EXPLOSION AND INTENSIFICATION OF MAGNETIC-FLUX TUBES

A magnetic flux tube anchored at the bottom of the solar convection zone and rising toward the surface as a result of an undulatory instability can be affected by a sudden catastrophic expansion and weakening of the magnetic field at its apex if the original field strength is below a few times 10(4) G. Such an ''explosion'' occurs if the flux tube evolves close enough to (adiabatic) hydrostatic equilibrium along the magnetic field lines in a superadiabatically stratified environment. This condition is satisfied if the diameter of the tube is small enough for the drag force to dominate the dynamical evolution. For example, rising flux tubes with equipartition field (10(4) G) at their basis explode in the middle of the convection zone if their magnetic flux is below similar or equal to 10(21) Mr. Apart from preventing flux tubes with equipartition held from reaching the surface, the explosion process may have other consequences for the evolution of magnetic fields in the convection zone: (a) it provides a source of weak field to be acted upon by the convective flows in the course of a turbulent dynamo process; (b) upflow of matter into the inflated top of a loop intensifies the magnetic field in the submerged part of the flux tube at the bottom of the convection zone. This might constitute a mechanism to produce the strong azimuthal fields in the overshoot region suggested by recent studies of the undular instability of magnetic flux tubes.