Since their initial discovery with AIA on board the Solar Dynamics Observatory (SDO), HCs have been generally regarded as proxies for magnetic flux ropes (MFRs; volumetric plasma structures with magnetic field lines that wrap around a central axis). This is supported by the following observational studies: (1) Cheng et al. (2014a) observed an HC that showed helical threads winding around an axis. Simultaneously, cool filamentary materials descended spirally down to the chromosphere, providing direct observational evidence of an intrinsic helical structure for the HC. (2) Cheng et al. (2011) reported that an HC can grow during an eruption, similar to the MFR growth process according to the classic magnetic reconnection scenario in eruptive flares. Song et al. (2014a) presented the formation process of an HC during a coronal mass ejection (CME) and found that the HC was formed from coronal arcades through magnetic reconnection. These works further support the idea that an HC is an MFR structure based on the relation between the HC and magnetic reconnection. (3) Cheng et al. (2014b) found that an HC was initially cospatial with a prominence. Then a separation of the HC top from that of the prominence was observed during the eruption initiated by the ideal kink instability (Török et al. 2004). It is widely accepted that a prominence/filament can exist at the dip of a flux rope (Rust & Kumar 1994). Therefore, this observation offered further important support for the idea that an HC is an MFR; beside an HC, several lines of observations in the lower corona have also been proposed as MFRs, including sigmoid structures in an active region (Titov & Démoulin 1999; McKenzie & Canfield 2008) and coronal cavities in quiescent regions (Wang & Stenborg 2010). A sigmoid has either a forward or reverse S-shape with enhanced X-ray emissions (implying an entity of high temperature) with its center straddling along the polarity inversion line of the hosting active region. Zhang et al. (2012) showed that the HC initially appeared as a sigmoidal structure and then changed to a semi-circular shape. Therefore, a sigmoid and an HC might represent the same structure, and their different shapes are likely from different perspectives and evolution phases. Both structures feature a high temperature, a possible result of flare magnetic reconnection (e.g., Song et al. 2014a, 2014b). A coronal cavity, on the other hand, which is observed as a dark circular or oval structure above the solar limb in coronal images with temperatures close to the background corona (Fuller et al. 2008; Gibson et al. 2010; Kucera et al. 2012), is also interpreted as an MFR. As mentioned, the long-studied feature of solar filaments/prominences shown best in Hα images has been interpreted as being situated along the dip in the MFR. Therefore, a prominence lying in the dip of a coronal cavity is not rare. The eruption of a coronal cavity (or filament) from a quiescent region does not show a high-temperature signature like an HC, which might be attributed to a lack of obvious heating acquired from the weak magnetic reconnection (e.g., Song et al. 2013).
Citation: Török et al. 2004
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