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- Singh, Nishant K., et al.
(author)
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HIGH-WAVENUMBER SOLAR f-MODE STRENGTHENING PRIOR TO ACTIVE REGION FORMATION
- 2016
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In: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 832:2
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Journal article (peer-reviewed)abstract
- We report a systematic strengthening of the local solar surface or fundamental f- mode one to two days prior to the emergence of an active region (AR) in the same (corotating) location. Except for a possibly related increase in the kurtosis of the magnetic field, no indication can be seen in the magnetograms at that time. Our study is motivated by earlier numerical findings of Singh et al., which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f- mode fans out in the diagnostic kw diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and show for six isolated ARs, 11130, 11158, 11242, 11105, 11072, and 11768, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f- mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. Furthermore, we study two ARs, 12051 and 11678, apart from a magnetically quiet patch lying next to AR. 12529, to demonstrate the challenges in extracting such a precursor signal when a newly forming AR emerges in a patch that lies in close proximity to. one or several already existing ARs, which are expected to pollute neighboring patches. We then discuss plausible procedures for extracting precursor signals from regions with crowded environments. The idea that the f- mode is perturbed days before any visible magnetic activity occurs at the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.
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| 2. |
- Singh, Nishant K.
(author)
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Moffatt-drift-driven large-scale dynamo due to a fluctuations with non-zero correlation times
- 2016
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In: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 798, s. 696-716
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Journal article (peer-reviewed)abstract
- We present a theory of large-scale dynamo action in a turbulent flow that has stochastic, zero-mean fluctuations of the a parameter. Particularly interesting is the possibility of the growth of the mean magnetic field due to Moffatt drift, which is expected to he finite in a statistically anisotropic turbulence. We extend the Kraichnan Moffatt model to explore effects of finite memory of a fluctuations, in a spirit similar to that of Sridhar & Singh (Mon. Not. R. Astron. Soc., vol. 445, 2014, pp. 3770-3787). Using the first-order smoothing approximation, we derive a linear integro-differential equation governing the dynamics of the large-scale magnetic field, which is non-perturbative in the alpha-correlation time tau(alpha), We recover earlier results in the exactly solvable white-noise limit where the Moffatt drift does not contribute to the dynamo growth/decay. To study finite-memory effects, we reduce the integro-differential equation to a partial differential equation by assuming that tau(alpha). be small but non-zero and the large-scale magnetic field is slowly varying. We derive the dispersion relation and provide an explicit expression for the growth rate as a function of four independent parameters. When tau(alpha) not equal 0, we find that: (i) in the absence of the Moffatt drift, but with finite Kraichnan diffusivity, only strong a fluctuations can enable alpha mean-field dynamo (this is qualitatively similar to the white-noise case); (ii) in the general case when also the Moffatt drift is non-zero, both weak and strong a fluctuations can lead to a large-scale dynamo; and (iii) there always exists a wavenumber (k) cutoff at sonic large k beyond which the growth rate turns negative, irrespective of weak or strong a fluctuations. Thus we show that a finite Moffatt drift can always facilitate large-scale dynamo action if sufficiently strong, even in the case of weak alpha fluctuations, and the maximum growth occurs at intermediate wavenumbers.
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