R. S. Bogart et al.: Spatially-resolved analysis of the the upper convection-zone

1. Introduction

Localized plane-wave (``ring-diagram'') analysis of high-degree modes in data from the SOI/MDI instrument on SOHO permits us to determine spatial and temporal variations of the structure and dynamics of the upper convection zone, to a depth of about 0.025 solar radius (~15 Mm) below the photosphere. Because the analysis is efficient and robust, it can be applied to areas covering much of the visible disc, and the results can potentially be compared with those of more difficult phase-based procedures such as time-distance and acoustic imaging. The spatial resolution achieved for localized flows with full-disc data is at least 15 heliographic degrees (180 Mm), and the temporal resolution is of order 1 day. Data useful for such analysis cover at least two full Carrington rotations in each year since 1996 (see figure 1). (Additional data with three times the spatial resolution over a small portion of the disc are available for shorter durations at various times, but the results reported here are based exclusively on the full-disc data.) Analyses of the full-disc data from the earlier years have already revealed systematic patterns in the global meridional flow and flows associated with active regions during the early phase of the solar cycle. Here we report on variations and trends seen in the flows as the activity level of the cycle approaches maximum.