Cosmic ray variability and geomagnetic activity: A statistical study

We availed hourly data of four middle and high cut-off rigidity neutron monitors recorded over 21 years and then discussed various measures of the CR variability and simplified estimates of the CR anisotropy in relation to the Dst time history. The correlation of Dst with the prehistory of CR variability at these cut-offs is shown. A different distribution of the CR indices for 24 h before the sharp Dst decreases in comparison with that for geomagnetically quiet periods is found. The relation of CR variability with the solar wind speed appeared but neither with the solar wind density nor with the Bz component of the interplanetary magnetic field. The multiple regression of Dst with the prehistory of CR variability resulted to be significant, suggesting that the parameters describing CR variability/anisotropy are potentially useful to be implemented into the schemes of forecasting of geomagnetic activity, performed with interplanetary medium data obtained before their arrival at the Earth (e.g. at the L1 point).