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Coronal mass ejections are solar eruptions driven by a sudden release of magnetic energy stored in the Sun’s corona. In many cases, this magnetic energy is stored in long-lived, arched structures called magnetic flux ropes. When a flux rope destabilizes, it can either erupt and produce a coronal mass ejection or fail and collapse back towards the Sun. The prevailing belief is that the outcome of a given event is determined by a magnetohydrodynamic force imbalance called the torus instability. This belief is challenged, however, by observations indicating that torus-unstable flux ropes sometimes fail to erupt. This contradiction has not yet been resolved because of a lack of coronal magnetic field measurements and the limitations of idealized numerical modelling. Here we report the results of a laboratory experiment that reveal a previously unknown eruption criterion below which torus-unstable flux ropes fail to erupt. We find that such ‘failed torus’ events occur when the guide magnetic field (that is, the ambient field that runs toroidally along the flux rope) is strong enough to prevent the flux rope from kinking. Under these conditions, the guide field interacts with electric currents in the flux rope to produce a dynamic toroidal field tension force that halts the eruption. This magnetic tension force is missing from existing eruption models, which is why such models cannot explain or predict failed torus events.

A study of the magnetic configuration and evolution of a long-lasting quiescent coronal sigmoid is presented. The sigmoid was observed by Hinode/XRT and Transition Region and Coronal Explorer (TRACE) between 2007 February 6 and 12 when it finally erupted. We construct nonlinear force-free field models for several observations during this period, using the flux-rope insertion method. The high spatial and temporal resolution of the X-Ray Telescope (XRT) allows us to finely select best-fit models that match the observations. The modeling shows that a highly sheared field, consisting of a weakly twisted flux rope embedded in a potential field, very well describes the structure of the X-ray sigmoid. The flux rope reaches a stable equilibrium, but its axial flux is close to the stability limit of about 5 x 1020 Mx. The relative magnetic helicity increases with time from February 8 until just prior to the eruption on February 12. We study the spatial distribution of the torsion parameter a in the vicinity of the flux rope, and find that it has a hollow-core distribution, i.e., electric currents are concentrated in a current layer at the boundary between the flux rope and its surroundings. The current layer is located near the bald patch separatrix surface (BPSS) of the magnetic configuration, and the X-ray emission appears to come from this current layer/BPSS, consistent with the Titov and Demoulin model. We find that the twist angle Ph of the magnetic field increases with time to about 2p just prior to the eruption, but never reaches the value necessary for the kink instability.

The X-Ray Telescope (XRT) aboard Hinode has revealed the fine structure of solar X-ray jets. One of the fine structures observed by XRT is an expanding loop. The loop appeared near the footpoint of the jet when footpoint brightening was observed. Additionally, we have found that the X-ray jets began just after the expanding loop “breaks”. Other fine structures discovered by XRT are thread-like features along the axis of the jets. XRT has shown that these thread structures compose the cross-section of jets. The fine structures and their motions strongly support an X-ray jet model based on magnetic reconnection, and also suggest that we must consider the three-dimensional configuration of the magnetic field to understand the jet phenomenon. We also investigated the reverse jet associated with the X-ray jet in the quiet Sun, and propose that the reverse jet is produced by heat conduction, or a MHD wave subsequent to the main jet.

Hinode/SOHO campaign 7197 is the most extensive study of polar jet formation and evolution from within both the north and south polar coronal holes so far. For the first time, this study showed that the appearance of X-ray jets in the solar coronal holes occurs at very high frequency–about 60 jets d$^{-1}$on average. Using observations collected by the X-Ray Telescope on Hinode, a number of physical parameters from a large sample of jets were statistically studied. We measured the apparent outward velocity, the height, the width and the lifetime of the jets. In our sample, all of these parameters show peaked distributions with maxima at 160 km s$^{-1}$for the outward velocity, 5$\times$10$^4$km for the height, 8$\times$10$^3$km for the width, and about 10 min for the lifetime of the jets. We also present the first statistical study of jet transverse motions, which obtained transverse velocities of 0–35 km s$^{-1}$. These values were obtained on the basis of a larger (in terms of frequency) and better sampled set of events than what was previously statistically studied (Shimojo et al. 1996, PASJ, 48, 123). The results were made possible by the unique characteristics of XRT. We describe the methods used to determine the characteristics and set some future goals. We also show that despite some possible selection effects, jets preferably occur inside the polar coronal holes.