68 research outputs found
The Sun's Preferred Longitudes and the Coupling of Magnetic Dynamo Modes
Observations show that solar activity is distributed non-axisymmetrically,
concentrating at "preferred longitudes". This indicates the important role of
non-axisymmetric magnetic fields in the origin of solar activity. We
investigate the generation of the non-axisymmetric fields and their coupling
with axisymmetric solar magnetic field. Our kinematic generation (dynamo) model
operating in a sphere includes solar differential rotation, which approximates
the differential rotation obtained by inversion of helioseismic data, modelled
distributions of the turbulent resistivity, non-axisymmetric mean helicity, and
meridional circulation in the convection zone. We find that (1) the
non-axisymmetric modes are localised near the base of the convection zone,
where the formation of active regions starts, and at latitudes around
; (2) the coupling of non-axisymmetric and axisymmetric modes
causes the non-axisymmetric mode to follow the solar cycle; the phase relations
between the modes are found. (3) The rate of rotation of the first
non-axisymmetric mode is close to that determined in the interplanetary space.Comment: 22 pages, 18 figures. Accepted for publication in the Astrophysical
Journa
Response to: "Critical Analysis of a Hypothesis of the Planetary Tidal Influence on Solar Activityâ by S. Poluianov and I. Usoskin
Coupled spin models for magnetic variation of planets and stars
Geomagnetism is characterized by intermittent polarity reversals and rapid
fluctuations. We have recently proposed a coupled macro-spin model to describe
these dynamics based on the idea that the whole dynamo mechanism is described
by the coherent interactions of many small dynamo elements. In this paper, we
further develop this idea and construct a minimal model for magnetic
variations. This simple model naturally yields many of the observed features of
geomagnetism: its time evolution, the power spectrum, the frequency
distribution of stable polarity periods, etc. This model has coexistent two
phases; i.e. the cluster phase which determines the global dipole magnetic
moment and the expanded phase which gives random perpetual perturbations that
yield intermittent polarity flip of the dipole moment. This model can also
describe the synchronization of the spin oscillation. This corresponds to the
case of sun and the model well describes the quasi-regular cycles of the solar
magnetism. Furthermore, by analyzing the relevant terms of MHD equation based
on our model, we have obtained a scaling relation for the magnetism for
planets, satellites, sun, and stars. Comparing it with various observations, we
can estimate the scale of the macro-spins.Comment: 16 pages, 9 figure
Analytical determination of coronal parameters using the period ratio P<sub>1</sub>/2P<sub>2</sub>
<p>Context. In transverse coronal loop oscillations, two periodicities have been measured simultaneously and are interpreted as the fundamental
kink mode (with period P1) and the first harmonic (with period P2). Deviations of the period ratio P1/2P2 from unity provide
information about the extent of longitudinal structuring within the loop.</p>
<p>Aims. Here we develop an analytical approximation that describes the shift in P1/2P2 in terms of the ratio L/Îc of the length 2L of a
coronal loop and the density scale height Îc.</p>
<p>Methods. We study the MHD wave equations in a low ÎČ plasma using the thin tube approximation. Disturbances are described by a
differential equation which may be solved for various equilibrium density profiles, obtaining dispersion relations in terms of Bessel
functions. These dispersion relations may be used to obtain analytical approximations to the periods P1 and P2. We also present a
variational approach to determining the period ratio and show how the WKB method may be used.</p>
<p>Results. Analytical approximations to the period ratio P1/2P2 are used to shed light on the magnitude of longitudinal structuring in
a loop, leading to a determination of the density scale height. We apply our formula to the observations in Verwichte et al. (2004) and
Van Doorsselaere et al. (2007), obtaining the coronal density scale height.</p>
<p>Conclusions. Our simple formula and approximate approaches highlight a useful analytical tool for coronal seismology. We demonstrate
that P1/2P2 is linked to the density scale height, with no need for estimates of other external parameters. Given the accuracy of
current observations, our formula provides a convenient means of determining density scale heights.</p>
Alpha effect due to buoyancy instability of a magnetic layer
A strong toroidal field can exist in form of a magnetic layer in the
overshoot region below the solar convection zone. This motivates a more
detailed study of the magnetic buoyancy instability with rotation. We calculate
the alpha effect due to helical motions caused by a disintegrating magnetic
layer in a rotating density-stratified system with angular velocity Omega
making an angle theta with the vertical. We also study the dependence of the
alpha effect on theta and the strength of the initial magnetic field. We carry
out three-dimensional hydromagnetic simulations in Cartesian geometry. A
turbulent EMF due to the correlations of the small scale velocity and magnetic
field is generated. We use the test-field method to calculate the transport
coefficients of the inhomogeneous turbulence produced by the layer. We show
that the growth rate of the instability and the twist of the magnetic field
vary monotonically with the ratio of thermal conductivity to magnetic
diffusivity. The resulting alpha effect is inhomogeneous and increases with the
strength of the initial magnetic field. It is thus an example of an
"anti-quenched" alpha effect. The alpha effect is nonlocal, requiring around
8--16 Fourier modes to reconstruct the actual EMF based on the actual mean
field.Comment: 14 pages, 19 figures 3 tables (submitted to A & A
Instability-driven interfacial dynamo in protoneutron stars
The existence of a tachocline in the Sun has been proven by helioseismology.
It is unknown whether a similar shear layer, widely regarded as the seat of
magnetic dynamo action, also exists in a protoneutron star. Sudden jumps in
magnetic diffusivity and turbulent vorticity , for example at
the interface between the neutron-finger and convective zones, are known to be
capable of enhancing mean-field dynamo effects in a protoneutron star. Here we
apply the well-known, plane-parallel, MacGregor-Charbonneau analysis of the
Solar interfacial dynamo to the protoneutron star problem and calculate the
growth rate analytically under a range of conditions. It is shown that, like
the Solar dynamo, it is impossible to achieve self-sustained growth if the
discontinuities in , , and shear are coincident and the magnetic
diffusivity is isotropic. In contrast, when the jumps in and
are situated away from the shear layer, self-sustained growth is possible for
ms (if the velocity shear is located at ) or ms (if the velocity shear is located at ). This translates into
stronger shear and/or -effect than in the Sun. Self-sustained growth is
also possible if the magnetic diffusivity if anisotropic, through the
effect, even when the , , and shear
discontinuities are coincident.Comment: 14 pages, 5 figures, 1 tabl
Magnetoconvection and dynamo coefficients: Dependence of the alpha-effect on rotation and magnetic field
We present numerical simulations of three-dimensional compressible
magnetoconvection in a rotating rectangular box that represents a section of
the solar convection zone. The box contains a convectively unstable layer,
surrounded by stably stratified layers with overshooting convection. The
magnetic Reynolds number, Rm, is chosen subcritical, thus excluding spontaneous
growth of the magnetic field through dynamo action, and the magnetic energy is
maintained by introducing a constant magnetic field into the box, once
convection has attained a statistically stationary state. Under the influence
of the Coriolis force, the advection of the magnetic field results in a
non-vanishing contribution to the mean electric field, given by uxb. From this
electric field, we calculate the alpha-effect, separately for the stably and
the unstably stratified layers, by averaging over time and over suitably
defined volumes. From the variation of alpha we derive an error estimate, and
the dependence of alpha on rotation and magnetic field strength is studied.
Evidence is found for rotational quenching of the vertical alpha-effect, and
for a monotonic increase of the horizontal alpha-effect with increasing
rotation. For Rm~30, our results for both vertical and horizontal alpha-effect
are consistent with magnetic quenching by a factor 1/[1+Rm(B_0/B_eq)^2]. The
signs of the small-scale current helicity and of the vertical component of
alpha are found to be opposite to those for isotropic turbulence.Comment: 14 pages, 11 figures; to appear in Astronomy & Astrophysics
(accepted
A possible influence on standard model of quasars and active galactic nuclei in strong magnetic field
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