Effect of solar wind disturbances on small-amplitude simultaneous Forbush events during solar cycle 23

Abstract

The transient and rapid reduction in galactic cosmic ray (GCR) flux intensity, commonly referred to as Forbush decrease (FD), is widely used as a mediator variable in searching for solar-terrestrial weather linkages. Whereas literature in the field is replete with high-amplitude (FD(%) ⩾ 3 ) FD-based analyses, the empirical connections between low-amplitude (FD(%) ⩽ 3 ) events and Sun-Earth variables are seldom tested. It is recently observed that the dearth of such studies might be due to the daunting task of accurate detection/precise timing of FDs and other transient astrophysical events. Using a sensitive version of the FD detection algorithm, we selected 264, 267, and 206 low amplitude (FD(%) ⩽ 3 ) FDs from the daily GCR data of Hermanus (HRMS), Newark (NWRK) and Oulu (OULU) neutron monitor stations respectively during solar cycle 23. A total of 80 simultaneous small-amplitude FDs were identified from the three stations with a simple coincident computer code. Statistically significant correlations at 95% confidence level exist between the planetary K-index (Kp), disturbance storm time index (Dst), planetary A-index (Ap), and the corresponding simultaneous small-amplitude FDs at OULU station, but not found in HRMS and NWRK data. These significant links between the weak FDs and the geomagnetic indices (Kp, Dst, and Ap) at OULU station suggest that the parameters (Kp, Dst, and Ap) are likely connected with some common solar activities.