The role of interplanetary magnetic field and solar wind in modulating both galactic cosmic rays and geomagnetic activity

We show that the enhancements in both the upper limiting rigidity of the cosmic ray solar diurnal variation and the geomagnetic activity during the decline of solar activity are related to high values of the product of the solar wind speed and the interplanetary magnetic field strength (VB). The upper limiting rigidity correlates very well with the magnitude of VB. This reflects the effect of both interplanetary magnetic field (IMF) and solar wind speed in modulating cosmic rays. The upper limiting rigidity is correlated with the geomagnetic activity as well. However, it is the value of the product VB that directly modulates the geomagnetic activity. The product VB is more important for both cosmic ray and geomagnetic activity modulations rather than the IMF alone.