# Reconnection nanojets in the solar corona

### Subjects

A Publisher Correction to this article was published on 06 October 2020

## Abstract

The solar corona is shaped and mysteriously heated to millions of degrees by the Sun’s magnetic field. It has long been hypothesized that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. Misaligned magnetic field lines can break and reconnect, producing nanoflares in avalanche-like processes. However, no direct and unique observations of such nanoflares exist to date, and the lack of a smoking gun has cast doubt on the possibility of solving the coronal heating problem. From coordinated multi-band high-resolution observations, we report on the discovery of very fast and bursty nanojets, the telltale signature of reconnection-based nanoflares resulting in coronal heating. Using state-of-the-art numerical simulations, we demonstrate that the nanojet is a consequence of the slingshot effect from the magnetically tensed, curved magnetic field lines reconnecting at small angles. Nanojets are therefore the key signature of reconnection-based coronal heating in action.

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## Data availability

The IRIS and Hinode observations used in the instrument data figures are available at https://bit.ly/3gcI2Wt. Other data used in this Article are available from the corresponding author on reasonable request.

## Code availability

PLUTO is a modular Godunov-type code for solving mixed hyperbolic/parabolic systems of partial differential equations (conservation laws) targeting high Mach number flows in astrophysical fluid dynamics. Equations are discretized and solved on a structured mesh that can be either static or adaptive through the Adaptice Mesh Refinement (AMR) interface. PLUTO is distributed freely under the GNU general public license and can be downloaded at http://plutocode.ph.unito.it/.

## Change history

• ### 06 October 2020

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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## Acknowledgements

P.A. acknowledges STFC support from grant numbers ST/R004285/2 and ST/T000384/1 and support from the International Space Science Institute, Bern, Switzerland to the International Teams on ‘Implications for coronal heating and magnetic fields from coronal rain observations and modeling’ and ‘Observed Multi-Scale Variability of Coronal Loops as a Probe of Coronal Heating’. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 647214). P.T. was also supported by contracts 8100002705 and SP02H1701R from Lockheed-Martin to the Smithsonian Astrophysical Observatory (SAO), and NASA contract NNM07AB07C to the SAO. P.A. thanks I. De Moortel, R. Rutten and B. De Pontieu for valuable discussion and J. A. McLaughlin for the suggested name of the nanojet. 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 co-operation with ESA and NSC (Norway). 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. SDO is part of NASA’s Living With a Star Program. All data used in this work are publicly available through the websites of the respective solar missions. This work used the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (https://www.dirac.ac.uk). The DiRAC@Durham equipment was funded by BEIS capital funding via STFC capital grants ST/P002293/1 and ST/R002371/1, Durham University and STFC operations grant ST/R000832/1. The DiRAC component of CSD3 was funded by BEIS capital funding via STFC capital grants ST/P002307/1 and ST/R002452/1 and STFC operations grant ST/R00689X/1. DiRAC is part of the National e- Infrastructure.

## Author information

Authors

### Contributions

P.A. was responsible for the planning, coordination, image processing, analysis of the observations and writing of most of the manuscript. P.P. was responsible for the numerical simulations and wrote the analysis of the numerical results. P.T. was responsible for the differential emission measure analysis and wrote the corresponding section in the manuscript. A.P. and F.R. provided the numerical code and the initial condition for the numerical simulation. A.P., F.R. and P.T. also provided feedback on the analysis of the results and the writing of the manuscript.

### Corresponding author

Correspondence to Patrick Antolin.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Nanojet N4.

Snapshots of nanojet N4 and the time–distance diagram along the nanojet axis (see Fig. 2 for further details). Part of the process is captured by the IRIS slit at position 1 (marked in the figure). Note the rapid succession of 2 nanojets, whose slopes in the time–distance diagrams are seen in all channels. To spatially and temporally locate this nanojet within the global loop structure and during the loop expansion see Extended Data Figs. 5 and 6, respectively. See Supplementary Video 5.

### Extended Data Fig. 2 Nanojet N5.

Snapshots of nanojet N5 and the time–distance diagram along the nanojet axis (see Fig. 2 for further details). The IRIS slit at position 2 (marked in the figure) captures the strongest feature, with a Doppler peak at ≈ −120 km s−1 and a long velocity tail down to ≈ −10 km s−1. Positions 3 and 4 capture the subsequent evolution. Note that the large blueshift seen in the Si iv line is almost absent in the Mg ii line. To spatially and temporally locate this nanojet within the global loop structure and during the loop expansion see Extended Data Figs. 5 and 6, respectively. See Supplementary Video 6.

### Extended Data Fig. 3 EUV signatures of nanojets N1, N2 and N3.

ac, Time–distance maps in all AIA and IRIS channels (SOT stopped observing at this time) along cuts corresponding to the axis of nanojets N1 (a), N2 (b) and N3 (c) shown, respectively, in Figs. 2, 3 and 4. The temporal and spatial extent shown is the same as that of the time–distance diagrams shown in those figures. The N1 and N2 cuts trace part of the first clustered appearance of nanojets and the ejection of plasmoids. All cuts show an intensity enhancement that is similar across most EUV channels, indicating very fast and substantial heating.

### Extended Data Fig. 4 EUV signatures of nanojets N4 and N5.

a,b, Time–distance maps in all AIA and IRIS channels (SOT stopped observing at this time) along cuts corresponding to the axis of nanojets N4 (a) and N5 (b), shown, respectively, in Extended Data Figs. 1 and 2. The temporal and spatial extent shown is the same as that of the time–distance diagrams shown in those figures.

### Extended Data Fig. 5 Formation of strands.

ad, Five moments during the expansion period of the loop are selected that show the global evolution of the structure in the IRIS 1,400 Å channel (a) and AIA 171 Å channel (b), together with the corresponding running difference images in c and d. The cuts for the nanojets ‘N1’ to ‘N13’ are overlaid in groups, according to their time of occurrence. See Extended Data Fig. 6 for a more precise time location of these nanojets during the evolution. Note the formation of very fine and long coronal strands that originate from the nanojets.

### Extended Data Fig. 6 UV & EUV lightcurves.

a, A curved slab containing the loop of interest is selected (avoiding most bright structures along the LOS present in the hot channels), and the total over the slab of the intensity relative to a time prior to the loop expansion (t = 76 min) is shown for the IRIS channels and AIA 304 Å, 171 Å and 193 Å channels, normalised to their maxima over the last 22min of observation. The expansion of the loop starts from t ≈ 84 min. We mark with a small vertical line and corresponding nanojet number the location in time of the nanojets ‘N1’ to ‘N13’. b, Standard deviation of the total intensity calculated in a. The unit is DN, a representative of the number of photons (see Fig. 2 for details). Note that the values for AIA 171 and 193 have been divided by 6 in order to fit in the same axis. See the legend in the figure for the colour corresponding to each channel. The level of noise in each channel, as measured from the standard deviation over a quiet region off-limb in the IRIS 1,400, 2,796 and AIA 304, 171, 193 is, respectively, 1, 1.1, 1.4, 18.9 and 65.6 DN.

### Extended Data Fig. 7 Statistics of nanojets.

a, Total velocity distribution of IRIS slit pixels having a total intensity above 200 DN (equal to ≈ 1.4 times the total intensity average during the quiet, non-expansion period of the loop). The total velocity is defined as the sum in absolute value of all Doppler components and non-thermal velocities for each pixel. b, Total intensity distribution of IRIS slit pixels having a total velocity higher than 100 km s−1. The slit pixels corresponding to the nanojets are defined as those having both a total intensity higher than 200 DN and a total velocity larger than 100 km s−1 (both ranges are marked by a vertical dashed line in the top histograms). c, Distribution of the nanojet pixels along the slit and for a time interval focusing on the last 4 min of observation (time in which the slit captures the nanojets best). The time evolution is shown with different colours. The total number of nanojet pixels within this time interval is 436. Histograms for each quantity are shown at the bottom of each plot. Note that the nanojets are characterised by increases in intensity, Doppler velocity and non-thermal velocity that are very localised in time and space. A few of the nanojets are marked with arrows.

### Extended Data Fig. 8 Multi-wavelength view of the coronal structure with Hinode/SOT, IRIS and SDO/AIA.

a, From top left to bottom right we show in order of increasing temperature formation, the common FOV of the coordinated observation at the same time shown in Fig. 1. Chromospheric temperatures are represented by the SOT Ca ii H and IRIS 2,796 Å channels ($${\mathrm{log}}\,T\approx 4-4.2$$). Lower transition region temperatures are represented by IRIS 1,400 Å and AIA 304 Å ($${\mathrm{log}}\,T\approx 4.8-5$$). Upper transition region temperatures are represented by AIA 131 Å and 171 Å ($${\mathrm{log}}\,T\approx 5.7-5.9$$). Coronal temperatures are best observed by AIA 193 Å and 211 Å ($${\mathrm{log}}\,T\approx 6.2-6.3$$). A radial filter has been applied to the SOT, IRIS 2,796 Å and IRIS 1,400 Å images to decrease the disk intensity and to make the off-limb features more visible.b, Same as in a but at the last IRIS observation time (SOT has stopped observing at this time). Note the appearance of the coronal loop following the occurrence of nanojets. See Supplementary Video 7.

### Extended Data Fig. 9 Hot plasma emission measure in the loop.

A DEM analysis is performed (see Methods) for which the temperature range is binned in intervals of $${\mathrm{log}}\,T=0.2$$. The FOV is the same as in Extended Data Fig. 8. a, A time towards the end of the loop expansion is selected (t = 96.8min) and the emission measure of the region for each bin is shown. Note the presence of hot plasma along the loop at temperatures of $${\mathrm{log}}\,T=6.3-6.5$$ (and possibly at higher temperatures as well). See Supplementary Video 8. b, To see better the appearance of this hot component in the loop the difference between the same snapshot as in a and a snapshot 10 min before is shown. The bins are regrouped to temperature widths of $${\mathrm{log}}\,T=0.3$$ for better visualisation. Note the distinct appearance of plasma at $${\mathrm{log}}\,T=6.15-6.45$$ and possibly also at $${\mathrm{log}}\,T=6.45-6.75$$.

### Extended Data Fig. 10 Evolution of twist and braiding along the loop.

A slab containing the loop is selected and the slab is interpolated into a rectangle in order to eliminate the loop curvature and better observe the presence of twist (x and y scales are different in the image). Several times during the expansion of the loop are selected and the Mg ii k Doppler velocity (extrema among the multiple components at each pixel) measured from the IRIS slit is overlaid. The evolution goes from panels a to h. a shows the state of the loop just prior to the loop expansion. Note the presence of braiding. Particularly, 2 rain strands can be seen intersecting each other in the POS (white arrow in panel). Minutes later, b shows that one of the 2 intersecting strands has brightened and moved rapidly outward (and is blueshifted, contrary to the rest of the loop), while the other strand exhibits the clustered appearance of nanojets and plasmoid ejections. The following snapshots (c to h) indicate the rapid outward motion of the strands, local brightening events and the gradual reduction of twist and braiding along the loop. During this motion the strands at the lower part of the loop are on average redshifted while the upper part of the loop is blueshifted, suggesting an untwisting motion sketched in Supplementary Fig. 14. See Supplementary Video 9.

## Supplementary information

### Supplementary Information

Supplementary Figs. 1–23 and discussion.

### Supplementary Video 1

Video of Fig. 2: a nanojet cluster with plasmoid ejecta. Left six-set column, occurrence of a nanojet cluster and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1 and 2) in the figures. Please refer to Fig. 2 for details.

### Supplementary Video 2

Video of Fig. 3: a single nanojet. Left six-set column, occurrence of a single nanojet and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamond marked 1 in the figures. Please refer to Fig. 3 for details.

### Supplementary Video 3

Video of Fig. 4: spectral features of a nanojet and coronal loop formation. Left six-set column, formation of coronal strands and the running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (2 and 3) in the figures. Please refer to Fig. 4 for details.

### Supplementary Video 4

Video of Fig. 5: a nanojet in the numerical model. Magnetic field lines representative of the two loops (magenta and green) are displayed. Magnetic reconnection is localized around z = 0. The magnetic tension from the reconnected magnetic field lines produces a high-velocity (up to 200 km s−1), bidirectional jet collimated along the y axis. The width of the region with high velocities (>100 km s−1) along the x and z axes is less than 1 Mm and 3 Mm, respectively. Note also that the z velocities are only on the order of 20 km s−1.

### Supplementary Video 5

Video of Extended Data Fig. 1: nanojet N4. Left six-set column, occurrence of nanojet N4 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1 and 2) in the figures. Please refer to Fig. 2 and Extended Data Fig. 1 for details.

### Supplementary Video 6

Video of Extended Data Fig. 2: nanojet N5. Left six-set column, occurrence of nanojet N5 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1 and 2) in the figures. Please refer to Fig. 2 and Extended Data Fig. 2 for details.

### Supplementary Video 7

Video of Extended Data Fig. 8: multi-wavelength view of the coronal structure. From top left to bottom right in order of increasing temperature formation, we show the common FOV of the coordinated observation: Hinode/SOT Ca ii H, IRIS 2,796 Å (logT ≈ 4–4.2), IRIS 1,400 Å, AIA 304 Å (logT ≈ 4.8–5), AIA 131 Å and 171 Å (logT ≈ 5.7–5.9), AIA 193 Å and 211 Å (logT ≈ 6.2–6.3). Please refer to Extended Data Fig. 8 for details.

### Supplementary Video 8

Video of Extended Data Fig. 9: hot plasma emission measure in the loop. A DEM analysis is performed (see Methods) for which the temperature range is binned in intervals of logT = 0.2. The FOV is the same as in Extended Data Fig. 8. The emission measure of the region for each bin is shown. Note the presence of hot plasma along the loop at temperatures of logT = 6.3–6.5 (and possibly at higher temperatures as well). Please refer to Extended Data Fig. 9 for details.

### Supplementary Video 9

Video of Extended Data Fig. 10: evolution of twisting and braiding along the loop. A slab containing the loop is selected and the slab is interpolated into a rectangle to eliminate the loop curvature and better observe the presence of twist (x and y scales are different in the image). The Mg ii k Doppler velocity extrema measured from the IRIS slit is overlaid. The nanojets appear mostly vertical in this configuration. Note the reduction of twisting and braiding with time. Please refer to Extended Data Fig. 10 for details.

### Supplementary Video 10

Video of Supplementary Fig. 1: EUV absorption for density calculation. Left six-set column, from top to bottom the panels show a pair of a snapshot and its running difference version of a 34 arcsec × 34 arcsec portion at the apex of the loop in the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cuts A and B (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 1 for details.

### Supplementary Video 11

Video of Supplementary Fig. 2: nanojet N6. Left six-set column, occurrence of nanojet N6 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1 and 2) in the figures. Please refer to Supplementary Fig. 2 for details.

### Supplementary Video 12

Video of Supplementary Fig. 3: nanojet N7. Left six-set column, occurrence of nanojet N7 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1, 2 and 3) in the figures. Please refer to Supplementary Fig. 3 for details.

### Supplementary Video 13

Video of Supplementary Fig. 4: nanojet N8. Left six-set column, occurrence of nanojet N8 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1) in the figures. Please refer to Supplementary Fig. 4 for details.

### Supplementary Video 14

Video of Supplementary Fig. 5: nanojet N9. Left six-set column, occurrence of nanojet N9 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Middle three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Right two-set column, Mg ii k and Si iv 1,402.77 Å spectra at the yellow diamonds (1 and 2) in the figures. Please refer to Supplementary Fig. 5 for details.

### Supplementary Video 15

Video of Supplementary Fig. 6: nanojet N10. Left six-set column, occurrence of nanojet N10 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 6 for details.

### Supplementary Video 16

Video of Supplementary Fig. 7: nanojet N11. Left six-set column, occurrence of nanojet N11 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 7 for details.

### Supplementary Video 17

Video of Supplementary Fig. 8: nanojet N12. Left six-set column, occurrence of nanojet N12 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 8 for details.

### Supplementary Video 18

Video of Supplementary Fig. 9: nanojet N13. Left six-set column, occurrence of nanojet N13 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 9 for details.

### Supplementary Video 19

Video of Supplementary Fig. 16: nanojet N14 before loop expansion. Left six-set column, occurrence of nanojet N14 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels, respectively. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 16 for details.

### Supplementary Video 20

Video of Supplementary Fig. 17: nanojet N15 prior to loop expansion. Left six-set column, occurrence of nanojet N15 and its running difference version. From top to bottom: the IRIS 1,400 Å, AIA 304 Å and AIA 171 Å channels. Right three-set column, time–distance diagrams along cut ‘N’ (white dashed lines). The time of the snapshots is indicated by the white vertical dashed line, measured from the start of the IRIS observation. Please refer to Supplementary Fig. 17 for details.

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Antolin, P., Pagano, P., Testa, P. et al. Reconnection nanojets in the solar corona. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1199-8