Phase distribution of the first harmonic of the cosmic ray anisotropy during the initial phase of Forbush effects

Abstract

Phase distribution and amplitude-phase dependence of the first harmonic of the cosmic ray anisotropy in the initial phase of Forbush decreases are studied in the events during the time period 1957-2013. Statistical analysis of all Forbush decreases with a sudden onset for this period showed that the specific features of the phase distribution of the first harmonic of the cosmic ray anisotropy exist throughout the initial phase of the Forbush decreases, starting one hour before the shock, and ending at the time of maximum amplitude of anisotropy. Amplitude of the cosmic ray vector anisotropy is higher than in quiet periods already before the shock arrival, and it gradually increases as Earth enters deeper into the interplanetary disturbance, which creates the Forbush decrease.