Geomagnetic storm dependence on the solar flare class
  Content. Solar flares are often used as precursors of geomagnetic storms. In particular, Howard and Tappin (2005) recently published in A&A a dependence between X-ray class of solar flares and Ap and Dst indexes of geomagnetic storms which contradicts to early published results.   Aims. We compare published results on flare-storm dependences and discuss possible sources of the discrepancy.   Methods. We analyze following sources of difference: (1) different intervals of observations, (2) different statistics and (3) different methods of event identification and comparison.   Results. Our analysis shows that magnitude of geomagnetic storms is likely to be independent on X-ray class of solar flares. 
Yu. I. Yermolaev and M. Yu. Yermolaev