Sudden Expansion of the Corona Following a Large Solar Flare and the Attendant Magnetic Field and Cosmic-Ray Effects.

Abstract

We show from the hydrodynamic equations that the 4 X 106 K temperatures observed in the solar corona after a large solar flare can produce a hydrodynamic blast wave, moving out through interplanetary space with velocities of 1500 km/sec and densities from a few times 105/cm3 to 105/cm3 at the orbit of earth. We suggest that it is this hydrodynamic explosion of the enhanced corona that is the "accelerating mechanism" for the "plasma clouds" or "enhanced solar corpuscular emission" responsible for the geomagnetic storm, the cosmic-ray decrease, the low4atitude aurorae, etc, observed at earth a day or two following large solar flares on the visible hemisphere of the sun. We compute the interplanetary magnetic field in the blast wave, assuming an initial quiet-day solar wind of a few hundred km/sec ahead of the wave. The spiral field of the quiet-day wind is sheared by the blast wave, and its density, of the order of 2 X 10- gauss at the orbit of earth, may be increased to 5 X 10- gauss or more for a period of several hours. The effect of the outward-sweeping magnetic shear in the blast wave is to decrease the cosmic-ray intensity, occasionally perhaps by as much as 40 per cent, in the inner solar system behind the blast wave. The onset of the decrease has a characteristic time of several hours, and the relaxation many hours or days. The energy dependence of the decrease may go inversely with particle rigidity, or it may be flatter, depending on the details of the magnetic configuration in and around the blast wave. Thus the cosmic-ray decrease produced by the blast wave is identical with the Forbush-type cosmic-ray decrease observed in association with "enhanced solar corpuscular radiation." Thus, besides producing the observed 1500 km/sec ejection of matter from the sun responsible for the geomagnetic storm, the hydrodynamic expansion of the enhanced corona produces the observed Forbush decrease. We suggest, therefore, that the simple dynamical model of the enhanced corona developed in this paper is not unlike what actually occurs following a large solar flare.