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  • 1
    Publication Date: 2016-09-23
    Description: The impact of the St. Patrick's 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 (5̊ 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.
    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
    Publication Date: 2015-07-01
    Description: The earlier evolved method for the forecast of the spatio-temporal 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 LT 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2009-07-10
    Description: An open chromatin largely devoid of heterochromatin is a hallmark of stem cells. It remains unknown whether an open chromatin is necessary for the differentiation potential of stem cells, and which molecules are needed to maintain open chromatin. Here we show that the chromatin remodelling factor Chd1 is required to maintain the open chromatin of pluripotent mouse embryonic stem cells. Chd1 is a euchromatin protein that associates with the promoters of active genes, and downregulation of Chd1 leads to accumulation of heterochromatin. Chd1-deficient embryonic stem cells are no longer pluripotent, because they are incapable of giving rise to primitive endoderm and have a high propensity for neural differentiation. Furthermore, Chd1 is required for efficient reprogramming of fibroblasts to the pluripotent stem cell state. Our results indicate that Chd1 is essential for open chromatin and pluripotency of embryonic stem cells, and for somatic cell reprogramming to the pluripotent state.〈br /〉〈br /〉〈a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3891576/" target="_blank"〉〈img src="https://static.pubmed.gov/portal/portal3rc.fcgi/4089621/img/3977009" border="0"〉〈/a〉   〈a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3891576/" target="_blank"〉This paper as free author manuscript - peer-reviewed and accepted for publication〈/a〉〈br /〉〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Gaspar-Maia, Alexandre -- Alajem, Adi -- Polesso, Fanny -- Sridharan, Rupa -- Mason, Mike J -- Heidersbach, Amy -- Ramalho-Santos, Joao -- McManus, Michael T -- Plath, Kathrin -- Meshorer, Eran -- Ramalho-Santos, Miguel -- DP2 OD004698/OD/NIH HHS/ -- R01 GM080783/GM/NIGMS NIH HHS/ -- R01 GM080783-01/GM/NIGMS NIH HHS/ -- R01 GM080783-02/GM/NIGMS NIH HHS/ -- R01 GM080783-03/GM/NIGMS NIH HHS/ -- R01 GM080783-04/GM/NIGMS NIH HHS/ -- R01 GM080783-05/GM/NIGMS NIH HHS/ -- England -- Nature. 2009 Aug 13;460(7257):863-8. doi: 10.1038/nature08212. Epub 2009 Jul 8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Ob/Gyn and Pathology, Center for Reproductive Sciences and Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research, University of California, San Francisco, 513 Parnassus Avenue, San Francisco, California 94143-0525, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19587682" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; Biomarkers ; Cell Proliferation ; Cells, Cultured ; Cellular Reprogramming ; *Chromatin Assembly and Disassembly ; DNA-Binding Proteins/deficiency/genetics/*metabolism ; Embryonic Stem Cells/*cytology/*metabolism ; Endoderm/metabolism ; Euchromatin/genetics/*metabolism ; Fibroblasts/cytology/metabolism ; GATA6 Transcription Factor/genetics/metabolism ; Histones/metabolism ; Methylation ; Mice ; Neurogenesis ; Neurons/cytology/metabolism ; Octamer Transcription Factor-3/genetics ; Pluripotent Stem Cells/*cytology/*metabolism ; Promoter Regions, Genetic/genetics ; RNA Interference
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 4
    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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  • 5
    Publication Date: 2013-01-31
    Description: [1]  We have studied the potential inaccuracies in the geopotential height, its manifestation in the altitude profile of temperature measured by balloon-borne GPS radiosonde (GPS sonde), and its association with the variations in ionospheric plasma density, particularly in the Indian low latitude region. Using results from a GPS sonde inter-comparison campaign, conducted at Gadanki (13.5°N, 79.2°E, 6.4°N geomagnetic latitude) at different local times of a day, we show that differences in temperature at different geopotential heights vary with respect to the local time of the day in October 2010. However, differences in temperature become negligible and within the limits of experimental uncertainty if the comparison is made with respect to the time of measurements during the ascent rather than with respect to the GPS-derived geopotential height. We show that the origin of the mismatch rests with the inaccurate estimation of geopotential height by the two GPS sondes and not due to any inaccuracy in their measurements itself. Differences in the geopotential height estimates are larger during late afternoon, resulting in larger temperature differences between the sondes, and least during late evening/early morning hours. The uncertainty in the geopotential height estimation remains well synchronized with the magnitude and variations in the total electron content (TEC) of the ionosphere. This study exhorts us to account for the local ionospheric variations while extracting the geopotential height profiles of temperature from GPS sonde measurements, especially in the low and equatorial regions where there are large spatial and temporal variations in the ionospheric densities.
    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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  • 6
    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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  • 7
    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
    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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  • 9
    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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  • 10
    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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