An experiment to measure the solar ℓ = 1 rotational frequency splitting

Abstract

To date, only integrated light experiments have attained the high signal-to-noise ratio and frequency resolution necessary to measure the rotational frequency splitting of low degree solar p-modes. These experiments, however, are limited by the finite mode linewidths coupled with the inability of non-imaging experiments to unambiguously separate prograde and retrograde modes. In particular, the separation of the prograde and retrograde mode frequencies of the very important ℓ = 1 spherical harmonic, dictates that the experiment have the capability to coarsely resolve the eastern from the western hemisphere of the solar disk. Initial attempts to attain the desired image resolution by masking the solar image at the focal plane of the telescope and chopping the two hemispheres on the detector have been unsuccessful due to the high velocity noise introduced by the solar rotation through image motions and guiding instabilities. In this paper we present the concept of what we call “spectroscopic masking,” which provides the ability to filter oscillation modes spectroscopically, and without the need to image the Sun. This results in an optical configuration which is insensitive to image motions and guiding errors while still providing adequate spatial resolution to separate prograde and retrograde ℓ = 1 modes. A conceptual study will be presented along with a test observing run showing the quality of the achievable data.