induced oscillations, monitored using the mesospheric OH airglow emission over Kolhapur (16.8° N, 74.2° E), India, during January to April 2010 and January to December 2011, have been characterized using the Krassovsky method. The nocturnal variability reveals prominent wave signatures with periods ranging from 5.2 to 10.8 h as the dominant nocturnal wave with embedded short-period waves having wave periods of 1.5–4.4 h. The results show that the magnitude of the Krassovsky parameter, viz. |η|, ranged from 2.1 to 10.2 h for principal or long nocturnal waves (5.2–10.8 h observed periods), and from 1.5 to 5.4 h for the short waves (1.5–4.4 h observed periods) during the years of 2010 and 2011, respectively. The phase (i.e., Φ) values of the Krassovsky parameters exhibited larger variability and varied from −8.1 to −167°. The deduced mean vertical wavelengths are found to be approximately −60.2 ± 20 and −42.8 ± 35 km for long- and short-period waves for the year 2010. Similarly, for 2011 the mean vertical wavelengths are found to be approximately −77.6 ± 30 and −59.2 ± 30 km for long and short wave periods, respectively, indicating that the observations over Kolhapur were dominated by upward-propagating waves. We use a full-wave model to simulate the response of OH emission to the wave motion and compare the results with observed values.

Nighttime thermospheric meridional winds as inferred from ionosonde parameters over Indian region and their plausible effects on plasma irregularities

In this paper, we present nighttime thermospheric meridional winds at Indian low latitude station namely Hyderabad during March–May 2013 derived using (a) h’F method and (b) hpF2 method based on ionosondes. The estimation of meridional winds using h’F method employs the basic principle that the vertical drift at magnetic equator is purely due to ExB drift, while for a station slightly away from magnetic equator, meridional wind also contributes for the vertical drift in addition to diffusion along the field lines. Assuming the EXB drifts are the same at both the locations, the vertical drift contribution is subtracted from the equatorial vertical drifts and then meridional winds are derived, whereas in hpF2 method, winds are derived using the SERVO theory. The winds so derived are compared with HWM07 wind model. The comparison suggests that magnitude of winds derived using h’F method are in better agreement with HWM07 model than hpF2 derived winds where their polarities are better comparable to HWM07 model than hpF2 method. The poor agreement of hpF2 method could be due to uncertainties involved in estimating various ionospheric parameters. Also the winds derived using h’F method showed signatures of the well known phenomenon of Midnight Temperature Maximum (MTM). Based on these results, we suggest that h’F method is better suited for meridional wind estimates over low latitudes than hpF2 method. Accordingly, we further investigated the role of thermospheric meridional winds in equatorial spread F (ESF) occurrence. Our results suggest that equatorward winds along with post sunset height increase could enhance the Rayleigh Taylor (RT) instability growth rate and thereby leading to the generation of ESF/scintillations. The Superposed Epoch Analysis (SEA) technique has been applied to understand the general trend of h’F and meridional winds around the ESF onset time. The study showed reduction in poleward winds, half an hour prior to the onset of ESF. Hence indicating the role of meridional winds in the generation of plasma irregularities/scintillations in addition to post sunset height rise.

Ionospheric irregularities at low latitude using VHF scintillations during extreme low solar activity period (2008–2010)

In the present study, we have used 250 MHz radio signal radiated by geostationary satellite UFO-02 to study the occurrence characteristics of very high frequency scintillations associated with ionospheric irregularities during recent extreme low solar activity period from 2008 to 2010 at low latitude Indian station Varanasi (Geomag. latitude = 14°55′N, long. = 154°E, Dip angle = 37.3°, Sub-ionospheric dip = 34°). The impact of this recent extreme low solar activity period on ionosphere is investigated. It is observed that the scintillation occurrence is low having maximum percentage occurrence during pre-midnight periods. With increasing interest in understanding the behavior of ionospheric irregularities, an effort has been made to examine also the influence of solar and magnetic activity over the occurrence of scintillations. During the extreme low solar activity years the scintillation occurrences do not vary linearly with the sunspot number. The inhibition and generation of irregularities during enhanced magnetic activity period are explained by considering changes in the electric field. The spectral analysis provide spectral index for irregularities which varied between −1.5 and −8 and characteristic length of irregularities varied between 400 and 1200 m which confirms that 250 MHz scintillations observed over Varanasi were associated with intermediate scale irregularities.

Remote sensing of D-region ionosphere using multimode tweeks

Lightning discharges radiate electromagnetic waves in a wide frequency range, with maximum energy in extremely low frequency/very low frequency band. A part of the radiated extremely low frequency/very low frequency wave energy is trapped in the Earth–ionosphere waveguide and travels thousands of kilometers in different modes with lower attenuation. Amplitude, frequency and phase of these waves are used to study the less explored D-region ionosphere at lower latitudes. Extremely low frequency/very low frequency observations are recorded continuously by automatic whistler detector setup installed at low-latitude Indian station Lucknow (Geom. lat. 17.6°N; long. 154.5°E). In total, 149 cases of tweeks having modes ranging from 3 to 6 have been recorded by automatic whistler detector during December 2010 and analyzed. Result shows that the propagation distance in the Earth–ionosphere waveguide lies between 1.1 and 9.4 Mm. The electron density in the lower D-region varies between 25 and 150 cm−3. The upper boundary of the waveguide varies between 80 and 95 km. The reported results are in good agreement with the earlier measurements at different latitudes and longitudes.

Observation of discrete VLF emissions at Indian low-latitude ground station Srinagar (L = 1.28)

During the routine analysis of recorded very low-frequency data during the period November, 2012 to February, 2013, at an Indian low-latitude ground station Srinagar (geomag. lat., 24°10′N; L = 1.28), we observed some interesting discrete emissions during quiet period on February 5, 2013, which has been presented in the present study. The spectral analysis of the observed discrete very low-frequency emissions consisting of risers, fallers, hooks and oscillating tones has been presented. To explain the observed dynamic spectra of the discrete chorus emissions, a possible generation mechanism has been presented based on the recent nonlinear theory. On the basis of this theory, frequency sweep rate of discrete chorus emissions has been computed and compared with those of our experimentally observed values.

A statistical comparison of IRI TEC prediction with GPS TEC measurement over Varanasi, India

The widely used International Reference Ionosphere (IRI) model in simulation/modeling of ionospheric studies specially at low latitudes require performance evaluation. To achieve this objective, we have compared total electron content (TEC) derived from the recent version of IRI model (IRI-2012) over low latitude Indian station Varanasi (Geographic: 25°16′N, 82°59′E; Geomagnetic: 14°55′N, 154°E) with TEC measured from the GPS receiver installed at Varanasi. The results are in good agreement with correlation coefficient ~0.9, and root-mean square deviations generally around 25–70% for diurnal comparisons. The seasonal variability of TEC derived by using the different topside options of the IRI-2012 model (IRI-2001, NeQuick and IRI01-corr) are studied and validated for the three seasons. The IRI-2012 model with all the three topside options significantly overestimates TEC for the station during the winter season of 2012–2013, which has been attributed to discrepancy in foF2 parameter as predicted by the model during the winter season of 2012–2013. This has been confirmed by comparing the foF2 data from COSMIC with IRI model.

Solar flares induced D-region ionospheric and geomagnetic perturbations

The D-region ionospheric perturbations caused by solar flares which occurred during January 2010–February 2011, a low solar activity period of current solar cycle 24, have been examined on NWC transmitter signal (19.8kHz) recorded at an Indian low latitude station, Allahabad (Geographic lat. 25.75°N, long. 81.85°E). A total of 41 solar flares, including 21 C-class, 19 M-class and 01 X-class, occurred during the daylight part of the NWC-Allahabad transmitter receiver great circle path. The local time dependence of solar flare effects on the change in the VLF amplitude, time delay between VLF peak amplitude and X-ray flux peak have been studied during morning, noon and evening periods of local daytime. Using the Long Wave Propagation Capability code V 2.1 the D-region reference height (H/) and sharpness factor (β) for each class of solar flare (C, M and X) have been estimated. It is found that D-region ionospheric parameters (H/, β) strongly depend on the local time of flare's occurrence and their classes. The flare time electron density estimated by using H/ and β shows maximum increase in the electron density of the order of ~80 times as compared to the normal day values. The electron density was found to increase exponentially with increase in the solar flux intensity. The solar flare effect on horizontal component (H) of the Earth's magnetic field over an equatorial station, Tirunelveli (Geographic lat., 8.7°N, long., 77.8°E, dip lat., 0.4°N), shows a maximum increase in H of ~8.5% for M class solar flares. The increase in H is due to the additional magnetic field produced by the ionospheric electrojet over the equatorial station.

Ionospheric vertical drifts over an Indian low latitude station and its comparison with IRI-2007 vertical drift model

The vertical plasma drift velocity (Vz=dh′Fdt) of the Ionospheric F-Region is calculated using the Ionosonde data over an Indian low latitude station, Waltair (17.7°N, 83.3°E, dip 20°N). The vertical drift velocities thus computed are compared with those of the IRI-2007 vertical drift model during the post sunset hours of the high sunspot period of 2001–2002. The vertical drift values are entirely downward between 2000 and 0700hLT for the IRI-2007 vertical drift model compared to those derived using Ionosonde. The root mean square errors (RMSE) in the vertical drifts during the post sun-set hours computed between the IRI vertical drift model and Ionosonde measured values are found to be 13.54, 21.68 and 22.39m/s for summer, equinox and winter seasons respectively. The average vertical drifts derived using Ionosonde showing pre-reversal enhancements during evening hours (1700–1900hLT) over Waltair are found to be 8, 16 and 12m/s, whereas the IRI-2007 model derived values show 15, 35 and 36m/s during summer, equinox and winter seasons respectively. The correlation analysis of peak drift velocities derived using Ionosonde and those from IRI model with Rz and F10.7 shows more or less similar variation while they are differed in their variation with Ap index. A good correlation is observed between the vertical plasma drift obtained using Ionosonde and Equatorial Electrojet Strength (EEJ) except during winter, while the correlation coefficients computed using IRI model are little higher than those obtained using Ionosonde with maximum values during summer season. The percentage of deviation for IRI-model correlation coefficient and that of Ionosonde-inferred drift for equinoctial and summer months is around 50% and 25% respectively where as it is more than 100% for winter.