Solar Sources of Geoeffective CMEs: a SOHO/EIT View

Abstract

Observations of the low solar corona, in particular in the EUV, are an eectiv e means of identifying the solar sources of coronal mass ejections (CMEs). SOHO/EIT, with its continuous 24 hours per day coverage, is well suited to perform this task. Source regions and start times of frontside full and partial halo CMEs (that may be geoeectiv e) can thus be determined. The most frequent EUV signatures of CMEs are coronal dimmings. EIT waves, eruptive lamen ts and post-eruption arcades are also reliable signatures. Frontside halo CMEs with source regions close to the solar disc center have the strongest chance to hit the Earth. The inspection of the EIT data together with photospheric magnetograms may give an idea about the ejected interplanetary ux rope magnetic eld and, in particular, about the presence or absence of southward (geoeectiv e) eld. If a source region is situated close to the solar limb, the corresponding CME also may be geoeectiv e, as the CME-driven shocks have large angular extent. In this case the storm can be produced by the sheath plasma behind the shock, provided it contains strong enough southward interplanetary magnetic eld. Some implications for the operational space weather forecast are discussed. EIT and LASCO are capable to identify the solar sources of the most of geomagnetic storms. In some cases, however, the identication is uncertain, so the observations by the future STEREO mission will be needed for the investigation of similar events.