First Detection of Kink Oscillations with Solar Orbiter

From RHESSI Wiki

Nugget
Number:	436
1st Author:	Sihui ZHONG et al.
2nd Author:
Published:	19 September 2022
Next Nugget:	TBD
Previous Nugget:	Energetic Neutral Hydrogen from Large Solar Flares
List all

Contents 1 Introduction 2 Simultaneous Detection of Decayless Kink Oscillations by EUI and AIA 3 Conclusions 4 Authorship 5 References

Introduction

Similar to a rubber band or a violin strinG, coronal plasma loops are essentially elastic, i.e., they respond periodically (or quasi-periodically) to an external impulsive perturbation. Among natural oscillatory modes of a loop are kink oscillations periodic transverse displacements of the loop axis. Kink oscillations are perhaps the most intensively studied phenomenon among all the coronal waves and oscillations (Ref. [1]]; they were first discovered with high-resolution EUV imaging from the TRACE spacecraft. Kink oscillations of coronal loops appear in both decaying and "decayless" regimes. Impulsively triggered by e.g., coronal eruptions (flares/CMEs), the decaying kink oscillations have large amplitude and decay very rapidly, in a few oscillation cycles. The other, decayless regime is characterised by a low amplitude, lack of systematic damping, and no association with impulsive energy releases. This type of oscillation has been discovered recently in AIA EUV imaging data, as persistent motions detected via time-series imaging. In both decaying and decayless regimes, the oscillation periods scale linearly with the lengths of the oscillating loop, indicating that kink oscillations are standing wave-modes. Both the regimes can be seen in different periods of time in the same loop. Decayless kink oscillations have been found to be ubiquitous in quiescent active regions, with characteristic periods of 1.2-11 min. The decayless nature suggests that there must be some continuous energy input into the loop plasma to compensate the energy losses. Several mechanisms for which have been proposed, such as massaging the loop footpoints by quasi-steady or random flows, Alfvénic vortex shedding, or insufficient resolution.

The confident detection of coronal wave phenomena creates a solid ground for estimating various important parameters of the oscillating plasma (which cannot be measured otherwise) by the method of magnetohydrodynamic (MHD) seismology. In particular, kink oscillations are widely applied to probe such coronal plasma parameters as the magnetic field strength and magnetic twist, atmospheric density stratification, and cross-field fine structuring. With decaying kink oscillations, seismological diagnostics can be only performed after the energy releases which excite them. In contrast, the omnipresence of decayless kink oscillations, which exist independently of flares, allows for the seismological plasma diagnostics almost at any time (Ref. [2]). One of promising applications is the estimation of the amount of free magnetic energy stored in coronal active regions prior to flares, which is crucial for the tasks of space weather forecasting. In addition, the decayless behaviour of a dissipative system attracts attention in the context of the enigmatic coronal heating problem.

For the last decade, decaying kink oscillations have been successfully studied using images taken by Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). However, the typical amplitude of decayless kink oscillation is smaller than one pixel of AIA images, restricting, together with limited time resolution, the range of detected oscillatory phenomena. The recent commissioning of the new spacecraft Solar Orbiter (SolO) with the Extreme Ultraviolet Imager (EUI) onboard is expected to become a game-changer in the field of coronal waves and oscillations in general, and for studies of kink oscillations in particular. Due to its close proximity to the Sun, EUI has higher spatial resolution than AIA (up to 100 km in the perihelion vs. 400 km) and higher temporal resolution (2-5 s vs. 12 s), which is very promising for studying short-period, low-amplitude decayless kink oscillations.

Do SolO data agree with the earlier observations?

Simultaneous Detection of Decayless Kink Oscillations by EUI and AIA

Decayless kink oscillations of an ensemble of loops are captured simultaneously by the High Resolution Imager (HRI) of EUI and the AIA from 22:58 UT on 5 November to 00:27 UT on 6 November 2021; see the video animation showing the oscillating loops (note that the amplitudes have been enhanced by a factor of three).. This is the first time that HRI/EUI has detected kink oscillations. During the observations, the vantage points of SolO and SDO were almost identical. The images taken by HRI have spatial resolution of 306 km/pixel and time cadence of 5 s, while that for AIA images is 435 km/pixel and 12 s, respectively.

During the processing, some artificial oscillations caused by the spacecraft jitter were identified in the HRI 174 &Acircle; image sequence and removed by a cross-correlation technique. Then, both data sets were co-aligned based on their common features and processed with the motion magnification technique of Ref. [3] to reveal the subtle transverse movements in the plane of the sky caused by the oscillations.

Situated in an active region near disk centre, the loop bundle of interest is around 51 Mm in length, among which several distinguishable threads are seen to host decayless kink oscillations (see the video animation). The time-distance maps of Figure 1 show that oscillations of the analysed threads last for several cycles, with displacement amplitudes of 2500 km and periods of 60-120 s, which consistent with previously detected decayless kink oscillations. After compensation to the light travel time difference from the Sun to each instrument (66 s), the cross-correlation coefficient between the oscillatory signals detected with HRI and AIA varies from 0.82 to 0.97, indicating that both instruments observe the same phenomenon.

Conclusions

This work demonstrates that SolO/HRI is capable of observing the same oscillatory phenomena as AIA, which opens up promising perspectives for the detection and studying coronal wave modes with shorter, previously unresolved, time scales with EUI. In addition, our results indicate the need for and robustness of the de-jittering procedure in the study of kink oscillations with HRI.

Authorship

Sihui ZHONG, Valery M. NAKARIAKOV, Dmitrii Y. KOLOTKOV (University of Warwick), Cis VERBEECK, and David BERGHMANS (Royal Observatory of Belgium)

References

[1] "Kink Oscillations of Coronal Loops"

[2] "Motion Magnification in Coronal Seismology"