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A theoretical model for tilts of bipolar magnetic regions

DSilva, S and Choudhuri, AR (1993) A theoretical model for tilts of bipolar magnetic regions. In: Astronomy and Astrophysics, 272 (2). pp. 621-633.

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Abstract

Joy's law (Hale et al. 1919) states that bipolar magnetic regions (BMRs) are inclined to the latitudinal line, with the p-shot (preceding spot) of the BMR closer to the equator and the tilt angle increasing with latitude. It is believed that the solar dynamo operates in the overshoot region just below the convection zone and the BMRs are produced by the flux loops rising from there due to magnetic buoyancy. These rising loops are expected to be twisted by the Coriolis force so that they eventually emerge on the solar surface with a tilt. The authors extend the numerical calculations of Choudhuri (1989) to study the tilts produced on the rising flux loops by the Coriolis force. They find that the theoretically calculated tilts match the observations only if the magnetic field of the flux loops lies in the range between 60 and 160 kG. For such flux loops, the tilt has the correct magnitude and also varies correctly with the latitude. If the magnetic fields were stronger than 160 kG, then Coriolis force is much weaker than magnetic buoyancy and is only able to produce tilts which are very small in overall magnitude, though they still vary correctly with latitude. On the other hand, if the fields were smaller than 60 kG, then the Coriolis force would have been so overpowering that the flux loops would move parallel to the rotation axis as found earlier (Choudhuri 1989). Such flux loops appear only in high latitudes and do not obey Joy's law. On changing the drag on the flux tube, these conclusions are not changed. If we change the footpoint separation of the flux loop, then we find that magnetic tension may halt the rise of the flux loop if the footpoint separation is below a critical value. However, for flux tubes which are able to reach the surface, the range from 60 to 160kG for the magnetic field still approximately holds. Thus our calculations seem to rule out either equipartition fields (about lOkG) or very strong megagauss fields.

Item Type: Journal Article
Additional Information: Copyright of this article belongs to European Southern Observatory.
Keywords: Sun;Magnetic fields-sunspots–rotation;Hydromagnetics
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Date Deposited: 27 Sep 2006
Last Modified: 19 Sep 2010 04:31
URI: http://eprints.iisc.ernet.in/id/eprint/8368

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