On the multi-threaded nature of solar spicules

Abstract

A dominant constituent in the dynamic chromosphere are spicules. Spicules at the limb appear as relatively small and dynamic jets that are observed to everywhere stick out. Many papers emphasize the important role spicules might play in the energy and mass balance of the chromosphere and corona. However, many aspects of spicules remain a mystery. In this Letter we shed more light on the multi-threaded nature of spicules and their torsional component. We use high spatial, spectral and temporal resolution observations from the Swedish 1-m Solar Telescope in the Hα spectral line. The data targets the limb and we extract spectra from spicules far out from the limb to reduce the line-of-sight superposition effect. We discover that many spicules display very asymmetric spectra with some even showing multiple peaks. To quantify this asymmetry we use a double Gaussian fitting procedure and find an average velocity difference between the single Gaussian components to be between 20-30 km s${}^{-1}$ for a sample of 57 spicules. We observe that spicules show significant sub-structure where one spicule consists of many 'threads'. We interpret the asymmetric spectra as line-of-sight superposition of threads in one spicule and therefore have a measure for a perpendicular flow inside spicules which will be important for future numerical model to reproduce. In addition we show examples of λ-x-slices perpendicular across spicules and find spectral tilts in individual threads providing further evidence for the complex dynamical nature of spicules.

Accepted by APJ Letters

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• IRIS Mg II Observations and Non-LTE Modeling of Off-limb Spicules in a Solar Polar Coronal Hole
• On Modeling the Kelvin--Helmholtz Instability in Solar Atmosphere
• Evidence of multithermal nature of spicular downflows. Impact on solar atmospheric heating
• ALMA Observations of Solar Spicules in a Polar Coronal Hole
• The group height of spicules links their acceleration and velocity