Fermi acceleration in plasmoids interacting with fast shocks of reconnection via fractal reconnection.

We propose the particle acceleration model coupled with multiple plasmoid ejections in a solar flare. Unsteady reconnection produces plasmoids in a current sheet and ejects them out to the fast shocks, where particles in a plasmoid are reflected upstream the shock front by magnetic mirror effect. As the plasmoid passes through the shock front, the reflection distance becomes shorter and shorter driving Fermi acceleration, until it becomes proton Larmor radius. The fractal distribution of plasmoids may also have a role in naturally explaining the power-law spectrum in nonthermal emissions.

Superflares on solar-type stars.

Solar flares are caused by the sudden release of magnetic energy stored near sunspots. They release 10(29) to 10(32) ergs of energy on a timescale of hours. Similar flares have been observed on many stars, with larger 'superflares' seen on a variety of stars, some of which are rapidly rotating and some of which are of ordinary solar type. The small number of superflares observed on solar-type stars has hitherto precluded a detailed study of them. Here we report observations of 365 superflares, including some from slowly rotating solar-type stars, from about 83,000 stars observed over 120 days. Quasi-periodic brightness modulations observed in the solar-type stars suggest that they have much larger starspots than does the Sun. The maximum energy of the flare is not correlated with the stellar rotation period, but the data suggest that superflares occur more frequently on rapidly rotating stars. It has been proposed that hot Jupiters may be important in the generation of superflares on solar-type stars, but none have been discovered around the stars that we have studied, indicating that hot Jupiters associated with superflares are rare.

Magneto-thermal convection in solar prominences.

Coronal cavities are large low-density regions formed by hemispheric-scale magnetic flux ropes suspended in the Sun's outer atmosphere. They evolve over time, eventually erupting as the dark cores of coronal mass ejections. Although coronal mass ejections are common and can significantly affect planetary magnetospheres, the mechanisms by which cavities evolve to an eruptive state remain poorly understood. Recent optical observations of high-latitude 'polar crown' prominences within coronal cavities reveal dark, low-density 'bubbles' that undergo Rayleigh-Taylor instabilities to form dark plumes rising into overlying coronal cavities. These observations offered a possible mechanism for coronal cavity evolution, although the nature of the bubbles, particularly their buoyancy, was hitherto unclear. Here we report simultaneous optical and extreme-ultraviolet observations of polar crown prominences that show that these bubbles contain plasma at temperatures in the range (2.5-12) × 10(5) kelvin, which is 25-120 times hotter than the overlying prominence. This identifies a source of the buoyancy, and suggests that the coronal cavity-prominence system supports a novel form of magneto-thermal convection in the solar atmosphere, challenging current hydromagnetic concepts of prominences and their relation to coronal cavities.

Chromospheric anemone jets as evidence of ubiquitous reconnection.

The heating of the solar chromosphere and corona is a long-standing puzzle in solar physics. Hinode observations show the ubiquitous presence of chromospheric anemone jets outside sunspots in active regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their velocity is 10 to 20 kilometers per second. These small jets have an inverted Y-shape, similar to the shape of x-ray anemone jets in the corona. These features imply that magnetic reconnection similar to that in the corona is occurring at a much smaller spatial scale throughout the chromosphere and suggest that the heating of the solar chromosphere and corona may be related to small-scale ubiquitous reconnection.

Filamentary structure on the Sun from the magnetic Rayleigh-Taylor instability.

Magnetic flux emerges from the solar surface as dark filaments connecting small sunspots with opposite polarities. The regions around the dark filaments are often bright in X-rays and are associated with jets. This implies plasma heating and acceleration, which are important for coronal heating. Previous two-dimensional simulations of such regions showed that magnetic reconnection between the coronal magnetic field and the emerging flux produced X-ray jets and flares, but left unresolved the origin of filamentary structure and the intermittent nature of the heating. Here we report three-dimensional simulations of emerging flux showing that the filamentary structure arises spontaneously from the magnetic Rayleigh-Taylor instability, contrary to the previous view that the dark filaments are isolated bundles of magnetic field that rise from the photosphere carrying the dense gas. As a result of the magnetic Rayleigh-Taylor instability, thin current sheets are formed in the emerging flux, and magnetic reconnection occurs between emerging flux and the pre-existing coronal field in a spatially intermittent way. This explains naturally the intermittent nature of coronal heating and the patchy brightenings in solar flares.

Extraction of black hole rotational energy by a magnetic field and the formation of relativistic jets.

Using numerical simulations, we modeled the general relativistic magnetohydrodynamic behavior of a plasma flowing into a rapidly rotating black hole in a large-scale magnetic field. The results show that a torsional Alfvén wave is generated by the rotational dragging of space near the black hole. The wave transports energy along the magnetic field lines outward, causing the total energy of the plasma near the hole to decrease to negative values. When this negative energy plasma enters the horizon, the rotational energy of the black hole decreases. Through this process, the energy of the spinning black hole is extracted magnetically