Solar rotation measurements at Mount Wilson

Abstract

This paper describes a thorough reevaluation of the procedures for reducing the data acquired at the Mt. Wilson Observatory synoptic program of solar observations at the 150-foot tower. We also describe a new program of acquiring as many scans per day as possible of the solar magnetic and velocity fields. We give a new fitting formula which removes the background velocity field from each scan. An important new feature of our reduction algorithm is our treatment of the limb shift which permits time variation in this function. We identify the difference between the limb shift along the north-south axis and the east-west axis as potentially being a result of meridional circulation. Our analysis interprets the time variation in the east-west limb shift as being the result of changes in a vertical component of the meridional circulation.The performance of the system improved in 1982 as a result of the installation of a new exit slit assembly. The amplitude of the limb shift variations found prior to 1982 is larger than is easily explained with simple ideas of meridional circulation. However, we have not been able to firmly identify instrumental causes for the variations although small changes in the band-pass of the exit slit assembly could have contributed.We have established a correlation between the observed stray light in the system and a component of the velocity field which is antisymmetric with respect to the solar central meridian. We remove this stray light effect by adding an additional term to the fitting function.Finally, we show that the inclusion of the above improvements allows us to study the torsional oscillations at high latitude using a procedure which can retain the longitude dependent information about the velocity pattern.

The differential rotation of G dwarfs

“…For instance, the solar equator rotates with a shorter period than the solar polar caps. The difference of 132 nHz between the rotation rates found by Ulrich et al (1988) corresponds to a difference of 0.07 rad/day between the respective angular velocities or a lapping time of 88 days. Helioseismology has found that this pattern persists throughout the whole solar convection zone but not in the radiative zone below (Thompson et al 2003).…”

Meridional motions and Reynolds stress from SDO/AIA coronal bright points data

“…Doppler measurements give similar results for rotation (Howard & Harvey 1970;Ulrich et al 1988;Snodgrass & Ulrich 1990), but analyses of meridional motions and torsional oscillation differ significantly between tracer and Doppler Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/587/A29 measurements. Analyses of tracer data show that meridional flow is going out of the centre of activity (Howard & Gilman 1986;, while Doppler measurements usually show poleward meridional flow for all latitudes (Duvall 1979;Hathaway 1996).…”

A New Component of Solar Dynamics: North-South Diverging Flows Migrating toward the Equator with an 11 Year Period

“…Torsional oscillations and meridional flows have been studied for over two decades (e.g., Duvall 1979;LaBonte & Howard 1982;Ulrich 1988). The meridional flow is generally agreed to have a poleward flow in both hemispheres with an amplitude of 10-20 m s Ϫ1 ; the torsional oscillations (also termed "zonal flows") consist of latitudinal bands of alternating faster and slower rotation that migrate toward the equator over the solar cycle and are superposed on top of the differential rotation.…”

“…However, Nesme-Ribes, Meunier, & Vince (1997) have studied meridional flows using sunspots as tracers and have identified a covariance of east-west and north-south motions consistent with angular momentum transport, which would sustain differential rotation. Time variations of the meridional flow were noticed by Ulrich (1988) and Hathaway (1996). Haber et al (2002) have studied meridional flows using ring-diagram analysis.…”