Polarisation of decayless kink oscillations of solar coronal loops

(Solar Orbiter nugget #15 by S. Zhong¹, V. M. Nakariakov¹, D. Y. Kolotkov^1,2, L.P. Chitta³, P. Antolin⁴, C. Verbeeck⁵ & D. Berghmans⁵ ​)

Introduction

The unequivocal and persistently growing interest in studying wave and oscillatory phenomena in the corona is motivated by their use for plasma diagnostics [1], and for their potential contribution to the enigmatic coronal heating [2]. Recently discovered low-amplitude decayless kink oscillations [3] are among the most promising candidates for both those applications, as they are evidently observed almost in every coronal active region, especially during their quiescent phases (i.e., periods excluding solar eruptions).

However, the mechanism by which energy is continuously supplied to compensate the rapid damping typical for kink oscillations, is uncertain. Several theories have been proposed. Among them is a self-oscillatory model that invokes interaction between loops and quasi-steady flows [4-6], and the other being a model driven by random flows [7-9]. The quasi-steady external flows required by the self-oscillatory model could be granulation or supergranulation flows. The alternative interpretation is an apparent brightness caused by the Kelvin-Helmholtz Instability (KHI) and resonant absorption [9]. These possible mechanisms can be discriminated by the polarisation of the oscillations. The self-oscillatory model leads to linear polarisation of kink oscillations [5-6]; while in the random-driver model, oscillations are randomly polarised [8-9]. Yet the detection of the polarisation of decayless kink oscillations is challenging, as it needs either a combination of high-resolution imaging and spectral data or high-resolution stereoscopic observations. The latter approach became possible with the Extreme Ultraviolet Imager (EUI; [11]) on Solar Orbiter (SoLO).

In this study, we present the first detection of the polarisation of decayless kink oscillations in coronal loops, and infer the signature to be a direct proxy of the energy supply to the corona. The detection is conducted with EUI and, simultaneously, with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), taking images of the oscillating loop from two lines of sight separated by 104 degrees (see Figure 1). The HRIEUV (High Resolution Imager in the 17.4 nm passband of EUI) images have high spatial resolution down to 123 km/pixel, which allowed us to resolve low-amplitude decayless oscillations and analyse their properties.

Figure 1. The location of the two spacecraft during the observations, and images of the regions of interest.

Observational signatures

As shown in Figure 2, we detected clearly the oscillatory transverse displacements of a loop in both the HRIEUV and AIA data. Fourier power spectra of the oscillatory signals from the two instruments share the same power peak at about 4 min, which agrees with the fundamental kink harmonic.

After correction for the light travel time difference, the oscillatory signals detected by HRIEUV and AIA, are found to be nearly in anti-phase (Figure 3a and b). The mutual hodograms of the signals are highly elliptical with a high eccentricity and negative inclination. The histogram of the distribution of phase trajectories in the minor axis is approximately Gaussian.

The determination of the oscillation polarisation is based on (1) the phase difference between the oscillations detected with HRIEUV and AIA, and (2) the localisation of phase trajectories along the minor axis of an elliptic hodogram. Forward modelling demonstrates these two features could be reconciled with oscillations exhibiting linear polarisation.

Figure 2. Decayless kink oscillations of the analysed loop bundle. The slits across the loop are used to make time—distance plots (panels b-d, f-h for HRIEUV and AIA, respectively). The AIA data set is processed with a motion magnification coefficient of 5.

Figure 3. Phase shifts between the kink oscillations detected with HRIEUV and AIA. The signals are averaged over 5 slits near the apex.

Forward Modelling

To determine the type and plane of the oscillation polarisation, we designed a forward model mimicking the observational manifestation of differently polarised kink oscillations (i.e., horizontal, vertical, oblique linear, circular, and elliptical with respect to the loop’s plane) of a 3D loop, in the PoS of HRIEUV and AIA.

The modelled signals from two LoS, and their phase difference and hodograms are shown in Figure 4. Only the models with horizontal and oblique linear polarisations can reproduce the observed anti-phase behaviour together with the localisation of most phase trajectories near the origin. The discrimination between the horizontal and oblique linear polarisations is based on the skewness of the distribution in the hodogram. But such a minor difference is hard to spot in observations due to noise. Therefore, we conclude that the detected kink oscillation is linearly polarised with the polarisation plane being highly horizontal.

Figure 4. Simulated oscillations with different types of polarisations seen in two LoS similar to that of the observations. Left: The correlation between oscillatory signals from two LoS. Right: The distribution of projected phase trajectories in the mutual hodograms of the oscillatory signals, where signals from the AIA LoS are displayed in x-axis and from the HRIEUV LoS in y-axis.

Conclusions

The identified linear polarisation of oscillations favours a self-oscillatory nature of the decayless kink oscillations, where the energy is taken from quasi-steady plasma flows. As demonstrated in ref. [12], solar atmospheric dynamics are characterised by coloured noises. According to our finding, the energy sustaining the decayless oscillations in the corona comes from the low-frequency part of the spectrum. Thus, the phenomenon of decayless kink oscillations of coronal loops is a smoking gun indicating the transfer of the low-frequency atmospheric motions to the corona, potentially heating the coronal plasma.

This study has been published in Zhong et al., Nat Commun 14, 5298, 2023. https://doi.org/10.1038/s41467-023-41029-8

Acknowledgements:

The following fundings are gratefully acknowledged: China Scholarship Council-University of Warwick joint scholarship (S.Z.), the EUI Guest Investigatorship (S.Z. and V.M.N.), the STFC consolidated grant ST/X000915/1 (D.Y.K), the Latvian Council of Science Project No. lzp2022/1-0017 (D.Y.K. and V.M.N.), the STFC Ernest Rutherford Fellowship No. ST/R004285/2 (P.A.), and the European Union funding (ERC, ORIGIN, 101039844) (L.P.C.). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000134088, 4000112292, 4000117262, and 4000134474), the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO).

Affiliations:

¹Centre for Fusion, Space and Astrophysics, Physics Department, University of Warwick, Coventry CV4 7AL, UK

²Engineering Research Institute “Ventspils International Radio Astronomy Centre (VIRAC)” of Ventspils, University of Applied Sciences, Inzenieru iela 101, Ventspils, LV-3601, Latvia

³Max Planck Institute for Solar System Research, D-37077 Göttingen, Germany

⁴Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle Upon Tyne, NE1 8ST, UK

⁵Solar-Terrestrial Centre of Excellence - SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, Brussels, 1180, Belgium

References

[1] Nakariakov & Kolotkov, 2020, ARAA, 58, 441

[2] Van Doorsselaere et al., 2020, SSRv 216, 140

[3] Nakariakov et al., 2021, SSRv, 217, 73

[4] Nakariakov et al., 2016, A&A, 591, L5

[5] Karampelas & Van Doorsselaere, 2020, ApJL, 897, L35

[6] Karampelas & Van Doorsselaere, 2021, ApJL, 908, L7

[7] Afanasyev et al., 2020, A&A, 633, L8

[8] Ruderman & Petrukhin, 2021, MNRAS, 501, 3017

[9] Ruderman et al., 2021, SoPh, 296, 124

[10] Antolin et al., 2016, ApJL, 830, L22

[11] Rochus et al., 2020, A&A, 642, A8

[12] Kolotkov et al., 2016, 592, A153