Direct Evidence for Magnetic Reconnection in a Solar EUV Nanoflare

Abstract

We provide observational evidence that the mechanism of solar EUV nanoflares may be close to the standard flare model. The object of our study was a nanoflare on 25 February 2011, for which we determined a plasma temperature of 3.1 MK, a total thermal energy of $6.2 \times 10^{25}~\mbox{erg}$ , and an electric-current distribution that reaches its maximum at a height of ${\approx}\,1.5~\mbox{Mm}$ . Despite the lack of spatial resolution, we reconstructed the 3D magnetic configuration for this event in the potential and non-linear force-free-field interpolations. As a result, we identified four null-points, two of which were coincident with the region of maximal energy release. The nanoflare was initiated by a new small-scale magnetic flux, which appeared on the photosphere about 15 – 20 minutes before the nanoflare. The total free energy stored in the region before the nanoflare was ${\approx}\,8.9 \times 10^{25}~\mbox{erg}$ . Only about two-thirds of this amount was transferred into the plasma heating and EUV radiation. We posit that the remaining energy could be transferred during particle acceleration and plasma motions, which are still inaccessible for direct observations in nanoflares.