The sunspot cycle variations of the neutral line on the source surface

Abstract

We refine the earlier presentation method of the sunspot cycle variations of the neutral line on the source surface by introducing the view longitude. It is shown that the neutral line seen from the view longitude and the equivalent dipole (determined by Hoeksema, 1984) showed a trend of rotation throughout the sunspot cycle. However, the surface bounded by the neutral line is generally far from a circle. In fact, our combined presentation of both the equivalent dipole and the neutral line indicates graphically that such a simple description of the rotating (equivalent) dipole may be misleading. The concept of an inclined dipole with respect to the rotation axis may also be misleading.

A representation of the magnetic neutral line on the solar source surface in terms of the Sun's axial dipole at the center and two equatorial dipoles in the photosphere

“…As pointed out by Saito and Akasofu [1987] and Saito et al [1989], it may be speculated that there is some similarity between the solar situation described in the above sections and the Earth's situation. The source surface and the photosphere correspond to the Earth's surface and the core's surface, respectively.…”

“…Such a field polarity could be represented by a single dipole at the solar center lying horizontally and rotating with the Sun. However, as discussed by Saito et al [1989], and as will be elaborated somewhat in later sections, such an equivalent dipole representation may not be physically meaningful.…”

Dependence of the IMF sector structure on the solar dipole tilt angle

“…The persistence of the sector structure is consistent with an equatorial dipole, which suggests the solar dipole simply rotates through the equator to the opposite pole [ Saito and Akasofu , 1987]. Saito et al [1989] diagramed the geometry of the magnetic equator on the sun and the related heliospheric current sheet along with its variation over an entire solar cycle.…”

The source surface and photospheric magnetic field models

“…In a series of papers, Saito and Akasofu [ 1987] and Saito et al [1989] demonstrated that the basic features of the neutral line configuration on the source surface (a concentric sphere of 2.5 solar radii) and their variations during a sunspot cycle can be represented fairly well in terms of an axial main dipole at the center of the Sun and a few fictitious dipoleS on the photosphere. The location and magnitude of the fictitious dipoleS are not necessarily unique.…”