Sun-as-a-star observations: Evidence for degree dependence of changes in damping of low-ℓ p modes along the solar cycle

Abstract

Aims. We use 9.5-yr of BiSON Sun-as-a-star data to search for dependence of solar-cycle parameter changes on the angular degree, ℓ, of the data. The nature of the Sun-as-a-star observations is such that for changes measured at fixed frequency, or for changes averaged across the same range in frequency, any ℓ dependence present carries information on the latitudinal distribution of the agent (i.e., the activity) responsible for those changes. Methods. We split the 9.5-yr timeseries into contiguous 108-d pieces, and determine mean changes in the damping of, power in, and energy supplied to the modes through the solar cycle. We also apply a careful correction to account for the deleterious effects of the ground-based BiSON window function on the results. Results. From our full analysis we obtain a marginally significant result for the damping parameter, where the mean change is found to be weakest at ℓ = 0. The other parameters show hints of some dependence in ℓ. Conclusions. Our main conclusion is that the mean fractional solar-cycle change in the ℓ = 0 damping rates is approximately 50% smaller than was previously assumed. It had been common practice to use an average over all low-ℓ modes; our downward revision of the radial-mode value has implications for comparisons with models of the global solar cycle changes, which are usually based on a spherically symmetric geometry. © ESO 2007.