Vikram Sarabhai Space Centre

About: Vikram Sarabhai Space Centre is a facility organization based out in Thiruvananthapuram, India. It is known for research contribution in the topics: Aerosol & Ultimate tensile strength. The organization has 2092 authors who have published 3058 publications receiving 47975 citations. The organization is also known as: VSSC.

Impacts of atmospheric ultrafast Kelvin waves on radio scintillations in the equatorial ionosphere

Abstract: [1] We present a statistical analysis of the amplitudes of GPS scintillations (S4 index) observed throughout 2008–2010 using the satellite radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). Here, for the first time, periodic variability in the occurrence of S4 is investigated using these data. Significant variations of S4 with periods of 2.5–4 days (quasi-3 days) are identified from the observations during postsunset hours (1900–2400 local time) between 15°S– 15°N magnetic latitude during this 3 year interval. Coherence analyses of these variations with the geomagnetic Ap index, solar EUV irradiance, and atmospheric wind measurements from an equatorial mesosphere meteor radar at Thumba, India ( 8.5°N, 77°E) are performed, providing a measure of the relationship between variations in the scintillations and potential drivers. The quasi-3 day variations in S4 are found to covary with the variations of the three drivers examined. In particular, the S4 signatures are found to be coherent with the atmospheric ultrafast Kelvin (UFK) planetary waves characterized by the zonal wind measurements of the radar. This study shows that these UFK waves are as important as the solar and geomagnetic drivers in forcing the day-to-day variations of the occurrence of equatorial spread F.

A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

Abstract: The height-time-concentration map of neutral sodium (Na) atoms measured by a Na lidar during the night of 18 to 19 March 2007 over Gadanki, India (13.5° N, 79.2° E) reveals an unusual structure in the Na layer for around 30 min in the altitude range of 92 to 98 km which is similar to the usual ‘C’ type structures observed at other locations. In order to understand the physical mechanism behind the generation of this unusual event, an investigation is carried out combining the data from multiple instruments that include the meteor wind radar over Thiruvananthapuram, India (8.5° N, 77° E) and the SABER instrument onboard the TIMED satellite. The temperature and wind profiles from the data set provided by these instruments allow us to infer the Richardson number which is found to be noticeably less than the canonical threshold of 0.25 above 92 km over Thiruvananthapuram suggesting the plausible generation of Kelvin-Helmholtz (KH) billows over southwestern part of the Indian subcontinent. Based on the average wind speed and direction over Thiruvananthapuram, it is proposed that the KH-billow structure was modified due to the background wind and was advected with it in nearly ‘frozen-in’ condition (without significant decay) in the northeastward direction reaching the Na lidar location (Gadanki). This case study, therefore, presents a scenario wherein the initially deformed KH-billow structure survived for a few hours (instead of a few minutes or tens of minutes as reported in earlier works) in an apparently ‘frozen-in’ condition under favorable background conditions. In this communication, we suggest a hypothesis where this deformed KH-billow structure plays crucial role in creating the abovementioned unusual structure observed in the Na layer over Gadanki.

Characterisation of Absorbing Aerosols Using Ground and Satellite Data at an Urban Location, Hyderabad

Abstract: In the present study we have attempted to characterize aerosols using their optical properties over a tropical urban location of Hyderabad, India. We have analyzed three years of in-situ data on aerosol absorption from Aethalometer and scattering from Nephelometer measurements. Satellite based absorption measurements from ozone monitoring instrument, absorbing aerosol index are also analyzed to investigate the role of long range transport of dust. Further, the Cloud-Aerosol Lidar Pathfinder Satellite Observations (CALIPSO) data is used to study the vertical extent of aerosol particles as well as their sphericity using its particulate depolarization ratio. The study revealed that irrespective of seasonal variation, local anthropogenic fossil fuel aerosols form the predominant aerosol type over this site. Biomass/dust aerosols in their pure form are not present during the study period; however the spread of frequency distribution of scattering Angstrom exponent and absorption Angstrom exponent suggested their possible existence in mixed condition with local anthropogenic aerosols. The analysis of columnar aerosol absorption data during pre-monsoon period showed the dominance of UV absorbing dust aerosols in the study region. CALIPSO data analysis over study area showed that majority aerosols are confined within 2 km from the surface during winter while in pre-monsoon particles are distributed throughout the profile (~6 km) with extinction coefficient varying between 0.1-0.2 km^(-1). As the season shift from winter to pre-monsoon a change in sphericity of particle is observed. Cluster mean trajectory analysis revealed that during premonsoon majority of air mass movements (~68%) are from western side passing through dust source region like Persian Gulf and Thar Desert before entering into Indian region. During post-monsoon (~70%) and winter (~65%), majority of the air masses are coming from north-west and north-east side of the study area where biomass burning is quite frequent during this period.

Rheology of HTPB Propellant: Development of generalized correlation and evaluation of pot life

Abstract: Les propergols solides a base de polybutadienes a terminaison hydroxy (HTPB) sont devenus les precurseurs parmi les propergols dans le monde moderne des moteurs de fusees. En raison de leur taux de remplissage eleve, de tels propergols ont des proprietes rheologiques tres complexes. On a deja etudie et publie des rapports sur l'effet de la substance de remplissage, de la taille des particules de l'oxydant et de la teneur en Al, ainsi que sur l'effet de la temperature, la cinetique du melange et le temps de cycle lors du melange. Dans la presente etude, on a developpe une correlation generalisee pour la vitesse de deformation de la contrainte de cisaillement sur la base des parametres rheologiques tels que la contrainte d'ecoulement, l'indice de consistance et l'indice de pseudoelasticite, qui sont respectivement independants de la fraction de volume solide, de la duree de vulcanisation t et de la temperature; le coefficient de correlation avec les valeurs determinees experimentalement est de 0,94. Un terme communement utilise dans l'industrie des propergols, la duree de conservation, a egalement ete defini et depouille pour le systeme de propergol HTPB.

Effect of powder oxidation on densification and properties of vacuum hot pressed Cu–Cr–Nb alloy

Abstract: The densification behavior of oxidized Cu-8 at% Cr-4 at%Nb alloy powders by vacuum hot pressing and subsequent mechanical properties of alloy compares well with that of fresh powders with no oxides and hydrogen reduced powders sintered using same conditions. The reason for such observation is the presence of high vacuum and carbon atmosphere due to graphite die at the furnace temperature helping in the decomposition of oxide layer. Reduction of oxides of copper controls the overall densification process. A new mechanism for the densification of air-oxidized powders during vacuum hot pressing is also put forward. Present work indicates that reduction of powders is not needed if consolidated by vacuum hot pressing.