Migration of Zonal Flows Detected Using Michelson Doppler Imager f-Mode Frequency Splittings

The small-scale zonal flows in the outer part of the solar convection zone were recently detected by Kosovichev & Schou using f-mode frequency splittings. Results from five 72 day time series (Schou et al.) show a slight drift of the pattern toward the equator in a manner similar to that seen in the so-called torsional oscillation observed using surface Doppler shift measurements. Here I describe results using 12 72 day time series of the Michelson Doppler Imager medium-l data. These results show a clear drift of the pattern toward the equator. Also, the near-surface rotation rate close to the solar poles is observed to be slower than expected from an extrapolation from lower latitudes and to be changing with time.