A STUDY OF FAST FLARELESS CORONAL MASS EJECTIONS

Abstract

Two major processes have been proposed to convert the coronal magnetic energy\ninto the kinetic energy of a coronal mass ejection (CME): resistive magnetic\nreconnection and ideal macroscopic magnetohydrodynamic instability of magnetic\nflux rope. However, it remains elusive whether both processes play a comparable\nrole or one of them prevails during a particular eruption. To shed light on\nthis issue, we carefully studied energetic but flareless CMEs, \\textit{i.e.},\nfast CMEs not accompanied by any flares. Through searching the Coordinated Data\nAnalysis Workshops (CDAW) database of CMEs observed in Solar Cycle 23, we found\n13 such events with speeds larger than 1000 km s$^{-1}$. Other common\nobservational features of these events are: (1) none of them originated in\nactive regions; they were associated with eruptions of well-developed long\nfilaments in quiet-Sun regions, (2) no apparent enhancement of flare emissions\nwas present in soft X-ray, EUV and microwave data. Further studies of two\nevents reveal that (1) the reconnection electric fields, as inferred from the\nproduct of the separation speed of post-eruption ribbons and the photospheric\nmagnetic field measurement, were in general weak; (2) the period with a\nmeasurable reconnection electric field is considerably shorter than the total\nfilament-CME acceleration time. These observations indicate that, for these\nfast CMEs, the magnetic energy was released mainly via the ideal flux rope\ninstability through the work done by the large scale Lorentz force acting on\nthe rope currents rather than via magnetic reconnections. We also suggest that\nreconnections play a less important role in accelerating CMEs in quiet Sun\nregions of weak magnetic field than those in active regions of strong magnetic\nfield.\n