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

Abstract

It is shown from the hydrodynamic equations that the 4 x 10/sup 6/ deg 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 102 to 105/cm/sup 3/ at the orbit of earth. It is suggested that this hydrodynamic explosion of the enhanced corona is the accelerating mechanism for the plasma clouds or enhanced solar corpuscular emission responsible for the geomagnetic storm, the cosmic-ray decrease, the low-latitude aurorae, etc., observed at earth a day or two following large solar flares on the visible hemisphere of the sue. The interplanetary magnetic field is computed 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 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/sup -5/ gauss at the orbit of earth, maymore » be increased to 5 x 10/sup -4/ 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, by as much as 40%, 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. (auth)« less