A persistent quiet-Sun small-scale tornado

Abstract

Context. Vortex flows have been extensively observed over a wide range of spatial and temporal scales in different spectral lines, and thus layers of the solar atmosphere, and have been widely found in numerical simulations. However, signatures of vortex flows have only recently been reported in the wings of the Hα, but never so far in the Hα line centre. Aims. We investigate the appearance, characteristics, substructure, and dynamics of a 1.7 h persistent vortex flow observed from the ground and from space in a quiet-Sun region in several lines/channels covering all atmospheric layers from the photosphere up to the low corona. Methods. We use high spatial and temporal resolution CRisp Imaging SpectroPolarimeter (CRISP) observations in several wavelengths along the Hα and Ca II 8542 Å line profiles, simultaneous Atmospheric Imaging Assembly (AIA) observations in several Ultraviolet (UV) and Extreme ultraviolet (EUV) channels and Helioseismic and Magnetic Imager (HMI) magnetograms to study a persistent vortex flow located at the south solar hemisphere. Doppler velocities were derived from the Hα line profiles. Our analysis involves visual inspection and comparison of all available simultaneous/near-simultaneous observations and detailed investigation of the vortex appearance, characteristics and dynamics using time slices along linear and circular slits. Results. The most important characteristic of the analysed clockwise rotating vortex flow is its long duration (at least 1.7 h) and its large radius (~3″). The vortex flow shows different behaviours in the different wavelengths along the Hα and Ca II 8542 Å profiles reflecting the different formation heights and mechanisms of the two lines. Ground-based observations combined with AIA observations reveal the existence of a funnel-like structure expanding with height, possibly rotating rigidly or quasi-rigidly. However, there is no clear evidence that the flow is magnetically driven as no associated magnetic bright points have been observed in the photosphere. Hα and Ca II 8542 Å observations also reveal significant substructure within the flow, manifested as several individual intermittent chromospheric swirls with typical sizes and durations. They also exhibit a wide range of morphological patterns, appearing as dark absorbing features, associated mostly with mean upwards velocities around 3 km s−1 and up to 8 km s−1, and occupying on average ~25% of the total vortex area. The radial expansion of the spiral flow occurs with a mean velocity of ~3 km s−1, while its dynamics can be related to the dynamics of a clockwise rigidly rotating logarithmic spiral with a swinging motion that is, however, highly perturbed by nearby flows associated with fibril-like structures. A first rough estimate of the rotational period of the vortex falls in the range of 200–300 s. Conclusions. The vortex flow resembles a small-scale tornado in contrast to previously reported short-lived swirls and in analogy to persistent giant tornadoes. It is unclear whether the observed substructure is indeed due to the physical presence of individual intermittent, recurring swirls or a manifestation of wave-related instabilities within a large vortex flow. Moreover, we cannot conclusively demonstrate that the long duration of the observed vortex is the result of a central swirl acting as an “engine” for the vortex flow, although there is significant supporting evidence inferred from its dynamics. It also cannot be excluded that this persistent vortex results from the combined action of several individual smaller swirls further assisted by nearby flows or that this is a new case in the literature of a hydrodynamically driven vortex flow.

Unveiling the magnetic nature of chromospheric vortices

“…The two structures rotate clockwise over the whole observational period, with lower rotation rates than those previously reported for chromospheric vortices (Wedemeyer-Böhm & Rouppe van der Voort 2009;Tziotziou et al 2018) but with a speed consistent with the rotation rates observed in photospheric vortices (Bonet et al 2010;Vargas Domínguez et al 2011). Near their centres the two structures rotate as a rigid body.s…”

“…Previous studies have treated solar vortices in isolation or at least composed of individual substructures that form and reappear in and around vortex flow (Tziotziou et al 2018). Opposite polarity flux sources associated with a vortex are often found close to each other (Wedemeyer-Böhm et al 2012;Shetye et al 2019;Liu et al 2019c).…”

“…Vortices of a few hundred kilometers in size have been identified across the solar atmosphere using modern telescopes supported by adaptive optics systems (e.g. Bonet et al 2010;Attie et al 2009;Vargas Domínguez et al 2011;Wedemeyer-Böhm et al 2012;Wedemeyer-Böhm & Rouppe van der Voort 2009;Tziotziou et al 2018). Quiet-Sun photospheric vortices have a typical size of 0.5−2 Mm and a mean lifetime up to 15 min (Bonet et al 2010;Wedemeyer et al 2013;Liu et al 2019a).…”

“…Photospheric vortex flows are usually correlated with network magnetic elements at supergranular vertices (Requerey et al 2018). In the quiet-Sun chromosphere, vortices have typical sizes of 1.5−4.4 Mm and a lifetime of about 10−15 min up to 1.7 h (Wedemeyer-Böhm & Rouppe van der Voort 2009;Tziotziou et al 2018). They are seen to rotate as quasi-rigid bodies (Wedemeyer-Böhm et al 2012;Tziotziou et al 2018).…”

Solar Vortex Tubes: Vortex Dynamics in the Solar Atmosphere

“…Vortical flows in the photosphere have been investigated using velocity fields derived from both observations (Bonet et al 2010;Attie et al 2009;Bonet et al 2008;Balmaceda et al 2010;Giagkiozis et al 2018;Requerey et al 2018;Tziotziou et al 2018Tziotziou et al , 2019Shetye et al 2019) and magnetohydrodynamics (MHD) simulations (see e.g. Kitiashvili et al 2012b;Moll et al 2012;Wedemeyer-Böhm et al 2012;Wedemeyer & Steiner 2014;Kato & Wedemeyer 2017).…”