Zonal modes of Cosmic Microwave Background temperature maps

All-sky maps of the cosmic microwave background temperature fluctuations are usually represented by a spherical harmonic decomposition, involving modes labelled by their degree l and order m (where -l <= m <= +l). The zonal modes (i.e. those with m = 0) are of particular interest because they vary only with galactic latitude; any anomalous behaviour in them might therefore be an indication of erroneous foreground substraction. We perform a simple statistical analysis of the modes with low l for sky maps derived via different cleaning procedures from the Wilkinson Microwave Anisotropy Probe, and show that the zonal modes provide a useful diagnostic of possible systematics.