Abstract
The large-scale photospheric magnetic field is commonly thought to be mainly dipolar during sunspot minima, when magnetic fields of opposite polarity cover the solar poles. However, recent studies show that the octupole harmonics contribute comparably to the spatial power of the photospheric field at these times. Also, the even harmonics are non-zero, indicating that the Sun is hemispherically asymmetric with systematically stronger fields in the south during solar minima. We present here an analytical model of two eccentric axial dipoles of different strength, which is physically motivated by the dipole moments produced by decaying active regions. With only four parameters, this model closely reproduces the observed large-scale photospheric field and all significant coefficients of its spherical harmonics expansion, including the even harmonics responsible for the solar hemispheric asymmetry. This two-dipole model of the photospheric magnetic field also explains the southward shift of the heliospheric current sheet observed during recent solar minima.
Highlights
It is known since the early 20th century that sunspot activity is often different in the two solar hemispheres
In this paper we present a novel, analytical model of the large-scale photospheric magnetic field, which very accurately reproduces the observed distribution of photospheric magnetic fields during solar minima, and naturally explains the observed hemispheric asymmetry
An asymmetric photospheric field affects the largescale structure of the solar coronal magnetic field. We have investigated this by computing the potential-field source-surface (PFSS) model of the coronal field (Wang & Sheeley, 1992) between the solar surface and the coronal source surface at 2.5 solar radii, using the magnetic field of the two-dipole model as an inner boundary condition
Summary
It is known since the early 20th century that sunspot activity is often different in the two solar hemispheres. The whole large-scale pattern of magnetic fields on the solar surface (photosphere) has been found to be systematically north–south asymmetric (Zhao et al, 2005; Virtanen & Mursula, 2014; Petrie, 2015; Zieger et al, 2019) This is true for the solar polar fields, which are unipolar around solar minima. Several studies have verified that the southern polar fields were stronger than the northern fields around solar minima during the recent solar cycles (21–24) Neither the global structure of the photospheric field nor its hemispheric asymmetry is only of academic or theoretical interest They affect the distribution of solar eruptions, flares and coronal mass ejections, as well as the occurrence of high-speed solar wind streams.
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