[Paper Review] A persistent quiet-Sun small-scale tornado III. Waves

Tziotziou, Kostas, Tsiropoula, Georgia+1 more|arXiv (Cornell University)|Oct 13, 2020

Solar and Space Plasma DynamicsPhysics and Astronomy21 citations

TL;DR

This study investigates wave dynamics in a persistent quiet-Sun vortex using high-resolution Hα and Ca ii 8542 Å time series from the SST/CRISP. It identifies upward-propagating fast kink waves (20–30 km s⁻¹) and localized torsional Alfvén waves, with evidence of a standing wave pattern likely from wave reflection at the transition region, suggesting vortex flows as conduits for chromospheric and coronal energy transport.

ABSTRACT

Vortex flows can foster a variety of wave modes. A recent oscillatory analysis of a persistent 1.7 h vortex flow with a significant substructure has suggested the existence of various types of waves within it. We investigate the nature and characteristics of waves within this quiet-Sun vortex flow to better understand its physics and dynamics. We used a cross-wavelet spectral analysis between pairs of Ha and Ca II 8542 intensity time series at different wavelengths and, hence, atmospheric heights, acquired with CRISP/SST, as well as the derived Ha Doppler velocity and full width at half maximum (FWHM) time series. We constructed halftone frequency-phase difference plots and investigated the existence and propagation characteristics of different wave modes. Our analysis suggests the existence of upwards propagating Alfvenic type waves with phase speeds of ~20-30 km/s. The dominant wave mode seems to be the fast kink wave mode; however, our analysis also suggests the existence of localised Alfvenic torsional waves related to the dynamics of individual chromospheric swirls that characterise the substructure of the vortex flow. The Ha V-I phase difference analysis seems to imply the existence of a standing wave pattern possibly arising from the interference of upwards propagating kink waves with downwards propagating ones that are reflected at the transition region or the corona. Moreover, the results provide further evidence that the central chromospheric swirl drives the dynamics of the vortex flow. This is the first exhaustive phase difference analysis within a vortex flow that explores the nature and dynamics of different wave modes within it. The questions, however, of whether, and how, the dissipation of the derived wave modes occurs and if vortex flows ultimately play a role in the energy budget of the upper layers of the solar atmosphere remain open.

Motivation & Objective

• To understand the nature and dynamics of wave modes within a persistent quiet-Sun vortex flow.
• To determine the propagation characteristics and phase relationships of waves across multiple atmospheric layers.
• To investigate whether wave modes in vortices contribute to energy transport and heating in the solar atmosphere.
• To examine the role of individual chromospheric swirls in driving wave activity within the vortex structure.

Proposed method

• Conducted cross-wavelet spectral analysis between Hα and Ca ii 8542 Å intensity time series at varying atmospheric heights.
• Utilized Doppler velocity and FWHM time series derived from CRISP observations at the Swedish Solar Telescope.
• Constructed half-tone frequency-phase difference plots to analyze wave propagation and interference patterns.
• Compared phase differences across different atmospheric layers to infer wave mode types and propagation direction.
• Applied oscillatory analysis techniques to identify wave modes and their coherence across the vortex structure.
• Evaluated the implications of phase differences for wave reflection and standing wave formation at the transition region or corona.

Experimental results

Research questions

• RQ1 What types of MHD wave modes are present within a persistent quiet-Sun vortex flow?
• RQ2 How do wave modes propagate through the chromosphere and into the upper solar atmosphere?
• RQ3 What is the origin of the observed standing wave pattern in the Hα minimum intensity region?
• RQ4 To what extent do individual chromospheric swirls contribute to localized torsional Alfvén wave activity?
• RQ5 How do wave reflections and interference patterns influence wave energy transport and dissipation?

Key findings

• Fast kink wave modes propagate upward through the chromosphere at phase speeds of approximately 20–30 km s⁻¹.
• Localized torsional Alfvénic waves are associated with individual chromospheric swirls, indicating localized energy transfer.
• A standing wave pattern is inferred from Hα V-I phase difference analysis, likely due to interference between upward and downward propagating kink waves.
• Wave reflection at the transition region or corona is suggested as a mechanism for the formation of the observed standing wave pattern.
• The central chromospheric swirl is identified as the primary driver of the vortex flow dynamics and associated wave activity.
• The results indicate that vortex structures may act as waveguides for Alfvénic and magneto-acoustic waves, potentially contributing to chromospheric and coronal heating.