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  • 1
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    A Twenty Year Decline in Solar Photospheric Magnetic Fields: Inner-Heliospheric Signatures and Possible Implications? (2015)
    Wiley
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    Publication Date: 2015-06-13
    Description: We report observations of a steady 20 year decline of solar photospheric fields at latitudes ≥45° starting from ~1995. This prolonged and continuing decline, combined with the fact that Cycle 24 is already past its peak, implies that magnetic fields are likely to continue to decline until ~2020, the expected minimum of the ongoing solar Cycle 24. In addition, interplanetary scintillation (IPS) observations of the inner heliosphere for the period 1983–2013 and in the distance range 0.2–0.8 AU, have also shown a similar and steady decline in solar wind micro-turbulence levels, in sync with the declining photospheric fields. Using the correlation between the polar field and heliospheric magnetic field (HMF) at solar minimum, we have estimated the value of the HMF in 2020 to be between 3.8 (±0.5) and 4.0 (±0.6) nT and a floor value of the HMF of ~3.2 (±0.4) nT. Given this floor value for the HMF, our analysis suggests that the estimated peak sunspot number for solar Cycle 25 is likely to be between ~ 56 and 69 (±12).
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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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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  • 3
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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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  • 4
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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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  • 5
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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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  • 6
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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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  • 7
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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
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    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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  • 8
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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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  • 9
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    Chd1 regulates open chromatin and pluripotency of embryonic stem cells (2009)
    Nature Publishing Group (NPG)
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    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
    Published by Nature Publishing Group (NPG)
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  • 10
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    Further refinements to the Spatio-temporal forecast model for L-band scintillation based on comparison with C/NOFS observations (2017)
    Wiley
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    Publication Date: 2017-04-28
    Description: 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 N max 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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