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The origin of reconnection-mediated transient brightenings in the solar transition region

Nature Astronomy volume 5, pages 237–245 (2021)Cite this article

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Abstract

The ultraviolet emission from the solar transition region is dominated by dynamic, low-lying magnetic loops. The enhanced spatial and temporal resolution of the solar observation satellite Interface Region Imaging Spectrograph (IRIS) has made it possible to study these structures in fine detail. IRIS has observed ‘transient brightenings’ in these loops, associated with strong excess line broadenings1,2 providing important clues to the mechanisms that heat the solar atmosphere. However, the physical origin of the brightenings is debated. The line broadenings have been variously interpreted as signatures of nanoflares3, magneto-hydrodynamic turbulence4, plasmoid instabilities5 and magneto-acoustic shocks6. Here we use IRIS slit-jaw images and spectral data, and the Atmospheric Imaging Assembly of the Solar Dynamics Observatory spacecraft, to show that the brightenings are consistent with magnetic-reconnection-mediated impulsive heating at field-line braiding sites in multi-stranded transition-region loops. The spectroscopic observations present evidence for preferential heating of heavy ions from the transition region and we show that this is consistent with ion cyclotron turbulence caused by strong currents at the reconnection sites. Time-dependent differential emission measure distributions are used to determine the heating frequency7,8,9 and to identify pockets of faintly emitting ‘super-hot’ plasma. The observations we present and the techniques we demonstrate open up a new avenue of diagnostics for reconnection-mediated energy release in solar plasma.

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Fig. 1: Co-aligned IRIS, SDO/AIA and SDO/HMI images of the observed loop brightenings and the associated spectra at the pixel locations of the brightenings.
Fig. 2: Pixel-by-pixel analysis of the physical parameters along the magnetic loop structure observed at location 1.
Fig. 3: Pixel-by-pixel analysis of the physical parameters along the magnetic loop structure observed at location 3 .
Fig. 4: Temporal evolution of the light curves and the DEMs at the locations of loop brightenings.
Fig. 5: Heating rate due to the ion cyclotron instability for the most abundant heavy species (O, Si, S, Mg and Fe) in the transition region.

Data availability

The observational data used for this study are designated as AR12396 tracking OBS 3860109180 (2015-08-06 15:19:21-16:41:59) and are publicly available for download from the Lockheed Martin Solar and Astrophysics Laboratory at https://iris.lmsal.com/search/. The co-aligned SDO data are also available at the above-mentioned web source. All data that support the findings of this study are available on reasonable request from the corresponding author.

Code availability

Details of the algorithms used to create the main figures, especially the unsharp masking and the calculation of ion cyclotron instability, is available in the Methods. The codes required to reproduce the results of these algorithms are available on reasonable request from the corresponding author. For the calculation of the DEM, an IDL routine written by Hannah and Kontar26 is implemented and can be found at https://github.com/ianan/demreg.

References

  1. De Pontieu, B. et al. The Interface Region Imaging Spectrograph (IRIS). Sol. Phys. 289, 2733–2779 (2014).

    Article  ADS  Google Scholar 

  2. Hansteen, V. et al. The unresolved fine structure resolved: IRIS observations of the solar transition region. Science 346, 1255757 (2014).

    Article  Google Scholar 

  3. Testa, P. et al. Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares. Science 346, 1255724 (2014).

    Article  Google Scholar 

  4. De Pontieu, B. et al. On the prevalence of small-scale twist in the solar chromosphere and transition region. Science 346, 1255732 (2014).

    Article  Google Scholar 

  5. Innes, D. E., Guo, L. J., Huang, Y. M. & Bhattacharjee, A. IRIS Si IV line profiles: an indication for the plasmoid instability during small-scale magnetic reconnection on the Sun. Astrophys. J. 813, 86 (2015).

    Article  ADS  Google Scholar 

  6. De Pontieu, B., McIntosh, S., Martinez-Sykora, J., Peter, H. & Pereira, T. M. D. Why is non-thermal line broadening of spectral lines in the lower transition region of the Sun independent of spatial resolution? Astrophys. J. Lett. 799, L12 (2015).

    Article  ADS  Google Scholar 

  7. Warren, H. P., Brooks, D. H. & Winebarger, A. R. Constraints on the heating of high-temperature active region loops: observations from Hinode and the Solar Dynamics Observatory. Astrophys. J. 734, 90 (2011).

    Article  ADS  Google Scholar 

  8. Winebarger, A. R. et al. Defining the ‘blind spot’ of Hinode EIS and XRT temperature measurements. Astrophys. J. Lett. 746, L17 (2012).

    Article  ADS  Google Scholar 

  9. 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).

    Article  ADS  Google Scholar 

  10. Cirtain, J. W. et al. Energy release in the solar corona from spatially resolved magnetic braids. Nature 493, 501–503 (2013).

    Article  ADS  Google Scholar 

  11. Bradshaw, S. J. & Testa, P. Quantifying the influence of key physical processes on the formation of emission lines observed by IRIS. I. Non-equilibrium ionization and density-dependent rates. Astrophys. J. 872, 123 (2019).

    Article  ADS  Google Scholar 

  12. Lemen, J. R. et al. in The Solar Dynamics Observatory (eds Chamberlin, P. et al.) 17–40 (Springer, 2011).

  13. Viall, N. M. & Klimchuk, J. A. Evidence for widespread cooling in an active region observed with the SDO atmospheric imaging assembly. Astrophys. J. 753, 35 (2012).

    Article  ADS  Google Scholar 

  14. Lionello, R., Alexander, C. E., Winebarger, A. R., Linker, J. A. & Mikić, Z. Can large time delays observed in light curves of coronal loops be explained in impulsive heating? Astrophys. J. 818, 129 (2016).

    Article  ADS  Google Scholar 

  15. Viall, N. M. & Klimchuk, J. A. The transition region response to a coronal nanoflare: forward modeling and observations in SDO/AIA. Astrophys. J. 799, 58 (2015).

    Article  ADS  Google Scholar 

  16. Bradshaw, S. J., Klimchuk, J. A. & Reep, J. W. Diagnosing the time-dependence of active region core heating from the emission measure. I. Low-frequency nanoflares. Astrophys. J. 758, 53 (2012).

    Article  ADS  Google Scholar 

  17. Barnes, W. T., Cargill, P. J. & Bradshaw, S. J. Inference of heating properties from ‘hot’ non-flaring plasmas in active region cores. I. Single nanoflares. Astrophys. J. 829, 31 (2016).

    Article  ADS  Google Scholar 

  18. Cargill, P. J. Active region emission measure distributions and implications for nanoflare heating. Astrophys. J. 784, 49 (2014).

    Article  ADS  Google Scholar 

  19. 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).

    Article  ADS  Google Scholar 

  20. Cargill, P. J. Some implications of the nanoflare concept. Astrophys. J. 422, 381–393 (1994).

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  22. Forme, F. R. E., Wahlund, J. E., Opgenoorth, H. J., Persson, M. A. L. & Mishin, E. V. Effects of current driven instabilities on the ion and electron temperatures in the topside ionosphere. J. Atmos. Terr. Phys. 55, 647–666 (1993).

    Article  ADS  Google Scholar 

  23. Satyanarayana, P., Chaturvedi, P. K., Keskinen, M. J., Huba, J. D. & Ossakow, S. L. Theory of the current‐driven ion cyclotron instability in the bottomside ionosphere. J. Geophys. Res. Space Phys. 90, 12209–12218 (1985).

    Article  ADS  Google Scholar 

  24. Marsch, E., Vocks, C. & Tu, C. Y. On ion-cyclotron-resonance heating of the corona and solar wind. Nonlin. Process. Geophys. 10, 101–112 (2003).

    Article  ADS  Google Scholar 

  25. Del Zanna, G., Dere, K. P., Young, P. R., Landi, E. & Mason, H. E. CHIANTI – An atomic database for emission lines. Version 8. Astron. Astrophys. 582, A56 (2015).

    Article  Google Scholar 

  26. Hannah, I. G. & Kontar, E. P. Differential emission measures from the regularized inversion of Hinode and SDO data. Astron. Astrophys. 539, A146 (2012).

    Article  ADS  Google Scholar 

  27. Metcalf, T. R., Jiao, L., McClymont, A. N., Canfield, R. C. & Uitenbroek, H. Is the solar chromospheric magnetic field force-free? Astrophys. J. 439, 474–481 (1995).

    Article  ADS  Google Scholar 

  28. Ashbourn, J. M. A. & Woods, L. C. Energy transport in the solar transition layer. Proc. Math. Phys. Eng. Sci. 457, 1873–1888 (2001).

    Article  Google Scholar 

  29. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

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Acknowledgements

This research was supported by the National Aeronautics and Space Administration (grant number NNX15AF97A). We acknowledge the contributions of the LMSAL/IRIS science team and the NASA/SDO including the AIA and HMI science teams for the observational data. IRIS is a NASA Small Explorer Mission developed and operated by LMSAL with mission operations executed at NASA Ames Research Center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre.

Author information

Authors and Affiliations

  1. Laboratory of Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA

    Shah Mohammad Bahauddin

  2. Department of Physics and Astronomy, Rice University, Houston, TX, USA

    Stephen J. Bradshaw

  3. Heliophysics and Planetary Science Office, NASA Marshall Space Flight Center, Huntsville, AL, USA

    Amy R. Winebarger

Authors
  1. Shah Mohammad Bahauddin
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  2. Stephen J. Bradshaw
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  3. Amy R. Winebarger
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Contributions

S.M.B. carried out the project as part of research towards the doctoral degree, including data analysis, interpretation and curation, formal analysis, and developing the necessary methodologies and validation schemes. S.J.B. co-conceived the project and is a co-investigator of the funding grant, undertook formal analysis, and supervised this work as part of S.M.B.’s doctoral thesis at Rice University. A.R.W. co-conceived the project and is the principal investigator of the funding grant. The original draft was written by S.M.B., with revisions by S.J.B. and A.R.W.

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Correspondence to Shah Mohammad Bahauddin.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Number density and Sulfur IV Doppler evolution of the candidate loop brightenings seen in the IRIS 1,400 Å channel.

a, b, (Left to Right) IRIS spectrograph slit positions co-spatial with the loop brightening examined at location 1 (a, top row) and location 3 (b, bottom row) of Fig. 1; spatiotemporal evolution of intensity, number density, and the single component S iv Doppler velocity is calculated at each slit position.

Extended Data Fig. 2 DEM-weighted temperature T for the candidate loops.

a, DEM-weighted temperature T for the candidate loops at location 1 (solid) and location 3 (dotted) shown in Fig. 1. b, T for narrower and hot temperature ranges: 107-107.5K and 106.5-107K. c, T for cooler temperature ranges: 106–106.5 K and 105.5–106 K. The onset times of impulsive heating events and their associated brightenings are shown by vertical arrows.

Extended Data Fig. 3 λ-t Diagram of candidate loops.

λ-t diagram of loop brightenings observed at location 1 (a) and location 3 (b) from Fig. 1. Left to right: temporal evolution of IRIS Si iv and Mg iv spectra where intensity is integrated along the x–axis of the IRIS image; temporal evolution of IRIS Si iv and Mg iv spectra where intensity is integrated along both the x and y axes of the IRIS image. In all cases, the integrated intensity is normalized by the pixel array.

Extended Data Fig. 4 Absence of self-absorption lines in candidate loop at location 1.

a, Single pixel array spectra (averaged over y axis) and b, spatially averaged (over x-y area) spectra of candidate loop at location 1 in search of self-absorption lines due to chromospheric reconnection-mediated line broadening. Vertical lines indicate possible self-absorption lines present in this channel such as S I 1,401.515 Å (green) and Fe II 1,401.774 Å, 1,403.101 Å, 1,403.255 Å (red).

Extended Data Fig. 5 Absence of self-absorption lines in candidate loop at location 3.

a, Single pixel array spectra (averaged over y axis) and b, spatially averaged (over x–y area) spectra of candidate loop at location 3 in search of possible self-absorption lines. S I 1,401.515 Å (green) and Fe II 1,401.774 Å, 1,403.101 Å, 1,403.255 Å (red) indicated by vertical lines.

Extended Data Fig. 6 Candidate loop brightenings observed in SDO/AIA channels.

Left to right: candidate loop brightenings at location 1 (top) and location 3 (bottom) discussed in Fig. 1 observed in 6 SDO/AIA channels: 131 Å, 171 Å, 193 Å, 211 Å, 335 Å and 94 Å, respectively.

Supplementary information

Supplementary Information

Supplementary Figs. 1–10, Discussion and Tables 1–3.

Supplementary Video 1

Temporal evolution of the loop at position 1 of Fig. 1.

Supplementary Video 2

Temporal evolution of the loop at position 3 of Fig. 1.

Supplementary Video 3

Existence of super-hot plasma (AIA 131 Å) at the site of reconnection-mediated brightening (IRIS 1,400 Å).

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Bahauddin, S.M., Bradshaw, S.J. & Winebarger, A.R. The origin of reconnection-mediated transient brightenings in the solar transition region. Nat Astron 5, 237–245 (2021). https://doi.org/10.1038/s41550-020-01263-2

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