Abnormal Stokes V profiles observed by Hinode in a sunspot

Detection of hidden photospheric fine-structures

¹ Physics Department, Faculty of Science, Imam Khomeini International University, Qazvin 34149-16818, Islamic Republic of Iran
² Physics Department, Sharif University of Technology, Tehran P.O. 11365-9161, Islamic Republic of Iran

^⋆ Corresponding authors; vafa@sci.ikiu.ac.ir, reza.rezaei@sharif.edu

Received: 6 May 2024
Accepted: 26 September 2024

Abstract

Context. Hidden magnetic components in a sunspot are present as small-scale structures that are absent in low-resolution observations.

Aims. We search for traces of the hidden magnetic components in spectro-polarimetric observations of a mature sunspot close to the disk center recorded by Hinode.

Methods. To find extra humps in the far blue and red lobes of Stokes V, we examined the sign reversal in the second derivative of the profile in the umbra and penumbra. We also looked for the hump signature in the Stokes I and the linear polarization profiles.

Results. The amplitudes of the humps are small compared to the main component. More than half of the profiles show one extra hump, while 21% show an extra hump on both the blue and the red lobe of the 630.15 nm line with the same magnetic polarity as the sunspot. The location of the pixels where the extra hump is seen on both lobes has a pseudo-grainy structure in the single wavelength Stokes V magnetograms. This type of profile is better detected in darker parts of the penumbra, as well as in the umbra-penumbra border toward the umbra. The spectral distance between the two humps averaged over elliptical rings levels off in the umbra, decreases toward the penumbra, and levels off again there. We find no correlations between the wavelength positions of the two humps.

Conclusions. We discuss two scenarios that could potentially produce the simultaneously observed blue and red humps: one in which a single hidden magnetic component is responsible for the two humps, and another in which the two humps emanate from two hidden magnetic components.