EFFECTS OF SOLAR SURFACE MAGNETIC FIELDS ON THE TIME-DISTANCE ANALYSIS OF SOLAR SUBSURFACE MERIDIONAL FLOWS

The solar meridional flow in the deep convection zone can be measured with time-distance analysis. The difference between northward and southward acoustic wave travel times relates to the speed of meridional flow in the solar convection zone. We study the effects of surface magnetic field on the measured travel time difference in time-distance analysis by comparing the results using data with and without removing surface magnetic regions. Two results are significantly different if the field strength threshold used to remove magnetic regions is small enough, such as 50 G. The difference represents the surface magnetic effects. This difference strongly correlates with the sunspot number. The range of travel distance in this study is 7 degrees-75 degrees, corresponding to a depth range of 0.54-0.96 R-circle dot. The difference depends on the travel time distance, and is greatest for the travel distance of 11 degrees-20 degrees. The study with different field strength thresholds indicates that a threshold of about 50 G can remove most surface magnetic effects. The measured surface magnetic effects can be explained by an effective downflow inside magnetic regions. These signals are not related to the large-scale meridional flow; they need to be considered in the measured travel time difference even when activity is low, if the travel time difference is used to infer meridional flow signals.