Properties of magnetic clouds and geomagnetic storms associated with eruption of coronal sigmoids

We study 46 solar coronal eruptions associated with sigmoids seen in images from the Yohkoh Soft X‐ray Telescope (SXT). We relate the properties of the sigmoids to in situ measurements at 1 AU and geomagnetic storms. Our primary result is that erupting sigmoids tend to produce geoeffective magnetic clouds (MCs): 85% of the erupting sigmoidal structures studied spawned at least a “moderate” (| Dst | ≥ 50 nT) geomagnetic storm. A collateral result is that MCs associated with sigmoids do not show the same solar‐terrestrial correlations as those associated with filaments and, as such, form a distinct class of events. First, rather than reversing with the global solar dipole (at solar maximum), the leading field in MCs weakly (2:1) shows a solar cycle (Hale polarity) based correlation (reversing at solar minimum). Second, whereas the handedness of MCs associated with filament eruptions is strongly (95%) related to their launch hemisphere, that of MCs associated with sigmoid eruptions is only weakly (∼70%) so related. Finally, we are unaware of any model of the magnetic fields of sigmoids and their eruption that gives a useful prediction of the leading field orientation of their associated MC.