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  • 2010-2014  (45)
  • 1
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    Satellite Based Augmentation Systems – A Novel and Cost Effective Tool For Ionospheric and Space Weather Studies (2014)
    Wiley
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    Publication Date: 2014-12-06
    Description: 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.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Scattering function for energetic neutral hydrogen atoms off the lunar surface (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: The Sub-keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan-1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti-sunward flux and shallower scattering. We analyzed more than 290′000 measurements and derived a mathematical description of the features and their dependencies on the SZA.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Planetary wave-tidal interactions over the equatorial mesosphere-lower thermosphere region and their possible implications for the equatorial electrojet (2011)
    Wiley
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    Publication Date: 2011-01-27
    Description: Optically measured daylight mean mesopause temperatures over a dip equatorial station, Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), have been analyzed in conjunction with simultaneously measured equatorial electrojet (EEJ)–produced magnetic field at the surface. The signature of planetary wave-tidal interactions in the mesosphere–lower thermosphere (MLT) region has been observed for the first time in the day-to-day variability in the EEJ, i.e., the time of its peaking and the duration, as inferred from the EEJ-produced magnetic field on the ground. The present study shows that the planetary wave of quasi 16 day periodicity plays an important role in causing these variabilities, especially during the winter months. The quasi 16 day wave is found to be modulating the mesopause temperature (MT), duration, and time of the maximum EEJ intensity (DEEJ and TEEJ). During positive excursions of the planetary wave, TEEJ showed a shift toward evening, while the MT showed an increase and DEEJ showed a broadening. Similarly, all these parameters exhibited an opposite trend during negative excursions. The planetary wave-tidal interactions and subsequent modification of the tidal components have been shown to be responsible for the observed variations. This study presents a new perspective addressing the day-to-day variability of the EEJ.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Daytime upper mesospheric energetics over a tropical station, Trivandrum (8.5°N, 77°E): An investigation using the multiwavelength dayglow photometry (2011)
    Wiley
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    Publication Date: 2011-01-14
    Description: This paper presents the first systematic measurements of “daytime” upper mesosphere temperatures over Trivandrum (8.5°N, 77°E), a near equatorial station in India. The measurements were made using a unique meridional scanning multiwavelength dayglow photometer. The first ever observation of the short-scale (within a day), seasonal (during a year), and spatiotemporal (spanning over a region of ∼300 km centered at Trivandrum) variation of the daytime upper mesospheric temperature have been discussed. In general, the daytime mesospheric temperatures over this latitude exhibit large variability in their short scale seasonal, and spatiotemporal characteristics. The seasonal variation of temperature during the year 2005 exhibited a clear-cut semiannual oscillation pattern with minima around day numbers 75 and 250, which corroborate well with the earlier measurements of temperature over Thumba using the sounding rockets.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Low-latitude ionospheric-thermospheric response to storm time electrodynamical coupling between high and low latitudes (2011)
    Wiley
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    Publication Date: 2011-01-13
    Description: Using multi-instrumental and multistation data, we present low-latitude ionospheric-thermospheric behavior during the geomagnetic storm of 15 May 2005. The diurnal pattern of total electron content (TEC) at a chain of equatorial to low-latitude stations shows strong positive ionospheric storm on 15 May. Latitudinal variation of TEC shows development of strong equatorial ionization anomaly (EIA) on the same day. Evidence, in terms of equatorial electrojet (EEJ) and magnetogram signatures, is presented for the prompt penetration of interplanetary electric field (IEF) as the cause of the positive ionospheric storm. Consequent to the storm time circulation resulting from the extra energy deposition via Joule heating over high latitudes, compositional changes occur in the global thermosphere. TEC enhancements on 16 May are attributed to enhancement of atomic oxygen at equatorial and low latitudes and the negative ionospheric storm on 17 May observed beyond certain low latitudes is explained in terms of enhancement of molecular species because of the storm time neutral composition changes. Strong ESF plume structures on range time intensity (RTI) map and L-band scintillation and TEC depletions in GPS measurements are observed in the longitude sectors where the local time of sudden storm commencement (SSC) falls after the post sunset hours. The ionospheric zonal electric fields are altered by the combined effects of eastward disturbance dynamo electric fields and direct prompt penetration of eastward electric fields associated with the northward turning of interplanetary magnetic field (IMF) Bz leading to subsequent development of ESF after midnight.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Structural basis for the sequence-specific recognition of human ISG15 by the NS1 protein of influenza B virus [Biochemistry] (2011)
    National Academy of Sciences
    Details
    Publication Date: 2011-08-18
    Description: Interferon-induced ISG15 conjugation plays an important antiviral role against several viruses, including influenza viruses. The NS1 protein of influenza B virus (NS1B) specifically binds only human and nonhuman primate ISG15s and inhibits their conjugation. To elucidate the structural basis for the sequence-specific recognition of human ISG15, we determined the crystal structure of the complex formed between human ISG15 and the N-terminal region of NS1B (NS1B-NTR). The NS1B-NTR homodimer interacts with two ISG15 molecules in the crystal and also in solution. The two ISG15-binding sites on the NS1B-NTR dimer are composed of residues from both chains, namely residues in the RNA-binding domain (RBD) from one chain, and residues in the linker between the RBD and the effector domain from the other chain. The primary contact region of NS1B-NTR on ISG15 is composed of residues at the junction of the N-terminal ubiquitin-like (Ubl) domain and the short linker region between the two Ubl domains, explaining why the sequence of the short linker in human and nonhuman primate ISG15s is essential for the species-specific binding of these ISG15s. In addition, the crystal structure identifies NS1B-NTR binding sites in the N-terminal Ubl domain of ISG15, and shows that there are essentially no contacts with the C-terminal Ubl domain of ISG15. Consequently, NS1B-NTR binding to ISG15 would not occlude access of the C-terminal Ubl domain of ISG15 to its conjugating enzymes. Nonetheless, transfection assays show that NS1B-NTR binding of ISG15 is responsible for the inhibition of interferon-induced ISG15 conjugation in cells.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 7
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    Evolutionary phases of equatorial spread F including L band scintillations and plumes in the context of GPS total electron content variability: A case study (2011)
    Wiley
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    Publication Date: 2011-10-04
    Description: The evolution of large-scale (few kilometers), medium-scale (few hundreds of meters), and small-scale (meters) size plasma density irregularities in the postsunset equatorial F region, in the context of characteristic GPS total electron content (GTEC) variations, are reported from Indian longitudes. The ionograms and GTEC from a GPS receiver installed as a part of the GPS Aided Geo Augmentation Network (GAGAN) project for satellite-based navigation are obtained from an equatorial station at Trivandrum (8.5°N, 76.91°E, dip latitude 0.5°N). The variations in the GTEC with respect to TEC are considered to represent the seed perturbations for the plasma instability that results in the equatorial spread F (ESF) irregularities and are treated as a perturbation factor (P). The VHF radar at Gadanki (13.5°N, 79.17°E, dip latitude 6.4°N) provided the small-scale structures of ESF. The background thermospheric conditions that affect the growth of the plasma instability through ion-neutral collision frequency (νin) are estimated using the F region base height (h′F)and the representative scale height of the neutral atmosphere and are represented by a growth factor (G). The present case study reveals a close coupling between the background ionospheric conditions and the baseline perturbations in deciding the evolutionary phases of ESF. It has been shown that although large-scale (kilometer scale) irregularities are formed without any constraints when the background ionospheric-thermospheric conditions are favorable in the presence of fluctuations in GTEC, consistently, the medium-scale and small-scale irregularities show remarkable similarity with the variations in the product of the perturbation and growth factors.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Effects of prolonged southward IMF Bz on low latitude ionospheric electron density (2014)
    Wiley
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    Publication Date: 2014-06-17
    Description: The present work describes the low latitude ionospheric variability during an unusually prolonged (~33 hours) geomagnetically disturbed condition that prevailed during 15-16 July 2012. The low latitude electron density in summer hemisphere, investigated using ground and satellite based observations, responded to this by generating strong negative ionospheric storm on 16 July. The maximum electron density on 16 July over Indian low latitudes was reduced by more than 50% compared to that on a geomagnetically quiet day (14 July 2012). In contrast to the extreme reduction in total electron content (TEC) in the northern hemisphere, TEC from a winter hemispheric station revealed substantial (~23 TECU) enhancements on the same day. This contrasting hemispherical response in TEC is suggested to be due to the combined effects of strong interhemispheric, and solar driven day-night winds. Further, very weak equatorial electrojet (EEJ) strength on 16 July indicated that the westward electric field perturbations in the low latitude ionosphere were possibly due to the disturbance dynamo effect associated with meridional circulation from polar to equatorial latitudes. Interestingly, despite reduction in the integrated EEJ strength on 15 July, the low latitude electron density showed substantial enhancement highlighting the significant effect of the positive ionospheric storm on the low latitude ionosphere. The roles of electro-dynamical/neutral-dynamical and compositional disturbances are discussed in view of these observations to understand low latitude ionospheric response when geomagnetic disturbance persists for longer duration.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Gravity wave signatures in the dip equatorial ionosphere –thermosphere system during the annular solar eclipse of 15 January 2010 (2014)
    Wiley
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    Publication Date: 2014-05-08
    Description: The present work pertains to the eclipse induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. ENWi rocket payload measured horizontal winds and electron densities at E region altitudes and ionosonde derived foF1 and foF2 parameters are used to analyze the characteristics of the eclipse induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30-100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave induced winds in generating blanketing Es over the equator is also examined.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Pre-assessment of the “strength” and “latitudinal extent” of L-band scintillation: A case study (2013)
    Wiley
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    Publication Date: 2013-01-28
    Description: [1]  Forecasting the strength of nighttime L-band scintillation well in advance of their actual occurrence by making use of the background conditions has been attempted over the Indian zone with an eye on operational forecasting capability. It has been shown that the base height of the equatorial F -region as early as 1930 h LT shows a linear relation to the maximum altitudinal extent of medium-scale irregularities that are responsible for the L-band scintillation during the course of the night and these in turn get mapped to latitudes away from the equator defining the latitude range that gets affected by their presence. On the other hand, the average value of the vertical total electron content between 1830 h LT and 1930 h LT on a given day is shown to control the maximum strength of the scintillation represented by S 4 index on the day. The above analysis, although it has been carried out for the equatorial station Trivandrum, is applicable to other latitudes as well. Having demonstrated the control of the background ionospheric-thermospheric conditions, it has been shown that the averaged vertical total electron content around the prereversal enhancement (1830 h LT to 1930 h LT) along with the actual base height information at 1930 h LT over the magnetic equator would enable one to forecast the maximum possible S 4 in a given latitudinal region as early as 1930 h and issue necessary forewarning alerts wherever needed.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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