scholarly journals The Sunspot Cycle and Solar Magnetic Fields. I. The Mechanism as Inferred from Observation

1985 ◽  
Vol 38 (6) ◽  
pp. 1045 ◽  
Author(s):  
Ronald G Giovanelli
Keyword(s):  
Magnetic Fields ◽  
Differential Rotation ◽  
Torsional Oscillation ◽  
Sunspot Cycle ◽  
Polar Field ◽  
Lower Latitude ◽  
Polar Regions ◽  
Sunspot Maximum ◽  
Field Reversal ◽  
Opposite Hemisphere

Observations of solar magnetic and velocity fields can be used to derive the course of events involved in the solar cycle. These differ in three important respects from those of conventional dynamo theories: (i) Polar field reversal. Following the outbreak of a new cycle, magnetic flux released by sunspots diffuses initially by Leighton's random-walk process, but this is soon dominated by the observed poleward flow of about 20 m s - 1 which carries flux to polar regions in about 12 months. Since follower spots lie about 2� higher in latitude than leaders, follower flux arrives in polar regions some two weeks ahead of leader flux, providing a net inflow of follower polarity there until sunspot maximum, reversing the polar field from the previous sunspot cycle and building it up to a maximum. After sunspot maximum, the flux arriving in polar regions is predominantly of follower polarity until or unless spots occur at latitudes so low that flux can diffuse towards and across the equator, predominantly from the lower latitude leader; the effect is doubled by a complementary migration from the opposite hemisphere. This prevents the change in polar flux over the cycle from dropping to zero, and leaves the polarity there reversed at the end of the cycle. (ii) The sunspot cycle. A slow, deeper counterflow, essential for continuity, carries flux strands down in the polar zones and then equatorwards. The concentration of strands is increased continually by differential rotation, and they are dragged continually into contact. Reconnection occurs rapidly except between tubes that are inclined at very small angles. This results in the formation of ropes of flux strands twisted very gently. At some stage they are large enough to float, forming sunspots. The mean sunspot latitude decreases continuously as the flux is carried equatorwards, dying out as the flux ropes become exhausted. The whole process repeats, once again reversing the polar and spot group magnetic fields. Hale's polarity laws follow immediately, and Sporer's law requires only minor adjustments to the predicted velocity of the deep equatorward counterflow. The estimated velocity of this flow is compatible with the observed sunspot and magnetic cycles of 11 and 22 years. (iii) The torsional oscillation. Shear by differential rotation increases the concentration of flux strands; the reaction to strongly sheared flux strands is a tendency to reduce differential rotation. This results in cyclic variations of differential rotation, the phase with respect to sunspot formation being in good agreement with the torsional oscillation observations of Howard and LaBonte (1981) at all latitudes up to 50-55�.

scholarly journals The Sunspot Cycle and Solar Magnetic Fields. II. The Interaction of Flux Tubes with the Convection Zone

1985 ◽  
Vol 38 (6) ◽  
pp. 1067 ◽  
Author(s):  
Ronald G Giovanelli
Keyword(s):  
Convection Zone ◽  
Torsional Oscillation ◽  
Sunspot Cycle ◽  
Solar Magnetic Fields ◽  
Field Reversal ◽  
Flux Tubes ◽  
Dynamo Mechanism ◽  
Floating Criteria ◽  
New Type ◽  
Major Cycle

Mechanisms of interaction between flux tubes or ropes and the convection zone are examined insofar as they are relevant to the sunspot cycle. These include floating, transport, and the penetration of gas from outside the tubes. It is found that all previous studies contain one or more major errors of physics which render their conclusions invalid. The errors include invariably the assumption that Archimedes' principle is applicable to flux ropes, that gas entry can be disregarded, and usually that floating criteria depend solely or primarily on local phenomena. Some of the results presented here are explanations of (i) the transport of flux tubes by the slow observed poleward motions and the even slower systems which carry extensions of these tubes downwards to depths of ~ 150 Mm and then equatorwards; (ii) their magnetic field strengths ( ~ 104 G at a depth 10 Mm to (6-12) x 104 G at ~ 150 Mm); and (iii) the amplitudes of the torsional oscillation. Taken in conjunction with Part I, where the mechanism of polar field reversal is described and the variation of the phase of the torsional oscillation explained, all major cycle observations are accounted for in what turns out to be a new type of dynamo mechanism.

scholarly journals Flux expulsion with dynamics

2016 ◽  
Vol 791 ◽  
pp. 568-588 ◽  
Author(s):  
Andrew D. Gilbert ◽  
Joanne Mason ◽  
Steven M. Tobias
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lorentz Force ◽  
Differential Rotation ◽  
Time Scale ◽  
Scaling Laws ◽  
Dynamical Regime ◽  
Kinematic Evolution ◽  
Flux Expulsion ◽  
The Magnetic Field

In the process of flux expulsion, a magnetic field is expelled from a region of closed streamlines on a $TR_{m}^{1/3}$ time scale, for magnetic Reynolds number $R_{m}\gg 1$ ($T$ being the turnover time of the flow). This classic result applies in the kinematic regime where the flow field is specified independently of the magnetic field. A weak magnetic ‘core’ is left at the centre of a closed region of streamlines, and this decays exponentially on the $TR_{m}^{1/2}$ time scale. The present paper extends these results to the dynamical regime, where there is competition between the process of flux expulsion and the Lorentz force, which suppresses the differential rotation. This competition is studied using a quasi-linear model in which the flow is constrained to be axisymmetric. The magnetic Prandtl number $R_{m}/R_{e}$ is taken to be small, with $R_{m}$ large, and a range of initial field strengths $b_{0}$ is considered. Two scaling laws are proposed and confirmed numerically. For initial magnetic fields below the threshold $b_{core}=O(UR_{m}^{-1/3})$, flux expulsion operates despite the Lorentz force, cutting through field lines to result in the formation of a central core of magnetic field. Here $U$ is a velocity scale of the flow and magnetic fields are measured in Alfvén units. For larger initial fields the Lorentz force is dominant and the flow creates Alfvén waves that propagate away. The second threshold is $b_{dynam}=O(UR_{m}^{-3/4})$, below which the field follows the kinematic evolution and decays rapidly. Between these two thresholds the magnetic field is strong enough to suppress differential rotation, leaving a magnetically controlled core spinning in solid body motion, which then decays slowly on a time scale of order $TR_{m}$.

scholarly journals Hydromagnetic dynamos in rotating spherical fluid shells in dependence on the Prandtl number, density stratification and electromagnetic boundary conditions

2014 ◽  
Vol 44 (4) ◽  
pp. 293-312 ◽  
Author(s):  
Tomáš Šoltis ◽  
Ján Šimkanin
Keyword(s):  
Magnetic Field ◽  
Electrical Conductivity ◽  
Magnetic Fields ◽  
Prandtl Number ◽  
Inner Core ◽  
Polar Regions ◽  
Number Density ◽  
Magnetic Diffusion ◽  
Spherical Fluid ◽  
The Magnetic Field

Abstract We present an investigation of dynamo in a simultaneous dependence on the non-uniform stratification, electrical conductivity of the inner core and the Prandtl number. Computations are performed using the MAG dynamo code. In all the investigated cases, the generated magnetic fields are dipolar. Our results show that the dynamos, especially magnetic field structures, are independent in our investigated cases on the electrical conductivity of the inner core. This is in agreement with results obtained in previous analyses. The influence of non-uniform stratification is for our parameters weak, which is understandable because most of the shell is unstably stratified, and the stably stratified region is only a thin layer near the CMB. The teleconvection is not observed in our study. However, the influence of the Prandtl number is strong. The generated magnetic fields do not become weak in the polar regions because the magnetic field inside the tangent cylinder is always regenerated due to the weak magnetic diffusion.

Diagnostics of Polar Field Reversal in Solar Cycle 23 Using a Flux Transport Dynamo Model

2004 ◽  
Vol 601 (2) ◽  
pp. 1136-1151 ◽  
Author(s):  
Mausumi Dikpati ◽  
Giuliana de Toma ◽  
Peter A. Gilman ◽  
Charles N. Arge ◽  
Oran R. White
Keyword(s):  
Solar Cycle ◽  
Polar Field ◽  
Dynamo Model ◽  
Flux Transport ◽  
Field Reversal ◽  
Solar Cycle 23

scholarly journals Could bow-shaped magnetic morphologies surround filamentary molecular clouds?

2019 ◽  
Vol 632 ◽  
pp. A68 ◽  
Author(s):  
M. Tahani ◽  
R. Plume ◽  
J. C. Brown ◽  
J. D. Soler ◽  
J. Kainulainen
Keyword(s):  
Magnetic Field ◽  
Monte Carlo ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Molecular Clouds ◽  
Monte Carlo Analysis ◽  
Line Of Sight ◽  
Field Reversal ◽  
Filamentary Structure ◽  
Western Side

Context. A new method based on Faraday rotation measurements recently found the line-of-sight component of magnetic fields in Orion-A and showed that their direction changes from the eastern side of this filamentary structure to its western side. Three possible magnetic field morphologies that can explain this reversal across the Orion-A region are toroidal, helical, and bow-shaped morphologies. Aims. In this paper, we constructed simple models to represent these three morphologies and compared them with the available observational data to find the most probable morphology(ies). Methods. We compared the observations with the models and used probability values and a Monte Carlo analysis to determine the most likely magnetic field morphology among these three morphologies. Results. We found that the bow morphology had the highest probability values, and that our Monte-Carlo analysis suggested that the bow morphology was more likely. Conclusions. We suggest that the bow morphology is the most likely and the most natural of the three morphologies that could explain a magnetic field reversal across the Orion-A filamentary structure (i.e., bow, helical and toroidal morphologies).

scholarly journals A Quantitative Study of Magnetic Flux Transport on the Sun

1983 ◽  
Vol 102 ◽  
pp. 273-278 ◽  
Author(s):  
N.R. Sheeley ◽  
J.P. Boris ◽  
T.R. Young ◽  
C.R. DeVore ◽  
K.L. Harvey
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Quantitative Study ◽  
Differential Rotation ◽  
Meridional Circulation ◽  
Diffusion Constant ◽  
The Sun ◽  
Flux Transport ◽  
Field Reversal ◽  
Best Fit

A computational model, based on diffusion, differential rotation, and meridional circulation, has been developed to simulate the transport of magnetic flux on the Sun. Using Kitt Peak magnetograms as input, we have determined a best-fit diffusion constant by comparing the computed and observed fields at later times. Our value of 730 ± 250 km2/s is consistent with Leighton's (1964) estimate of 770–1540 km2/s and is significantly larger than Mosher's (1977) estimate of 200–400 km2/s. This suggests that diffusion may be fast enough to account for the observed polar magnetic field reversal without requiring a significant assist from meridional currents.

scholarly journals Some patterns in the development of centers of solar activity, 1962–66

1968 ◽  
Vol 35 ◽  
pp. 56-63 ◽  
Author(s):  
Helen W. Dodson ◽  
E. Ruth Hedeman
Keyword(s):  
Solar Activity ◽  
Radio Emission ◽  
Differential Rotation ◽  
Graphical Representation ◽  
Past History ◽  
The Sun ◽  
Active Centers ◽  
Opposite Hemisphere ◽  
The Past ◽  
History Of

A graphical representation of the 66 solar rotations (Carrington) between January 1, 1962 and December 31, 1966 has been prepared. It includes all centers of activity for which the calcium plage attained an area of at least 1000 millionths of the solar hemisphere and/or intensity 3 (McMath scale). In this study the antecedents, descendents, and neighbors of each region can easily be discerned. The work shows clearly that zones of activity, apparently closely related and much larger than single plages existed for long intervals of time. For example, the significant increases in solar activity in February, May, and October of 1965 occurred in a ‘family’ of calcium plages apparently related through similarities of position and strong radio emission.The members of ‘families’ of centers of activity are found at systematically changing longitudes. For some ‘families’ the change of longitude appears to be primarily a consequence of differential rotation; for others, the pattern of formation of active centers dominates.According to the data for 1962–66 a meaningful study of the development of a center of activity may require consideration not only of the past history of the zone of the Sun in which it occurs but also of the zone approximately 180° away on the opposite hemisphere.

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