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Detection of long-lasting aurora-like radio emission above a sunspot

Abstract

Auroral radio emissions in planetary magnetospheres typically feature highly polarized, intense radio bursts, usually attributed to electron cyclotron maser emission from energetic electrons in the planetary polar region that features a converging magnetic field. Similar bursts have been observed in magnetically active low-mass stars and brown dwarfs, often prompting analogous interpretations. Here we report observations of long-lasting solar radio bursts with high brightness temperature, wide bandwidth and high circular polarization fraction akin to these auroral and exo-auroral radio emissions, albeit two to three orders of magnitude weaker than those on certain low-mass stars. Spatially, spectrally and temporally resolved analysis suggests that the source is located above a sunspot where a strong, converging magnetic field is present. The source morphology and frequency dispersion are consistent with electron cyclotron maser emission due to precipitating energetic electrons produced by recurring nearby flares. Our findings offer new insights into the origin of such intense solar radio bursts and may provide an alternative explanation for aurora-like radio emissions on other flare stars with large starspots.

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Fig. 1: Aurora-like radio source seen from a sunspot associated with solar flares.
Fig. 2: The relationship between X-ray and radio emissions.
Fig. 3: Synoptic radio spectra of the NOAA 12529 AR.
Fig. 4: 3D modelling of the NOAA 12529 AR and the associated ECM emission sites.
Fig. 5: Schematic sketch of the sunspot auroral radio emission.

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Data availability

The data that support the plots and other findings within this paper are available at https://data.nrao.edu/portal (VLA; Project ID: 16A-377), https://www.sws.bom.gov.au/World_Data_Centre (RSTN), https://solar.nro.nao.ac.jp/norp (NoRP), https://www.swpc.noaa.gov/products/goes-x-ray-flux (GOES) and http://jsoc.stanford.edu (SDO), or from the corresponding author upon reasonable request.

Code availability

The magnetic field extrapolation56 software packages are available through IDL SolarSoft at https://sohowww.nascom.nasa.gov/solarsoft. The regularized inversion code for differential emission measure (DEM) calculation63 is available at https://github.com/ianan/demreg. Public software packages used in this study include SunCASA https://github.com/suncasa/suncasa-src, CASA64https://casa.nrao.edu, SunPy65https://sunpy.org and Astropy66https://www.astropy.org.

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Acknowledgements

The VLA is operated by the National Radio Astronomy Observatory (NRAO), a facility of the National Science Foundation (NSF) operated under a cooperative agreement by Associated Universities, Inc. S.Y. and B.C. are supported by NASA HSOC and ECIP grants (80NSSC20K1283/SV0-09025 and 80NSSC21K0623) and NSF grant AGS-1654382 to the New Jersey Institute of Technology. R.S. acknowledges the support of the Swiss National Foundation, under grant 200021_175832. R.S. also acknowledges S. Krucker and A. Csillaghy, FHNW, for their support. D.E.G. acknowledges support from NASA HSR grant 80NSSC18K1128. We acknowledge A. Benz for the useful discussions.

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Authors and Affiliations

Authors

Contributions

S.Y. conceived the study, performed the data reduction, analysis, visualization, interpretation and manuscript preparation. B.C. led the VLA observing proposal (VLA/16A-377) and planned and conducted the VLA observations. T.S.B. and D.E.G. participated in the VLA proposal and the observing programmes. R.S. conducted an independent VLA data reduction and cross checked the results. B.C., R.S., T.S.B., S.M. and D.E.G. contributed to the theoretical interpretation. S.Y. and B.C. led the manuscript writing and all authors discussed the results and commented on the manuscript.

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Correspondence to Sijie Yu.

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Nature Astronomy thanks Harish Vedantham, Donald Melrose and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Dynamic spectra of the right circularly polarized radio emission from the sunspot, captured by VLA on April 9, 2016.

The time domain depicted in Fig. 2 is segmented into four equal-duration intervals, sequentially presented in panel a–d.

Extended Data Fig. 2 Radio and X-ray flux of the radio-burst-hosting active region (NOAA 12529 AR) observed by RSTN, NoRP, and GOES on 2016 April 9–10.

Top panel: The background subtracted solar radio flux at discrete frequencies monitored by RSTN, NoRP and VLA. The horizontal dotted lines in corresponding colours indicate equally spaced zero-lines for each frequency channel, set at 20 × 104 Jy (20 sfu) intervals. The error bars overlaid on the zero-flux-lines in corresponding colours show the 3σrms uncertainties of each frequency channel. The upward arrows indicate the most prominent examples of sunspot radio aurorae, while the downward arrows indicate the occurrence of flare events. The inset shows a blowup of the 1 GHz flux simultaneously observed by NoRP and VLA, denoted by the dashed box. No zero-line offset was applied for the inset. Bottom panel: Simultaneous time profiles of GOES 1.5–12 keV (black line) and 3–24 keV X-ray (grey line), with corresponding GOES X-ray flare classes indicated. The arrows are the same as the downward arrows in the top panel.

Extended Data Fig. 3 Flux profiles of radio-hosting active region (NOAA 12529 AR) over one solar rotation cycle.

a, SDO-HMI continuum image sequence showing the transiting sunspot at times marked by the vertical arrows in (b). b–e, Time history of the heliocentric angle θ of the sunspot, the radio, and X-ray emission of the Sun over one solar rotation. f, Blowup of the synoptic dynamic spectrum of Stokes I obtained by NoRP and RSTN in (c), showing the arcs (red dashed lines) of radio emission. g–h, Hierarchical substructures of the emission arcs shown in (f) obtained by VLA.

Extended Data Fig. 4 Photospheric images of NOAA 12529 AR.

a–b, Continuum intensity image at 6173 Å at the solar surface, obtained by the HMI on board SDO. c–d, Radial component of the photospheric magnetic field strength. In the right column, the coherent (L-band) and incoherent (S-band) radio sources are shown as yellow and purple, respectively. Contours for the two radio sources at 50%, 70%, and 90% are overlaid as black solid and blue dashed lines. The size of the synthesized beams for the L- and S-band images are displayed in the upper-left corner with the corresponding line styles. The L- and S-band radio images were made using VLA observation on 2016 April 9 at 22:08 and 21:53 UT, respectively. The flare loops and their footprints, as observed by SDO-AIA at 94 and 1600 Å, are shown in blue and red, respectively.

Extended Data Fig. 5 Physical characteristics in a 2D slice on the XZ plane of the 3D model of NOAA 12529 AR.

The location of the XZ plane is indicated by the semitransparent grey shade in Extended Data Fig. 4a. a, electron plasma frequency νpe. b, electron cyclotron frequency νce. The red contours from top to bottom show the frequencies of 0.125, 0.25, 0.5, 1.0, 2.0 GHz. % at below 1.5 GHz (0.245, 0.41, 0.61, 1.0, 1.415 GHz.) c, ratio of the electron plasma frequency to electron cyclotron frequency R = νpe/νce. The green contour lines show the ratio R of 0.25, 0.5, 1.0, 1.5, and 2.0, from bottom to top. The same contours are shown in panel (b). d, magnetic field scale height LB. e, magnetic field angle θ relative to the line-of-sight. f, gyro-resonance opacity τ of O mode emission at νGHz = 1.0 along the line-of-sight. The black arrow indicates the direction towards an Earth-based observer. The dotted, solid, and dashed lines indicate the direction with θ equals to 30°, 45°, and 60°. The corona that is transparent to the s = 3 gyro-resonance absorption layer is outlined as the tilted white shade. The green contour denotes the s = 2 ECM source that is inferred from the observed spatial distribution of the source locations of the coherent radio bursts. The yellow cross marks the approximate location of the solar flares.

Extended Data Fig. 6 Distribution of the frequency-dependent source centroid locations for seven individual 20-second time integrations.

Top panel: The frequency distribution of radio source locations for the example times (same as these used for Fig. 4a). The close-to-linear distribution of radio sources in space as a function of frequency is attributed to radio sources along the respective loop, which is outlined by a solid line. Bottom panel: the plane-of-the-sky projected distance d(ν) of radio source locations from the source centroid at 1 GHz along the direction of the flux tubes. The predicted d(ν) dependence along flux tubes of three orientations with viewing angle θ equals to 30°, 45°, and 60° as the dotted, solid, and dashed lines, respectively.

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Yu, S., Chen, B., Sharma, R. et al. Detection of long-lasting aurora-like radio emission above a sunspot. Nat Astron 8, 50–59 (2024). https://doi.org/10.1038/s41550-023-02122-6

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