34 research outputs found
Predicting solar cycle 24 with a solar dynamo model
Whether the upcoming cycle 24 of solar activity will be strong or not is
being hotly debated. The solar cycle is produced by a complex dynamo mechanism.
We model the last few solar cycles by `feeding' observational data of the Sun's
polar magnetic field into our solar dynamo model. Our results fit the observed
sunspot numbers of cycles 21-23 extremely well and predict that cycle~24 will
be about 35% weaker than cycle~23.Comment: 10 pages 1 table 3 figure
The origin of grand minima in the sunspot cycle
One of the most striking aspects of the 11-year sunspot cycle is that there
have been times in the past when some cycles went missing, a most well-known
example of this being the Maunder minimum during 1645-1715. Analyses of
cosmogenic isotopes (C14 and Be10) indicated that there were about 27 grand
minima in the last 11,000 yr, implying that about 2.7% of the solar cycles had
conditions appropriate for forcing the Sun into grand minima. We address the
question how grand minima are produced and specifically calculate the frequency
of occurrence of grand minima from a theoretical dynamo model. We assume that
fluctuations in the poloidal field generation mechanism and the meridional
circulation produce irregularities of sunspot cycles. Taking these fluctuations
to be Gaussian and estimating the values of important parameters from the data
of last 28 solar cycles, we show from our flux transport dynamo model that
about 1-4% of the sunspot cycles may have conditions suitable for inducing
grand minima.Comment: Accepted for publication in Physical Review Letter
Towards A Mean-Field Formulation Of The Babcock-Leighton Type Solar Dynamo. I. Alpha Coefficient Versus Durney's Double Ring Approach
We develop a model of the solar dynamo in which, on the one hand, we follow
the Babcock-Leighton approach to include surface processes like the production
of poloidal field from the decay of active regions, and, on the other hand, we
attempt to develop a mean field theory that can be studied in quantitative
detail. One of the main challenges in developing such models is to treat the
buoyant rise of toroidal field and the production of poloidal field from it
near the surface. We build up a dynamo model with two contrasting methods of
treating buoyancy. In one method, we incorporate the generation of the poloidal
field near the solar surface by Durney's procedure of double ring eruption. In
the second method, the poloidal field generation is treated by a positive
alpha-effect concentrated near the solar surface, coupled with an algorithm for
handling buoyancy. The two methods are found to give qualitatively similar
results.Comment: 32 pages, 27 figures, uses aastex.cls and epsfig.st
Quenching of Meridional Circulation in Flux Transport Dynamo Models
Guided by the recent observational result that the meridional circulation of
the Sun becomes weaker at the time of the sunspot maximum, we have included a
parametric quenching of the meridional circulation in solar dynamo models such
that the meridional circulation becomes weaker when the magnetic field at the
base of the convection zone is stronger. We find that a flux transport solar
dynamo tends to become unstable on including this quenching of meridional
circulation if the diffusivity in the convection zone is less than about 2 *
10^{11} cm^2/s. The quenching of alpha, however, has a stabilizing effect and
it is possible to stabilize a dynamo with low diffusivity with sufficiently
strong alpha-quenching. For dynamo models with high diffusivity, the quenching
of meridional circulation does not produce a large effect and the dynamo
remains stable. We present a solar-like solution from a dynamo model with
diffusivity 2.8 * 10^{12} cm^2/s in which the quenching of meridional
circulation makes the meridional circulation vary periodically with solar cycle
as observed and does not have any other significant effect on the dynamo.Comment: Consistent with the published version. Solar Physics, in pres
Faraday effect : a field theoretical point of view
We analyze the structure of the vacuum polarization tensor in the presence of
a background electromagnetic field in a medium. We use various discrete
symmetries and crossing symmetry to constrain the form factors obtained for the
most general case. From these symmetry arguments, we show why the vacuum
polarization tensor has to be even in the background field when there is no
background medium. Taking then the background field to be purely magnetic, we
evaluate the vacuum polarization to linear order in it. The result shows the
phenomenon of Faraday rotation, i.e., the rotation of the plane of polarization
of a plane polarized light passing through this background. We find that the
usual expression for Faraday rotation, which is derived for a non-degenerate
plasma in the non-relativistic approximation, undergoes substantial
modification if the background is degenerate and/or relativistic. We give
explicit expressions for Faraday rotation in completely degenerate and
ultra-relativistic media.Comment: 20 pages, Latex, uses axodraw.st
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
A model of the polarization position-angle swings in BL Lacertae objects
The polarization position-angle swings that have been measured in a number of BL Lacertae objects and highly variable quasars are interpreted in terms of shock waves which illuminate (by enhanced synchrotron radiation) successive transverse cross sections of a magnetized, relativistic jet. The jet is assumed to have a nonaxisymmetric magnetic field configuration of the type discussed in the companion paper on the equilibria of force-free jets. For a jet that is viewed at a small angle to the axis, the passage of a shock will give rise to an apparent rotation of the polarization position angle whose amplitude can be substantially larger than 180 deg. The effects of freely propagating shocks are compared with those of bow shocks which form in front of dense obstacles in the jet, and specific applications to 0727 - 115 and BL Lacertae are considered. In the case of 0727 - 115, it is pointed out that the nonuniformity of the swing rate and the apparent oscillations of the degree of polarization could be a consequence of relativistic aberration
The influence of the Coriolis force on flux tubes rising through the solar convection zone
In order to study the effect of the Coriolis force due to solar rotation on rising magnetic flux, the authors consider a flux ring, azimuthally symmetric around the rotation axis, starting from rest at the bottom of the convection zone, and then follow the trajectory of the flux ring as it rises. If it is assumed that the flux ring remains azimuthally symmetric during its ascent, then the problem can be described essentially in terms of two parameters: the value of the initial magnetic field in the ring when it starts, and the effective drag experienced by it. For field strengths at the bottom of the convection zone of order 10,000 G or less, it is found that the Coriolis force plays a dominant role and flux rings starting from low latitudes at the bottom are deflected and emerge at latitudes significantly poleward of sunspot zones