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  • 1
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    Right-handed Circularly Polarized Dispersive Alfvén Wave: Localization and Turbulence in Solar Wind (2015)
    Wiley
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    Publication Date: 2015-07-08
    Description: The elucidation of the turbulent spectrum in the solar wind region in the dispersive range of scales is not yet completely established. The observational analysis of many spacecrafts report that the fluctuations on the order of (or smaller scales) proton inertial length λ   i  =  c / ω   p i show disparity with respect to the large scale fluctuations. We present a parallel propagating dispersive Alfvén wave that becomes dispersive as a result of the finite frequency of the wave, when subjected to transverse instability can be used to study the steepening in power spectrum around proton inertial length. The transverse density perturbations of acoustic wave can couple nonlinearly with parallel propagating pump and the driven ponderomotive force consecutively leads to growth of perturbations. We have studied the evolution of localized structures with time and the power spectral density when the system reaches quasi steady state. The application of the results in the solar wind turbulence is also discussed.
    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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    Aperture‐Synthesis Radar Imaging With Compressive Sensing for Ionospheric Research (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Inverse methods involving compressive sensing are tested in the application of two‐dimensional aperture‐synthesis imaging of radar backscatter from field‐aligned plasma density irregularities in the ionosphere. We consider basis pursuit denoising, implemented with the fast iterative shrinkage thresholding algorithm, and orthogonal matching pursuit (OMP) with a wavelet basis in the evaluation. These methods are compared with two more conventional optimization methods rooted in entropy maximization (MaxENT) and adaptive beamforming (linearly constrained minimum variance or often “Capon's Method.”) Synthetic data corresponding to an extended ionospheric radar target are considered. We find that MaxENT outperforms the other methods in terms of its ability to recover imagery of an extended target with broad dynamic range. Fast iterative shrinkage thresholding algorithm performs reasonably well but does not reproduce the full dynamic range of the target. It is also the most computationally expensive of the methods tested. OMP is very fast computationally but prone to a high degree of clutter in this application. We also point out that the formulation of MaxENT used here is very similar to OMP in some respects, the difference being that the former reconstructs the logarithm of the image rather than the image itself from basis vectors extracted from the observation matrix. MaxENT could in that regard be considered a form of compressive sensing.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Non linear effects associated with Kinetic Alfvén Wave (2015)
    Wiley
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    Publication Date: 2015-03-24
    Description: The nonlinear phenomena are of striking importance in understanding the particle acceleration, heating and turbulence in the interplanetary space. Kinetic Alfvén wave (KAW) is one of the strong candidates responsible for accelerating the solar wind and powering the solar wind turbulence. Therefore, the nonlinear properties of KAW are attracting a good attention. In the present work we have investigated the non linear effects associated with KAW in the solar wind plasma at around 1 A.U. The ponderomotive force of (relatively high frequency, high power) pump KAW may be used to excite the low frequency KAW (LKAW) also. For this purpose we have derived the dynamical equations to analyze the nonlinear dynamics of relatively high frequency pump KAW in the presence of LKAW perturbation. The numerical solution has been carried out for the coupled system of equations by using the pseudospectral method for space integration and finite difference method along with the predictor corrector scheme for the evolution in time. The coupled system of nonlinear dynamical equations has been analyzed to study the nonlinear effects associated with pump KAW and the resulting turbulent spectra at 1 A.U.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Localized Structures and Turbulent Spectra in the Magnetopause (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Kinetic Alfvén wave (KAW) and whistler wave play a key role in the framework of turbulence and reconnection. There are lot of observations of these wave in the magnetopause region. The present paper deals with the nonlinear evolution of KAW and a weak whistler wave through the pre‐existing magnetic reconnection site. For this study the dynamical evolution equations are derived by taking into account the ponderomotive force driven density modification and magnetic field fluctuations due to shear field modelled by the Harris sheet. Furthermore, these equations have been solved numerically as well as semi‐analytically. For numerical integrations we have used the pseudospectral method and finite difference method and for semi‐analytically Runge Kutta method. Simulated results have shown the evolution of coherent structures or current sheets, which are capable to energy transfer efficiently. These structures have scale size around ion gyroradius as well as electron inertial length as calculated analytically. At a later time the chaotic structures arise in this reconnection site. This gives the signature of turbulence generation. Therefore, the magnetic power spectrum with scaling is also presented in this manuscript and their relevance with the observed spectrum (calculated from the cluster data (Chaston 2008)) is also pointed out.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
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  • 5
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    Super-Alfvénic Propagation and Damping of Reconnection Onset Signatures (2017)
    Wiley
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    Publication Date: 2017-12-06
    Description: The quadrupolar out-of-plane Hall magnetic field generated during collisionless reconnection propagates away from the x-line as a kinetic Alfvén wave (KAW). While it has been shown that this KAW carries substantial Poynting flux and propagates super-Alfvenically, how this KAW damps as it propagates away from the x-line is not well understood. In this study, this damping is examined using kinetic particle-in-cell simulations of antiparallel symmetric magnetic reconnection in a one dimensional current sheet equilibrium. In the reconnection simulations, the KAW wave vector has a typical magnitude comparable to an inverse fluid Larmor radius (effectively an inverse ion Larmor radius) and a direction of 85 - 89 degrees relative to the local magnetic field. We find that the damping of the reconnection KAW is consistent with linear Landau damping results from a numerical Vlasov dispersion solver. This knowledge allows us to generalize our damping predictions to regions in the magnetotail and solar corona where the magnetic geometry can be approximated as a current sheet. For the magnetotail, the KAW from reconnection will not damp away before propagating the approximately 20 Earth radii associated with global magnetotail distances. For the solar corona, on the other hand, these KAWs will completely damp before reaching the distances comparable to the flare loop length.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Bedload tracer mobility in a mixed bedrock/alluvial channel (2017)
    Wiley
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    Publication Date: 2017-03-09
    Description: The presence of bare or partially-covered rock in an otherwise alluvial river implies a downstream change in transport capacity relative to supply. Field investigations of this change and what causes it are lacking. We used two sets of magnet-tagged tracer clasts to investigate bedload transport during the same sequence of floods in fully alluvial, bare rock, and partial-cover reaches of an upland stream. High-flow shear stresses in different reaches were calculated using stage loggers. Tracers seeded in the upstream alluvial channel moved more slowly than elsewhere until the frontrunners reached bare rock and sped up. Tracers seeded on bare rock moved rapidly off it and accumulated just upstream from, and later in, a partial-cover zone with many boulders. The backwater effect of the boulder-rich zone is significant in reducing tracer mobility. Tracer movement over full or partial sediment cover was size selective but dispersion over bare rock was not. Along-channel changes in tracer mobility are interpreted in terms of measured differences in shear stress and estimated differences in threshold stress.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Localization of Whistler Wave and Turbulent Spectra in the Magnetotail Region (2017)
    Wiley
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    Publication Date: 2017-01-31
    Description: Observations show that whistler waves play a very crucial role both in magnetic reconnection and turbulence. In this context, linear and nonlinear dynamical equations of whistler wave have been derived and further, these equations have been solved numerically as well as semi-analytically. For application purpose, magnetotail parameters have been chosen. The obtained results reveal that the nonlinearity as well as field perturbation results in the formation of localized structures. These structures contribute to the turbulence and lead to the formation of current sheets in the magnetotail. Finally, the power spectrum has been evaluated and compared with the observed spectra in the magnetotail ion diffusion region as reported [Eastwood et al., 2009].
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel (2017)
    Wiley
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    Publication Date: 2017-03-03
    Description: Assumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision, paleoflood estimation, flood routing, and river engineering. Field data on bedrock flow resistance are very limited and calculations generally use standard alluvial-river assumptions such as a fixed value of Manning's n . To help inform future work we measured how depth, velocity and flow resistance vary with discharge in four short reaches of a small bedrock channel, one with an entirely rock bed and the others with 20%-70% sediment cover, and in the alluvial channel immediately upstream. As discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the Darcy-Weisbach friction factor f . The bare-rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macro-roughness of its rock walls. Flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive. Apart from the effect of rough rock walls, the flow resistance trends are qualitatively consistent with logarithmic and variable-power equations and with non-dimensional hydraulic geometry, but quantitative agreement using sediment D 84 as the roughness height is imperfect. This article is protected by copyright. All rights reserved.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
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  • 9
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    Nonlinear effects associated with quasi-electrostatic whistler waves relevant to that in radiation belts (2017)
    Wiley
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    Publication Date: 2017-01-04
    Description: A model is proposed to study the dynamics of high amplitude quasi-electrostatic whistler waves propagating near resonance cone angle and their interaction with low frequency kinetic Alfvén waves (KAWs)in Earth's radiation belts. The wave dynamics clearly indicates the whistlers having quasi-electrostatic character when propagating close to resonance cone angle. A high amplitude whistler wave packet is obtained using the present analysis which has also been observed by S/WAVES instrument onboard STEREO. A numerical simulation technique has been employed to study the localization of quasi-electrostatic whistler waves in radiation belts. The ponderomotive force of pump quasi-electrostatic whistlers (high frequency) is used to excite low frequency waves (KAWs).The turbulent spectrum obtained using the analysis suggests the presence of quasi-electrostatic whistlers and density fluctuations associated with KAW in radiation belts plasma. The wave localization and steeper spectra could be responsible for particle energization or heating in radiation belts.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    The nonlinear evolution of kinetic Alfvén wave with the ion acoustic wave and turbulent spectrum in the magnetopause region. (2015)
    Wiley
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    Publication Date: 2015-02-06
    Description: The mutual nonlinear interplay of kinetic Alfvén wave (KAW) and ion acoustic wave, for the high-β plasma (i.e. m e / m i  ≪  β  ≪ 1, where β is thermal to magnetic pressure ratio) in the magnetopause has been considered in the present study. A set of dimensionless nonlinear Schrödinger equation (NLS) has been derived taking into account the finite frequency as well as ion temperature corrections. The dynamical equation of the ion acoustic wave (propagating at an angle with respect to the background magnetic field) in the presence of ponderomotive nonlinearity due to KAW is also derived. Numerical simulation has been carried out to study the effect of nonlinear interaction between these waves which results in the formation of localized structures and turbulent spectrum, applicable to the high-β plasmas like magnetopause regions. Results reveal that due to the nonlinear interplay between these waves, natures of the formation of localized structures are complex and intense in nature in quasi steady state. From the results, we have found that spectral index follows the scaling at large scale and spectral index follows at small scale. We also found the steepening in the turbulent spectrum. Steepening in the turbulence spectrum has been reported by the THEMIS spacecraft across the magnetopause and results are found to be consistent with spacecraft observation.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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