The solar coronal sheet during the period of sunspot maximum

Abstract

It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.