Comparing Global Solar Rotation Results from MDI and GONG

The GONG (Global Oscillations Network Group) project and the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) instrument aboard the SOHO spacecraft have jointly accumulated more than five years of data on medium-degree solar p-modes, including nearly four years of contemporaneous observations. The inferences of interior solar rotation from the two projects are broadly consistent and show similar temporal variations, but there are also significant systematic differences. We report here on the results of an ongoing attempt to cross-compare the results and analysis techniques of the two projects. Three 108-day periods, at low, medium and high solar activity epochs, have been analysed, with both MDI and GONG analysis being applied to each data set, and the results are compared.