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

¹ National Solar Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA e-mail: dsalabert@nso.edu
² High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA
³ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁴ Faculty of Arts, Computing, Engineering and Sciences, Sheffield Hallam University, Sheffield S1 1WB, UK

Received: 19 September 2006
Accepted: 4 December 2006

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, , 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  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 . The other parameters show hints of some dependence in .

Conclusions.Our main conclusion is that the mean fractional solar-cycle change in the damping rates is approximately 50% smaller than was previously assumed. It had been common practice to use an average over all low- 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.