The processes that heat the solar and stellar coronae to several million kelvins, compared with the much cooler photosphere (5,800 K for the Sun), are still not well known1. One proposed mechanism is heating via a large number of small, unresolved, impulsive heating events called nanoflares2. Each event would heat and cool quickly, and the average effect would be a broad range of temperatures including a small amount of extremely hot plasma. However, detecting these faint, hot traces in the presence of brighter, cooler emission is observationally challenging. Here we present hard X-ray data from the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2), which detected emission above 7 keV from an active region of the Sun with no obvious individual X-ray flare emission. Through differential emission measure computations, we ascribe this emission to plasma heated above 10 MK, providing evidence for the existence of solar nanoflares. The quantitative evaluation of the hot plasma strongly constrains the coronal heating models.
We acknowledge the FOXSI instrument team for the development of the experiment. The X-ray focusing optics were provided by a team at NASA/Marshall Space Flight Center led by B. Ramsey. The focal plane detectors were developed by a team at ISAS/JAXA led by T. Takahashi and S. Watanabe. This work was supported through KAKENHI grants 24244021 and 20244017 from the Japan Society for the Promotion of Science. FOXSI was funded by NASA’s Low-Cost Access to Space program, grant NNX11AB75G. L.G. is supported by an NSF grant (AGS-1429512). Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in cooperation with ESA and NSC (Norway). The FOXSI team thanks NASA’s Sounding Rockets Program Office and the NSROC teams who helped to attain a successful flight.