The pre-coronal mass ejection (pre-CME) structure is of great importance to understanding the origin of CMEs, which, however, has been largely unknown for CMEs originating from active regions. In this paper, we investigate this issue using the wavelet-enhanced EUV Imaging Telescope (EIT) observations combined with the Large Angle and Spectrometric Coronagraph, Michelson Doppler Imager, and GOES soft X-ray observations. Selected for studying are 16 active-region coronal arcades whose gradual inflation lead up to CMEs. Twelve of them clearly build upon post-eruptive arcades resulting from a preceding eruption; the remaining four are located high in the corona in the first place and/or have existed for days. The observed inflation lasts for 8.7 ± 4.1 hr, with the arcade rising from 1.15 ± 0.06 R⊙ to 1.36 ± 0.07 R⊙ within the EIT field of view (FOV). The rising speed is less than 5 km s-1 most of the time. Only at the end of this quasi-static stage does it increase to tens of kilometers per second over tens of minutes. The arcade then erupts out of the EIT FOV as a CME with similar morphology. This pre-CME structure is apparently unaffected by the flares occurring during its quasi-static inflation phase, but is closely coupled with the flare occurring during its acceleration phase. For four events that are observed on the disk, it is found that the gradual inflation of the arcade is accompanied by significant helicity injection from the photosphere. In particular, a swirling structure, which is reminiscent of a magnetic flux rope, was observed in one of the arcades over 4 hr prior to the subsequent CME, and the growth of the arcade is associated with the injection of helicity of opposite sign into the active region via flux emergence.We propose a four-phase evolution paradigm for the observed CMEs, i.e., a quasi-static inflation phase which corresponds to the buildup of magnetic free energy in the corona, followed by the frequently observed three-phase paradigm, including an initial phase, an acceleration phase, and a gradual phase.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- Sun: corona
- Sun: coronal mass ejections (CMEs)
- Sun: flares