Cross-scale nature of decayless waves in the solar corona

(Solar Orbiter Nugget #66 by A. K. Shrivastav¹, V. Pant¹, D. Berghmans², R. Kumar³, A. N. Zhukov², T. Van Doorsselaere⁴, E. Petrova⁴, D. Banerjee⁵, D. Lim^3,4, and C. Verbeeck²)

1. Introduction

The long-standing problem of coronal heating focuses on probing sources that can continuously supply energy to sustain million-degree temperatures. Magnetohydrodynamic (MHD) waves play a crucial role because their dissipation in the solar corona can supply the necessary energy to compensate for radiative losses. Moreover, MHD waves offer an effective tool for diagnosing local physical conditions through coronal seismology. Elucidating the characteristics of MHD waves across various coronal regions and at different spatial scales is crucial in the context of coronal heating and its driving mechanisms. Decayless transverse kink waves are a type of MHD wave mode in coronal loops with no significant decay in the wave amplitude over multiple cycles. These are omnipresent in active regions [1] which makes them a viable candidate for coronal heating [2]. Additionally, they  provide an essential tool to diagnose coronal magnetic fields in the quiet active region corona [3] .

Although decayless waves in coronal loops spanning several hundred megameters have been extensively studied, their characteristics in small-scale loops (~ 5-50 Mm) remain relatively unexplored. In this study, we leverage high-resolution observations (100-200 km pixel scale) from the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter, to detect decayless waves in short coronal loops rooted in quiet Sun regions, coronal holes and active regions. The properties of these waves across different coronal environments highlight the cross-scale nature of decayless oscillations and suggest their potential roles in coronal heating.

2. Properties of decayless waves in quiet Sun and coronal holes

Figure 1:  Examples of a few selected loops in coronal holes and the quiet Sun region (Left panel). The red artificial slits are placed to detect the decayless waves. The oscillations are seen to persist for more than two cycles without any decay in their amplitudes (middle and right panels) .

The High Resolution Imager (HRIEUV) onboard Solar Orbiter captured observations of coronal regions (active regions, quiet Sun and coronal holes) with a cadence of 3 and 5 seconds which is ideal for analyzing dynamics in small-scale coronal loops. From these observations, we identified several decayless oscillations in short loops in different coronal regions (a few examples are given in Figure 1).

Due to the higher spatial resolution of HRIEUV compared to previous imaging instruments, the average loop lengths in our study are notably shorter (see Figure 2) than those reported in earlier statistical analyses [1,4]. The period distribution reveals the presence of both short and long period decayless oscillations in short loops rooted in different coronal regions (see Figure 2). To our knowledge, this is the first observation of decayless oscillations occurring in coronal holes. 

Figure 2: The distribution of the loop length, the period, the displacement amplitude, and the velocity amplitude for the detected decayless waves in different coronal regions.

3. Wave mode uncertainty and a second branch in Loop Length vs Period Relation
 

A significant correlation between loop length and period indicate the presence of standing modes in coronal loops [1,4]. Unlike previous findings for large-scale loops [1], we found no significant correlation (see Figure 3, left panel), which challenges the interpretation of these waves as standing modes. However, phase lags observed along the loops support standing wave behavior, leaving the wave mode uncertain but standing modes remain a possibility [A,B]. Combining our results with earlier studies reveals a secondary branch for short loops that deviates from the trend seen in longer loops [1,4]. This secondary branch, also observed in active regions, suggests similarities in decayless oscillations across different coronal environments. The presence of this branch is a novel finding that raises new questions about the nature and driving mechanisms of these waves.

Figure 3. The relationship between loop length and the periods of the detected oscillations obtained in this study (Left panel). The same relation after combining data from both small- and large-scale loops, revealing distinct behaviors across different loop sizes (Right panel).

4. Magnetic field estimates and role in coronal heating

Assuming that the observed oscillations can also be standing wave modes, coronal seismology provides an estimation of the kink speeds and associated magnetic field strengths (see Figure 4). This indicates the potential of decayless oscillations in coronal holes as a valuable diagnostic tool for probing magnetic field strengths in these regions [5]. The distribution of energy flux reveals that only a limited number of oscillations exhibit high energy flux to compensate for energy losses in different coronal regions (see Figure 4, last panel), however recent statistical studies with more number of oscillations suggest their significant contribution to coronal heating [6,7,8].

Figure 4. The panels show the distributions of kink speed, magnetic field strength, and energy flux derived from the properties of the detected decayless waves in quiet Sun and coronal holes.

5. Conclusions

Our study presents the detection of decayless waves across various coronal regions, offering important insights into the role of decayless kink waves in the solar atmosphere. Leveraging high-resolution observations from the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter, we examined decayless oscillations in short coronal loops in different coronal regions and compared them with previously well-studied long loops, thereby highlighting their cross-scale characteristics. The availability of such high-resolution data has been instrumental in identifying these wave signatures and underscores the critical importance of Solar Orbiter observations in probing small-scale structures and their dynamics. The persistent nature of decayless waves and their presence across various coronal environments raise important questions about their role in coronal heating. Moreover, our findings underscore the potential of decayless waves as valuable diagnostics for probing the magnetic field in coronal holes.

The publications related to these works can be found here,

(A). Statistical investigation of decayless oscillations in small-scale coronal loops observed by Solar Orbiter/EUI

(B). On the Existence of Long-period Decayless Oscillations in Short Active Region Loops

Affiliations

1. Aryabhatta Research Institute of Observational Sciences, Nainital, India

2. Solar-Terrestrial Centre of Excellence - SIDC, Royal Observatory of Belgium, Belgium

3. Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata, India

4. Centre for mathematical Plasma Astrophysics, Mathematics Department, KU Leuven, Belgium

5. Indian Institute of Space Science and Technology, Thiruvananthapuram, India, 

Acknowledgements

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 4000112292 and 4000134088); the Centre National d’Études 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). A.K.S was supported by funds of the Council of Scientific & Industrial Research (CSIR), India, under file no. 09/079(2872)/2021-EMR-I. V.P. is supported by SERB start-up research grant (File no. SRG/2022/001687). A. N. Z. thanks the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX Programme of the European Space Agency (ESA) under contract number 4000136424. T.V.D. was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 724326), the C1 grant TRACEspace of Internal Funds KU Leuven, and a Senior Research Project (G088021N) of the FWO Vlaanderen.

References 

[1] Anfinogentov, S. A., Nakariakov, V. M., & Nisticò, G. 2015, A&A, 583, A136

[2] Van Doorsselaere, T., Srivastava, A. K., Antolin, P., et al. 2020, Space Sci. Rev., 216, 140

[3] Anfinogentov, S. A., & Nakariakov, V. M. 2019, ApJ, 884, L40

[4] Zhong, S., Nakariakov, V. M., Kolotkov, D. Y., Verbeeck, C., & Berghmans, D. 2022, MNRAS, 516, 5989

[5] Yang, Z., Tian, H., Tomczyk, S., et al. 2020, Sci. Chin. E Technol. Sci., 63, 2357

[6] Petrova, E., Magyar, N., Van Doorsselaere, T., & Berghmans, D. 2023, ApJ, 946, 36

[7] Lim, D., Van Doorsselaere, T., Berghmans, D., et al. 2023, ApJL, 952, L15

[8] Lim, D., Van Doorsselaere, T., Berghmans, D., & Petrova, E. 2024, A&A, 689, A16