Abstract
Magnetic flux ropes (MFRs) are important physical features closely related to solar eruptive activities with potential space weather consequences. Studying MFRs in the low solar atmosphere can shed light on their origin and subsequent magnetic structural evolution. In recent years, observations of solar photosphere and chromosphere reached a spatial resolution of 0.1 to 0.2 arcsec with the operation of meter class ground-based telescopes, such as the 1.6m Goode Solar Telescope at Big Bear Solar Observatory and the 1m New Vacuum Solar Telescope at Yunnan Observatory. The obtained chromospheric Halpha filtergrams with the highest resolution thus far have revealed detailed properties of MFRs before and during eruptions, and the observed pre-eruption structures of MFRs are well consistent with those demonstrated by nonlinear force-free field extrapolations. There are also evidences that MFRs may exist in the photosphere. The magnetic channel structure, with multiple polarity inversions and only discernible in high-resolution magnetograph observations, may be a signature of photospheric MFRs. These MFRs are likely formed below the surface due to motions in the convection zone and appear in the photosphere through flux emergence. Triggering of some solar eruptions is associated with an enhancing twist in the low-atmospheric MFRs.
Highlights
Magnetic flux ropes are generally defined as a bundle of magnetic fields that are twisted about each other and wrap around a common axis
For the sake of completeness, we note that New Vacuum Solar Telescope (NVST) Hα images display twisted structures of two active-region eruptive filaments in Yan et al (2015), where the authors identified the filaments as magnetic flux ropes with non-linear force-free field (NLFFF) modeling and suggested that they are formed after horizontal magnetic fields are twisted as a result of sunspot rotation
Flux ropes are one of the most important targets in solar physics research due to their potential space weather effects caused by their eruptions
Summary
Magnetic flux ropes are generally defined as a bundle of magnetic fields that are twisted about each other and wrap around a common axis. The genesis and evolution of solar magnetic flux ropes is an intriguing and challenging question, the answer of which can benefit from a quantitative characterization of solar magnetic fields in terms of physical parameters (e.g., magnetic twist, electric current). This can be routinely obtained by magnetic field modeling methods, such as the non-linear force-free field (NLFFF) extrapolations, based on the advanced vector magnetograph observations such as from Hinode and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). We discuss the future prospects and challenges in this important research field
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