Solar rotation measurements at Mount Wilson

We examine the background velocity fields of the Sun as observed at Mount Wilson. The method of velocity reduction of the full-disk Mount Wilson data is outlined. We describe a number of tests that have been carried out in order to find an instrumental origin for short-term rotation variations and a large-scale background line-shift - the ears. No instrumental cause can be found for this ear effect, although such a cause cannot yet be ruled out.Operated jointly by the Carnegie Institution of Washington and the California Institute of Technology.     

Solar rotation measurements at Mount Wilson

Possible sources of systematic error in solar Doppler rotational velocities are examined. Scattered light is shown to affect the Mount Wilson solar rotation results, but this effect is not enough to bring the spectroscopic results in coincidence with the sunspot rotation. Interference fringes at the spectrograph focus at Mount Wilson have in two intervals affected the rotation results. It has been possible to correlate this error with temperature and thus correct for it. A misalignment between the entrance and exit slits is a possible source of error, but for the Mount Wilson slit configuration the amplitude of this effect is negligibly small. Rapid scanning of the solar image also produces no measurable effect.     

Solar rotation measurements at Mount Wilson

The ears velocity pattern described in Paper I (Howard et al., 1980) had no physical explanation. A reanalysis shows that the large scale solar velocity patterns are better described by a nonmonotonic limbshift and a meridional flow. The results of the new analysis imply that the study of solar velocity pattern at the level of a few ms^–1 required that magnetic regions be treated separately from nonmagnetic regions.Now at: Institute for Astronomy, University of Hawaii, Honolulu, Hawaii 96822, U.S.A.     

Solar rotation measurements at Mount Wilson

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.     

SID flare production is independent of Mt. Wilson magnetic class

We have analyzed the observed frequency distributions of SID flare days for the different sunspot magnetic classes, taking flare days as well as no flare days into account. We find that there is no significant difference between any observed distribution for a specific magnetic class, and the Poisson random theoretical distribution, computed for that class.St. Augustine, Trinidad, West Indies.Cave Hill, St. Michael, Barbados, West Indies.     

Solar rotation results at Mount Wilson

We publish here rotation results from Doppler velocity measurements made at Mount Wilson over a period of more than 14 years. Altogether data from 188 rotations are presented. These results are displayed in various tables and figures. Measurements of scattered light along with its effect on the measured rotation rate are shown.     

SID flare production and Mt. Wilson magnetic class: An alternative interpretation

The statistical results presented by Achong and Stahl (1984) may alternatively be interpreted as demonstrating a strong dependence of SID flare production on Mt. Wilson magnetic class of the parent sunspot group.     

Recalibration of Mount Wilson Doppler measurements

A new calibration of the spectrograph at the Mount Wilson 150-foot Tower Telescope demonstrates that all reported solar Doppler rates to date measured at 5250.2 with this instrument are too high by a factor of 0.55%.     

CCD-Doppler measurements of solar differential rotation

Using the AT1 CCD camera at the Echelle spectrograph of the GCT at Tenerife, solar Doppler rotation measurements in the photospheric lines Fe I 6301.5 Å and 6302.5 Å and in the chromospheric line Na-D₂ 5890.0 Å have been made. The line shifts measured at different heliographic latitudes around the limb were corrected for observer motion and converted into sidereal rotation rates. At the equator the observed chromospheric rotation rate is about 8 % larger than the photospheric rate, and the average observed Doppler rotation rate is not very much different from the mean rotation rates deduced from all published tracer works and all published Doppler works. Near the poles (where tracer methods rely on extrapolation) both the chromospheric and the photospheric rotation rate are slightly smaller than the all Doppler rate and are considerably smaller than the extrapolated all tracer rate. If all previous measurements of solar rotation are taken into account, a surface rotation law with lower error bounds than previously possible can be derived.     

Straylight correction to doppler rotation measurements

The data published by Pierce and LoPresto (1984) are corrected for straylight. This correction increases the observed equatorial rotation velocity from 1977 to 2004 m s ^–1. The correction has an uncertainty of approximately 10 m s^–1 because the accurate form of the straylight function is not available.     

Additional measurements of the high-latitude sunspot rotation rate

We report measurements of the sunspot rotation rate at high sunspot latitutdes for the years 1966–1968. Ten spots at ¦latitude¦ 28 deg were found in our Mees Solar Observatory H patrol records for this period that are suitable for such a study. On the average we find a sidereal rotation rate of 13.70 ± 0.07 deg day^-1 at 31.05 ± 0.01 deg. This result is essentially the same as that obtained by Tang (1980) for the succeeding solar cycle, and significantly larger than Newton and Nunn's (1951) results for the 1934–1944 cycle. Taken together, the full set of measurements in this latitude regime yield a rotation rate in excellent agreement with the result =14^°.37₇–2^°.77 sin², derived by Newton and Nunn from recurrent spots predominatly at lower latitudes throughout the six cycles from 1878–1944.Summer Research Assistant.     

Telluric water vapor contamination of the Mount Wilson solar Doppler measurements

It has been shown that the solar line 5250.2 (Fei) is weakly blended with a telluric line in the water vapor spectrum, and that magnetograms taken using this line are therefore inaccurate. We investigate the effects of this contamination on the Mount Wilson synoptic magnetograph data, which is based on 5250.2. Using spectrum scans taken at Kitt Peak, we model the contamination and develop a procedure that would correct for it, whenever the slant water vapor along the line of sight to the Sun is known. As this information is not available for the data collected thus far at Mount Wilson, we use the variation of determined quantities with airmass to obtain an average, or first-order, correction. Concentrating on the fitted coefficients for the solar rotation, the correction is found to be very slight, 0.5%, raising the value for the A coefficient, averaged over the period 3 December, 1985 to 22 July, 1990, from 2.8289 to 2.8422 rad s^-1, The correction also removes a slight annual variation that has become discernible in the data collected since 1986.Now at Oregon Heath Sciences University, Portland, OR, U.S.A.Now at Department of Astronomy, University of Minnesota, U.S.A.     

The standard systems of stellar rotation measurements (II)

The limb darkening effect on the measurement of stellar rotation is discussed in this paper. It is shown that this effect plays an important role in the measurement of V_e sin i. In the extreme case with the limb darkening coefficient of 1.0, it may cause a difference up to 17%. As a sequel to paper [1], this work presents the following new explanation for the systematic differences between Slettebak's new and old systems. The main causes of the systematic differences are: (1) The old system made an inadequate twice repeated correction for the limb darkening. (2) Owing to the historical reason, the old system used too large limb darkening coefficients.     

An Explanation of the Differences Between the Sunspot Area Scales of the Royal Greenwich and Mt. Wilson Observatories,and the SOON Program

Several studies have shown that the sunspot areas recorded by the Royal Greenwich Observatory (RGO) between 1874?–?1976 are about 40?–?50 % larger than those measured by the NOAA/USAF Solar Observing Optical Network (SOON) since 1966. We show here that while the two measurement sets provide consistent total areas for large spots, the impossibility of recording small spots as anything except dots in the SOON drawings leads to an underestimate of small spot areas. These are more accurately recorded by the RGO and other programs that use photographic or CCD images. The large number of such small spots is often overlooked. A similar explanation holds for the RGO umbral areas, which amount to 40 % more than those measured from Mt. Wilson data between 1923 and 1982. The neglected small spots have a much lower photometric contrast. Our explanation suggests, therefore, that the adjustment to spot irradiance blocking at the 1976 transition from RGO to SOON areas is smaller than the almost 50 % correction advocated by some recent, purely statistical, studies.     

The mean coronal magnetic field determined from HELIOS Faraday rotation measurements

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r ^–, where = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 ^–5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.     

The mean coronal magnetic field determined from HELIOS Faraday rotation measurements

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r ^?α, where α = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 ^?5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.     

Measurement and analysis of rotation in close binaries. I. Observations and results

A set of close binaries have been observed using the same instrumental configuration at Kitt Peak National Observatory, USA. The rotation velocities for 99 components in 75 binaries, derived by two methods, are given here, including 55 components for the first time. These high-accuracy velocities provide a reliable observational basis for investigating synchronism in binaries and the various synchronizing mechanisms.     

A comparison of simultaneous measurements of the polar magnetic fields made at Crimea and Mount Wilson

Measurements of the polar magnetic fields of the sun made in August 1968 with the Crimean and Mt Wilson magnetographs are compared. The agreement between the results obtained at the two observatories is rather satisfactory. The correlation coefficient between the Crimean and Mt Wilson values of the observed average field strength at different latitudes is 0.7 for the north and 0.5 for the south polar region. The earlier conclusion based on the Mt Wilson material that a polarity reversal of the field occurred at latitudes +70° and -55° in the north and south hemispheres (Stenflo, 1970) is confirmed by the Crimean data.     

C.T.R. Wilson and the cloud chamber

A brief history of C.T.R. Wilson’s invention of the cloud chamber is presented. Initially, the research programme concerned the origin of the atmospheric phenomena he had experienced as a temporary meteorological observer on Ben Nevis. As his understanding of the physical conditions under which the condensation of water droplets took place from supersaturated water vapour developed, he realised that the technique provided the first way of imaging the tracks of the charged particles released by X-rays, in radioactive decays and in nuclear interactions.