Helioseismic sensing of the solar cycle

Abstract

All quantities observed with helioseismic methods, such as frequencies, width, and amplitudes of acoustic waves, vary with the solar cycle. However, they relate to the dynamics and structure in different parts of the solar convection zone. The rotation rate varies with the solar cycle, showing the so-called torsional oscillation pattern, from the surface throughout most of the convection zone. Near the tachocline, the current observations do not show this solar-cycle variation, but there is some evidence that there is a 1.3-year variation of the rotation rate. The meridional flow, observed in the outer 2% of the solar radius, varies with the solar cycle showing flows converging toward the mean latitude of magnetic activity at depths less than about 10 Mm and flows diverging at greater depth. There is some evidence for a counter-cell in the northern hemisphere during epochs of high activity. Structure inversions show variations in asphericity near the surface where the sound speed varies with the distribution of surface activity. There are hints but no conclusive evidence that such variations exist in the convection zone. The damping of acoustic modes increases and the mode energy decreases with increasing activity. Their variation with time and latitude shows that even global modes sense the local distribution of the surface magnetic activity.