Observations of fan-spine topology by Solar Orbiter/EUI: Rotational motions and indications of Alfvén waves

Abstract

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. We present a torsional Alfvén wave detected in an inverted Y-shaped structure observed with the HRIEUV telescope of the EUI instrument on board Solar Orbiter in its 174 AA 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. We made use of the simultaneous observations from two other instruments on board Solar Orbiter. The first one is the Polarimetric and Helioseismic Imager, which is used to derive the magnetic configuration of the observed feature. The second is the Spectral Imaging of the Coronal Environment (SPICE) instrument, which 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. The difference movie constructed from the HRIEUV images shows clear signatures of propagating rotational motions in the spine. We measured propagation speeds of 136 km/s km/s which are consistent with the expected Alfvén speeds. 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 a 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. We conclude that the presented observations are compatible with an interpretation of either propagating torsional Alfvén waves in a low coronal structure or the untwisting of a flux rope. This is the first time we have seen signatures of propagating torsional motion in the corona as observed by the three instruments on board Solar Orbiter.