Interactions Between Distant Centres Around the Sun

Until comparatively recently solar activity was discussed in terms of independent centres around the Sun’s surface. These centres, which form a constantly but slowly changing pattern, were thought to be linked only beneath the photosphere by tangled magnetic fields, themselves dominated by the mechanical energy of mass motions at photospheric and lower levels. Recent discoveries now indicate that active centres interact with one another above photospheric level and especially through linkages within the corona. The linkages appear to be of at least two distinctive kinds: magneto-hydrodynamic shock waves capable of travelling vast distances through the corona at ∼10³ km/s and triggering eruptions at distant centres; and magnetic field lines which loop high and wide in the corona and convey energy in the form of fast particles. These two kinds of interaction will be considered in turn.