Surendra Sunda

Impact of the perturbation zonal velocity variation on the spatio/temporal occurrence pattern of L band scintillation—A case study

The earlier evolved method for the forecast of the spatiotemporal variation of L band scintillation based on the expected variation of the perturbations, under favorable ionospheric/thermospheric conditions, has been refined by duly accounting for the local time variation of the zonal velocity of the perturbations. The unique combination of the two geostationary satellites (GSAT-8 and GSAT-10) over the Indian zone has been used to estimate the typical local time dependence of the perturbation velocities by closely following identifiable features in the scintillation pattern. The measured velocities, that registered a steady decrease with the progression of night, had been shown to significantly alter the forecast pattern of the scintillations with respect to longitude and local time. The significant improvement in the forecast pattern has been demonstrated through a case study putting the forecast method on a firmer footing.

Satellite-based augmentation systems: A novel and cost-effective tool for ionospheric and space weather studies

Satellite-Based Augmentation Systems (SBASes) are designed to provide additional accuracy and robustness to existing satellite-based radio navigation systems for all phases of a flight. However, similar to navigation systems such as GPS which has proven its worth for the investigation of the ionosphere, the SBASes do have certain advantages. In the present paper, we propose and demonstrate SBAS applicability to ionospheric and space weather research in a novel and cost-effective way. The recent commissioning of the Indian SBAS, named GPS Aided Geo Augmented Navigation (GAGAN), covering the equatorial and low-latitude regions centered around the Indian longitudes provides the motivation for this approach. Two case studies involving different ionospheric behavior over low-latitude regions vindicate the potential of SBAS over extended areas.

Local time, seasonal, and solar cycle dependency of longitudinal variations of TEC along the crest of EIA over India

[1] The global wave number 4 structure in the Indian longitudinal region spanning from ~70 to 95°E forming the upward slope of the peak in the total electron content (TEC) are reported along the crest of equatorial ionization anomaly (EIA). The continuous and simultaneous measurements from five GPS stations of GPS Aided Geo Augmented Navigation (GAGAN) network are used in this study. The long-term database (2004–2012) is utilized for examining the local time, seasonal, and solar cycle dependency on the longitudinal variations of TEC. Our results confirm the existence of longitudinal variations of TEC in accordance with wave number 4 longitudinal structure including its strength. The results suggest that these variations, in general, start to develop at ~09 LT, achieve maximum strength at 12–15 LT, and decay thereafter, the decay rate depending on the season. They are more pronounced in equinoctial season followed by summer and winter. The longitudinal variations persist beyond midnight in equinox seasons, whereas in winter, they are conspicuously absent. Interestingly, they also exhibit significant solar cycle dependence in the solstices, whereas in the equinoxes, they are independent of solar activity. The comparison of crest-to-trough ratio (CTR) in the eastern (92°E) and western (72°E) extreme longitudes reveals higher CTR on the eastern side than over the western extreme, suggesting the role of nonmigrating tides in modulating the ExB vertical drift and the consequential EIA crest formation.

The impact of the 17 March 2015‐ St. Patrick's Day storm on the evolutionary pattern of Equatorial Ionization Anomaly over the Indian longitudes using high resolution spatio‐temporal TEC maps – New insights

The impact of the St. Patricks Day storm (17 March 2015) on the major equatorial electro-dynamical process viz., the Equatorial Ionization Anomaly (EIA) has been assessed using 2D (5 ° lat. x 5 ° long.) total electron content (TEC) maps generated from the ground based SBAS (Satellite Based Augmentation System) enabled receiver data. The various aspects of EIA specifically the i) evolution/devolution, ii) longitudinal structure, and iii) its variability during different phases of a geomagnetic storm, have been brought out. These 2D TEC maps, which have a large latitudinal (5 S-45° N) and longitudinal (55-110° E) coverage, show the complete reversal in the longitudinal structure/pattern of EIA during the recovery phase of the storm as compared to the quiet day. These results have been explained in the light of the combined effects of the storm associated processes such as i) the penetration electric fields of magnetosphere origin, ii) storm-induced thermospheric winds, and, iii) activation of the consequent disturbance dynamo effectively distorting the longitudinal wave number 4 (WN4) structure of the EIA. It has been shown unambiguously that even a separation of ~10°-15° longitude could experience significantly different forcings. The relevance and the far reaching consequences of the study in the light of the current trends and requirements for reliable satellite based navigation are highlighted.

The Response Time of Equatorial Ionization Anomaly Crest: A Unique Precursor to the Time of Equatorial Spread F Initiation

The time delay with which the magnitude and location of the Total Electron Content (TEC) at the Equatorial Ionization Anomaly (EIA) crest responds to the mean height of F - layer (hmF2) is examined seasonally using the ionosonde data at Trivandrum and the GPS data at different stations in the EIA region for low and high solar activity years. The study brings out the fact that for the low solar activity year, the crest TEC responds fastest in winter solstice (ws) compared to other seasons. Further, the response time of the crest is found to decrease with solar activity. The seasonal variation in the EIA response time is attributed to the modulation by meridional neutral winds whereas the solar activity variation seems to be basically controlled by the diffusion times and the background ionization gradients in the respective epochs. The derived relationship between EIA crest location/ magnitude and hmF2 for any season can be used for the prediction purposes. Furthermore, this study for the first time, establishes the relation of EIA response time to Equatorial Spread - F (ESF) start time, for days when the F layer is in the neutral dynamically controlled domain. The deleterious effects of ESF irregularities on communication and navigation systems are well-known and the above result is significant in this context. The present study has the potential to be extended into a model to predict the ESF start time, from the EIA response time, earlier in the day.

Map matching algorithm: curve simplification for Frechet distance computing and precise navigation on road network using RTKLIB

Map matching is the process which work for location measurement and also find out appropriate route for smooth traveling on the road network, but map matching accuracy fully depends on GPS trajectory data and electronic map of the road network. Unfortunately, GPS data is not accurate because, it’s through weak GPS signaling and low quality GPS devices. So, it is difficult to calculate accurate location of movements.This work provides, a map mapping algorithm for simplifying appropriate path, including curves on road network and also, we proposed Frechet distance computing algorithm with road Preliminaries for network simplification.

Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and Low‐Latitude Ionosphere of the Indian Longitudes: A Case Study

Using the Equatorial Electrojet (EEJ) induced surface magnetic field and Total Electron Content (TEC) measurements, we investigated the impact of the Sudden Stratospheric Warming (SSW) of January 2009 on the equatorial electrodynamics and low-latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low-latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behaviour well before the onset of the SSW. This is found to be coincided with the beginning of enhanced planetary wave (PW) activity over high-latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moon. The response of TEC to SSW is found to be latitude dependent as the near-equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low-latitude regions, appearing as an ‘ionization hole’, well after the SSW peak. The investigation revealed the existence of a quasi-16 day wave in the TEC over low-latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favourable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.

Origin of the ahead of tsunami traveling ionospheric disturbances during Sumatra tsunami and offshore forecasting

The presence of ionospheric disturbances associated with Sumatra 2004 tsunami that propagated ahead of tsunami itself has previously been identified. However their origin remains unresolved till date. Focusing on their origin mechanism, we document these ionospheric disturbances referred as Ahead of tsunami Travelling Ionospheric Disturbances (ATIDs). Using TEC data from GPS Aided GEO Augmented Navigation (GAGAN) GPS receivers located near the Indian east coast, we first confirm the ATIDs presence in TEC that appear ~90 minutes ahead of the arrival of tsunami at the Indian east coast. We propose here a simulation study based on Tsunami-Atmospheric-Ionospheric coupling that considers tsunamigenic Acoustic Gravity Waves (AGWs) to excite these disturbances. We explain the ATIDs generation based on the dissipation of transverse mode of the primary AGWs. The simulation corroborates the excitation of ATIDs with characteristics similar to the observations. Therefore, we offer an alternative theoretical tool to monitor the offshore ATIDs where observations are either rare or not available and could be potentially important for the tsunami early warning.

Further refinements to the spatiotemporal forecast model for L-band scintillation based on comparison with C/NOFS observations

The model generated spatio-temporal maps to forecast the occurrence pattern of plasma density irregularities in the night side equatorial F-region that are responsible for the L-band scintillations have been put to test, both in space and time, by comparing it with actual observations by the C/NOFS (Communication/Navigation Outage Forecasting System) satellite. The forecast model is based on i) the temporal variations of the density perturbations during daytime in the Nmax region and ii) a-priori knowledge of zonal velocity of the perturbations in the post sunset hours. The present study not only substantiates the hypothesis used for the generation of the scintillation forecast but also suggests that the equatorial plasma bubbles remain tied-up with the initial perturbations which trigger the primary Rayleigh-Taylor instability. The outcome highlights the need to take into account the altitudinal profile of the topside F-region electron density as it could modify the zonal extent of the plasma bubbles that support the generation of the density irregularities and the consequent L-band scintillations. The present study takes us one more step closer towards realization of an operational forecast system for satellite based navigation.

SBAS-derived TEC maps: a new tool to forecast the spatial maps of maximum probable scintillation index over India

With the increase in the use of satellite-based navigation services, the forecasting of L band scintillation has turned out to be of paramount importance as it affects their accuracy and availability. Forecasting the time of occurrence or non-occurrence, strength and probable location of scintillation enables the service providers and users to take appropriate action to mitigate the effects and optimize the services. We use the recently developed method to retrieve TEC from the ionospheric correction data transmitted by the Indian satellite-based augmentation system (SBAS)–GAGAN. By making use of the established linear relation between the dusk time TEC and the maximum probable scintillation intensity (S4max), scintillation forecast maps have been generated as early as 1930 LT. The superposition of actual S4 measurements, obtained from the GAGAN network of receivers, on the forecasted S4max map shows that the actual measurements are less than the predicted S4max except on very few occasions. The potential of the simple technique to predict the 2 D maps of maximum probable scintillation index for the whole night has been demonstrated which with more refinements could evolve into a viable forecast or forewarning system.