Ubiquitous hundred-Gauss magnetic fields in solar spicules

Abstract

<p><em>[eng] Aims. We aim to study the magnetic field in solar spicules using high-resolution spectropolarimetric observations in the Ca ii 8542Å</em></p><p><em>line obtained with the Swedish 1-m Solar Telescope.</em></p><p><em>Methods. The equations that result from the application of the weak field approximation (WFA) to the radiative transfer equations</em></p><p><em>were used to infer the line-of-sight (LOS) component of the magnetic field (BLOS). Two restrictive conditions were imposed on the</em></p><p><em>Stokes I and V profiles at each pixel before they could be used in a Bayesian inversion to compute its BLOS.</em></p><p><em>Results. The LOS magnetic field component was inferred in six data sets totalling 448 spectral scans in the Ca ii 8542Å line and</em></p><p><em>containing both active region and quiet Sun areas, with values of hundreds of Gauss being abundantly inferred. There seems to be no</em></p><p><em>di erence, from a statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region.</em></p><p><em>On the other hand, the BLOS distributions present smaller values on the disc than o -limb, a fact that can be explained by the e ect of</em></p><p><em>superposition on the chromosphere of on-disc structures. We show that on-disc pixels in which the BLOS is determined are possibly</em></p><p><em>associated with spicular structures because these pixels are co-spatial with the magnetic field concentrations at the network boundaries</em></p><p><em>and the sign of their BLOS agrees with that of the underlying photosphere. We find that spicules in the vicinity of a sunspot have a</em></p><p><em>magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the e ect of o -limb</em></p><p><em>overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is</em></p><p><em>equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler</em></p><p><em>velocities, and line intensities of these two structures leads in '50% of the cases to Stokes I and V parameters that are unsuitable to</em></p><p><em>be used with the WFA.</em></p><p><em>Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as</em></p><p><em>evidence against the existence of these magnetic field strengths in spicules. This fact possibly arises as the consequence of signal</em></p><p><em>superposition and noise in the data. We also suggest that the failure of previous works to infer the strong magnetic fields in spicules</em></p><p><em>detected here is their coarser spatial and/or temporal resolution.</em></p>