Context. The energy released during solar flares is believed to be stored in non-potential magnetic fields associated with electric currents flowing in the corona. While no measurements of coronal electric currents are presently available, maps of photospheric electric currents can now be derived from SDO/HMI observations. Photospheric electric currents have been shown to be the tracers of the coronal electric currents. Particle acceleration can result from electric fields associated with coronal electric currents. We revisit here some aspects of the relationship between particle acceleration in solar flares and electric currents in the active region. Aims. We study the relation between the energetic electron interaction sites in the solar atmosphere, and the magnitudes and changes of vertical electric current densities measured at the photospheric level, during the X2.2 flare on February 15, 2011, in AR NOAA 11158. Methods. X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are overlaid on magnetic field and electric current density maps calculated from the spectropolarimetric measurements of the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) using the UNNOFIT inversion and Metcalf disambiguation codes. X-ray images are also compared with extreme ultraviolet (EUV) images from the SDO Atmospheric Imaging Assembly (AIA) to complement the flare analysis. Results. Part of the elongated X-ray emissions from both thermal and non-thermal electrons overlay the elongated narrow current ribbons observed at the photospheric level. A new X-ray source at 50-100 keV (produced by non-thermal electrons) is observed in the course of the flare and is cospatial with a region in which new vertical photospheric currents appeared during the same period (an increase of 15%). These observational results are discussed in the context of the scenarios in which magnetic reconnection (and subsequent plasma heating and particle acceleration) occurs at current-carrying layers in the corona.
Bibliographical noteFunding Information:
We thank Mikola Gordovskyy, Guillaume Aulanier, Miho Janvier, Anna Massone and Brigitte Schmieder for their useful comments, as well as the RHESSI team for providing free access to data. SDO data were made available courtesy of the NASA/SDO AIA and HMI science teams. The authors thank the anonymous referee for his suggestions which led to major improvements in this manuscript. Sophie Musset acknowledges the CNES and the LABEX ESEP (N 2011-LABX-030) for the PhD funding, and thanks the French State and the ANR for their support through the “Investissements d’avenir” programm in the PSL initiative (convention No. ANR-10-IDEX-0001-02).
© ESO, 2015.
- Acceleration of particles
- Sun: X-rays, gamma rays
- Sun: flares
- Sun: magnetic fields
- Sun: particle emission