Spatial variation and evolution of heliospheric sector structure

Abstract

The magnetic sector polarity at the Sun and in interplanetary space has been surveyed during approximately three quarters of sunspot cycle 21 using photospheric magnetic field observations from the Wilcox Solar Observatory at Stanford University and spacecraft observations by Voyagers 1 and 2, Pioneer Venus Orbiter, and, to a lesser extent, Helios A, IMP 8, and ISEE 3. During the more than 8‐year period of the study, late 1977–1985, the Voyagers measured the interplanetary magnetic field (IMF) over a heliocentric distance range of 1–25 AU and a heliolatitude range of −5°–27°. The photospheric data were used together with a potential field (PF) model to calculate the location of the heliospheric current sheet near the Sun throughout the same period, which included sunspot maximum and much of the declining phase. The large‐scale magnetic polarity structure was found to be in almost continuous evolution throughout the period of study. This included an alternation between two‐sector and four‐sector patterns, with a mean duration in each case of approximately seven solar rotations. Although the azimuth of the IMF was found by the Voyager spacecraft to be oriented more than 60° from the theoretical Parker spiral angle 20% of the time, application of smoothing techniques produced IMF polarity patterns which could be compared with the pattern derived at the Sun. Such comparisons revealed very good agreement (82%) between solar and IMF sector patterns observed by Pioneer Venus in the inner heliosphere and significantly poorer agreement (61–64%) between solar and outer heliosphere patterns observed by Voyagers 1 and 2. This may be the result of dynamical processes in the solar wind which alter the position and shape of the current sheet beyond a few astronomical units. Sector pattern stability was found to decrease with increasing heliocentric distance, also. Evidence was found for the more frequent occurrence of four‐sector structure near the Sun than in the outer heliosphere. In 1985, when Voyager 1 reached a heliolatitude of 25°, it observed the disappearance of the sector structure, consistent with the tilt of the current sheet deduced from photospheric magnetic field measurements.