arXiv subjects

Rekha Jain

Publications and source records attributed to Rekha Jain.

25 records · Page 2Linked to original sources

arXiv2014

Opinion Mining In Hindi Language: A Survey

Opinions are very important in the life of human beings. These Opinions helped the humans to carry out the decisions. As the impact of the Web is increasing day by day, Web documents can be seen as a new source of opinion for human beings. Web contains a huge amount of information generated by the users through blogs, forum entries, and social networking websites and so on To analyze this large amount of information it is required to develop a method that automatically classifies the information available on the Web. This domain is called Sentiment Analysis and Opinion Mining. Opinion Mining or Sentiment Analysis is a natural language processing task that mine information from various text forms such as reviews, news, and blogs and classify them on the basis of their polarity as positive, negative or neutral. But, from the last few years, enormous increase has been seen in Hindi language on the Web. Research in opinion mining mostly carried out in English language but it is very important to perform the opinion mining in Hindi language also as large amount of information in Hindi is also available on the Web. This paper gives an overview of the work that has been done Hindi language.

cs.IR↗

arXiv2013

An Interpretation of Flare-Induced and Decayless Coronal-Loop Oscillations as Interference Patterns

We present an alternative model of coronal-loop oscillations which considers that the waves are trapped in a 2D waveguide formed by the entire arcade of field lines. This differs from the standard 1D model which treats the waves as the resonant oscillations of just the visible bundle of field lines. Within the framework of our 2D model, the two types of oscillations that have been observationally identified, flare-induced waves and "decayless" oscillations, can both be attributed to MHD fast waves. The two components of the signal differ only because of the duration and spatial extent of the source that creates them. The flare-induced waves are generated by strong localized sources of short duration, while the decayless background can be excited by a continuous, stochastic source. Further, the oscillatory signal arising from a localized, short-duration source can be interpreted as a pattern of interference fringes produced by waves that have traveled diverse routes of various pathlengths through the waveguide. The resulting amplitude of the fringes slowly decays in time with an inverse square root dependence. The details of the interference pattern depend on the shape of the arcade and the spatial variation of the Alfvén speed. The rapid decay of this wave component, which has previously been attributed to physical damping mechanisms that remove energy from resonant oscillations, occurs as a natural consequence of the interference process without the need for local dissipation.

astro-ph.SR↗

arXiv2013

Equilibrium models of coronal loops that involve curvature and buoyancy

We construct magnetostatic models of coronal loops in which the thermodynamics of the loop is fully consistent with the shape and geometry of the loop. This is achieved by treating the loop as a thin, compact, magnetic fibril that is a small departure from a force-free state. The density along the loop is related to the loop's curvature by requiring that the Lorentz force arising from this deviation is balanced by buoyancy. This equilibrium, coupled with hydrostatic balance and the ideal gas law, then connects the temperature of the loop with the curvature of the loop without resorting to a detailed treatment of heating and cooling. We present two example solutions: one with a spatially invariant magnetic Bond number (the dimensionless ratio of buoyancy to Lorentz forces) and the other with a constant radius of curvature of the loop's axis. We find that the density and temperature profiles are quite sensitive to curvature variations along the loop, even for loops with similar aspect ratios.

astro-ph.SR↗

arXiv2013

Reconnection at 3D Magnetic Null Points: Effect of Current Sheet Asymmetry

Asymmetric current sheets are likely to be prevalent in both astrophysical and laboratory plasmas with complex three dimensional (3D) magnetic topologies. This work presents kinematic analytical models for spine and fan reconnection at a symmetric 3D null with asymmetric current sheets. Asymmetric fan reconnection is characterized by an asymmetric reconnection of flux past each spine line and a bulk flow of plasma across the null point. In contrast, asymmetric spine reconnection is inherently equal and opposite in how flux is reconnected across the fan plane. The higher modes of spine reconnection also include localized wedges of vortical flux transport in each half of the fan. In this situation, two definitions for reconnection rate become appropriate: a local reconnection rate quantifying how much flux is genuinely reconnected across the fan plane and a global rate associated with the net flux driven across each semi-plane. Through a scaling analysis it is shown that when the ohmic dissipation in the layer is assumed to be constant, the increase in the local rate bleeds from the global rate as the sheet deformation is increased. Both models suggest that asymmetry in the current sheet dimensions will have a profound effect on the reconnection rate and manner of flux transport in reconnection involving 3D nulls.

physics.plasm-ph↗

arXiv2012

Kink Oscillations of a Curved, Gravitationally Stratified Coronal Loop

Loops of magnetic field in the corona are observed to oscillate and these oscillations have been posited to be the superposition of resonant kink waves. To date, most analyses of these oscillations have concentrated on calculating the frequency shifts that result from spatial variation in the kink wave speed. Further, most have ignored gravity and treated the loop as a straight tube. Here we ignore spatial variation in the wave speed, but self-consistently include the effects of gravity and loop curvature in both the equilibrium loop model and in the wave equation. We model a coronal loop as an isolated, thin, magnetic fibril that is anchored at two points in the photosphere. The equilibrium shape of the loop is determined by a balance between magnetic buoyancy and magnetic tension, which is characterized by a Magnetic Bond Number ε, that is typically small |ε| << 1. This balance produces a loop that has a variable radius of curvature. The resonant kink waves of such a loop come in two polarizations that are decoupled from each other: waves with motion completely within the plane of the loop (normal oscillations) and waves with motions that are completely horizontal, perpendicular to the plane of the loop (binormal oscillations). We solve for the eigensolutions of both polarizations using perturbation theory for small Magnetic Bond Number. For modes of the same order, normal oscillations have smaller eigenfrequencies than binormal oscillations. The additional forces of buoyancy and magnetic tension from the curvature of the loop increase and decrease the mode frequencies, respectively. The ratio of the frequencies of the first overtone to the fundamental mode is modified by the inclusion of buoyancy and curvature. We find that the normal polarization possesses a frequency ratio that exceeds the canonical value of 2, whereas the binormal polarization has a ratio less than 2.

astro-ph.SR↗

arXiv2011

Axisymmetric Scattering of p Modes by Thin Magnetic Tubes

We examine the scattering of acoustic p-mode waves from a thin magnetic fibril embedded in a gravitationally stratified atmosphere. The scattering is mediated through the excitation of slow sausage waves on the magnetic tube, and only the scattering of the monopole component of the wavefield is considered. Since such tube waves are not confined by the acoustic cavity and may freely propagate along the field lines removing energy from the acoustic wavefield, the excitation of fibril oscillations is a source of acoustic wave absorption as well as scattering. We compute the mode mixing that is achieved and the absorption coefficients and phase shifts. We find that for thin tubes the mode mixing is weak and the absorption coefficient is small and is a smooth function of frequency over the physically relevant band of observed frequencies. The prominent absorption resonances seen in previous studies of unstratified tubes are absent. Despite the relatively small absorption, the phase shift induced can be surprisingly large, reaching values as high as 15 degree for f modes. Further, the phase shift can be positive or negative depending on the incident mode order and the frequency.

astro-ph.SR↗

arXiv2008

The Generation of Coronal Loop Waves below the Photosphere by p-Mode Forcing

Recent observations of coronal-loop waves by TRACE and within the corona as a whole by CoMP clearly indicate that the dominant oscillation period is 5 minutes, thus implicating the solar p modes as a possible source. We investigate the generation of tube waves within the solar convection zone by the buffeting of p modes. The tube waves--in the form of longitudinal sausage waves and transverse kink waves--are generated on the many magnetic fibrils that lace the convection zone and pierce the solar photosphere. Once generated by p-mode forcing, the tube waves freely propagate up and down the tubes, since the tubes act like light fibers and form a waveguide for these magnetosonic waves. Those waves that propagate upward pass through the photosphere and enter the upper atmosphere where they can be measured as loop oscillations and other forms of propagating coronal waves. We treat the magnetic fibrils as vertically aligned, thin flux tubes and compute the energy flux of tube waves that can generated and driven into the upper atmosphere. We find that a flux in excess of 10^5 ergs/cm^2/s can be produced, easily supplying enough wave energy to explain the observations. Furthermore, we compute the associated damping rate of the driving p modes and find that the damping is significant compared to observed line widths only for the lowest order p modes.

astro-ph↗