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


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 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


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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



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.

Corresponding author

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).

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