Magnetic flux ropes (MFRs) are believed to be the core structure in solar eruptions, nevertheless, their formation remains intensely debated. Here we report a rapid buildup process of an MFR-system during a confined X2.2 class flare occurred on 2017 September 6 in NOAA AR 12673, three hours after which the structure erupted to a major coronal mass ejection (CME) accompanied by an X9.3 class flare. For the X2.2 flare, we do not find EUV dimmings, separation of its flare ribbons, or clear CME signatures, suggesting a confined flare. For the X9.3 flare, large-scale dimmings, separation of its flare ribbons, and a CME show it to be eruptive. By performing a time sequence of nonlinear force-free fields (NLFFFs) extrapolations we find that: until the eruptive flare, an MFR-system was located in the AR. During the confined flare, the axial flux and the lower bound of the magnetic helicity for the MFR-system were dramatically enhanced by about 86% and 260%, respectively, although the mean twist number was almost unchanged. During the eruptive flare, the three parameters were all significantly reduced. The results evidence the buildup and release of the MFR-system during the confined and the eruptive flare, respectively. The former may be achieved by flare reconnection. We also calculate the pre-flare distributions of the decay index above the main polarity inversion line (PIL) and find no significant difference. It indicates that the buildup of the magnetic flux and helicity of the MFR-system may play a role in facilitating its final eruption.

We establish the largest eruptive/confined flare database to date and analyze 322 flares of Geostationary Operational Environmental Satellite class M1.0 and larger that occurred during 2010–2019, i.e., almost spanning all of solar cycle 24. We find that the total unsigned magnetic flux ( ) of active regions (ARs) is a key parameter governing the eruptive character of large flares, with the proportion of eruptive flares exhibiting a strong anticorrelation with . This means that an AR containing a large magnetic flux has a lower probability that the large flares it produces will be associated with a coronal mass ejection (CME). This finding is supported by the high positive correlation we obtained between the critical decay index height and , implying that ARs with a larger have a stronger magnetic confinement. Moreover, the confined flares originating from ARs larger than Mx have several characteristics in common: stable filament, slipping magnetic reconnection, and strongly sheared post-flare loops. Our findings reveal new relations between the magnetic flux of ARs and the occurrence of CMEs in association with large flares. The relations obtained here provide quantitative criteria for forecasting CMEs and adverse space weather, and have important implications for “superflares” on solar-type stars and stellar CMEs.

Sub-ion magnetic holes are rich in the terrestrial plasma sheet and magnetosheath. Here, we statistically investigate 60 sub-ion magnetic holes in the solar wind at 1 AU using the high-resolution data measured by the Magnetospheric Multiscale mission. We find that they are observed with a duration of 0.1-0.5 s, and the lengths of their cross-section are~0.1-0.6 ion gyroradius. These structures prefer to occur in the slow solar wind with a weak ambient magnetic field strength. They also prefer to occur in the marginally mirror stable or unstable environments. Electron vortices as well as an enhancement of the electron perpendicular temperature and electron fluxes at~90°pitch angle tend to be observed inside some magnetic holes with a large ambient magnetic field strength. By contrast, there are no clearly observational electron vortices as well as the electron fluxes at~90°pitch angle inside some magnetic holes with a weak ambient magnetic strength. The current density with a value of~10-50 nA/m 2 reveals that the corresponding maximum electron velocity is <10 km/s inside some magnetic holes, lower than the level of the observational electron velocity noise, which prevents the detection of the weak electron vortex. We suggest that electron vortices exist inside all the sub-ion magnetic holes in the solar wind. The generation of these sub-ion magnetic holes can be explained by the electron magnetohydrodynamics soliton and the electron vortex magnetic hole.

The coronal heating region is able to generate mirror mode structures by ion mirror instabilities. Linear magnetic holes are believed to be the remnants of mirror mode structures, thus they are believed to be messengers from the coronal heating region. They can be convected to ∼9 au with the solar wind flow, indicating that a stabilizing mechanism is necessary to make the magnetic holes survive for such a long time. Here, we investigate a magnetic hole with a size of ∼6.7 ρ i in the solar wind based on observations by the Magnetospheric Multiscale mission. The unprecedented high-resolution data enable us to reveal the existence of the ion vortex inside the structure for the first time. Such an ion vortex forms a ring-like current, which is consistent with the magnetic field depression. The self-consistent structure of the magnetic hole contributed by the ion vortex can help to further shed light on the mechanism of the long-term survival of magnetic holes in the astrophysical plasma.

With the aim of understanding the physical mechanisms of confined flares, we selected 18 confined flares during 2011-2017, and classified the confined flares into two types based on their different dynamic properties and magnetic configurations. "Type I" of confined flares are characterized by slipping reconnection, strong shear, and stable filament. "Type II" flares have nearly no slipping reconnection, and have a configuration in potential state after the flare. Filament erupts but is confined by strong strapping field. "Type II" flares could be explained by 2D MHD models while "type I" flares need 3D MHD models. 7 flares of 18 (∼39 %) belong to "type I" and 11 (∼61 %) are "type II" confined flares. The post-flare loops (PFLs) of "type I" flares have a stronger non-potentiality, however, the PFLs in "type II" flares are weakly sheared. All the "type I" flares exhibit the ribbon elongations parallel to the polarity inversion line (PIL) at speeds of several tens of km s −1 . For "type II" flares, only a small proportion shows the ribbon elongations along the PIL. We suggest that different magnetic topologies and reconnection scenarios dictate the distinct properties for the two types of flares. Slipping magnetic reconnections between multiple magnetic systems result in "type I" flares. For "type II" flares, magnetic reconnections occur in anti-parallel magnetic fields underlying the erupting filament. Our study shows that "type I" flares account for more than one third of the overall large confined flares, which should not be neglected in further studies.