Structure and evolution of the photospheric magnetic field in 2010–2017: comparison of SOLIS/VSM vector field and B_LOS potential field

¹ ReSoLVE Centre of Excellence, Space Climate research unit, University of Oulu, PO Box 3000 90014 Oulu, Finland
e-mail: ilpo.virtanen@oulu.fi, kalevi.mursula@oulu.fi
² National Solar Observatory, Boulder, CO 80303, USA
e-mail: apevtsov@nso.edu
³ Pulkovo Astronomical Observatory, Russian Academy of Sciences, Saint Petersburg 196140, Russia

Received: 17 December 2018
Accepted: 25 February 2019

Abstract

Context. The line-of-sight (LOS) component of the large-scale photospheric magnetic field has been observed since the 1950s, but the daily full-disk observations of the full vector magnetic field started only in 2010 using the SOLIS Vector Stokes Magnetograph (VSM) and the SDO helioseismic and magnetic imager (HMI). Traditionally, potential field extrapolations are based on the assumption that the magnetic field in the photosphere is approximately radial. The validity of this assumption has not been tested yet.

Aims. We investigate here the structure and evolution of the three components of the solar large-scale magnetic field in 2010–2017, covering the ascending to mid-declining phase of solar cycle 24, using SOLIS/VSM vector synoptic maps of the photospheric magnetic field.

Methods. We compare the observed VSM vector magnetic field to the potential vector field derived using the VSM LOS magnetic field observations as an input. The new vector field data allow us to derive the meridional inclination and the azimuth angle of the magnetic field and to investigate their solar cycle evolution and latitudinal profile of these quantities.

Results. SOLIS/VSM vector data show that the photospheric magnetic field is in general fairly non-radial. In the meridional plane the field is inclined toward the equator, reflecting the dipolar structure of the solar magnetic field. Rotationally averaged meridional inclination does not have significant solar cycle variation. While the vector radial component B_r and the potential radial component B^PFSS_r are fairly similar, the meridional and zonal components do not agree very well. We find that SOLIS/VSM vector observations are noisy at high latitudes and suffer from the vantage point effect more than LOS observations. This is due to different noise properties in the LOS and transverse components of the magnetic field, which needs to be addressed in future studies.