Collapsograms: measurement of low signal-to-noise-ratio solar p modes in spatially-resolved helioseismic data

Date and time
21 May 2008 - 00:00 Europe/London


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1.- I will give an overview of WeCAPP (Wendelstein Calar Alto Pixellensing Project), a microlensing survey towards the Andromeda galaxy. WeCAPP monitored the central region of M31 for nearly a decade and used a difference imaging approach to identify variable sources in the dataset. I will outline the methods used to detect and classify objects like pulsating variable stars, novae, and microlensing events, and will discuss the basic properties of the over 20000 identified variable sources. 2.- I present an adaptation of the rotation-corrected, m-averaged spectrum technique designed to observe low signal-to-noise, low-frequency solar p modes. The frequency shift of each of the 2l+1 m-spectra of a given (n,l) multiplet is chosen that yields the narrowest mode in the m-averaged spectrum. In the case of spatially-resolved helioseismic data (such as HMI, MDI, and GONG observations), m-averaged spectra appear to offer a powerful tool, since for a given multiplet (n,l), there exist 2l+1 individual-m spectra, so that even if the individual-m spectra have a very low signal-to-noise ratio (SNR), which would preclude finding modes, once the individual-m spectra are corrected for the rotation- and structure-induced shifts the m-averaged spectrum can result in a SNR >> 1. We apply the technique to GONG and MDI data and show that it allows us to measure modes with lower frequencies than those obtained with classic peak-fitting analysis of the individual-m spectra. We measure their central frequencies, splittings, asymmetries, lifetimes, and amplitudes. The low-frequency, low- and intermediate-angular degrees rendered accessible by this new method correspond to modes that are sensitive to the deep solar interior and to the radiative interior. Moreover, the low-frequency modes have deeper upper turning points, and are thus less sensitive to the turbulence and magnetic fields of the outer layers, as well as uncertainties in the nature of the external boundary condition. As a result of their longer lifetimes (narrower line widths) at the same SNR the determination of the frequencies of lower-frequency modes is more accurate, and the resulting inversions should be therefore more precise.