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  • 1
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    Low latitude Pi2 oscillations observed by polar Low Earth Orbiting satellite (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: Low-latitude Pi2 pulsations in the topside ionosphere are investigated using vector magnetic field measurements from LEO satellite, CHAMP and underneath ground station. Substorm-associated Pi2s are initially identified using high-resolution data from Indian station Shillong, during 2007-2009, and are further classified into three sub-groups of Pi2-band (6-15mHz), based on its frequency. During nighttime, coherent in-phase oscillations are observed in the compressional component at satellite and horizontal-component at underneath ground-station for all the Pi2 events, irrespective of the Pi2 frequency. We observe that the identification of daytime Pi2s at CHAMP (compressional component) depends on the frequency of Pi2 oscillation i.e., 40%, 45% and 100% of Pi2 events observed in dayside ground station with frequency between 6-10mHz, 10-15mHz and 15-25mHz were identified at satellite respectively. At CHAMP during daytime, the presence of a dominant power in the lower frequencies of Pi2-band, which is unique to satellite is consistently observed and can modify the Pi2-oscillations. Pi2s having frequency 〉15mHz are less affected by these background frequencies and a clear signature of daytime Pi2s at CHAMP is possible to observe, provided contribution from non-Pi2 frequencies at satellite from the lower end of Pi2 band is eliminated. Daytime Pi2s identified in the topside ionosphere showed coherent but mostly opposite-phase oscillations with underneath ground station, and satellite-to-ground amplitude ratio is in general found to be less than one. Present results indicate that a combination of fast-cavity-mode oscillations and an instantaneous transmission of Pi2 electric field from high-to-low latitude ionosphere is responsible for the observation of daytime Pi2s.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    Insight into the Growth Reaction Mechanism of Ceramic Co3TeO6: Synchrotron Structural and Thermal Analysis (2016)
    Wiley
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    Publication Date: 2016-06-23
    Description: A two-step solid-state reaction is proposed to synthesize monophasic cobalt tellurate Co 3 TeO 6 (CTO), a type II multiferroic, using Co 3 O 4 and TeO 2 as the starting reagents. First step of the reaction results in the secondary monoclinic ( P 2 1 / c ) CoTeO 4 compound, which on further calcination (second step) leads to the primary monoclinic ( C 2/ c ) Co 3 TeO 6 phase. High-resolution synchrotron X-ray diffraction and the subsequent Rietveld analysis are used to probe different phases present in the synthesized CTO and to achieve its single phase. X-ray absorption near-edge structure studies at Co K and Te L edges reveal mixed oxidation states (Co 2+/3+ ) of Co and hexavalent Te, respectively. Charge imbalance due to mixed valence Co ions has been attributed to cations vacancies. Enhanced multiferroic properties, such as effective magnetic moment, spin phonon coupling, etc., have been attributed to the aforementioned observations in grown ceramic CTO via proposed synthesis route.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 3
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    Modulation of electromagnetic ion cyclotron waves by Pc5 ULF waves and energetic ring current ions (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We present the ground observation of the modulation of strong electromagnetic ion cyclotron (EMIC) waves by short and long periodicities at Indian Antarctic station, Maitri. The signatures of these waves were evident in the magnetic field variations recorded by an induction coil magnetometer during the interval 4.7‐7.2 UT on 17 September 2011, a moderately disturbed day. These waves were preceded by a gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising tone EMIC waves were observed in the Pc1 frequency band (~0.5‐0.9 Hz). The investigation of the periodicities of the observed wave spectrogram shows the presence of short (≈2.4 minute) and long (≈ 39‐69 minute) periodicities. Our analysis shows that the short periodicities are associated with the Pc5 ULF waves generated by magnetic field line oscillations, while long periodicities might be associated with the ring current drifting ions. A new method, based on the cross‐correlation technique is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in the range of 0.44‐1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from the satellite observations of EMIC waves. We found that the higher sweep rates are associated with the stronger EMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that the theoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region is retained even during the propagation of these waves on the ground.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
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  • 4
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    Day side cosmic noise absorption at the equator-ward boundary of aurora oval as observed from Maitri, Antarctica (L = 5; CGM 62.450 S, 55.450 E) (2016)
    Wiley
    Details
    Publication Date: 2016-04-08
    Description: On 02 April 2011, a couple of cosmic noise absorptions (CNA) events were detected at Maitri, Antarctica (L = 5, CGM 63.14 0  S, 53.69 0 E) confining to night and day times. One of the two events that occurred during night hours was caused due to auroral substorm onset. The current study focuses on the later CNA event, which was recorded during daytime (1000-1300 MLT, MLT = UT-1, at Maitri, Antarctica). We refer to this CNA event as day side CNA (DCNA) event. Absence of westward electrojet during DCNA confirms its dissimilarity from Auroral Substorm absorption events. A comparison has been made between the DCNA event of 02 April 2011 with that of 14 July 2011, a day with substorm activity when Maitri is in day side but without DCNA event. The comparison has been made in the light of interplanetary conditions, imaging Riometer data, ground magnetic signatures, GOES electron flux density and associated pulsations. The study shows that stronger prolonged eastward interplanetary electric field favors the occurrence of DCNA event. It is concluded that DCNA event is due to the gradient curvature drift of trapped non-relativistic electrons in the equatorial plane. Estimated energy of trapped electrons using azimuthal drift time for a set of ground stations within the auroral oval confirms the enhancement in electron fluxes in the same energy band as recorded by geostationary satellites GOES 13 and GOES 15. The reason for precipitation of electrons is expected to be the loss cone scattering caused by wave-particle interaction triggered by ULF waves.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Geomorphic signatures of glacial activity in Alba Patera volcanic province: Implications for recent frost accumulation on Mars (2013)
    Wiley
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    Publication Date: 2013-07-26
    Description: [1]  Glacial/periglacial landforms lying within impact craters on Mars have led to the identification of two mechanisms for their formation, (1) intermittent deposition of atmospherically emplaced snow/ice during past spin-axis/orbital conditions and (2) flow of debris covered ice-rich deposits. The maximum presence of the young ice/snow-rich features (thermal-contraction crack polygons, gully, arcuate ridge, and lobate debris tongue) were observed on the pole-facing slope, indicating that pole-facing slope was the preferred site for ice/snow accumulation (during last 10 Ma). In this study, we have investigated 30 craters lying in Alba Patera volcanic province in the latitudinal bands between 45˚-32.4˚ N. The morphological comparison of these younger ice/snow-rich features in these craters have led us conclude that glacial/periglacial features in Alba Patera are mainly present within pole-facing slopes of craters lying within 45˚-39˚ N. The craters lying within 40.2˚-40˚ N have not shown any glacial/periglacial features. We suggest that the formation of these young ice/snow-rich features follow the same orientation trends as of other older (〉10 Ma) glacial features (debris covered ice/snow-rich large deposits at the base of crater wall) in the region. Present work has revealed that the onset of physical processes that result in the formation of glacial/periglacial landforms is also dependent on the changes in elevation ranges of the investigated craters in Alba Patera. Our results confirm the past inferences for accumulation of ice/snow on Mars and suggest that the period of ice/snow accumulation activity in Alba Patera occurred throughout Amazonian and lasted till the recent past, i.e. 2.1-0.4 Ma.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Enhancement and modulation of cosmic noise absorption in the afternoon sector at sub-auroral location (L = 5) during the recovery phase of 17th March 2015 geomagnetic storm (2017)
    Wiley
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    Publication Date: 2017-08-04
    Description: The present study has focused on the intense production of cosmic noise absorption(CNA) at Maitri, Antarctica ( L = 5;CGM −62 ∘ S 55 ∘ E ) during the early recovery phase of the largest storm of the current solar cycle commenced on 17th March,2015 St. Patrick Day. The enhancement of CNA during 15 − 18 UT(14 − 17 MLT); (MLT=UT-1 at Maitri) was as large as the CNA enhancement occurred during the main phase of the storm. During this time the CNA pattern also exhibits oscillation in the Pc5(2 − 7 mHz) range and is in simultaneity with geomagnetic pulsations in the same frequency range. We observed the amplitude of CNA pulsation is well correlated with the level of CNA production. High amplitude Pc5 oscillations were observed in the vicinity of auroral oval near Maitri. Absence of Electro-Magnetic Ion-Cyclotron(EMIC) waves is marked suggesting the possible role of VLF waves in precipitation. The reason for the intense CNA production is found to be the precipitation caused mainly by hiss-driven sub-relativistic electrons. The CNA enhancement event is located well inside the dusk plasmaspheric bulge region as suggested by Tsurutani et al. [2015]. Signature of enhanced eastward electrojet at Maitri during 14 − 17 MLT could be an additional factor for such large CNA. In order to establish the cause and effect relationship between the geomagnetic and CNA oscillations at Maitri, Transfer Entropy method has been used, which confirmed the modulation of CNA by geomagnetic pulsations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Amazonian fluvial outflow channels in Jovis Tholus region, Mars (2017)
    Wiley
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    Publication Date: 2017-05-03
    Description: Outflow channels are prevalent on the surface of Mars and they are one of the strongest lines of evidence for fluvial activity. The purported presence of some of the channel networks on the Amazonian surface provides evidence for recent surficial fluvial activity. We describe morphological evidence collectively diagnostic of fluvial origin for the channels located northwest (NW) of Jovis Tholus, within the Tharsis volcanic province. The fluvial relationships are deciphered from 1) morphological analysis of catastrophic effusion associated with the graben system superposed over NW portion of an unnamed crater ejecta (19.85° N, 118.03° W), and 2) crater-size frequency distributions of unnamed crater ejecta revealing the tentative period of the fluvial activity. Channels, streamlined/curvilinear islands, terraces, divide crossing, crossover channels, networks of braided like channels and eroded ejecta are collectively suggestive of fluvial outflow that emanated from the graben. Chronological analysis revealed that the unnamed crater formed at ~3.4 Ga. Subsequently, the NW part of crater ejecta was incised by a channel network originated from the graben. Furthermore, we infer that the current channel floor has undergone volcanic and aeolian resurfacing to some extent, and the estimated modeled resurfaced channel age is ~0.2-0.5 Ga. This reveals that the fluvial activity possibly occurred after the crater formation, but before channel floor resurfacing, which is during Early-to-Middle Amazonian. Nevertheless, the potential source for the channel formation likely persisted under this region since the pre-Amazonian epoch. The suite of these fluvial features inferred within the Jovis Tholus region outflow channel system thus adds one another evidence for Amazonian fluvial activity.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    On the nocturnal downward and westward equatorial ionospheric plasma drifts during the 17 March 2015 geomagnetic storm (2018)
    Wiley
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    Publication Date: 2018-02-06
    Description: During quiet period, the nocturnal equatorial ionospheric plasma drifts eastward in the zonal direction and downward in the vertical one. This quiet time drifts pattern could be understood through dynamo processes in the night time equatorial ionosphere. The present case study reports the nocturnal simultaneous occurrence of the vertically downward and zonally westward plasma drifts over the Indian latitudes during the geomagnetic storm of 17 March 2015. After ~17:00 UT (~22:10 local time), the vertical plasma drift became downward which coincided with the westward zonal drift, a rarely observed feature of low latitude plasma drifts. The vertical drift turned upward after ~18:00 UT whilst the zonal drift became eastward. We mainly emphasize here the distinct bipolar type variations of vertical and zonal plasma drifts observed around 18:00 UT. We explain the vertical plasma drift in terms of the competing effects between the storm time prompt penetration and disturbance dynamo electric fields. Whereas the westward drift is attributed to the storm time local electrodynamical changes mainly through the disturbance dynamo field in addition to the vertical Pedersen current arising from the spatial (longitudinal) gradient of the field aligned Pedersen conductivity.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Anomalous diurnal variation of atmospheric potential gradient and air-Earth current density observed at Maitri, Antarctica (2016)
    Wiley
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    Publication Date: 2016-10-08
    Description: The scope of this paper is to explore the mechanisms operating over Maitri (70.76 o S, 11.74 o E, 117 m AMSL), a coastal Antarctic station, that produce an anomalous fair-weather diurnal pattern of the atmospheric electric Potential Gradient (PG) and air-Earth current density (AEC). The anomaly in the diurnal variations of AEC and the PG is displaying an ostensible minimum at ~10 UT and a diminished response to the thunderstorm over the African continent in the 14-16 UT time frame. The data sets (2005-2014, except 2012) of the PG, and to some extent, AEC, from Maitri, are used to explore this anomaly. It follows that the fair-weather electrical phenomena over Maitri can be ascribed to global electrified convection on the one hand and to regional phenomena like convection due to the replacement of warm air by katabatic winds on the other hand. The katabatic winds originate on the polar plateau, and blow from ~130° at Maitri are likely to transport various elements from the mountain slopes and space charge from the polar plateau is expected to produce various disturbances in the PG and AEC monitored over the coastal Antarctica. This mechanism may be responsible for peaks in the early UT hours and so also for the anomalous behavior of atmospheric electrical parameters observed at Maitri. Maitri data is compared with that of Carnegie cruise and Vostok to explain the source of anomaly.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    What Fraction of the Outer Radiation Belt Relativistic Electron Flux at L ≈ 3‐4.5 Was Lost to the Atmosphere During the Dropout Event of the St. Patrick's Day Storm of 2015? (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Observations of relativistic energetic electron fluxes in the outer radiation belt can show dropouts, that is, sudden electron flux depletions during the main phase of a geomagnetic storm. Many recent studies show that these dropouts typically involve a true loss of particles, that is, nonadiabatic losses in nature. Precipitation into the atmosphere of relativistic electrons driven into the bounce loss cone, through wave‐particle interactions, is envisaged as one of the primary loss mechanisms. Such precipitation can be studied using ground‐based observations such as VLF narrowband radio waves, due to the deposition of energy into the lower ionospheric D‐region, thereby modifying the subionospheric waveguide. The present study focuses on the dropout event observed during the St. Patrick's Day storm of March 2015. Perturbations lasting several hours were observed in the received VLF amplitude and phase of the NAA transmitter signal measured at Seattle and Edmonton and the NML transmitter signal received at St. John's and Edmonton. All these L ≈ 3–4.5 paths were located on the nightside of the Earth during dropout phase of the storm. Observations of relativistic electron characteristics from Van Allen Probes, and ionospheric perturbation characterization from VLF radio waves, are used to calculate that during the time interval of the dropout event, 〈0.5% of the relativistic fluxes involved in the dropout event were lost to the atmosphere. This leads to the conclusion that relativistic electron precipitation was not the major contributor to the observed dropout event at L ≈ 4 that occurred during the St. Patrick's Day storm of March 2015.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
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