Coronal mass ejections (CMEs) are often associated with erupting magnetic structures or disappearing filaments. The majority of CMEs headed directly toward the Earth are observed at 1 AU as magnetic clouds—the region in the solar wind where the magnetic field strength is higher than average and there is a smooth rotation of the magnetic field vectors. The three-dimensional structure of magnetic clouds can be represented by a force-free flux rope. When CMEs reach the Earth, they may or may not cause magnetic storms, alter Earth's magnetic field, or produce the phenomena known as auroras. The geoeffectiveness of a solar CME depends on the orientation of the magnetic field in it. Two M-class solar flares erupted on 2000 February 17. The second flare occurred near a small active region, NOAA Active Region 8872. This eruption was accompanied by a halo CME. However, the February 17 CME did not trigger any magnetic activity when it arrived at the Earth. Another powerful flare, on 2000 July 14, was also associated with a halo CME, which caused the strongest geomagnetic activity of solar cycle 23. Using ACE measurements of the interplanetary magnetic fields, we study the orientation of the magnetic flux ropes in both sets of magnetic clouds and compare them with the orientation of the solar magnetic fields and disappearing filaments. We find that the direction of the axial field and helicity of the flux ropes are consistent with those of the erupted filaments. Thus, the geoeffectiveness of a CME is defined by the orientation and structure of the erupted filament and by its magnetic helicity as well. We also suggest that the geoeffectiveness of a CME can be forecasted using daily full-disk Hα and Yohkoh images and MDI magnetograms as well.
展开▼
