Photospheric and coronal magnetic fields in six magnetographs

III. Photospheric and coronal magnetic fields in 1974–2017

ReSoLVE Centre of Excellence, Space Climate research unit, University of Oulu, POB 3000, 90014 Oulu, Finland
e-mail: ilpo.virtanen@oulu.fi, kalevi.mursula@oulu.fi

Received: 17 April 2019
Accepted: 7 May 2019

Abstract

Context. Solar photospheric magnetic fields have been observed since the 1950s and calibrated digital data are available from the 1970s onwards. Synoptic maps of the photospheric magnetic field are widely used in solar research, especially in the modeling of the solar corona and solar wind, and in studies of space weather and space climate. Magnetic flux density of the solar corona is a key parameter for heliospheric physics.

Aims. The observed photospheric magnetic flux depends on the instrument and data processing used, which is a major problem for long-term studies. Here we scale the different observations of the photospheric field to the same absolute level and form a uniform record of coronal magnetic flux since the 1970s.

Methods. We use a recently suggested method of harmonic scaling, which scales any pair of synoptic observations of any resolution to the same level. After scaling, we use the Potential Field Source Surface (PFSS) model to calculate the scaled magnetic field at various altitudes from photosphere to coronal source surface.

Results. Harmonic scaling gives effective, latitudinally dependent scaling factors, which vary over the solar cycle. When scaling low-resolution data to high-resolution data, effective scaling factors are typically largest at low latitudes in the ascending phase of solar cycle and smallest for unipolar polar fields around solar minima. The harmonic scaling method used here allows for the observations of the different data sets to be scaled to the same level and the scaled unsigned coronal flux densities agree very well with each other. We also find that scaled coronal magnetic fields show a slightly different solar cycle variation from that of the nonscaled fields.