LONG-TERM VARIATIONS OF THE TORSIONAL OSCILLATIONS OF THE SUN

Abstract

We investigated long-term variations of the differential rotation of the solar large-scale magnetic field on 1024 Hα charts in the latitude zones from +45° to -45° in the period 1915–1990. We used the expansion in terms of Walsh functions. It turns out that the rotation of the Sun becomes more rigid than average during the cycle maximum and the rotation is more differential during minimum. From 1915 to 1990, 7 bands of faster- and 7 bands of slower-than-average rotation are revealed showing an 11-year period. These bands drift towards the equator: 45° in 2.5 to 8 years. The time span of the bands varies from 4 to 6.8 years and is in anti-phase with long-term solar activity. The latitude span of the bands of torsional oscillations varies from 0.5 R _⊙ to 1.3 R _⊙ and shows a long-term variation of about 55 years. The poloidal component of velocity, V _θ varies from 2 ms ^-1 to 6 ms ^-1. The maximum rate of the equatorial drift occurs in the period between 1935 and 1955 and it develops prior to the highest maximum activity. At the modern epoch from 1965 to 1985, V _θ does not exceed 3 ms ^-1, but now it has a tendency to increase. The bands of slower-than-average rotation correspond to the evolution of the magnetic activity towards the equator in the butterfly diagram.