Petrova, E. ; Van Doorsselaere, T. ; Berghmans, D. ; Parenti, S. ; Valori, G. ; Plowman, J.

accepted to be published in AA (2024)

Abstract: Context. Torsional Alfvén waves do not produce any intensity variation and are, therefore, challenging to observe with imaging instruments. Previously, Alfvén wave observations were reported throughout all the layers of the solar atmosphere using spectral imaging. Aims. We present an observation of a torsional Alfvén wave detected in an inverted y-shape structure observed with the HRIEUV telescope of the EUI instrument onboard Solar Orbiter in its 174 Å channel. The feature consists of two footpoints connected through short loops and a spine with a length of 30 Mm originating from one of the footpoints. Methods. In the current work, we also make use of the simultaneous observations from two other instruments onboard Solar Orbiter. The first one is PHI (Polarimetric and Helioseismic Imager) that is used to derive the magnetic configuration of the observed feature. The second one is SPICE (Spectral Imaging of the Coronal Environment) that provided observations of intensity maps in different lines including Ne VIII and C III lines. We also address the issues of the SPICE point spread function and its influence on the Doppler maps via performed forward modeling analysis. Results. The difference movie shows clear signatures of propagating rotational motions in the spine. We measure propagation speeds of 136 km/s - 160 km/s which are consistent with the expected Alfvén speeds. The evidence of rotational motions in the transverse direction with velocities of 26 km/s - 60 km/s serves as an additional indication of torsional waves being present. Doppler maps obtained with SPICE show strong signal in the spine region. Under the assumption that the recovered point spread function is mostly correct, synthesized raster images confirm that this signal is predominantly physical. Conclusions. We conclude that the presented observations are compatible with an interpretation of either propagating torsional Alfvén waves in a low coronal structure or untwisting of a flux rope. This is the first time we see signatures of propagating torsional motion in corona as observed by the three instruments onboard Solar Orbiter.

Keyword(s): Solar Orbiter ; EUI ; Alfvèn waves
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