Models and observations have demonstrated that twisted flux ropes (TFRs) play a significant role in the structure and eruptive dynamics of active regions. Their role in the dynamics of the quiet Sun atmosphere has remained elusive, their fundamental relevance emerging mainly from theoretical models showing that they form and erupt as a result of flux cancellation. Here, Hinode high-resolution photospheric vector magnetic field measurements are integrated with advanced environment reconstruction models: TFRs develop on various scales and are associated with the appearance of mesospots. The developing TFRs contain sufficient free magnetic energy to match the requirements of the recently observed “campfires” discovered by Solar Orbiter in the quiet Sun. The free magnetic energy is found to be large enough to trigger eruptions, while the magnetic twist is large enough to trigger confined eruptions, heating the chromosphere. TFRs are also connected to larger-scale magnetic fields such as supergranulation loops, allowing the generation of Alfvén waves at the top of the chromosphere that can propagate along them. High-resolution magnetohydrodynamic simulations, incorporating subsurface dynamo activity at an unprecedented 30 km spatial resolution, confirm that TFRs are ubiquitous products of the permanent small-scale dynamo engine that feeds their formation, destabilization, eruption via flux emergence, submergence, and cancellation of their chromospheric feet, similar to the dynamics driving large-scale eruptive events. Future investigations, especially with the Daniel K. Inouye Solar Telescope and Solar Orbiter, will deepen our understanding of TFRs in the context of atmospheric heating.

Solar abundances have been historically assumed to be representative of cosmic abundances. However, our knowledge of the solar abundance of helium, the second most abundant element, relies mainly on models 1 and indirect measurements through helioseismic observations 2 , because actual measurements of helium in the solar atmosphere are very scarce. Helium cannot be directly measured in the photosphere because of its high first ionization potential, and measurements of its abundance in the inner corona have been sporadic 3 , 4 . In this Letter, we present simultaneous global images of the helium (out to a heliocentric distance of 3 R ⊙ (solar radii)) and hydrogen emission in the solar corona during the minimum of solar activity of cycle 23 and directly derive the helium abundance in the streamer region and surrounding corona (out to 2.2 R ⊙ ). The morphology of the He + corona is markedly different from that of the H corona, owing to significant spatial variations in helium abundance. The observations show that the helium abundance is shaped according to and modulated by the structure of the large-scale coronal magnetic field and that helium is almost completely depleted in the equatorial regions during the quiet Sun. This measurement provides a trace back to the coronal source of the anomalously slow solar wind observed in the heliosphere at the Sun–Earth Lagrangian point L1 in 2009, during the exceptionally long-lasting minimum of solar activity cycle 23.

Measurements and modelling of a large confined eruption on the Sun show that its evolution is controlled by a multilayer magnetic cage containing a twisted flux rope, which can sometimes be ejective.

Seven different models are applied to the same problem of simulating the Sun's coronal magnetic field during the solar eclipse on 2015 March 20. All of the models are non-potential, allowing for free magnetic energy, but the associated electric currents are developed in significantly different ways. This is not a direct comparison of the coronal modelling techniques, in that the different models also use different photospheric boundary conditions, reflecting the range of approaches currently used in the community. Despite the significant differences, the results show broad agreement in the overall magnetic topology. Among those models with significant volume currents in much of the corona, there is general agreement that the ratio of total to potential magnetic energy should be approximately 1.4. However, there are significant differences in the electric current distributions; while static extrapolations are best able to reproduce active regions, they are unable to recover sheared magnetic fields in filament channels using currently available vector magnetogram data. By contrast, time-evolving simulations can recover the filament channel fields at the expense of not matching the observed vector magnetic fields within active regions. We suggest that, at present, the best approach may be a hybrid model using static extrapolations but with additional energization informed by simplified evolution models. This is demonstrated by one of the models.

Modelling the solar magnetic field using observations of the photospheric field in the four-day period preceding a coronal mass ejection shows that the formation and later ejection of a twisted rope of magnetic flux provides the physical mechanism responsible for the ejection.

Nonlinear force-free field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically constructed Sun-like test cases, but they are less successful in application to real solar data. Different NLFFF models have been found to have markedly different field line configurations and to provide widely varying estimates of the magnetic free energy in the coronal volume, when applied to solar data. NLFFF models require consistent, force-free vector magnetic boundary data. However, vector magnetogram observations sampling the photosphere, which is dynamic and contains significant Lorentz and buoyancy forces, do not satisfy this requirement, thus creating several major problems for force-free coronal modeling efforts. In this article, we discuss NLFFF modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT, STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process illustrate the three such issues we judge to be critical to the success of NLFFF modeling: (1) vector magnetic field data covering larger areas are needed so that more electric currents associated with the full active regions of interest are measured, (2) the modeling algorithms need a way to accommodate the various uncertainties in the boundary data, and (3) a more realistic physical model is needed to approximate the photosphere-to-corona interface in order to better transform the forced photospheric magnetograms into adequate approximations of nearly force-free fields at the base of the corona. We make recommendations for future modeling efforts to overcome these as yet unsolved problems.