We present an analysis of the formation and eruption of a filament and fast coronal mass ejection associated with a flare that occurred in active region 11429 using observations in the ultraviolet, extreme ultraviolet, X-ray, and radio wavelength bands. Precursor activity began as an interaction between two filaments, F1 and F2, that are identified as having twisted magnetic flux ropes (MFRs). Transient brightenings in all wavelengths are observed as a result of this interaction, likely the result of magnetic reconnection between the two filaments. This interaction results in a reconfiguration of the two filaments into a long overlying filament and a shorter low-lying filament. The upper filament subsequently undergoes a partial confined eruption. Plasma flows originating near the east footpoint of F1 lead to an extension of the upper filament into the filament channel to the west, resulting in a new active region filament (ARF). This new filament begins a slow rise and expansion. During its slowly rising phase, the MFR in which the filament is embedded becomes visible, with both the filament and flux rope rising and expanding simultaneously. The twist of the magnetic rope is determined as four turns. The erupting configuration changes from a twisted arch shape to a reversed γ shape within ∼75 s at the beginning of the fast-rise phase, representing a transformation from twist to writhe. The observations provide a clear example of filament formation via the tether-cutting reconnection of two nearby filaments. A helical kink instability may be the trigger of the ARF eruption.

Studying the formation and dynamic evolution of the magnetic flux rope (MFR) is key to understanding the physics of most solar eruptions. In the present study, we investigate the dynamic evolution of four MFRs, which involve in a major eruption. The MFR1, which represents as filament (F1), first appears about 31 hr before the major eruption. The MFR2 appears as a hot-channel with a small filament (F2) in its east part. The hot-channel becomes intermittently visible about 2 hr before the major eruption. The MFR3 is formed by the reconnection between MFR1 and MFR2. The annular components of MFR3 along the MFR2 part appear as a hollow helical structure from the background with the temperature of several MK after the reconnection. Coronal material then flows along the hollow structure from MFR1 part to MFR2 part to form a new filament (F3), which is hosted by MFR3 and acts as the axial component of MFR3 in the following evolution. The MFR4 appears during MFR3's eruption phase as conjugated extreme ultraviolet (EUV) brightenings. Both brightenings extend outward into irregular ribbons with conjugate EUV dimmings inside. Meanwhile, an erupted hot-channel, which roots in the conjugated brightening, is identified. The morphology of the MFRs are also profiled by four associated flare ribbon pairs. The on-disk observations shed light on our understanding of the topology of the MFRs and their formation and eruption. Further work should lay emphasis on the magnetic environment for the MFR formation and evolution.

Using vector magnetic field data from the Helioseismic and Magnetic Imager instrument onboard the Solar Dynamics Observatory, we study the signs of helicity (magnetic twist α_av, z-component of current helicity H_c) and tilt angle of 85 sample active regions (ARs) that appeared on the central solar disc (within ±45° from disc centre) between December 2018 and November 2020. This time range spans the exchange period of Solar Cycle 24 and 25. The main findings are as follows: (1) As a whole, 62 per cent of sample ARs follow the helicity hemispherical sign rule, and our observational results do not show any hemispherical rule sign reversal at the end or beginning of a Solar Cycle. (2) Interestingly, there is no significant statistical relationship between helicity and tilt angle, as in contrast with the general idea on the conservation of magnetic helicity in the solar convection zone inferred by the relationship between the photospheric helicity and tilt angle. It is also found that the hemispherical tendency of helicity and tilt angle is more obvious for ARs at high latitudes, and the hemispherical preference is more obvious for ARs with magnetic twist and writhe of the opposite signs than for ARs with the same signs.

We studied trans-equatorial loops (TLs) statistically from December 2006 to 2020. During this period, 160 TLs were identified, and 12 per cent of all active regions are connected by TLs. The result shows that 74 per cent of TLs are connected to regions of preceding magnetic polarity (PTLs), and only 26 per cent of TLs are connected to regions of following magnetic polarity (FTLs). The dominance of PTLs is related to Joy’s law. Moreover, the average length of TLs is 20°, and their lengths decrease during the solar cycle. By comparing the number of TLs and the smoothed monthly mean number of sunspots from 2009 to 2018, 88 per cent of the TLs appeared near the solar maximum (2014 ± 2). We noted that the closer a sunspot is to the equator, the easier it is for a TL to form. Furthermore, the tilt angle and latitudinal asymmetry of TL foot-points are independent of the solar cycle.

We present a detailed analysis of a partial eruption of a sigmoid filament lying along the polarity inversion line (PIL) of the small active region (AR) NOAA 12734 (with an area of 1.44 $\times {10}^3$ square megameters). The active filament was rooted in a dipole sunspot of the AR. The eruption was associated with a C1.3 flare and subsequent large-scale coronal disturbances. During its solar disk passage before the flare, the AR had the following characteristics: (1) Most of the time, the magnetic field lines in the AR showed a sigmoidal structure (‘L1’) in the low corona and arc-shaped loops (i.e., ‘L2’) in the upper atmosphere. (2) An ‘X’-shaped structure was formed between the original ‘S’-shaped magnetic loop (‘L1’) and the newly rising one (‘L3’) between the main positive and negative magnetic polarities of the sunspots, and the intersection point of flux ropes ‘L1’ and ‘L3’ corresponds well with the area where the initial extreme-ultraviolet (EUV) 1600 Å brightening of the flare occurred. (3) The AR disobeyed the hemispherical helicity rule and had magnetic twist and writhe of the same signs, i.e., its magnetic helicity/current helicity were positive in the northern hemisphere. (4) Sustained magnetic emergence and cancellation occurred before the flare. Therefore, the magnetic reconnection of highly twisted helical flux ropes under the confinement of the overlying magnetic fields is probably responsible for the partial eruption of the filament.

Magnetic flux rope (MFRs) are the most probable core structure of coronal mass ejections (CMEs). Their footpoints on the photosphere are the only feature for which their magnetic properties can be accurately measured, and to some extent, determine the property of the whole MFR. So far, the magnetic properties of footpoints are still a mystery. Y. Zhang et al. presented the dynamic evolution of four MFRs involved in a major eruption. For this special event, the footpoints of the MFRs could be identified by extreme-ultraviolet (EUV) brightening and MFR morphology during the preeruption phase, and by conjunction, EUV dimming during the eruption phase. This is a new effort to identify the footpoints of MFRs during the preeruption phase. This identification discloses that all footpoints originate out of the umbra and penumbra of the sunspots. The footpoint areas and magnetic flux during the preeruption phase are about 10⁷ km² and around the order of 10¹⁹–10²⁰ Mx, respectively. During the eruption phase, the footpoint area and the magnetic flux are about 10⁸ km² and around the order of 10²⁰ Mx, respectively. The temporal evolution of the magnetic features around the footpoints disclose that all footpoint pairs belong to nonconjugated opposite polarities. This observational evidence will help us to understand the magnetic nature of MFRs and their associated CMEs.