The Implication of Enhanced Cosmic Ray Diurnal Anisotropy on the Global Simultaneity of Forbush Decreases: A Statistical Approach

Abstract

The short-term rapid CR flux depressions, generally referred to as Forbush decreases (FDs), are the most spectacular time-intensity CR variation. The need for analytical transformation of the observational CR time series data, to account for FDs and other recurrence tendencies such as periodicities and cycles, was noted since the 1930s'. Nevertheless, it has been recently observed that harmonic analysis, which is capable of transforming raw CR data into different frequencies, is rarely exploited. Predominant in the literature are the ordinary Fourier and power spectral analyses, generally used to calculate the positive vectors (amplitude and phase) of the periodic diurnal CR anisotropy. In the two approaches, the days immediately connected with FDs are frequently removed to minimize unusual changes in the amplitude of the vectors as well as a spurious time of maximum. However, there is a paucity of publications that adjust for the influence of enhanced CR diurnal anisotropy on the magnitude and timing of FDs. Recently, in an attempt to test the global FD event simultaneity, a combination of numerical filtering and fast Fourier transform techniques was deployed to account for these superposition tendencies in daily CR data, including the intractable CR diurnal anisotropy. However, an extremely sensitive version of the software would be required to analyze high-resolution CR hourly averages. As a way of achieving accurate detection and precise timing of FD signals, the computer algorithms were technically improved and employed in a long-term statistical investigation. To appreciate the implemented extremely sensitive statistical technique, several validation analyses, including FD-based solar-terrestrial correlation, comparison of FD catalogues, and Frobush event simultaneity test were conducted.