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Solar magnetic fields and convection

IV: Magnetic flux ropes and their fibres

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Abstract

The flux-rope model of solar magnetic fields is developed further by the use of a variety of observational results.

  1. (i)

    It is confirmed that magnetic fields emerging to form active regions are already in the form of helically twisted flux ropes.

  2. (ii)

    A flux rope is not a homogeneous structure but is made up of hundreds or thousands of flux fibres. These are individually twisted and isolated from one another by non-magnetic plasma. They have fields of ≈2000 G at the surface, ≈4000 G at depths of ≳700 km where they are fully compressed and also more tightly packed together.

  3. (iii)

    Convection occurs only between the fibres, and when they fill roughly half of the photospheric area this is halted and a pore forms. The kinematics of formation and decay of sunspots are described in terms of the rising of a flux-rope section and its subsequent unwinding and fraying to scattered fibres.

  4. (iv)

    These fibres are attached to the parent rope and so migrate in ordered fashion with random motions superposed. All surface fields occupy on an average ≲1% of the total area, and these and the underlying fibres and ropes make up a completely isolated plasma system with its own processes of mass and energy transfer. The remaining ≳99T of the convection zone is permanently non-magnetic and subject to normal convection except as in (iii) above.

  5. (v)

    The dynamics of a sunspot are discussed and a model developed which explains the observed radial decrease of magnetic field strength, umbral flashes and the umbral boundary, the penumbral plasma structure and the Wilson depression.

  6. (vi)

    A new, large-scale (up to ≳105 km) convective motion is described, originating below the penumbra and carrying away the sunspot energy deficit. The separate Evershed effect is explained in terms of dual flow both inside and between the penumbral flux fibres.

  7. (vii)

    In an appendix are listed some further difficulties met by the tranditional theory of solar magnetic fields, in which pre-spot fields are dominated by convection.

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Authors and Affiliations

  1. National Measurement Laboratory, CSIRO, Sydney, Australia

    J. H. Piddington

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Piddington, J.H. Solar magnetic fields and convection. Astrophys Space Sci 40, 73–90 (1976). https://doi.org/10.1007/BF00651189

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  • DOI: https://doi.org/10.1007/BF00651189

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