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This study presents a detailed comparative analysis of the two widely used percentage of occurrence methodologies used to compute the statistics of ionospheric scintillation activity based on the data of a Global Positioning System (GPS) Ionospheric Scintillation and (TEC) Total Electron Content Monitor (GISTM) receiver located at the northern Equatorial Ionization Anomaly (EIA) crest region Agartala (23.76°N, 91.26°E) of the Indian subcontinent, during 2012–2015 of the rising, peak and declining solar activity periods of solar cycle-24. Both the methodologies, one based on the number of scintillation event days and another based on the number of scintillation events, provide the correct and similar scintillation statistics qualitatively. But a rigorous comparative study shows significant quantitative differences between the statistics obtained by the two methodologies. The variation of scintillation occurrence obtained by the two methodologies with solar and geomagnetic activity shows that the methodology based on the number of scintillation events provides more accurate and physically acceptable results. Though this fact may be familiar to some researchers but still the scientific community has been using both the methodologies in parallel to compute scintillation statistics. So, from this comparative analysis future researchers can get a clear scenario of the advantages and disadvantages of these methodologies over one another before adopting any one of those for computing scintillation statistics. We also want to highlight the issue to the scientific community to come to a common platform by adopting the percentage of occurrence methodology based on the number of scintillation events for future studies.

The effect of the rocket exhaust products on the D-region of the ionosphere is investigated with the help of Very low frequency (VLF) electromagnetic wave propagation characteristics within the Earth-ionosphere waveguide. The changes in the electron density profile are computed from the observed VLF signal amplitude perturbations about 3 dB during the rocket launch. We find a localized electron depletion in the lower ionosphere at an altitude of around 58 km, that is thought to be originated by the attachment of ionospheric ion and molecular hydrogen along with water molecule in the exhaust product of first stage burn of Geosynchronous Launch Vehicle (GSLV) rocket at the time of GSLV launched from Sriharikota, India, on 27 August 2015 at 11:22 UT (16:52 IST). The ionospheric depletion perturbed the navigational VLF signal (VTX = 17 kHz) 134 s after the launch of the GSLV rocket.

We have studied the black carbon (BC) mass concentrations, contributions from Fossil Fuels (FF), and Biomass burning (BB) to total BC mass concentrations using a 7-wavelength Aethalometer (Model, AE-31; Make, Magee Scientific, USA) at a rural continental location in the northeastern India. We have taken the continuous measurements of BC from January 2011 to December 2017 (excluding the year 2016 due to nonavailability of data). The annual mean BC concentration at 880 nm is observed maximum 12.56±5.06 µgm-3 in the year 2011 with a minimum of 7.26±2.76 µgm-3 during the year 2015." is the final sentence. BC, BCff, and BCbb mass concentrations show significant variation during morning, afternoon, evening, and night hours. The significant monthly, seasonal, and annual variabilities in the BC concentration, equivalent BC from FF and BB, are due to seasonal change in the emission sources, boundary layer dynamics, and dispersion and dilution conditions. The determination of Angstrom exponent (α) for the BC emitted during burning of different biofuels by performing a burning experiment is an important part of the present study. We have conducted a survey in and around the study location to know the different biofuels used by the people for daily household activities, and those biofuels are used in the burning experiment. As a result of biomass burning experiment to determine Angstrom exponent, we have found the α values to be ranging from 1.20 to 2.37 for flaming and a range from 1.59 to 2.33 for smoldering conditions for different biofuels. Annual mean contributions of BCff and BCbb to the total BC are found to be dominated by FF emissions during the whole study period. The percentage contribution of BCff and BCbb is found to be 56% and 44% during winter season due to increase in wood and biomass burning activities for various purposes. We have found the higher percentage contribution from BCff (85%) during monsoon season due to reduction in usage of biofuels.

One of the extensively used methods for aerosol mass concentration measurement is the optical filter method used in Aethalometer. Due to the aerosol loading and multi scattering effect, there are uncertainties that can lead to underestimation or overestimation of aerosol concentration and measurement of the absorption coefficient. To overcome such uncertainties, a data correction method has been adopted in the present work. During the winter, pre-monsoon and post-monsoon seasons, Aethalometer underestimated black carbon (BC) concentration by 8 ± 1%, 11 ± 1%, and 10.5 ± 0.2%, due to the higher aerosol deposition. While in the monsoon season, underestimation of 5.5 ± 2% is observed because of the presence of bigger scattering nature aerosol. For 70% of days, found a positive correlation between the compensation parameter (k880) and wavelengths, which indicated the dominance of the smaller aerosol particles. While 30% of days have a negative correlation due to the bigger aerosol particle. In the winter, pre-monsoon and monsoon seasons, the absorption coefficient at 370 nm was 86%, 90%, and 93% higher than that of 880 nm due to the higher biomass burning emissions. The absorption Angstrom exponent (α370-880) was 1.02 for fossil fuel burning and the corresponding k880 was 0.0037. In the winter and post-monsoon seasons, BC emissions from fossil fuel combustion dominated over the BC emitted by biomass burning emissions with α370-880

In the present work, we are studying the dynamics of 11 tropical cyclones (TCs) over the Bay of Bengal (BoB) using the World Wide Lightning Location Network (WWLLN) lightning data during 2013–2017. Detailed investigation of Lightning Stroke Count (LSC) shows that the initial phase of the TC intensification over BoB is associated with high lightning activity both in the eye region and rainband region. The study of different category TCs shows the weak and moderate-intensity TCs during both in pre-monsoon and post-monsoon seasons are lightning populated (specifically inner bands) during and prior to the intensification phase. Peak Maximum Sustained Wind Speed (MSWS), which is linked to sea surface temperatures (SSTs), is preceded by the peak LSC with a time lag of 6–12 h. The LSC peaks in the eye region and rain band for different categories of TCs between small to moderate wind shear range. The present study highlights that the monitoring of real-time lightning activity could provide supportive information on the existing forecast techniques. But further analyses are required over BoB to quantify the relationship between TC intensification and lightning activity both in space and time.

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.

The spectral estimates of Aerosol Optical Depth (AOD, τ) were made by operating a Microtops-II sun photometer in the spectral range 0.380–0.870 μm over Tripura in north-eastern India and analyzed to infer the aerosol types and source characteristics in different seasons. The Angstrom exponent (α) derived from spectral AOD in different wavelength (λ) range and subsequent second order derivative of Angstrom exponent (i.e., α′) and their curvature analysis in the ln τ versus ln λ relationship has revealed the crucial information related to the dominance of different aerosol types and their characteristics in different seasons. The average AOD (mean ± standard deviation) at 0.5 μm in winter, pre-monsoon, monsoon and post-monsoon seasons are observed to be 0.70 ± 0.28, 0.74 ± 0.18, 0.55 ± 0.20, 0.44 ± 0.19 respectively; while the corresponding seasonal mean values of α (over 0.380–0.870 μm spectral range) are found to be 1.09 ± 0.17, 0.92 ± 0.24, 0.51 ± 0.27, 0.89 ± 0.38 respectively. Examination of the Angstrom exponents derived from satellite retrieved AOD by Moderate Resolution Imaging Spectroradiometer (MODIS) instrument operating onboard Terra satellite, along with MODIS fine mode aerosol fraction and aerosol types indicates broad seasonal features of aerosol size spectrum over the study region similar to those observed from ground-based measurements. The estimation of the values of α at different spectral ranges indicate that winter season is mainly influenced by the fine-mode aerosols having fine-mode fraction (FMF) ∼ 0.7; whereas coarse-mode aerosols dominate in the monsoon season having FMF ∼ 0.3. The pre-monsoon and post-monsoon seasons exhibit the presence of mixed type of aerosols, with slightly greater fraction of fine-mode aerosol in pre-monsoon. Curvature analyses of Angstrom exponent put insight into the consistency of observed features of seasonal aerosol types.

The latitudinal ionospheric response of the three strongest geomagnetic storms of 2015 of the current solar cycle 24 during 16–19 March 2015, 21–24 June 2015 and 19–22 December 2015 is investigated using the total electron content data derived from a latitudinal chain of Global Positioning System (GPS) receivers extending from 70°N to 70°S. The storm time perturbations of the ionosphere during main and recovery phase is presented by the GPS derived vertical total electron content (VTEC) data which is further supported by ionospheric F region critical frequency (foF2) and F region peak height (hmF2) data. We observed symmetrical hemispheric response of the ionosphere during the strongest 17th March (St. Patrick’s Day) storm whereas asymmetrical hemispheric response of the ionosphere during 22nd June and 20th December storm over the Asian-Australian sector. The observations are explained by the combined transport of background inter-hemispheric seasonal wind and storm time disturbed meridional wind and by the global thermospheric compositional variation [O/N2] data.

We investigate the solar flare effects on the D-region of the ionosphere with the help of VLF (Very Low Frequency) radio waves using a portable E-field system from Antarctica during the summer period of 34th Indian scientific expedition. Two GPS time synchronized VLF receivers, one located at Bharati, Antarctica (geographical latitude 69.40°S, longitude 76.18°E) and another located at Tripura, India (geographical latitude 23.84°N, longitude 91.28°E) were operated simultaneously to infer common mode changes in the lower ionosphere for a number of solar flares events. The two systems constantly monitored the carrier amplitude and phase of the MSK (Minimum Shift Keying) modulated navy transmitter located in Australia (Callsign: NWC, 19.8 kHz, geographical latitude 21.88°S, longitude 114.13°E), around 5.6 Mm great circle distance from the two receivers. The results are interpreted in terms of Earth-ionosphere wave-guide characteristics. A Long Wave Propagation Capability (LWPC) model study is also performed to infer the changes in the daytime electron density in polar D-region ionosphere during the solar flares. The exponential fit of the modeled electron density change with average X-ray flux change shows an excellent correlation (R 2 value 0.95). The exponential fit is utilized to infer the daytime electron density change in the polar ionosphere during solar flare events. The analyses indicate that small solar flares of class ‘C’ can be very effectively detected with the portable antenna system even if the receiver is located in polar coastal region compared to equatorial region. The expedition results also demonstrate the feasibility of using portable VLF receivers from the coastal stations for monitoring the polar lower ionosphere from Antarctica and open up new opportunities for long term exploration.

In the area of global climatic implications of atmospheric aerosols in the recent decades, accurate estimation of aerosol properties on long-time basis over distinct geographic regions and seasons are very essential In this regard, satellite remote sensing is an effective tool to monitor Aerosol Optical Depth (AOD), a parameter most important for assessing the radiative impact of aerosols, at the regional as well as global level. This study aims at comparing the satellite retrieved AOD from geostationary and polar-orbiting platforms with the ground-based measurements over the semi-continental site of northeastern India. For this, we have utilized AOD products derived from the imager of Indian National Satellite (INSAT)-3DR and the Moderate Resolution Imaging Spectro Radiometer (MODIS) on-board Terra and Aqua satellites. A handheld Sun Photometer (Microtops-II) provided the ground based estimates of AOD. The main goal is to validate and demonstrate the most accurate and suitable satellite retrieval of aerosols over the northeastern semi-continental site of India. In the present study, we used INSAT-3DR AOD observations at 650 nm over the region of interest and the level-3 daily AOD estimates from MODIS (both Aqua and Terra) at the same wavelength for the year of 2015. The inter-comparison of satellite retrieved AOD with ground based AOD is examined and statistical parameters, such as root mean square error (RMSE), coefficient of determination (R2) are determined for each set of data in different time blocks. Overall, MODIS on-board the polar orbiting platforms (aqua and terra) showed better correlation with the ground based sun-Photometer measurements. On the other hand, even though the bias in AOD retrieval from geostationary platform is higher than that of the polar orbiting ones, the diurnal features of aerosol were clearly visible in the INSAT-3DR data. This is important in order to understand the distinct influence local and synoptic source processes.

The lightning climatology over Nepal is analysed in detail for the first time. For the analysis, we utilised the satellite-based lightning imaging sensor data for the period from 1998 to 2013. A comparison of these climatological results is also performed with two ground-based lightning detection networks, namely, the World Wide Lightning Location Network and the Global Lightning Network for 3 yr from 2011 to 2013. On analysing the data obtained from the three sources, we conclude that the months of April and May are extremely vulnerable in the perspective of lightning hazards in Nepal, in contrast to the results reported previously which indicated that the maximum lightning activity occurred in the month of June. The central and eastern regions of the country receive the majority of lightning strikes during the months of April and May. The present finding is supported by the thunderstorm frequency data obtained from the disaster Information Management System, Nepal and also from thunder-day data from NOAA.

Lightning activities are distributed asymmetrically over the globe. Satellite images show that the Himalayan region is one of the prone zones of lightning activity. We do not understand such an uneven distribution of lightning activities as of today. To elaborate on the present-day understanding of lightning flashes over the Himalayan region, we have analyzed various atmospheric factors in association with Lightning Flash Density (LFD). For this purpose, we divided the Himalayan Range into three sections, namely, eastern, middle, and the western Himalayas. We explored the possible association of monthly mean Convective Available Potential Energy (CAPE), Surface Air Temperature (SAT), thermodynamic temperature of the top of the cloud; Cloud Top Temperature (CTT), Relative Humidity (RH), and Specific Humidity (SH) with LFD over the three sections of the Himalayan range. We observed that CAPE and SAT play a vital role in creating instability over that region. In contrast, moderate moisture (i.e., RH) is the most suitable condition for lightning activities over all three sections. The analysis shows that 50–60% RH at 700hpa is the most favorable condition for lightning over the Himalayan region.

Northeast India (NEI) is located within the boundary of the great Himalayas in the north and the Bay of Bengal (BoB) in the southwest, experiences the mixed influence of the westerly dust advection from the Indian desert, anthropogenic aerosols from the highly polluted Indo-Gangetic Plains (IGP) and marine aerosols from BoB. The present study deals with the estimation and characterization of aerosol radiative forcing over a semi-continental site Tripura, which is a strategic location in the western part of NEI having close proximity to the outflow of the IGP. Continuous long term measurements of aerosol black carbon (BC) mass concentrations and columnar aerosol optical depth (AOD) are used for the estimation of aerosol radiative forcing in each monthly time scale. The study revealed that the surface forcing due to aerosols was higher during both winter and pre-monsoon seasons, having comparable values of 32 W/m 2 and 33.45 W/m 2 respectively. The atmospheric forcing was also higher during these months due to increased columnar aerosol loadings (higher AOD ~ 0.71) shared by abundant BC concentrations (SSA ~ 0.7); while atmospheric forcing decreased in monsoon due to reduced magnitude of BC (SSA ~ 0.94 in July) as well as columnar AOD. The top of the atmosphere (TOA) forcing is positive in pre-monsoon and monsoon months with the highest positive value of 3.78 W/m 2 in June 2012. The results are discussed in light of seasonal source impact and transport pathways from adjacent regions.

Multi-year measurements of spectral properties of aerosol absorption are examined over four geographically distinct locations of northeastern India. Results indicated significant spatio-temporal variation in aerosol absorption coefficients (σabs) with highest values in winter and lowest in monsoon. The western parts of the region, close to the outflow of Indo-Gangetic Plains, showed higher values of σabs and black carbon (BC) concentration - mostly associated with fossil fuel combustion. But, the eastern parts showed higher contributions from biomass burning aerosols, as much as 20-25% to the total aerosol absorption, conspicuously during pre-monsoon season. This is attributed to a large number of burning activities over the Southeast Asian region, as depicted from MODIS fire count maps, whose spatial extent and magnitude peaks during March/ April. The nearly consistent high values of Aerosol Index (AI) and layer height from OMI indicates the presence of absorbing aerosols in the upper atmosphere. The observed seasonality has been captured fairly well by GOCART as well as WRF-Chem model simulations. The ratio of column integrated optical depths due to particulate organic matter (POM) and BC from GOCART showed good coincidence with satellite based observations, indicating the increased vertical dispersion of absorbing aerosols, probably by the additional local convection due to higher fire radiative power caused by the intense biomass burning activities. The WRF-Chem though underperformed by different magnitude in winter, the values are closer or overestimated near the burnt areas. Atmospheric forcing due to BC was highest (~30 Wm-2) over the western part associated with the fossil fuel combustion.

The effect of the rocket exhaust products on the D-region of the ionosphere is investigated with the help of Very Low Frequency (VLF) electromagnetic wave along the Earth-ionosphere waveguide. VLF is a very useful tool to study the Earth’s lower ionospheric disturbances. The changes in the electron density profile are computed from the observed VLF signal perturbations after the rocket launch. We report the effect of the rocket exhaust on the lower ionosphere at an altitude around 58 km. The effect is primarily caused by localized free-electron depletion by water molecules by the process of attachment, decreasing the electrical conductivity of the ionosphere. The electron depletion is supposed to be formed by the rocket exhaust products of the 1st stage burn of Geosynchronous Launch Vehicle (GSLV) rocket launched from Sriharikota, India, on 27th August 2015 at 11:22 UT (16:52 IST). A Long Wave Propagation Capability (LWPC) model study also supports our findings.

Mars Colour Camera (MCC) is a medium resolution camera onboard India’s first Mars Orbiter Mission (MOM). MCC captured 827 images of Mars from varying distances in the first two years of operation in 2014 and 2015 that includes more than 20 limb viewing images. In our analysis, we considered 10 images of high altitude limb-viewing clouds. Through image processing techniques, we computed the projected heights of the clouds. We developed codes for the calculation of Top of the Atmosphere (TOA) reflectance and $\alpha$ value for the high altitude limb clouds visible from MCC. We furnish below the key results highlighting the main findings from the analyzed results. Using spatial pixel resolution counts, projected heights for the limb viewing images is estimated around 33-93 km above the surface of the Mars. After the zenith angle and lens correction, the projected heights become 43 km to 123 km. Horizontal extension of the clouds in the Martian limb is found to be around 400km to 1300 km. Top of the Atmosphere (TOA) reflectance value is computed to be 0.18 for red wavelength and 0.13 for blue wavelength. This indicates a presence of hexagonal H 2 O ice crystal as well as CO 2 ice crystal in the Martian clouds. Angstrom exponent $(\alpha)$ value varies from 0.8 to 1.88 within the cloudy region of the atmosphere that may indicate the presence of coarse mode as well as fine mode particle. C/2013 A1, a cometary object passed through the Martian atmosphere on $14^{{\mathrm {th}}}$ October 2014 may indicate an extra-territorial origin of the dust clouds in the Martian atmosphere. Formation of clouds at Solar longitude Ls=200° to Ls=251° indicate an evidence that the origin of the cloud may also be dust storm in the Martian atmosphere or solar wind. We need more evidence to confirm the hypothesis for the formation of the high altitude clouds in the Martian atmosphere.

The high latitude southern polar cap ionospheric irregularities observed during the St. Patrick’s Day 2015 severe geomagnetic storm is reported using Total Electron Content (TEC) data of network of International Ground Stations (IGS) GPS receivers McMurdo-MCM4(-77.830 S, 166.660 E), Casey-CAS1 (-66.280 S, 110.520 E), Davis-DAV 1 (68.580 S, 77.970 E), Mawson-MAW1 (-77.830 S, 166.660 E), East Ongle Island-SYOG Mawson-MAW1 (-67.610 S, 62.870 E) and Rothera-ROTH (-67.570 N, 68.130 E), in the Antarctic Circle. The irregularities of TEC phase fluctuation i.e. Rate of TEC (ROT) and TEC rate index (ROTI) expressed in TECU/min was estimated during the period of 16-18 March, 2015.

Methodologies reported in the existing literature for identification of a region of interest (ROI) in medical thermograms suffer from over- and under-extraction of the abnormal and/or inflammatory region, thereby causing inaccurate diagnoses of the spread of an abnormality. We overcome this limitation by exploiting the advantages of a logarithmic transformation. Our algorithm extends the conventional region growing segmentation technique with a modified similarity criteria and a stopping rule. In this method, the ROI is generated by taking common information from two independent regions produced by two different versions of a region-growing algorithm that use different parameters. An automatic multi-seed selection procedure prevents missed segmentations in the proposed approach. We validate our technique by experimentation on various thermal images of the inflammation of affected knees and abnormal breasts. The images were obtained from three databases, namely the Knee joint dataset, the DBT-TU-JU dataset, and the DMR-IR dataset. The superiority of the proposed technique is established by comparison to the performance of state-of-the-art competing methodologies. This study performed temperature emitted inflammatory area segmentation on thermal images of knees and breasts. The proposed segmentation method is of potential value in thermal image processing applications that require expediency and automation.

Four years (2010–2014) of spectral aerosol optical depth (AOD) data from 4 Indian Space Research Organisation's ARFINET (Aerosol Radiative Forcing over India) stations (Shillong, Agartala, Imphal and Dibrugarh) in the North-Eastern Region (NER) of India (lying between 22–30°N and 89–98°E) are synthesized to evolve a regional aerosol representation, for the first time. Results show that the columnar AOD (an indicator of the column abundance of aerosols) is highest at Agartala (0.80 ± 0.24) in the west and lowest at Imphal (0.59 ± 0.23) in the east in the pre-monsoon season due to intense anthropogenic bio-mass burning in this region aided by long-range transport from the high aerosol laden regions of the Indo-Gangetic Plains (IGP), polluted Bangladesh and Bay of Bengal. In addition to local biogenic aerosols and pollutants emitted from brick kilns, oil/gas fields, household bio-fuel/fossil-fuel, vehicles, industries. Aerosol distribution and climatic impacts show a west to east gradient within the NER. For example, the climatological mean AODs are 0.67 ± 0.26, 0.52 ± 0.14, 0.40 ± 0.17 and 0.41 ± 0.23 respectively in Agartala, Shillong, Imphal and Dibrugarh which are geographically located from west to east within the NER. The average aerosol burden in NER ranks second highest with climatological mean AOD 0.49 ± 0.2 next to the Indo-Gangetic Plains where the climatological mean AOD is 0.64 ± 0.2 followed by the South and South-East Asia region. Elevated aerosol layers are observed over the eastern most stations Dibrugarh and Imphal, while at the western stations the concentrations are high near the surface. The climate implications of aerosols are evaluated in terms of aerosol radiative forcing (ARF) and consequent heating of the atmosphere in the region which follows AOD and exhibit high values in pre-monsoon season at all the locations except in Agartala. The highest ARF in the atmosphere occurs in the pre-monsoon season ranging from 48.6 Wm −2 in Agartala to 25.1 Wm −2 in Imphal. Winter radiative forcing follows that in pre-monsoon season at these locations. The heating rate is high at 1.2 K day −1 and 1.0 K day −1 over Shillong and Dibrugarh respectively in this season. However, Agartala experiences higher surface forcing (−56.5 Wm −2 ) and consequent larger heating of the atmosphere of 1.6 K day −1 in winter.

The sunrise effect is a characteristic feature of very low frequency and low frequency radio waves propagated over a large distance. The 40 kHz signal level, transmitted from Miyakoji station (37.4° N, 140.9° E), Japan, and received at Tripura University (23° N, 91.4° E), is found to be attenuated during sunrise with an enhancement before the decrease in the signal level. On the basis of the nature of attenuation of the observed records from 2005 to 2006, those are classified into four different types, namely, type A (three step attenuation), type B (two step attenuation), type C (one step attenuation), and type D (no attenuation). During geomagnetically active days, 84% of type D cases and 31% of type C cases are observed, whereas only 0.9% of type A cases and 7% of type B cases are observed during geomagnetically active days. The fade amplitude of type C fade is also found to maintain a good negative correlation of 77.3% with the geomagnetic Ap indices over the period of 2 years. From the model calculation it is found that in the altitude range from 65 to 80 km, on average the electron density increases by a factor of 5.22 times during geomagnetically active days versus normal days.

The effect of geomagnetic storms on low latitude ionosphere has been investigated with the help of Global Positioning System Total Electron Content (GPS-TEC) data. The investigation has been done with the aid of TEC data from the Indian equatorial region, Port Blair (PBR) and equatorial ionization anomaly region, Agartala (AGR). During the geomagnetic storms on 24th April and 15th July 2012, significant enhancement up to 150% and depression up to 72% in VTEC is observed in comparison to the normal day variation. The variations in VTEC observed from equatorial to EIA latitudes during the storm period have been explained with the help of electro-dynamic effects (prompt penetration electric field (PPEF) and disturbance dynamo electric field (DDEF)) as well as mechanical effects (storm-induced equatorward neutral wind effect and thermospheric composition changes). The current study points to the fact that the electro-dynamic effect of geomagnetic storms around EIA region is more effective than at the lower latitude region. Drastic difference has been observed over equatorial region (positive storm impact) and EIA region (negative storm impact) around same longitude sector, during storm period on 24th April. This drastic change as observed in GPS-TEC on 24th April has been further confirmed by using the O/N2 ratio data from GUVI (Global Ultraviolet Imager) as well as VTEC map constructed from IGS data. The results presented in the paper are important for the application of satellite-based communication and navigational system. Published version

The present work investigates the effects of long-duration geomagnetic storms on VLF signal during ionospheric sunrise time, commonly known as D Layer Preparation Time (DLPT) depth. The VLF signal at 19.8 kHz transmitted from Northwest Cape, Australia, and received at a low-latitude station, Tripura, India, is used for the present analysis. The data for the analysis are selected from November 2008 to October 2011. In the active period of the geomagnetic storms, the average DLPT depth is found to have a negative correlation coefficient of 0.91 with geomagnetic Ap index. It is also found that with each 10 unit increase of Ap index, the DLPT depth (the day and night asymmetry level) changes by 1.25 dB. The results are supported with modeled International Reference Ionosphere (IRI) electron density data and DLPT depth at 71 km height for the three positions, namely, receiver position, signal hop position, and the transmitter position along the total Great Circle Path. It is found that the receiver position electron density is the main controlling factor for DLPT depth. The correlation between IRI electron density and DLPT depth increases from −0.13 at transmitter position to −0.33 at the first hop position, to −0.46 at the receiver position, respectively. The percentage change of post storm electron density, at 71 km height, is found to increase by more than 100% at the receiver position. The results are discussed on the basis of the electron density changes over the signal propagation path, mainly caused by the geomagnetic storms.

This study presents the characteristics of aerosol black carbon (BC) from a rural continental site, Agartala, located in the North-Eastern part of India using two year measurements from September 2010 to September 2012. Diurnal and seasonal variations are examined in relation to the unique geographical location, changeable meteorological conditions and distinct source characteristics. Winter season is characterized by extremely high BC concentration (17.8 ± 9.2 µg/m 3 ) comparable to those seen in urban environments of India, dropping off to much lower values during the monsoon (2.8 ± 1.7 µg/m 3 ). Even this lowest seasonal mean is rather high, given the rural nature of Tripura. Examination of the spectral dependence of aerosol absorption coefficients indicates that the main source of aerosol to total BC burden at Agartala is the fossil fuel combustions. Concentration weighted trajectory (CWT) analysis indicate that the characteristic high BC during winter is mostly associated with the advection from the Indo-Gangetic Plains (IGP), while the air mass pattern is constricted to the oceanic region during monsoon making BC aloft due to local pollution only.

On human face, non-rigid facial movements due to facial expressions cause noticeable alterations in their usual shapes, which sometimes create occlusions in facial feature areas making face recognition as a difficult problem. The paper presents an automatic Region of Interest (ROI) detection technique of six universal expressive face images. The proposed technique is a facial geometric based hybrid approach. The localization accuracy was evaluated by rectangular error measure and was tested on Japanese Female Facial Expression (JAFFE) database. The average localization accuracy of all detected facial regions is 94%.

To investigate the precursory signature of earthquakes on low frequency (LF) signal propagation, six earthquakes, having magnitude greater than equal to 6.5 and depth less than equal to 30 km, are being studied. The base line level of 40 kHz signal, transmitted from JJY station, Japan, is analysed with respect to V d statistical parameter. Results show that the V d parameter values starts fluctuating from its ambient levels before and during the days of the earthquakes, with significant variation starting 1–3 days prior to the earthquake concerned. This present study is an approach for identifying the precursory signatures of earthquakes on LF signal propagation using a new methodology with V d parameter.

Equatorial and low latitude ionospheric response of the 8th September 2017 severe G4-class geomagnetic storm is investigated using the total electron content (TEC) data from a longitudinal chain of global positioning system (GPS) receivers over Asian, African and American sectors. During the main phase, a positive storm effect is observed over Asian sector, a complete negative storm effect over African sector and both are observed over American sector. A sharp increase in peak TEC is observed over the complete longitudinal chain during the recovery phase. The results show the decisive contribution of prompt penetration electric fields (PPEFs) and disturbance dynamo electric fields (DDEFs), storm time disturbed meridional (equatorward) wind as well as the neutral compositional changes over equatorial and low latitudes in the observed ionospheric storm effects.

The ionospheric response of two geomagnetic storms of 2016 occurred during spring equinox (5-8 March, 2016) and autumn equinox (12-15 October, 2016) is investigated using the total electron content (TEC) data derived from Global Positioning System (GPS) receivers located in the equatorial ionization anomaly (EIA) crest regions at northern hemisphere (Tripura University, Agartala, India) and southern hemisphere (Karratha, Australia). While in southern EIA station ionospheric responses for the two storms are found to be symmetric but in northern EIA station the responses are completely asymmetric. The observations are explained by the contribution of storm-time prompt penetration electric fields (PPEFs), disturb dynamo electric fields (DDEFs), disturbed meridional (equatorward) winds as well as the neutral compositional changes over low latitudes.

Electromagnetic waves from lightning activity, commonly known as atmospherics or sferics serve as an effective tool for studying the lower ionosphere as well as thunderstorm activity. It is also useful for locating lightning strokes regionally and globally. In this paper, we present the analysis of the Integrated Field Intensity of Sferics (IFIS) at six discrete VLF frequencies for 30 lightning-associated overhead thunderstorms in Tripura, within the period from August 2009 to October 2010. An ingeniously developed well calibrated GPS locked software VLF receiver, located at the Department of Physics, Tripura University (23.5°N, 91.25°E), is used for the present study. Two distinct types of variations of IFIS, (i) single peak and (ii) dual peak are found characterizing each thunderstorm and their occurrence show nearly inverse character. The spectral character of IFIS rise rate, fall rate and rate of enhancement for each type is studied searching for suitable frequencies in the VLF range to forecast a thunderstorm. It is concluded that VLF sferics from 3–10 kHz are the most effective in terms nowcasting an incoming thunderstorm well before 3–4 hours of its peak occurrence, when there may not be any visual indication of the thunderstorm.

Facial expressions in infrared imaging have been introduced to solve the problem of illumination, which is an integral constituent of visual imagery. The paper investigates facial skin temperature distribution on mixed thermal facial expressions of our created face database where six are basic expressions and rest 12 are a mixture of those basic expressions. Temperature analysis has been performed on three facial regions of interest (ROIs); periorbital, supraorbital and mouth. Temperature variability of the ROIs in different expressions has been measured using statistical parameters. The temperature variation measurement in ROIs of a particular expression corresponds to a vector, which is later used in recognition of mixed facial expressions. Investigations show that facial features in mixed facial expressions can be characterized by positive emotion induced facial features and negative emotion induced facial features. Supraorbital is a useful facial region that can differentiate basic expressions from mixed expressions. Analysis and interpretation of mixed expressions have been conducted with the help of box and whisker plot. Facial region containing mixture of two expressions is generally less temperature inducing than corresponding facial region containing basic expressions.

There are several reports of simultaneous optical flashes as well as audible sounds called electrophonics during the passage of meteors through the Earth’s atmosphere, but the exact physical mechanism behind these phenomena is still poorly understood. Some experiments performed during meteor showers indicate the presence of ELF-VLF electromagnetic waves generated simultaneously with the passage of a meteor, which after suitable transduction are converted into audible sound waves. In this context, the present paper investigates the temporal and spectral character of ELF electromagnetic waves produced during the Leonid meteor shower on 18th November, 2009. A well calibrated broadband GPS locked software receiver was used to perform the experiment at the Department of Physics, Tripura University, India (23.50° N 91.25° E). The analysis of temporal data and dynamic spectrogram reveals distinct electromagnetic pulses in the frequency range of a few Hz to 2 kHz during the predicted time of peak activity of the shower. The spectral character of these pulses is found to be completely different from the spectral signatures originating from lightning discharges. During the peak activity period, the meteor counts using ELF method is found to be around 11,000 meteors hr−1, which is around 137 times more than the worldwide reported visible rate. The observational results are discussed from the viewpoint of deposition of meteoric dust particles and its interaction with ionospheric plasma, leading to emission of electromagnetic waves during the passage of meteors through the Earth’s atmosphere.

The effects of 2009 Leonid Meteor Shower upon the two VLF subionospheric transmitted signals and vertical electric potential gradient from the ground surface have been studied from Kolkata (Lat: 22.56°N, Long: 88.5°E) on November 17, 2009. The received signals showed their peak values when ZHR (Zenithal Hourly Rate) was highest. Some typical variations in the outcome of these measurements during the meteor showers will be presented in this paper.

The detection of 2009 Leonid, Perseid and Geminid meteor showers over Agartala, Tripura, India (Lat: 23.0° N, Long: 91.4° E) will be reported here by using two VLF receivers tuned to subionospheric transmitted VLF signals at the frequency 16.4 kHz from Aldra Island, Norway (Lat: 66.42° N, Long: 13.13° E) and the other at 18.2 kHz from Vijayananarayanam, India (Lat: 8.4° N; Long: 77.7° E). The received signals exhibited their peak values on November 17, 2009 when ZHR (Zenithal Hourly Rate) was highest. Some typical variations which are observed in the records of amplitude during the 2009 Leonid, Perseid and Geminid meteor showers will be presented in this paper.

The variation of the first Schumann resonance (SR) frequency spectra observed from the recorded data over Kolkata (22.56°N, 88.5°E) during a solar proton event (SPE) on July 14, 2000 has been presented. It shows increase in frequency during X-ray bursts and decrease during the period of occurrence of an SPE. The results from some other locations for the same event are also reported. The severe X-ray bursts recorded just before the proton event exhibit enhancement in frequency of the first mode due to enhancement of ionization in the D-region of the ionosphere. Some attempts are made to explain the observed variation during solar proton events in terms of the perturbations within the Earth–ionosphere waveguide on the basis of two-layer-model.

Diurnal and seasonal behavior of amplitude of NWC 22.3 kHz signal transmitted from Australia (22049/S, 114023/E), and received at Kolkata (22034/N, 88024/E), India have been investigated The signal amplitude is remarkably low in October. The variations of daily maximum and daily minimum of signal amplitude have been analyzed. The signal on the present propagation path showed non-correlation with A p index, but showed a moderate but negative correlation with 10.7 cm solar flux.

During the month of March 2006, a short campaign was conducted to measure fair-weather atmospheric electricity parameters in Tripura, Northeast India (23.50°N, 91.25°E). The campaign was the first of its kind in this region of the globe. The main objective of the campaign was to characterize the diurnal variation of three parameters namely vertical potential gradient (E), vertical air-earth current density (J z ) and atmospheric electrical conductivity (σ) in fair-weather conditions. The diurnal variation of E and J z over sixteen fair-weather days shows two distinct maxima around 14UT and 20UT and a minimum around 03UT. The average vertical potential gradient is found to be 108V·m−1 and air-earth current density 1.85 pA·m−2. The average bipolar atmospheric electrical conductivity at the ground level is found to be 19.6 fS·m−1. An excellent positive correlation between E and J z is found, with a correlation coefficient of 0.96. A comparative study with Carnegie universal variation shows 70% correlation with observed variation of vertical potential gradient during the period of the campaign. The results are discussed in view of difficulty as well as possibility of getting global signatures in atmospheric electricity measurements made from tropical land stations.