THE ONSET TIMES OF FORBUSH-TYPE COSMIC RAY INTENSITY DECREASES

Abstract

The onset times of a number of Forbush-type decreases observed at four widely spaced stations are compared, and it is shown that appreciable differences occur. The stations selected were Hobart, Mawson, Ottawa, and Sulphur Mountain. It was found that a consistent pattern is obtained for the events studied when the onset times are plotted as a function of the direction of maximum sensitivity of the recorders relative to the earth-sun line. This is interpreted as being due to a directional anisotropy that exists in the mechanism producing the decreases, at least in the early stages. The depression occurs first for particles arriving from directions between 30° and 120° west of the earth–sun line. The relation between these observations and geomagnetic disturbances and the quiet-day daily variation is discussed.