Dependence of the meridional motions of sunspot groups on life spans and age of the groups, and on the phase of the solar cycle

Abstract

We have analyzed data on sunspot groups compiled during 1874-1981 and investigated the following: (i) dependence of the `initial' meridional motion (v_ini(τ)) of sunspot groups on the life span (τ) of the groups in the range 2-12 days, (ii) dependence of the meridional motion (v(t)) of sunspot groups of life spans 10-12 days on the age (t) of the spot groups, and (iii) variations in the mean meridional motion of spot groups of life span 2-12 days during the solar cycle. In each of the latitude intervals 0 deg-10 deg, 10 deg-20 deg and 20 deg-30 deg, the values of both v_ini(τ) and v(t) often differ significantly from zero. In the latitude interval 20 deg-30 deg, the forms of v_ini(τ) and v(t) are largely systematic and mutually similar in both the north and south hemispheres. The form of v(t) suggests existence of periodic variation in the solar meridional motion with period of 4 days and amplitude 10-20 m s^-1. Using the anchoring depths of magnetic structures for spot groups of different τ and t estimated earlier, (Javaraiah and Gokhale, 1997), we suggest that the forms of v_ini(τ) and v(t) may represent radial variation of meridional flow in the Sun's convection zone, rather than temporal variation of the flow. The meridional flows (v_e(t)) determined from the data during the last few days (i.e., age t: 10-12 days) of spot groups of life spans of 10-12 days are found to have magnitudes (10-20 m s^-1) and directions (poleward) similar to the those of the surface meridional plasma flows determined from the Dopplergrams and magnetograms. The mean meridional velocity of sunspot groups living 2-12 days seems to vary during the solar cycle. The velocity is not significantly different from zero during the rising phase of the cycle and there is a suggestion of equatorward motion (a few m s^-1 at lower latitudes and ~10 m s^-1 at higher latitudes) during the declining phase (last few years) of the cycle. The variation during the odd numbered cycles seems to anticorrelate with the variation during the even numbered cycles, suggesting existence of ~22-year periodicity in the solar meridional flow. The amplitude of the anticorrelation seems to be depending on latitude and the cycle phase. In the latitude interval 20 deg-30 deg the `surface plasma meridional motion', v_e(t), is found to be poleward during maximum years (v_e(t) ~20 m s^-1 at 4th year) and equatorward during ending years of the cycle (v_e(t) ~-17 m s^-1 at 10th year).