Letter | Published:

Detection of nanoflare-heated plasma in the solar corona by the FOXSI-2 sounding rocket

Nature Astronomyvolume 1pages771774 (2017) | Download Citation



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.

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  1. 1.

    Klimchuk, J. A. On solving the coronal heating problem. Sol. Phys. 234, 41–77 (2006).

  2. 2.

    Parker, E. N. Nanoflares and the solar X-ray corona. Astrophys. J. 330, 474–479 (1988).

  3. 3.

    Glesener, L., Krucker, S. & Christe, S. et al. The FOXSI solar sounding rocket campaigns. Proc. SPIE 9905, 99050E (2016).

  4. 4.

    Krucker, S., Christe, S. & Glesener, L. et al. First images from the Focusing Optics X-Ray Solar Imager. Astrophys. J. 793, L32 (2014).

  5. 5.

    Christe, S., Glesener, L. & Buitrago-Casas, C. et al. FOXSI-2: upgrades of the focusing optics X-ray solar imager for its second flight. J. Astron. Instrum. 5, 1640005 (2016).

  6. 6.

    Narukage, N., Sakao, T. & Kano, R. et al. Coronal-temperature-diagnostic capability of the Hinode/X-Ray Telescope based on self-consistent calibration. II. Calibration with on-orbit data. Sol. Phys. 289, 1029–1042 (2014).

  7. 7.

    Warren, H. P., Winebarger, A. R. & Brooks, D. H. A systematic survey of high-temperature emission in solar active regions. Astrophys. J. 759, 141 (2012).

  8. 8.

    Cargill, P. J. & Klimchuk, J. A. Nanoflare heating of the corona revisited. Astrophys. J. 605, 911–920 (2004).

  9. 9.

    Klimchuk, J. A., Patsourakos, S. & Cargill, P. J. Highly efficient modeling of dynamic coronal loops. Astrophys. J. 682, 1351–1362 (2008).

  10. 10.

    Cargill, P. J., Bradshaw, S. J. & Klimchuk, J. A. Enthalpy-based thermal evolution of loops. II. Improvements to the model. Astrophys. J. 752, 161 (2012).

  11. 11.

    Heyvaerts, J. & Priest, E. R. Coronal heating by phase-mixed shear Alfven waves. Astron. Astrophys. 117, 220–234 (1983).

  12. 12.

    Schmelz, J. T., Kashyap, V. L. & Saar, S. H. et al. Some like it hot: coronal heating observations from Hinode X-ray Telescope and RHESSI. Astrophys. J. 704, 863–869 (2009).

  13. 13.

    Klimchuk, J. A. Coronal loop models and those annoying observations! (Keynote). In Proc. ASP Conference ‘Second Hinode Science Meeting: Beyond Discovery-Toward Understanding’ 221–233 (Vol. 415, Astronomical Society of the Pacific, 2009).

  14. 14.

    Barnes, W. T., Cargill, P. J. & Bradshaw, S. J. Inference of heating properties from ‘hot’ non-flaring plasmas in active region cores. II. Nanoflare trains. Astrophys. J. 833, 217 (2016).

  15. 15.

    McTiernan, J. M. RHESSI/GOES observations of the nonflaring Sun from 2002 to 2006. Astrophys. J. 697, 94–99 (2009).

  16. 16.

    Hannah, I. G., Grefenstette, B. W. & Smith, D. M. et al. The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR. Astrophys. J. 820, L14 (2016).

  17. 17.

    Winebarger, A. R., Schmelz, J. T., Warren, H. P., Saar, S. H. & Kashyap, V. L. Using a differential emission measure and density measurements in an active region core to test a steady heating model. Astrophys. J. 740, 2 (2011).

  18. 18.

    Aschwanden, M. J. & Boerner, P. Solar corona loop studies with the Atmospheric Imaging Assembly. I. Cross-sectional temperature structure. Astrophys. J. 732, 81 (2011).

  19. 19.

    Schmelz, J. T., Saar, S. H. & DeLuca, E. E. et al. Hinode X-Ray Telescope detection of hot emission from quiescent active regions: a nanoflare signature? Astrophys. J. 693, L131–L135 (2009).

  20. 20.

    Winebarger, A. R., Warren, H. P. & Schmelz, J. T. et al. Defining the ‘blind spot’ of Hinode EIS and XRT temperature measurements. Astrophys. J. 746, L17 (2012).

  21. 21.

    Ishikawa, S., Glesener, L. & Christe, S. et al. Constraining hot plasma in a non-flaring solar active region with FOXSI hard X-ray observations.Publ. Astron. Soc. Jpn 66, S15 (2014).

  22. 22.

    Ramsey, B. D. The development of focusing optics for the hard-X-ray region. Adv. Space Res. 38, 2985–2988 (2006).

  23. 23.

    Ishikawa, S., Saito, S. & Tajima, H. et al. Fine-pitch semiconductor detector for the FOXSI mission. IEEE T. Nucl. Sci. 58, 2039–2046 (2011).

  24. 24.

    Ishikawa, S., Katsuragawa, M. & Watanabe, S. et al. Fine-pitch CdTe detector for hard X-ray imaging and spectroscopy of the Sun with the FOXSI rocket experiment. J. Geophys. Res. Space Phys. 121, 6009–6016 (2016).

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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.

Author information


  1. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan

    • Shin-nosuke Ishikawa
  2. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, 55455, USA

    • Lindsay Glesener
    •  & Juliana Vievering
  3. Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, 94720, USA

    • Säm Krucker
    •  & Juan Camilo Buitrago-Casas
  4. Institute of 4D Technologies, School of Engineering, University of Applied Sciences and Arts Northwestern Switzerland, 5210, Windisch, Switzerland

    • Säm Krucker
  5. NASA Goddard Space Flight Center, Greenbelt, MD, 20771-0001, USA

    • Steven Christe
  6. National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan

    • Noriyuki Narukage


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S.I., L.G., S.K., S.C. and J.B. contributed to the instrument development, analysis of the observational data, scientific discussions and text writing. N.N. contributed to the analysis of the data observed by the Hinode satellite and scientific discussions. J.V. contributed to software developments for the FOXSI data analysis.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Shin-nosuke Ishikawa.

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