Magnetic flux rope structures associated with filament channels: Two case studies

We report on two case studies regarding the magnetic flux rope (MFR) structure of coronal mass ejections (CMEs) near the Sun and at 1 au. The event is a stealth streamer blow-out CME on 2011 March 25 from AR11176 showing a typical three-part bubble-like flux rope structure. No apparent eruptive signatures (e.g., two-ribbon flare, and post-eruption arcade (PEA)) were detected in the solar disk images; but a faint eruptive prominence (EP) was seen at the limb in STEREO EUV images along with a few confined flares before the eruption. The second event is a CME on 2018 August 20 originating from a quiescent filament region with clear two-ribbon flare and PEA observed; and the prolonged CME acceleration coincides with the GOES X-ray flare light curve and reconnected (RC) flux (Gopalswamy et al., 2022). In both cases, a well-defined magnetic cloud (MC) is detected at 1 au after ∼5 days. The two MCs have similar magnetic field magnitude and poloidal flux, however, the RC fluxes estimated from the associated solar source regions are significantly different. Our detailed analysis suggests the following. 1) The two events share a similar kinematic pattern consisting of a slow-rise phase and a main acceleration phase. 2) In the first case, the RC flux of the associated trigger flare at AR11176 is much smaller than the MC poloidal flux at 1au, and a significant amount of poloidal flux has been added through reconnection during the eruption. 3) Both cases show that the post-eruption flux rope has a lower twist at the core and a higher twist at the edge of the flux rope. In addition, we find that the MCs may deviate from a force-free state at the edge of flux rope. A non-force-free, varying-twist Gold-Hoyle (GH) model provides a better fit to the boundaries of the flux rope than the Lundquist (LQ) fit.