The north-south coronal asymmetry with inferred magnetic quadrupole

Abstract

The quiet corona at times close to solar minimum shows a striking north-south asymmetry which suggests that neither dipole-like nor octupole-like fields are sufficient to describe the global coronal magnetic field. We believe that such phenomena reflect the asymmetry of the intrinsic magnetic field of the sun as a star; this weak field is usually obscured by active regions. Empirical models for spherical corona (at solar maximum) and for ellipsoidal corona (at solar minimum) have been established. We extend the existing classification to include an empirical model for the quiet solar corona with strong north-south asymmetry. We show examples of such asymmetric corona in the green line for three different solar minima and evolution of corona from almost ellipsoidal type to corona with strong north-south asymmetry “bald man with double beard”). A theoretical model (1984) of Osherovich et al. (1) relates such asymmetry to the existence of a significant quadrupole term in the global magnetic field of the sun. According to this model, the size of northern and southern polar coronal holes is affected differently by a quadrupole term which creates asymmetry in the magnetic and thermodynamic parameters as well as in the velocity of the outflow from the two polar regions.