Rotational motions and signatures of the Alfvén waves in a fan-spine topology

(Solar Orbiter Nugget #51 by E. Petrova¹ , T. Van Doorsselaere¹, D. Berghmans², S. Parenti³, G. Valori⁴, and J. Plowman⁵)

1. Introduction

Alfvén waves are of significant interest in the coronal heating problem due to their ability to transport energy from the lower layers of the Sun’s atmosphere to the corona [1]. However, their detection remains challenging because they are essentially rotational perturbations in plasma velocity and azimuthal components of the magnetic field, and are incompressible, and do not produce significant structural displacements [2]. As a result, they have primarily been observed using spectrometers across different layers of the solar atmosphere [4-8], while imaging instruments have struggled to capture them directly [3].
Recent advancements in solar imaging technology have drastically improved spatial and temporal resolution, enabling the detection of previously unresolved phenomena. In this study, we present new observations of propagating rotational motions within a fan-spine magnetic topology—a configuration consisting of a dome and a spine, formed when a magnetic bipole emerges into a preexisting unipolar field [9]. Numerical simulations suggest that this topology favours magnetic reconnection, which in turn generates propagating Alfvén waves [10-13]. Here, we use multiple instruments onboard Solar Orbiter to detect an event that has been extensively modelled in numerical studies.

2. Observational signatures of Alfvén waves

The images were obtained using three different instruments onboard Solar Orbiter: EUI/HRI_EUV (174 Å channel, hereafter simply HRI_EUV) for imaging observations, PHI for magnetic field data, and SPICE for spectral data (see Figure 1). 

Figure 1. Overview of the instrument’s FoVs as viewed from the Solar Orbiter FSI in 174 Å. Overlaid on top is the HRIEUV FoV, shown by red rectangle, and the SPICE and PHI FoVs are shown by blue and green rectangles, respectively.

At the time, Solar Orbiter was near perihelion (at a distance of 0.32 AU from the Sun), and HRI_EUV achieved a resolution of 100 × 100 km² with a temporal cadence of 5 seconds, allowing the detection of propagating pulses of rotational motion. The structure exhibiting these torsional motions consists of two footpoints connected by short loops and a 30 Mm-long spine originating from one footpoint (see Figure 2). A clear signature of torsional motions propagating along the spine is visible in both the original and difference images (the relative video showing the event's dynamics can be found here).

Figure 2. Sequence of images taken by HRIEUV. The left panel shows the original images and the right panel show a running difference images. A full movie can be found together with the full publication here.

Analysis of the time sequence reveals a southward longitudinal motion propagating at a speed consistent with local estimates of the Alfvén speed, ranging from approximately 130 to 180 km/s. Additionally, a transverse motion is observed with speeds of 26–56 km/s, constituting about 30% of the local Alfvén speed, suggesting a possible nonlinear regime. Given that SPICE data is available—and the Doppler velocity signal is typically the most compelling evidence for torsional Alfvén waves—we have also analysed the SPICE observations.

3. Doppler Velocity Analysis and Forward Modeling

SPICE Doppler maps in the C III channel revealed strong signals in the spine region (see Figure 3), a characteristic signature of torsional Alfvén waves.

Figure 3. Intensity and Doppler maps obtained with the SPICE C III line for three time frames (14:02:59, 14:14:28, and 14:25:57). Panels a and b show maps for the first time frame, panels c and d are for the second, and panels e and f are for the last frame.

However there is an additional, non-physical source of signal in the Doppler maps—an artefact caused by the point spread function (PSF) of SPICE. The PSF is elongated in both the spectral and slit-aligned directions and tilted by approximately 15 degrees. This tilt in the spatial-spectral plane creates an illusionary dim feature adjacent to a bright feature, leading to artificial Doppler signals that follow intensity gradients. To accurately distinguish between physical and artificial signals, we performed forward-modelling analysis.

Using HRIEUV data as input, we recreated SPICE-like data to mimic the dataset. Two simulations were conducted: one incorporating only the PSF and another including both the PSF and imposed rotational motions modelled as the rigid body rotation. These simulations accounted for typical broadening sources, both thermal and non-thermal. The resulting intensity maps (see Figure 4(a)) closely resemble SPICE observations, particularly in the Ne VIII channel, which has a temperature response closest to the HRIEUV. 

Figure 4. Synthesized spectral rasters obtained using forward-modelling analysis. Panel a shows the synthesized intensity, while the panels b and c show the Doppler maps (mimicking the SPICE CIII line) with and without local velocities imposed on top of the feature.

Panel (b) of Figure 4 confirms that a signal is present even without imposed motion, indicating that it arises purely from the PSF. However, panel (c) most closely resembles the SPICE data, suggesting that while the signal is modulated by the PSF—introducing various non-uniformities—it also contains strong blueshifts that align with SPICE observations. This alignment suggests that the detected signal is physical, assuming the recovered PSF is accurate.
Thus, the forward modelling provides additional support for interpreting the observed phenomenon as torsional motion, as it demonstrates the presence of a Doppler signal in the region corresponding to the spine.
 

4. Conclusions

Our study presents observational evidence of torsional Alfvén waves propagating within a coronal fan-spine magnetic topology. These findings provide crucial insights into the role of Alfvén waves in the solar atmosphere.
By leveraging high-resolution imaging from EUI and Doppler measurements from SPICE, we were able to confirm the presence of rotational motions along the spine structure. The synergy between different Solar Orbiter instruments has been pivotal in detecting these wave signatures, emphasising the importance of multi-instrument observations for understanding the Sun's dynamic plasma environment.
The ability of Alfvén waves to transport energy over large distances raises important questions about their role in coronal heating. If these waves contribute to energy redistribution, they could help sustain the high temperatures observed in the solar corona. Additionally, our findings highlight the potential for Alfvén waves to serve as diagnostics of the coronal magnetic field.
 

The full publication of this work can be found here: Petrova et al., A&A, 687, A13 (2024)

Affiliations

(1) Centre for mathematical Plasma Astrophysics, Mathematics Department, KU Leuven, Celestijnenlaan 200B Bus 2400, 3001 Leuven, Belgium

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

(3) Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

(4) Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany

(5) Southwest Research Institute, Boulder, CO 80302, USA
 

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); 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). We are grateful to the ESA SOC and MOC teams for their support. The German contribution to SO/PHI is funded by the BMWi through DLR and by MPG central funds. The Spanish contribution is funded by AEI/MCIN/10.13039/501100011033/ and European Union “NextGenerationEU/PRTR” (RTI2018-096886-C5, PID2021- 125325OB-C5, PCI2022-135009-2, PCI2022-135029-2) and ERDF “A way of making Europe”; “Center of Excellence Severo Ochoa” awards to IAA- CSIC (SEV-2017-0709, CEX2021-001131-S); and a Ramón y Cajal fellowship awarded to DOS. The French contribution is funded by CNES. The development of SPICE has been funded by ESA member states and ESA. It was built and is operated by a multi-national consortium of research institutes supported by their respective funding agencies: STFCRAL (UKSA, hardwarelead), IAS (CNES, operationslead), GSFC (NASA), MPS (DLR), PMOD/WRC (SwissS- paceOffice), SwRI (NASA), UiO (NorwegianSpaceAgency). T.V.D. was supported by the C1 grant TRACEspace of Internal Funds KU Leuven. E.P. has benefited from the funding of the FWO Vlaanderen through a Senior Research Project (G088021N).

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