Ubiquitous hundred-Gauss magnetic fields in solar spicules

TL;DR

This study uses high-resolution spectropolarimetric data and the Weak Field Approximation to measure magnetic fields in solar spicules, revealing widespread hundreds-Gauss fields with implications for solar magnetic topology.

Contribution

It provides the first large-scale statistical inference of magnetic field strengths in spicules using Bayesian inversion of spectropolarimetric data, including effects of off-limb structures.

Findings

Hundreds of Gauss magnetic fields inferred in spicules.

No significant difference between quiet Sun and active region spicules.

Magnetic polarity in spicules matches nearby sunspots.

Abstract

Aims. We use high-resolution spectropolarimetric observations in the Ca ii 8542 A line obtained with the SST to study the magnetic field in solar spicules. Methods. The equations that result from the application of the Weak Field Approximation (WFA) to the radiative transfer equations are used to infer the LOS component of the magnetic field (BLOS). Two restrictive conditions are imposed on the Stokes I and V profiles at each pixel before they can be used in a Bayesian inversion to compute its BLOS. Results. The LOS magnetic field component has been inferred in six data sets totalling 448 spectral scans in the Ca ii 8542 A line and containing both active region and quiet Sun areas, with values of hundreds of G being abundantly inferred. There seems to be no difference, from the statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region. On the other hand, the BLOS distributions present smaller values on the disk than off-limb, a fact that can be explained by the effect of superposition on the chromosphere of on-disk structures. We find that spicules in the vicinity of a sunspot have a magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the effect of off-limb overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler velocities and line intensities of these two structures leads in ~ 50% of the cases to Stokes I and V parameters unsuitable to be used with the WFA. Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as evidence against the existence of these magnetic field strengths in spicules.