How directions and helicity of erupted solar magnetic fields define geoeffectiveness of coronal mass ejections

Abstract

In this study we report on the relationship between the projected speed of CMEs, measured at 20R from SOHO/LASCO images, and the hourly averaged magnitude of the southwardly directed magnetic field, B _z, at the leading edge of interplanetary ejecta, as measured by the ACE magnetometer. We found that those CMEs that originate at the central part of the solar disk ( r < 0.6 R_⊙) are the most geoeffective and the intensity of B _z is an exponential function of the CME's speeds. We propose an approach to estimate the strength of the southward IMF at least one day in advance, immediately after a CME started. The predicted value of the B _z component can be then used to estimate the intensity of a geomagnetic storm caused by the eruption. The prediction method is based on the correlation between the speeds of CMEs and magnitudes of the southward IMF as well as the fact that the orientation and chirality of the erupted solar filaments correspond to the orientation and chirality of interplanetary ejecta.