Depth dependence of the periodicities in solar differential rotation

Abstract

Earlier, we showed existence of `20 yr' periodicity in the variations of the differential rotation coefficient B determined from Greenwich sunspot group data during 1879-1976 (Javaraiah and Gokhale, 1995). Recently, we analyzed data (1874-1939) of sunspot groups of life span up to 12 days and showed, (Javaraiah and Gokhale, 1997), that: (i) The magnetic structures of spot groups which live successively longer by 1 day are initially anchored successively deeper by ~21000 km. Structures which yield spot groups of life span 10-12 days are initially anchored near r ~0.7 Rodot. (ii) For a spot group which lives 10-12 days in latitudes 10o - 20o the `anchoring layer' of its magnetic structure also rises at a rate ~21000 km per day, as the spot group ages. In view of these results, we now study the variations in B by determining it separately for Greenwich data (1879-1939) samples defined as follows : (i) YLSG: young long-lived sunspot groups (t = 2, τ = 7-12, or t < 4, τ = 10-12), where τ and t represent life span and age of a spot group in days, (ii) SLSG: short-lived sunspot groups (tau = 2-4) and (iii) OLSG: old long-lived sunspot groups (t > 4, τ = 10-12). The Fourier power spectrum of B determined from YLSG shows existence of a prominent periodicity of ~21 yr. Spectra determined from the OLSG and from the SLSG show existence of a prominent periodicity of ~11 yr. Comparing the mean rotation frequencies of the YLSG, OLSG and SLSG with the radial dependence of solar rotation frequency determined from helioseismology, it is suggested that the periodicities ~21 yr and ~11 yr in B are dominant in the rotational perturbations near r ~0.73 Rodot and r > 0.95 Rodot respectively.