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Sanjay Kumar

Publications and source records attributed to Sanjay Kumar.

At least 73 records · Page 4Linked to original sources
arXiv2018

Dynamics of a polymer under multi-gradient fields

Effects of multi-gradient fields on the transport of a polymer chain have been investigated by using generalized Langevin dynamics simulations. We observe that the natural frequency of tumbling follows $Wi^{0.66}$ scaling, where $Wi$ is the Weissenberg number. Analysis of angular tumbling time distribution reveals that the tail of distribution follows exponential distribution and at high Weissenberg number, deviates from Poisson behaviour. Competition between velocity gradient which results shear flow in the system, and solvent quality gradient arising due to the interaction among monomers revealed that there is another scaling associated with the angular tumbling time distribution. Moreover, at low temperature, we observe unusual behaviour that at intermediate shear rates, decay rate $ν$ decreases with $Wi$.

cond-mat.soft↗
arXiv2018

Magnetohydrodynamic simulation of magnetic null-point reconnections in NOAA AR12192 initiated with an extrapolated non-force-free-field

Magnetohydrodynamics of the solar corona is simulated numerically. The simulation is initialized with an extrapolated non-force-free magnetic field using the vector magnetogram of the active region (AR) NOAA 12192 obtained on the solar photosphere. Particularly, we focus on the magnetic reconnections occurring close to a magnetic null-point that resulted in appearance of circular chromospheric flare ribbons on October 24, 2014 around 21:21 UT, after peak of an X3.1 flare. The extrapolated field lines show the presence of the three-dimensional (3D) null near one of the polarity inversion lines---where the flare was observed. In the subsequent numerical simulation, we find magnetic reconnections occurring near the null point, where the magnetic field lines from the fan-plane of the 3D null form a X-type configuration with underlying arcade field lines. The footpoints of the dome-shaped field lines, inherent to the 3D null, show high gradients of the squashing factor. We find slipping reconnections at these quasi-separatrix layers, which are co-located with the post-flare circular brightening observed at the chromospheric heights. This demonstrates the viability of the initial non-force-free field along with the dynamics it initiates. Moreover, the initial field and its simulated evolution is found to be devoid of any flux rope, which is in congruence with the confined nature of the flare.

astro-ph.SR↗
arXiv2018

Reduced Order Fractional Fourier Transform A New Variant to Fractional Signal Processing Definition and Properties

In this paper, a new variant to fractional signal processing is proposed known as the Reduced Order Fractional Fourier Transform. Various properties satisfied by its transformation kernel is derived. The properties associated with the proposed Reduced Order Fractional Fourier Transform like shift, modulation, time-frequency shift property are also derived and it is shown mathematically that when the rotation angle of Reduced Order Fractional Fourier Transform approaches 90 degrees, the proposed Reduced Order Fractional Fourier Transform reduces to the conventional Fourier transform. Also, the Reduced Order Fractional Fourier Transform of various kinds of signals is also derived and it is shown that the obtained analytical expressions of different Reduced Order Fractional Fourier Transform are a reduced form of the conventional fractional Fourier transform. It is also shown that proposed definition of Fractional Fourier Transform is easier to be handled analytically. Finally, the convolution theorem associated with the proposed Reduced Order Fractional Fourier Transform is derived with its various properties like shift convolution, modulation convolution, and time-frequency shift modulation properties. It has been shown that with this proposed new definition of fractional Fourier transform, the convolution theorem has been reduced to multiplication in the fractional frequency domain.

eess.SP↗
arXiv2018

Study of magnetic field topology of active region 12192 using an extrapolated non-force-free magnetic field

The solar active region (AR) 12192 was one of the most flare productive region of solar cycle 24, which produced many X-class flares; the most energetic being an X3.1 flare on October 24, 2014 at 21:10 UT. Customarily, such events are believed to be triggered by magnetic reconnection in coronal magnetic fields. Here we use the vector magnetograms from solar photosphere, obtained from Heliospheric Magnetic Imager (HMI) to investigate the magnetic field topology prior to the X3.1 event, and ascertain the conditions that might have caused the flare. To infer the coronal magnetic field, a novel non-force-free field (NFFF) extrapolation technique of the photospheric field is used, which suitably mimics the Lorentz forces present in the photospheric plasma. We also highlight the presence of magnetic null points and quasi-separatrix layers (QSLs) in the magnetic field topology, which are preferred sites for magnetic reconnections and discuss the probable reconnection scenarios.

astro-ph.SR↗
arXiv2018

Convolution, Product and Correlation Theorems for Simplified Fractional Fourier Transform: A Mathematical Investigation

The notion of fractional Fourier transform (FrFT) has been used and investigated for many years by various research communities, which finds widespread applications in many diverse fields of research study. The potential applications includes ranging from quantum physics, harmonic analysis, optical information processing, pattern recognition to varied allied areas of signal processing. Many significant theorems and properties of the FrFT have been investigated and applied to many signal processing applications, most important among these are convolution, product and correlation theorems. Still many magnificent research works related to the conventional FrFT lacks the elegance and simplicity of the convolution, product and correlation theorems similar to the Euclidean Fourier transform (FT), which for convolution theorem states that the FT of the convolution of two functions is the product of their respective FTs. The purpose of this paper is to devise the equivalent elegancy of convolution, product and correlation theorems, as in the case of Euclidean FT. Building on the seminal work of Pei et al. and the potential of the simplified fractional Fourier transform (SmFrFT), a detailed mathematical investigation is established to present an elegant definition of convolution, product and correlation theorems in the SmFrFT domain, along with their associated important properties. It has been shown that the established theorems along with their associated properties very nicely generalizes to the classical Euclidean FT.

eess.SP↗
arXiv2018

Automated Classification of Hand-grip action on Objects using Machine Learning

Brain computer interface is the current area of research to provide assistance to disabled persons. To cope up with the growing needs of BCI applications, this paper presents an automated classification scheme for handgrip actions on objects by using Electroencephalography (EEG) data. The presented approach focuses on investigation of classifying correct and incorrect handgrip responses for objects by using EEG recorded patterns. The method starts with preprocessing of data, followed by extraction of relevant features from the epoch data in the form of discrete wavelet transform (DWT), and entropy measures. After computing feature vectors, artificial neural network classifiers used to classify the patterns into correct and incorrect handgrips on different objects. The proposed method was tested on real dataset, which contains EEG recordings from 14 persons. The results showed that the proposed approach is effective and may be useful to develop a variety of BCI based devices to control hand movements.

q-bio.NC↗
arXiv2018

Fast Standing Modes in Transversely Nonuniform Solar Coronal Slabs: Effects of a Finite Plasma Beta

We examine the dispersive properties of linear fast standing modes in transversely nonuniform solar coronal slabs with finite gas pressure, or, equivalently, finite plasma beta. We derive a generic dispersion relation governing fast waves in coronal slabs for which the continuous transverse distributions of the physical parameters comprise a uniform core, a uniform external medium, and a transition layer (TL) in between. The profiles in the TL are allowed to be essentially arbitrary. Restricting ourselves to the first several branches of fast modes, which are of most interest from the observational standpoint, we find that a finite plasma beta plays an at most marginal role in influencing the periods ($P$), damping times ($τ$), and critical longitudinal wavenumbers ($k_{\rm c}$), when both $P$ and $τ$ are measured in units of the transverse fast time. However, these parameters are in general significantly affected by how the TL profiles are described. We conclude that, for typical coronal structures, the dispersive properties of the first several branches of fast standing modes can be evaluated with the much simpler theory for cold slabs provided that the transverse profiles are properly addressed and the transverse Alfvén time in cold MHD is replaced with the transverse fast time.

astro-ph.SR↗
arXiv2017

Simulated evolution of three-dimensional magnetic nulls leading to generation of cylindrically-shaped current sheets

The performed magnetohydrodynamic simulation examines the importance of magnetofluid evolution which naturally leads to current sheets in the presence of three-dimensional (3D) magnetic nulls. Initial magnetic field is constructed by superposing a 3D force-free field on a constant axial magnetic field. The initial field supports 3D magnetic nulls having classical spine axis and dome-shaped fan surface; and exerts non-zero Lorentz force on the magnetofluid. Importantly, the simulation identifies the development of current sheets near the 3D magnetic nulls. The morphology of the current sheets is similar to a cylindrical surface where the surface encloses the spine axis. The development is because of favorable deformation of magnetic field lines constituting the dome-shaped fan surface. The deformation of field lines is found to be caused from the flow generated by magnetic reconections at current sheets which are located away from the cylindrically-shaped current sheets.

astro-ph.SR↗
arXiv2017

Magnetohydrodynamic modeling of solar coronal dynamics with an initial non-force-free magnetic field

The magnetic fields in the solar corona are generally neither force-free nor axisymmetric and have complex dynamics that are difficult to characterize. Here we simulate the topological evolution of solar coronal magnetic field lines (MFLs) using a magnetohydrodynamic model. The simulation is initialized with a non-axisymmetric non-force-free magnetic field that best correlates with the observed vector magnetograms of solar active regions (ARs). To focus on these ideas, simulations are performed for the flaring AR 11283 noted for its complexity and well-documented dynamics. The simulated dynamics develops as the initial Lorentz force pushes the plasma and facilitates successive magnetic reconnections at the two X-type null lines present in the initial field. Importantly, the simulation allows for the spontaneous development of mass flow, unique among contemporary works, that preferentially reconnects field lines at one of the X-type null lines. Consequently, a flux rope consisting of low-lying twisted MFLs, which approximately traces the major polarity inversion line, undergoes an asymmetric monotonic rise. The rise is attributed to a reduction in the magnetic tension force at the region overlying the rope, resulting from the reconnection. A monotonic rise of the rope is in conformity with the standard scenario of flares. Importantly, the simulated dynamics leads to bifurcations of the flux rope, which, being akin to the observed filament bifurcation in AR 11283, establishes the appropriateness of the initial field in describing ARs.

astro-ph.SR↗
arXiv2017

Statistical mechanics of a polymer chain attached to the interface of a cone-shaped channel

A polymer chain confined in nano-scale geometry has been used to investigate the underlying mechanism of Nuclear Pore Complex (NPC), where transport of cargoes is directional. It was shown here that depending on the solvent quality (good or poor) across the channel, a polymer chain can be either inside or outside the channel or both. Exact results based on the short chain revealed that a slight variation in the solvent quality can drag polymer chain inside the pore and {\it vice versa} similar to one seen in NPC. Furthermore, we also report the absence of crystalline (highly dense) state when the pore-size is less than the certain value, which may have potential application in packaging of DNA inside the preformed viral proheads.

cond-mat.soft↗
arXiv2016

Shock-driven transition to turbulence: emergence of power-law scaling

We consider two cases of interaction between a planar shock and a cylindrical density interface. In the first case (planar normal shock), the axis of the gas cylinder is parallel to the shock front, and baroclinic vorticity deposited by the shock is predominantly two-dimensional (directed along the axis of the cylinder). In the second case, the cylinder is tilted, resulting in an oblique shock interaction, and a fully three-dimensional shock-induced vorticity field. The statistical properties of the flow for both cases are analyzed based on images from two orthogonal visualization planes, using structure functions of the intensity maps of fluorescent tracer pre-mixed with the heavy gas. At later times, these structure functions exhibit power-law-like behavior over a considerable range of scales. Manifestation of this behavior is remarkably consistent in terms of dimensionless time defined based on Richtmyer's linear theory within the range of Mach numbers from 1.1 to 2.0 and the range of gas cylinder tilt angles with respect to the plane of the shock front (0 to 30 degrees).

physics.flu-dyn↗
arXiv2016

Quaternionic Fock space in an Euclidean unit ball $\mathbb{B}^n$

In this paper, we study the quaternionic counterpart of complex Fock spaces $\mathfrak{F}_α^p ( 0<p<\infty$ and for some parameter $α$) of entire slice hyperholomorphic functions in an Euclidean unit ball $\mathbb{B}^n$ in $\mathbb{H}^n.$ We also study some basic properties of these spaces.

math.FA↗
arXiv2016

Quaternionic Fock space on slice hyperholomorphic functions

In this paper, we define the quaternionic Fock spaces $\mathfrak{F}_α^p$ of entire slice hyperholomorphic functions in a quaternionic unit ball $\mathbb{B}$ in $\mathbb{H}.$ We also study growth estimate and various results of entire slice regular functions in these spaces. The work of this paper is motivated by the recent work of \cite{alpa12} and \cite{marco}.

math.FA↗
arXiv2016

On the role of repetitive magnetic reconnections in evolution of magnetic flux-ropes in solar corona

Parker's magnetostatic theorem extended to astrophysical magnetofluids with large magnetic Reynolds number supports ceaseless regeneration of current sheets and hence, spontaneous magnetic reconnections recurring in time. Consequently, a scenario is possible where the repeated reconnections provide an autonomous mechanism governing emergence of coherent structures in astrophysical magnetofluids. In this work, such a scenario is explored by performing numerical computations commensurate with the magnetostatic theorem. In particular, the computations explore the evolution of a flux-rope governed by repeated reconnections in a magnetic geometry resembling bipolar loops of solar corona. The revealed morphology of the evolution process, including onset and ascent of the rope, reconnection locations and the associated topology of the magnetic field lines, agrees with observations, and thus substantiates physical realisability of the advocated mechanism.

astro-ph.SR↗
arXiv2016

Tangle Functors from Semicyclic Representations

Let $q$ be a $2N$th root of unity where $N$ is odd. Let $U_q(sl_2)$ denote the quantum group with large center corresponding to the lie algebra $sl_2$ with generators $E,F,K$, and $K^{-1}$. A semicyclic representation of $U_q(sl_2)$ is an $N$-dimensional irreducible representation $ρ:U_q(sl_2)\rightarrow M_N(\mathbb{C})$, so that $ρ(E^N)=aId$ with $a\neq 0$, $ρ(F^N)=0$ and $ρ(K^N)=Id$. We construct a tangle functor for framed homogeneous tangles colored with semicyclic representations, and prove that for $(1,1)$-tangles coming from knots, the invariant defined by the tangle functor coincides with Kashaev's invariant.

math.GT↗
arXiv2016

Effect of a Radiation Cooling and Heating Function on Standing Longitudinal Oscillations in Coronal Loops

Standing long-period (with the periods longer than several minutes) oscillations in large hot (with the temperature higher than 3 MK) coronal loops have been observed as the quasi-periodic modulation of the EUV and microwave intensity emission and the Doppler shift of coronal emission lines, and have been interpreted as standing slow magnetoacoustic (longitudinal) oscillations. Quasi-periodic pulsations of shorter periods, detected in thermal and non-thermal emissions in solar flares could be produced by a similar mechanism. We present theoretical modelling of the standing slow magnetoacoustic mode, showing that this mode of oscillation is highly sensitive to peculiarities of the radiative cooling and heating function. We generalised the theoretical model of standing slow magnetoacoustic oscillations in a hot plasma, including the effects of the radiative losses, and accounting for plasma heating. The heating mechanism is not specified and taken empirically to compensate the cooling by radiation and thermal-conduction. It is shown that the evolution of the oscillations is described by a generalised Burgers equation. Numerical solution of an initial value problem for the evolutionary equation demonstrates that different dependences of the radiative cooling and plasma heating on the temperature lead to different regimes of the oscillations, including growing, quasi-stationary and rapidly decaying. Our findings provide a theoretical foundation for probing the coronal heating function, and may explain the observations of decayless long-period quasi-periodic pulsations in flares. The hydrodynamic approach employed in this study should be considered with caution in the modelling of non-thermal emission associated with flares, as it misses potentially important non-hydrodynamic effects.

astro-ph.SR↗