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  1. Abstract Solar flares, driven by prompt release of free magnetic energy in the solar corona 1,2 , 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 flare 5 , 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 electrons 6 . 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 released 2 .
    Free, publicly-accessible full text available June 23, 2023
  2. Abstract The past decade has seen a dramatic increase in practical applications of microwave gyrosynchrotron emission for plasma diagnostics and three-dimensional modeling of solar flares and other astrophysical objects. This breakthrough became possible due to an apparently minor, technical development of fast gyrosynchrotron codes, which enormously reduced the computation time needed to calculate a single spectrum, while preserving the accuracy of the computation. However, the available fast codes are limited in that they can only be used for a factorized distribution over the energy and pitch angle, while the distribution of electrons over energy or pitch angle is limited to a number of predefined analytical functions. In realistic simulations, these assumptions do not hold; thus, the codes free from the mentioned limitations are called for. To remedy this situation, we extended our fast codes to work with an arbitrary input distribution function of radiating electrons. We accomplished this by implementing fast codes for a distribution function described by an arbitrary numerically defined array. In addition, we removed several other limitations of the available fast codes and improved treatment of the free–free component. The ultimate fast codes presented here allow for an arbitrary combination of the analytically and numerically defined distributions,more »which offers the most flexible use of the fast codes. We illustrate the code with a few simple examples.« less
  3. Abstract Nonpotential magnetic energy promptly released in solar flares is converted to other forms of energy. This may include nonthermal energy of flare-accelerated particles, thermal energy of heated flaring plasma, and kinetic energy of eruptions, jets, upflows/downflows, and stochastic (turbulent) plasma motions. The processes or parameters governing partitioning of the released energy between these components are an open question. How these components are distributed between distinct flaring loops and what controls these spatial distributions are also unclear. Here, based on multiwavelength data and 3D modeling, we quantify the energy partitioning and spatial distribution in the well-observed SOL2014-02-16T064620 solar flare of class C1.5. Nonthermal emission of this flare displayed a simple impulsive single-spike light curve lasting about 20 s. In contrast, the thermal emission demonstrated at least three distinct heating episodes, only one of which was associated with the nonthermal component. The flare was accompanied by upflows and downflows and substantial turbulent velocities. The results of our analysis suggest that (i) the flare occurs in a multiloop system that included at least three distinct flux tubes; (ii) the released magnetic energy is divided unevenly between the thermal and nonthermal components in these loops; (iii) only one of these three flaring loopsmore »contains an energetically important amount of nonthermal electrons, while two other loops remain thermal; (iv) the amounts of direct plasma heating and that due to nonthermal electron loss are comparable; and (v) the kinetic energy in the flare footpoints constitutes only a minor fraction compared with the thermal and nonthermal energies.« less
  4. 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 ~1028cubic 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.