Magnetic and Electric Phenomena in the Sun's Atmosphere associated with Sunspots

The magnetic field in the Sun's atmosphere near a sunspot, and the electric field resulting from changes in the magnetic field, are computed for an isolated sunspot growing in a non-conducting, and also in a uniformly conducting, medium. For a conductivity of 10 −8 e.m.u., which applies in the absence of a magnetic field, it is shown that the velocity of propagation of the magnetic field is very slow; the conductivity must be less than 10 −13 e.m.u. if the field is to penetrate to distances of the order of 3 × 10 9 cm. in the time of growth of the sunspot. It is shown, however, that a rapid propagation can be brought about in the atmosphere by the decrease in conductivity perpendicular to the magnetic field. The discussion is extended to include fields resulting from the growth of bipolar groups. If the magnetic field due to other sources is negligible the current density is low, as the electric field is everywhere perpendicular to the magnetic field, in which direction the conductivity is low. When an external magnetic field is taken into consideration very different results are obtained, and currents can circulate. For a general magnetic field inclined to the Sun's surface, the currents are of two types: ( a ) a current sheet in which all the current passes through the neighbourhood of a neutral point in the magnetic field, ( b ) a volume current, all of which passes through a sunspot. This latter type is possible because the conductivity across the magnetic field increases with depth inside a sunspot. Both types of current are discussed in some detail. The present theory forms the basis of a theory of flares which will be presented subsequently.