Study of Cosmic Ray Intensity in Relation to the Interplanetary Magnetic Field and Geomagnetic Storms for Solar Cycle 23

We investigate the association of the cosmic-ray intensity (CRI) with the interplanetary magnetic field (IMF, ) and geomagnetic storms (GS) for the period 1997 -aEuro parts per thousand 2006 (Solar Cycle 23). To do this, we conducted a Chree analysis by the superposed-epoch method. A transient decrease in CRI is found on the occurrence days of GS, and this decrease shows a similar pattern to that of the disturbance storm-time index (Dst). In addition, we show that the CRI decreases with the increase in IMF. The time lag between the decrease in CRI and increase in IMF is about one day or less. Furthermore, an increase in IMF is found with the decrease in Dst index. IMF and Dst index are highly anti-correlated to each other, while the sunspot number is not found to be correlated with IMF, Dst index, or CRI for the period studied. The IMF is found to be an effective parameter combination for producing GS and Forbush decrease. We also found two types of decrease in CRI for Solar Cycle 23: i) symmetric and ii) asymmetric decreases. The study of CRI decreases may be useful for studying space-weather effects.