Small-scale magnetic field effects on individual spectral line radial velocities in the photosphere and chromosphere of the Sun

¹ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
² Institute for Solar Physics, Dept. of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 August 2025
Accepted: 9 November 2025

Abstract

Context. Advancements in extreme-precision radial velocity (RV) observations for detecting low-mass exoplanets show that different spectral lines show different behaviours in response to stellar activity. Though this can be dealt with experimentally, why this is the case has not been studied. The Sun is a good test case for testing hypotheses as we can study spatially resolved observations with high-resolution spectropolarimetry to understand spectral line behaviour.

Aims. We aim to investigate whether the difference of spectral line behaviour can be attributed to the height of atoms in the solar atmosphere. It is expected that photospheric spectral lines will act differently from their chromospheric counterparts in response to magnetic fields.

Methods. We used a unique dataset using the CRisp Imaging SpectroPolarimeter (CRISP) looking at three spectral lines, two in the photosphere and one in the chromosphere, and measured their spatially resolved radial velocities, their transversal and longitudinal magnetic fields, their magnetic field strengths, and their source functions. We correlated the magnetic field measurements against the radial velocities and compared them against the case in which we destroyed the spatial resolution to mimic a normal stellar observation with high-resolution spectra.

Results. We find that the unsigned magnetic field is strongly correlated to the RV for both the photospheric and chromospheric spectral lines in the case where the observation is spatially resolved. When the spatial resolution is destroyed, this correlation changes. We find that for the photospheric spectral lines there still exists a correlation to both components of the magnetic field, but the chromospheric spectral lines do not show any significant correlation.