The last decade has seen an impressive improvement in the quality and quantity of helioseismic data. While much of the progress has come from a new generation of instruments, such as GONG and MDI, data analysis has also played a major role. In this review I will start with a brief discussion of how the basic analysis of helioseismic data is done. I will then discuss some of the data analysis problems, their influence on our inferences about the Sun and speculate on what improvements may be expected in the near future. Finally I will show a selection of recent results.

Introduction

Until recently most research in helioseismology has used modes in the low (l ≤ 3) and medium (3 < l ≤ 200) degree (l) ranges. Here I will concentrate on the methods and problems in the study of medium-degree modes as well as show selected results. Most studies of modes of high degree (l > 200) have used entirely different analysis methods, such as time-distance analysis, which is discussed elsewhere in this volume (Kosovichev 2003). However, I will touch on some of the issues regarding the analysis of the high-degree modes by methods similar to those used for the medium-degree modes. The reader is also referred to Haber et al. (2002) for results from a technique known as ring diagrams which also uses high-degree modes.

I will start by providing some background material on solar oscillations in Section 17.2.