[Paper Review] A persistent quiet-Sun small-scale tornado. II. Oscillations

Tziotziou, K., Tsiropoula, G.+1 more|arXiv (Cornell University)|Mar 12, 2019

Meteorological Phenomena and Simulations19 citations

TL;DR

This study investigates oscillatory dynamics in a persistent 1.7-hour quiet-Sun vortex resembling a small-scale tornado using high-resolution CRISP observations across Hα and Ca ii 8542 Å lines. It reveals significant 3–5 minute oscillations peaking at 4 minutes, with swaying motions at 200–220 s and rotation periods increasing with height (270 s in Hα, 215 s in Ca ii 8542 Å), indicating non-rigid rotation and the presence of evanescent, magnetoacoustic, and Alfvén waves, suggesting a magnetically supported structure with a central swirl acting as a 'central engine'.

ABSTRACT

Recently, the characteristics, and dynamics of a persistent 1.7 h vortex flow, resembling a small-scale tornado, have been investigated with ground-base and space-based observations and for the first time in the Ha line centre. The vortex flow showed significant substructure in the form of several intermittent chromospheric swirls. We investigate the oscillatory behaviour of various physical parameters in the vortex area, with a 2D wavelet analysis performed within the vortex flow area and in a quiet-Sun region (for comparison), using the same high spatial and temporal resolution Ha and Ca II 8542 CRISP observations, as well as Doppler velocities and FWHM derived from the Ha line profiles. The vortex flow shows significant oscillatory power in the 3-5 min range that peaks around 4 min and behaves differently than the reference quiet-Sun region. Oscillations reflect the cumulative action of different components such as swaying motions, rotation, and waves. The derived swaying motion periods are in the range of 200-220 s, and the rotation periods are ~270 s for Ha and ~215 s for Ca II. Periods increase with atmospheric height and seem to decrease with radial distance from the vortex centre, suggesting a deviation from a rigid rotation. The behaviour of power within the vortex flow as a function of period and height implies the existence of evanescent waves and the excitation of different types of waves, such as magnetoacoustic (e.g. kink) or Alfven waves. The vortex flow seems to be dominated by two motions: a transverse (swaying) motion, and a rotational motion while oscillations point to the propagation of waves within it. Nearby fibril-like flows could play an important role in the rotational modulation of the vortex flow. Indirect evidence exists that the structure is magnetically supported while the central swirl seems to be acting as a "central engine" to the vortex flow.

Motivation & Objective

• . To investigate the oscillatory behavior of physical parameters within a persistent quiet-Sun vortex flow.
• . To determine how oscillation periods vary with height and radial distance from the vortex center.
• . To identify the nature of waves excited within the vortex, including magnetoacoustic and Alfvénic modes.
• . To assess whether the vortex is magnetically supported and whether a central swirl acts as a driver for the vortex motion.
• . To provide the first comprehensive spectral analysis of oscillations in such a long-lived, small-scale solar vortex.

Proposed method

• . Conducted two-dimensional wavelet analysis on high-spatial- and high-temporal-resolution CRISP observations of Hα and Ca ii 8542 Å line profiles.
• . Analyzed Doppler velocities and full-width at half-maximum (FWHM) derived from Hα line profiles to extract oscillatory signals.
• . Performed spectral analysis along radial and circular slices within the vortex and in a reference quiet-Sun region for comparison.
• . Used wavelet power spectra to identify dominant oscillation periods and their spatial-temporal evolution across atmospheric heights.
• . Compared oscillation power in the vortex with that in a quiet-Sun region to isolate vortex-specific dynamics.
• . Investigated radial and height-dependent variations in oscillation periods to infer rotational and swaying motions.

Experimental results

Research questions

• RQ1 . What are the dominant oscillation periods within the vortex flow, and how do they vary with atmospheric height and radial distance from the center?
• RQ2 . Are the observed oscillations consistent with swaying, rotational, or wave-like motions, and what are their characteristic periods?
• RQ3 . What types of waves—e.g., magnetoacoustic or Alfvén—are excited within the vortex, and what evidence supports their presence?
• RQ4 . Is the vortex structure magnetically supported, and is there evidence of a central driver or 'central engine'?
• RQ5 . How do the oscillation properties in the vortex differ from those in the surrounding quiet-Sun region?

Key findings

• . The vortex flow exhibits significant oscillatory power in the 3–5 minute range, peaking at 4 minutes, with notable power extending up to 10 minutes at all atmospheric heights.
• . Swaying motions have periods of 200–220 seconds, indicating transverse oscillations within the vortex structure.
• . Rotation periods are approximately 270 seconds in Hα and 215 seconds in Ca ii 8542 Å, increasing with atmospheric height.
• . Oscillation periods decrease with radial distance from the vortex center, indicating a deviation from rigid rotation and suggesting quasi-rigid or differential rotation.
• . Evanescent wave behavior is indicated by power variation with period and height, suggesting wave trapping or reflection within the vortex structure.
• . The central swirl, marked by enhanced power in Hα and Ca ii 8542 Å line centers, dominates the oscillation spectrum and likely acts as a 'central engine' driving the vortex flow.