Time variations of directional cosmic ray intensity at low latitudes. III. Interpretation of solar daily variation and changes of East-West asymmetry

Abstract

The daily variation of cosmic ray intensity at low latitudes can under certain conditions be associated with an anisotropy of primary radiation. During 1957-8, this anisotropy had an energy spectrum of variation of the form a ε^-0.8±0.3 and corresponded to a source situated at an angle of 112±10° to the left of the earth-sun line. The daily variation which can be associated with a local source situated along the earth-sun line has an energy spectrum of variation of the form a ε⁰. Increases in east-west asymmetry and the associated daily variation for east and west directions can be explained by the acceleration of cosmic ray particles crossing beams of solar plasma in the neighbourhood of the earth. For beams of width 5×10¹² cm with a frozen magnetic field of the order of 10^-4 G, a radial velocity of about 1.5×10⁸ cm/s is required. The process is possible only if the ejection of beams takes place in rarefied regions of interplanetary space which extend radially over active solar regions. An explanation of Forbush type decreases observed at great distances from the earth requires similar limitation on the plasma density and conductivity of regions of interplanetary space. The decrease of east-west asymmetry associated with world-wide decreases of intensity and with SC magnetic storms is consistent with a screening of the low-energy cosmic ray particles due to magnetic fields in plasma clouds.