107 research outputs found
Magnetic helicity as a constraint on coronal dissipation
The Taylor hypothesis has provided a model for the relaxed magnetic configurations of not only laboratory plasmas, but also of astrophysical plasmas. However, energy dissipation is possible only for systems which depart from a strict Taylor state, and hence a parameter describing that departure must be introduced, when the Taylor hypothesis is used to estimate the dissipation. An application of the Taylor hypothesis to the problem of coronal heating provides an insight into this difficult problem. When particular sorts of footpoint motions put energy and helicity in the corona, the conservation of helicity puts a constraint on how much of the energy can be dissipated. However, on considering a random distribution of footpoint motions, this constraint gets washed away, and the Taylor hypothesis is probably not going to play any significant role in the actual calculation of relevant physical quantities in the coronal heating problem
A theoretical model of the variation of the meridional circulation with the solar cycle
Observations of the meridional circulation of the Sun, which plays a key role
in the operation of the solar dynamo, indicate that its speed varies with the
solar cycle, becoming faster during the solar minima and slower during the
solar maxima. To explain this variation of the meridional circulation with the
solar cycle, we construct a theoretical model by coupling the equation of the
meridional circulation (the component of the vorticity equation within
the solar convection zone) with the equations of the flux transport dynamo
model. We consider the back reaction due to the Lorentz force of the
dynamo-generated magnetic fields and study the perturbations produced in the
meridional circulation due to it. This enables us to model the variations of
the meridional circulation without developing a full theory of the meridional
circulation itself. We obtain results which reproduce the observational data of
solar cycle variations of the meridional circulation reasonably well. We get
the best results on assuming the turbulent viscosity acting on the velocity
field to be comparable to the magnetic diffusivity (i.e. on assuming the
magnetic Prandtl number to be close to unity). We have to assume an appropriate
bottom boundary condition to ensure that the Lorentz force cannot drive a flow
in the subadiabatic layers below the bottom of the tachocline. Our results are
sensitive to this bottom boundary condition. We also suggest a hypothesis how
the observed inward flow towards the active regions may be produced.Comment: 15 pages, 11 figures, accepted for publication in MNRA
Why do millisecond pulsars have weaker magnetic fields compared to ordinary pulsars?
Millisecond pulsars, with magnetic fields weaker by three to four orders
compared to those of ordinary pulsars, are presumed to be neutron stars spun up
by binary accretion. We expect the magnetic field to get screened by the
accreted material. Our simulation of this screening mechanism shows, for the
first time, that the field decreases by a purely geometric factor before freezing to an asymptotic value, where is the initial angular width of the polar cap. If
lies in the range --, then the magnetic field diminution factor
turns out to be -- in conformity with observational data.Comment: 11 pages, 3 figures, submitted to Current Scienc
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