LINE ASYMMETRY AND EXCITATION MECHANISM OF SOLAR OSCILLATIONS

Abstract

In his opening address at the conference Dr. Tim Brown posed the line asymmetry problem between velocity and intensity as a puzzle in helioseismology that has been resisting theoretical explanation for many years. It was the observations of Duvall et al. (1993) that for the first time indicated that the power spectrum of solar acoustic modes show varying amounts of asymmetry. In particular, the velocity and intensity power spectra revealed an opposite sense of asymmetry. Many doubted the correctness of the experiment and thought it to be a puzzling result (Abrams & Kumar, 1996). Many authors have investigated this problem theoretically and have found that there is an inherent asymmetry whenever there is a localized source exciting the solar oscillations (Gabriel, 1995; Roxburgh & Vorontsov, 1995; Abrams & Kumar, 1996; Nigam et al. 1997). This problem has important implications in helioseismology where the eigenfrequencies are generally determined by assuming that the power spectrum was symmetric and can be fitted by a Lorentzian. This leads to systematic errors in the determination of frequencies and, thus, affects the results of inversions (Rhodes et al. 1997). In this paper we offer an explanation for the difference in parity of the two asymmetries and estimate the depth and type of the sources that are responsible for exciting the solar p-modes.