Recent Progress in Understanding Particle Acceleration at the Sun

Efficient particle acceleration is produced in association with solar flares. These particles play a major role in the active Sun because they contain a large amount of the magnetic energy released during flares. Energetic electrons and ions interact with the solar atmosphere and produce high-energy X-rays and $\gamma$-rays. Energetic particles can also escape to the corona and interplanetary medium, produce radio emissions (electrons) and may eventually be detected aboard spacecrafts. It is currently admitted that solar flares are powered by magnetic energy previously stored in the coronal magnetic field and that magnetic energy release is likely to occur on coronal current sheets along regions of strong gradient of magnetic connectivity. However, understanding the connection between particle acceleration processes and the topology of the complex magnetic structures present in the corona is still a challenging issue. Particle transport from the acceleration region to the emitting sites should furthermore be considered to infer properties of the accelerated particles (and thus of the acceleration processes) from the observations of the radiation. In this talk, I will review the quantitative information on energetic particles (numbers, spectra,...)provided by the X-ray/$\gamma$-ray observations and illustrate how images in the X-ray and radio domains provide constraints on the location of the acceleration sites with respect to the magnetic structures observed in the flaring region. I will present recent results on the transport of energetic electrons in the solar corona as deduced from imaging spectroscopy in the X-ray domain and comment on the role of scattering in this process. Recent progress in the theories of particle acceleration in solar flares through the process of magnetic reconnection will be reviewed and the challenging issue of comparing results of the models with observations will be discussed. I will finally comment on the new constraints for particle acceleration at the Sun coming from the recent FERMI/LAT observations of the long duration production of GeV protons in connection with flares and/or coronal mass ejections.