3. SOLAR MAGNETIC FIELD


 

    All transient phenomena occurring in the solar atmosphere are connected with magnetic fields leading to a 22-year Hale cycle. Todate all observed phenomena due to subsurface solar magnetic fields are inferred from the laws of magnetohydrodynamics. In sunspots the magnetic-field lines are bundled and magnetic fields reach values of 2000 to 3000 Gauss. The mean magnetic-field intensity measurable at the solar surface is only approximately 1 Gauss. The small-scale features of magnetic activity on the solar surface are continously changing with a degree of randomness as a result of complicated turbulent and ordered convective motions in the envelope of the Sun. The large-scale sunspot cycle, however, shows a well-defined behavior as a result of convection and generation of poloidal and toroidal magnetic fields within the differentially rotating Sun. Near the base of the convection zone the magnetic field may reach an amplitude of 105 Gauss.

    The existence and generation of magnetic fields in the deep interior of the solar body is still a very controversial issue. The generally accepted view is that the convective envelope of the Sun is a converter of turbulence and differential rotation into an oscillating magnetic toroid and dipole. The magnetic field is confined to the convective envelope and is generated there by a dynamo mechanism, thereby consuming energy liberated by thermonuclear reactions in the gravitationally stabilized fusion reactor of the Sun. Energy generated in the core of the Sun is used to drive convection and differential rotation in the envelope of the Sun. Dynamo models successfully explain the periodic amplification of the solar magnetic field and the observed butterfly diagram of sunspots, respectively. Almost all these models rely on assumptions that employ stochastic mechanisms for the explanation of the 22-years solar activity cycle (Stix 1989).

    Contrary to the stochastic approach to the generation of the solar magnetic field, it is possible in principle to explain the magnetic field as the result of the collapse of the primitive solar nebula. The radiative core of the Sun may have conserved its primordial magnetic field, locked into matter. It can be supposed that the radiative core of the Sun has a high electric conductivity conserving its low-order magnetic multipoles. Because a magnetic dipole existing in a fluid conductor is unstable towards a splitting along its symmetry planes and rotation about 180o, the dominant magnetic field in the core has a quadrupole configuration. This quadrupole model for the solar magnetic field could explain many of solar magnetic activity phenomena, but has not yet been confirmed by observations (Kundt 1992).