Motions in the Sun at the photospheric level. VI. Large-scale motions in the equatorial region

A series of line-of-sight velocity measurements, made on spectra of the Sun obtained at Oxford in 1953 May, is described. Spectra some `5 mm in height, taken with a solar image of 8 mm diameter, covered an arc of length 4 x Io km in the surface extending inwards from the limb. Measurement was made of two plates, taken an hour apart at the East equatorial limb, each carrying four exposures obtained in rapid succession. Each of the eight spectra was measured at 140 pomts mm apart, corresponding to an average spatial resolution of 4 X I0 km on the solar surface. The error to be attributed to a velocity measurement is carefully considered ; with the fine- grain plates used in the present investigation, a value t 009(0)10 02(8) km is found for the error of measurement of a single velocity. An investigation of the equatorial velocity field is based on the 1120 measurements of velocity. Measurements from the four exposures on a plate were combined to give the best values of the velocity, thereby reducing the uncertainty of the velocity at any point on the surface to 1 (5) 1 (4) km sec-1. The residual velocities remaining after the limb effect and the component of solar rotation have been removed show a dispersion of + o 6( ) km sec-1. This dispersion exceeds the error of the velocity at a point on the surface by some eight times the uncertainty of the latter, thus providing conclusive proof of the existence of a velocity field. An irregular distribution of the velocity residuals over the solar surface is found ; correlographic analysis gives 26 X I0 km as the average linear extent of a velocity fluctuation. The stability of the field is established from the similarity in the velocity distribution at the beginning and end of the period of observation. Finally, a suggestion is put forward for the possible origin of the field.