Solar flare accelerates nearly all electrons in a large coronal volume.

Solar flares, driven by prompt release of free magnetic energy in the solar corona1,2, are known to accelerate a substantial portion (ten per cent or more)3,4 of available electrons to high energies. Hard X-rays, produced by high-energy electrons accelerated in the flare5, require a high ambient density for their detection. This restricts the observed volume to denser regions that do not necessarily sample the entire volume of accelerated electrons6. Here we report evolving spatially resolved distributions of thermal and non-thermal electrons in a solar flare derived from microwave observations that show the true extent of the acceleration region. These distributions show a volume filled with only (or almost only) non-thermal electrons while being depleted of the thermal plasma, implying that all electrons have experienced a prominent acceleration there. This volume is isolated from a surrounding, more typical flare plasma of mainly thermal particles with a smaller proportion of non-thermal electrons. This highly efficient acceleration happens in the same volume in which the free magnetic energy is being released2.

Decay of the coronal magnetic field can release sufficient energy to power a solar flare.

Solar flares are powered by a rapid release of energy in the solar corona, thought to be produced by the decay of the coronal magnetic field strength. Direct quantitative measurements of the evolving magnetic field strength are required to test this. We report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field. The field decays at a rate of ~5 Gauss per second for 2 minutes, as measured within a flare subvolume of ~1028 cubic centimeters. This fast rate of decay implies a sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. The decrease in stored magnetic energy is enough to power the solar flare, including the associated eruption, particle acceleration, and plasma heating.

Nucleation preexponential in dynamic Ising models at moderately strong fields.

A dynamic Ising model on a two-dimensional square lattice with nearest neighbor interactions is considered in the metastable region at low temperatures. A large number of low-energy cluster configurations is identified, and for those configurations a system of kinetic equations is written. Solution is obtained using symbolic computational approaches. This allows one to identify the full expression for the nucleation rate, including the preexponential. The treatment generalizes the earlier study of a different, lattice-gas spin-flip dynamics [V. A. Shneidman and G. M. Nita, Phys. Rev. Lett. 89, 025701 (2002)], for the cases of Glauber and Metropolis dynamics and for a broader region of fields. In addition, connection with the lowest-energy nucleation paths (which can be studied analytically, without computer assistance) is examined.

Collapse of transient nucleation fluxes in a cold Ising ferromagnet.

We report the time-dependent nucleation fluxes and associated nucleation rates in a metastable Ising ferromagnet on square lattice with Metropolis (Glauber-type) dynamics. It is discovered that, with lowering of the temperature, fluxes collapse into several representative transient curves corresponding to magic cluster sizes. Those can be associated with physical droplets, i.e., long-lived configurations which provide a link with the classical Becker-Döring picture.

On the critical cluster in the two-dimensional Ising model: computer-assisted exact results.

For a nearest-neighbor Ising model on a square lattice all cluster configurations with 17 or fewer spins are identified. In neglect of cluster-cluster interactions, critical sizes and barriers to nucleation are obtained as functions of temperature and magnetic field for two alternative definitions of a "critical cluster."

Modulation of the nucleation rate preexponential in a low-temperature Ising system.

A metastable lattice gas with nearest-neighbor interactions and continuous-time dynamics is studied using a generalized Becker-Döring approach in the multidimensional space of cluster configurations. The preexponential of the metastable-state lifetime (inverse of nucleation rate) is found to exhibit distinct peaks at integer values of the inverse supersaturation. Peaks are unobservable (infinitely narrow) in the strict limit T-->0, but become detectable and eventually dominate at higher temperatures.