A Study of Fast Flareless Coronal Mass Ejections

摘要

Two major processes have been proposed to convert the coronal magnetic energyinto the kinetic energy of a coronal mass ejection (CME): resistive magneticreconnection and ideal macroscopic magnetohydrodynamic instability of magneticflux rope. However, it remains elusive whether both processes play a comparablerole or one of them prevails during a particular eruption. To shed light onthis issue, we carefully studied energetic but flareless CMEs, \textit{i.e.},fast CMEs not accompanied by any flares. Through searching the Coordinated DataAnalysis Workshops (CDAW) database of CMEs observed in Solar Cycle 23, we found13 such events with speeds larger than 1000 km s$^{-1}$. Other commonobservational features of these events are: (1) none of them originated inactive regions; they were associated with eruptions of well-developed longfilaments in quiet-Sun regions, (2) no apparent enhancement of flare emissionswas present in soft X-ray, EUV and microwave data. Further studies of twoevents reveal that (1) the reconnection electric fields, as inferred from theproduct of the separation speed of post-eruption ribbons and the photosphericmagnetic field measurement, were in general weak; (2) the period with ameasurable reconnection electric field is considerably shorter than the totalfilament-CME acceleration time. These observations indicate that, for thesefast CMEs, the magnetic energy was released mainly via the ideal flux ropeinstability through the work done by the large scale Lorentz force acting onthe rope currents rather than via magnetic reconnections. We also suggest thatreconnections play a less important role in accelerating CMEs in quiet Sunregions of weak magnetic field than those in active regions of strong magneticfield.