Evidence for large-scale subsurface convection in the Sun

Abstract

A helioseismic statistical waveform analysis of subsurface flow was performed on two 720-day time series of SOHO/Michelson Doppler Imager medium-l spherical-harmonic coefficients. The time series coincide with epochs of high and low solar activity. Time-dependent coupling-strength coefficients b^t_s(n,l) of modes of the same radial order n and degree l, but different azimuthal order m, were inferred from the waveform analysis. These coefficients are sensitive to flows and general aspherical structure. For odd values of s ≪ l, the coefficient b^t_s(n,l) measures an average over depth of the amplitude of one spherical-harmonic (s, t) component of the toroidal flow velocity field. The depth-dependent weighting function defining the average velocity is the fractional kinetic energy density in radius of modes of the (n, l) multiplet. A mean-square (n, l)-dependent flow velocity was inferred from the b-coefficients for s in the range 5 through 35 for each n and l in the respective ranges 1 through 5 and 120 through 149 for the epochs of high and low activity. A further averaging, over l, yielded a root-mean-square flow velocity as a function of n for each epoch, which increases from about 20 m s^-1 at n = 1 to 35 m s^-1 at n = 5. The inferred velocities are consistent with (though perhaps do not demand) a cellular pattern of flow extending over the vertical range of mode sensitivity, estimated to be about 4 per cent of the solar radius below the photosphere.