Browsing by Author "Sur, Sharanya"

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Galactic dynamo action in presence of stochastic alpha and shear
(2008-10) Sur, Sharanya; Subramanian, Kandaswamy
Using a one-dimensional αω-dynamo model appropriate to galaxies, we study the possibility of dynamo action driven by a stochastic alpha effect and shear. To determine the ﬁeld evolution, one needs to examine a large number of different realizations of the stochastic component of α. The net growth or decay of the ﬁeld depends not only on the dynamo parameters but also on the particular realization, the correlation time of the stochastic α compared to turbulent diffusion timescale and the time over which the system is evolved. For dynamos where both a coherent and ﬂuctuating α are present, the stochasticity of α can help alleviate catastrophic dynamo quenching, even in the absence of helicity ﬂuxes. One can obtain ﬁnal ﬁeld strengths up to a fraction ∼ 0.01 of the equipartition ﬁeld Beq for dynamo numbers |D| ∼ 40, while ﬁelds comparable to Beq require much larger degree of α ﬂuctuations or shear. This type of dynamomay be particularly useful for amplifying ﬁelds in the central regions of disk galaxies.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Kinematic alpha effect in isotropic turbulence simulations
(2008-01) Sur, Sharanya; Brandenburg, Axel; Subramanian, Kandaswamy
Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivitywhose values are independent of the magnetic Reynolds number,Rm, provided Rm exceeds unity. These turbulent coefﬁcients are also consistent with expectations from the ﬁrst order smoothing approximation. For small values of Rm, alpha and turbulent diffusivity are proportional to Rm. Over ﬁnite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic ﬂuctuations. This suggests that small-scale dynamo-generated ﬁelds do not make a correlated contribution to the mean electromotive force.
Kinetic and magnetic alpha effects in nonlinear dynamo theory
(2007-01-19) Sur, Sharanya; Subramanian, Kandaswamy; Brandenburg, Axel
The backreaction of the Lorentz force on the α-effect is studied in the limit of small magnetic and ﬂuid Reynolds numbers, using the ﬁrst order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic ﬁelds can be expressed explicitly in terms of the forcing function. The nonlinear α-effect is then shown to be expressible in several equivalent forms in agreement with formalisms that are used in various closure schemes. On the one hand, one can express α completely in terms of the helical properties of the velocity ﬁeld as in traditional FOSA, or, alternatively, as the sum of two terms, a so-called kinetic α-effect and an oppositely signed term proportional to the helical part of the small scale magnetic ﬁeld. These results hold for both steady and time dependent forcing at arbitrary strength of the mean ﬁeld. In addition, the τ-approximation is considered in the limit of small ﬂuid and magnetic Reynolds numbers. In this limit, the τ closure term is absent and the viscous and resistive terms must be fully included. The underlying equations are then identical to those used under FOSA, but they reveal interesting differences between the steady and time dependent forcing. For steady forcing, the correlation between the forcing function and the small-scale magnetic ﬁeld turns out to contribute in a crucial manner to determine the net α-effect. However for delta-correlated time-dependent forcing, this force–ﬁeld correlation vanishes, enabling one to write α exactly as the sum of kinetic and magnetic α-effects, similar to what one obtains also in the large Reynolds number regime in theτ-approximation closure hypothesis. In the limit of strong imposed ﬁelds, B0, we ﬁnd α ∝ B−2 0 for delta-correlated forcing, in contrast to the well-known α ∝ B−3 0 behaviour for the case of a steady forcing. The analysis presented here is also shown to be in agreement with numerical simulations of steady as well as random helical ﬂows.
Role of the Yoshizawa effect in the Archontis dynamo
(2009-05-01) Sur, Sharanya; Brandenburg, Axel
The generation of mean magnetic ﬁelds is studied for a simple non-helical ﬂow where a net cross helicity of either sign can emerge. This ﬂow, which is also known as the Archontis ﬂow, is a generalization of the Arnold–Beltrami–Childress ﬂow, but with the cosine terms omitted. The presence of cross helicity leads to a mean-ﬁeld dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such ﬂows demonstrate the presence of magnetic ﬁelds on scales larger than the scale of the ﬂow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional to the mean magnetic ﬁeld and can therefore lead to its exponential instead of just linear ampliﬁcation for magnetic Reynolds numbers that exceed a certain critical value. Unlike α effect dynamos, it is found that the Yoshizawa effect is not noticeably constrained by the presence of a conservation law. It is argued that this is due to the presence of a forcing term in the momentum equation which leads to a nonzero correlation with the magnetic ﬁeld. Finally, the application to energy convergence in solar wind turbulence is discussed.