Oscillations and waves in a persistent quiet-Sun small-scale tornado
K.Tziotziou (1), G. Tsiropoula (1), I. Kontogiannis (2)
(1) IAASARS, NOA, Greece (2) AIP, Germany
Vortex flows can foster a variety of oscillations and wave modes. We investigate the oscillatory behaviour of a persistent 1.7-hour-long vortex flow with significant substructure, resembling a small-scale tornado, as well as the nature and propagation characteristics of waves within it. The observations were acquired with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope (SST) at several wavelengths along the Hα and Ca II 8542 A spectral line profiles. We apply a wavelet and cross-wavelet spectral analysis in intensity time series of both lines as well as Ha Doppler velocity and full width at half maximum (FWHM) time series. The vortex flow shows significant oscillatory power in the range of 3–5 min, peaking around 4 min with the derived oscillations reflecting the cumulative action of different components such as swaying motions, rotation, and waves. Our analysis based on frequency-phase difference halftone plots, suggests the existence of Alfvenic-type waves within the vortex flow, that propagate upwards with phase speeds of about 20-30 km/s. The dominant wave mode seems to be the fast kink wave mode. In addition, our analysis also suggests the existence of localized Alfvenic torsional waves, relating to the dynamics of individual chromospheric swirls that characterise the substructure of the vortex flow. The phase difference analysis seems also to imply the existence of a standing wave pattern, possibly arising from the interference of upwards propagating kink waves with downwards propagating ones, reflected at the transition region or the corona. Given the ubiquitous nature of such vortex structures on the solar surface, the dissipation of such waves can play an important role in the energy budget of the upper layers of the solar atmosphere.