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1.

Devlen, E.

et al.

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Ege, Fac Sci, Dept Astron & Space Sci, TR-35100 Izmir, Turkey.; Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.

This article presents a new model of the North Polar Coronal Hole (NPCH) with the aim of revealing the dissipative/propagative characteristics of magnetohydrodynamic (MHD) waves. We investigate the effects of isotropic viscosity and anisotropic heat conduction on the propagation characteristics of MHD waves in the NPCH. We first model the NPCH by considering differences in the radial direction as well as in the direction perpendicular to the line of sight (los) in temperature, particle number density and non-thermal velocities between plumes and interplume lanes, for the specific case of OVI ions. This model includes parallel and perpendicular (to the magnetic field) heat conduction and viscous dissipation. Next, we derive the dispersion relations for MHD waves in cases of the absence and presence of parallel heat conduction. In the case of the absence of parallel heat conduction, we find that MHD wave dissipation depends strongly on viscosity for modified acoustic and Alfven waves. The energy flux densities of acoustic waves vary between 10(4.7) and 10(7) erg cm-(2) s(-1), while the energy flux densities of Alfven waves turn out to be between 10(6) and 10(8.6) erg cm(-2) s(-1). When there is parallel heat conduction, we calculate the damping length-scales and the energy flux densities of magnetoacoustic waves. Our results suggest that modified magnetoacoustic waves may provide a significant source for the observed preferential acceleration and heating of OVI ions, thus coronal plasma heating, and an extra accelerating agent for the fast solar wind in the NPCH, depending on the values of the transport coefficients.

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Brandenburg, Axel

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Mitra, Dhrubaditya

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

A mean field dynamo from negative eddy diffusivity2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 432, no 2, 1651-1657 p.Article in journal (Refereed)

Abstract [en]

Using direct numerical simulations, we verify that Roberts-IV flow exhibits dynamo action dominated by horizontally averaged large-scale magnetic field. With the test-field method, we compute the turbulent magnetic diffusivity and find that it is negative and overcomes the molecular diffusivity, thus explaining quantitatively the large-scale dynamo for magnetic Reynolds numbers above approximate to 8. As expected for a dynamo of this type, but contrary to alpha-effect dynamos, the two horizontal field components grow independently of each other and have arbitrary amplitude ratios and phase differences. Small length-scales of the mean magnetic field are shown to be stabilized by the turbulent magnetic diffusivity becoming positive at larger wavenumbers. Oscillatory decaying or growing solutions have also been found in certain wavenumber intervals and sufficiently large values of the magnetic Reynolds number. For magnetic Reynolds numbers below approximate to 0.5, the turbulent magnetic diffusivity is confirmed to be positive, as expected for all incompressible flows. Earlier claims of a dynamo driven by a modified Taylor-Green flow through negative eddy diffusivity could not be confirmed.

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Devlen, Ebru

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Rädler, Karl-Heinz

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Brandenburg, Axel

KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.

Mean-field dynamo action from delayed transport2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, no 1, 116-126 p.Article in journal (Refereed)

Abstract [en]

We analyse the nature of dynamo action that enables growing horizontally averaged magnetic fields in two particular flows that were studied by Roberts in 1972, namely his flows II and III. They have zero kinetic helicity either pointwise (flow II), or on average (flow III). Using direct numerical simulations, we determine the onset conditions for dynamo action at moderate values of the magnetic Reynolds number. Using the test-field method, we show that the turbulent magnetic diffusivity is then positive for both flows. However, we demonstrate that for both flows large-scale dynamo action occurs through delayed transport. Mathematically speaking, the magnetic field at earlier times contributes to the electromotive force through the off-diagonal components of the a tensor such that a zero mean magnetic field becomes unstable to dynamo action. This represents a qualitatively new mean-field dynamo mechanism not previously described.