Solar Rotation Measurements at Mount Wilson over the Period 1990–1995

Abstract

One of the most fundamental properties of the Sun is its rate of rotation. Kinetic energy of large-scale circulation might interact with rotation and cause the surface rate to vary throughout the solar cycle. The solar wind carries off angular momentum from the Sun, and the coupling between the outer and inner parts of the heliosphere might produce effects that are evident in the photosphere and chromosphere. The quadrapole moment of Sun's gravitational potential depends on the rotation rate. The interaction between rotation, convection, and solar magnetic fields forms the solar dynamo which governs the solar cycle of activity. Although the rate of solar rotation has been measured for decades, several key questions remain: What is the rotation rate, and what is the uncertainty in this value? Does the rotation rate depend on the solar cycle? Can the gradient of rotation rate as a function of distance from the solar center be detected within the solar atmosphere? The synoptic program of solar observations carried out at the 150 foot tower of the Mount Wilson Observatory has long been a source of measurements which address these questions. Improvements in the facilities of this program over the past decade have led to a reduction in the errors of measurement which now permit a new examination of these questions. Key improvements were: 12/81?installation of a fiber-optic image reformattor to select the spectral sampling of the absorption lines; 2/86?multiple daily observations were begun; 12/87?the Cr II line at ?5237.3 was added to the regular program of observation; 9/90?the grating mount and alignment system were replaced; 7/91?the polarization analysis optics were placed in a new alignment box and index matching fluid was introduced around the KDP variable retarder; 11/95?antireflection optics were installed for the KDP end windows to reduce interference fringes. Each of these changes resulted in a noticeable reduction in the rms error in the measured rotation rate which is now at the level of 7 m s-1. Within this error we find that there is no solar cycle variation in the rotation rate and the rate is the same for both ?5250 and ?5237. We find that the synodic rotation rate is 2.84 ? 0.01 ?rad s-1. This value agrees well with most recent determinations.