Ubiquitous torsional motions in type II spicules

TL;DR

This study reveals that type II spicules exhibit significant torsional motions alongside known flows and swaying, which are crucial for understanding their dynamics and role in solar atmospheric energy and helicity transport.

Contribution

It provides the first strong evidence that torsional motions are ubiquitous in type II spicules, highlighting their importance in solar physics.

Findings

Type II spicules have torsional motions of 25-30 km/s.

Torsional motions are likely Alfvenic waves propagating outward.

Large torsional motions may contribute to spicule formation and helicity transport.

Abstract

Spicules are long, thin, highly dynamic features that jut out ubiquitously from the solar limb. They dominate the interface between the chromosphere and corona and may provide significant mass and energy to the corona. We use high-quality observations with the Swedish 1-m Solar Telescope to establish that so-called type II spicules are characterized by the simultaneous action of three different types of motion: 1. field-aligned flows of order 50-100 km/s, 2. swaying motions of order 15-20 km/s, and 3. torsional motions of order 25-30 km/s. The first two modes have been studied in detail before, but not the torsional motions. Our analysis of many near-limb and off-limb spectra and narrow-band images yields strong evidence that most, if not all, type-II spicules undergo large torsional modulation and that these motions, like spicule swaying, represent Alfvenic waves propagating outward at several hundred km/s. The combined action of the different motions explains the similar morphology of spicule bushes in the outer red and blue wings of chromospheric lines, and needs to be taken into account when interpreting Doppler motions to derive estimates for field-aligned flows in spicules and determining the Alfvenic wave energy in the solar atmosphere. Our results also suggest that large torsional motion is an ingredient in the production of type-II spicules and that spicules play an important role in the transport of helicity through the solar atmosphere.