The existence in the region around the sun of a field-free cavity in the galactic magnetic field seems indicated by the low-energy cosmic rays that reach the earth from the sun. Such a cavity would be produced by the solar corpuscular emission. A mean radius of the order of 200 times the distance from the sun to the earth may be estimated for this cavity by balancing the flux of momentum against the lateral pressure exerted by a field of ${10}^{-5\mathrm{}}$ gauss. Such a cavity would trap cosmic rays of energy less than 100 Bev for periods long compared to a sunspot cycle, but does not seem to make possible a solar origin of cosmic rays. Expected fluctuations in cavity size would explain the 4% fluctuation in cosmic-ray intensity observed by Forbush. A simple model of the cavity is considered in some detail, rates of escape from and entry to the cavity, acceleration by the Fermi mechanism, and change in energy density being estimated. More complicated models involving a solar magnetic field are considered briefly.

• Received 14 July 1955

DOI:https://doi.org/10.1103/PhysRev.100.1440