A critical reassessment of the empirical relationship between Forbush decreases and cosmic ray diurnal anisotropies

ABSTRACT Although there are some plausible theories on the temporal relationship between Forbush decreases (FDs) and diurnal cosmic ray (CR) anisotropy, the manual/semi-automated FD identification approaches which dominate the literature on the subject have provided little convincing empirical support. Despite considerable progress in CR data acquisition and storage, the identification of FDs and quantification of CR anisotropy is still largely performed using manual/visual techniques. The implications of this are many and varied. Leading the list of issues generated by the subjective methods are the difficulties of uniting/validating two FD catalogues prepared by external or even the same investigators. Since FDs and CR anisotropies are the most elusive parts of the naturally superposed signals in CR intensity flux, the results of analyses attempting to relate the two phenomena using a few isolated events may be viewed with skepticism. Comparative analyses of these events started several decades ago. But a critical look at the existing rich body of literature on the empirical relationship between the phenomena has yet to be published. In consideration of the numerous contradictory and disputable publications on the topic, coupled with some recent innovative and sophisticated computer analysis software employed in the analyses of FDs and CR anisotropies, a critical appraisal of the results of the past and recent methodologies are desirable and timely.