One picture of coronal mass ejection (CME) initiation relates these events to the expansion into space of previously closed coronal magnetic fields, often part of the helmet streamer belt. The work described here makes use of the potential field source surface model based on updated synoptic photospheric field maps to study the large-scale coronal field changes. We isolate those field lines that change from closed to open configurations (newly opening field lines) by comparing potential field source surface models from adjacent magnetograph observations, wherein the same starting foot points on the photosphere are used. If there are some newly opening field lines between the times of two maps, we assume there was a possibility for CME occurrence(s) between these times. In particular, if there are newly opening field lines near the solar disk center, an earthward directed CME may have been generated. Monitoring the coronal magnetic field behavior can in principle reinforce (or not) days in advance predictions of magnetic storms based on Solar and Heliospheric Observatory (SOHO) Large-Angle Spectrometric Coronagraph (LASCO) halo CMEs. Moreover, the coronal field over the visible hemisphere contains information about the possible geoeffectiveness of a particular CME because it shows the approximate orientation and location of the active arcades. By comparing halo CMEs with the newly opening field lines, the solar wind measurements from Wind and ACE spacecraft and the Dst index, we show that, like soft X-ray sigmoids, disappearing filaments, and Extreme ultraviolet Imaging Telescope (EIT) waves on the disk of the Sun, magnetograph observation-based coronal field models may provide additional information on the likelihood of CME effects at the Earth. ¿ 2001 American Geophysical Union