WN4 Variability in DMSP Ion Densities Across Season, Solar Cycle, and Local Time

Non-migrating tides are a major coupling mechanism between the different regions of the atmosphere and ionosphere. The WN4 pattern in the ionosphere has been recognized as originating primarily with the DE3 non-migrating tide in the troposphere, and significant effort has been devoted in recent years to understanding how tidal effects manifest in various physical parameters across a very wide range of altitudes. While DE3 and WN4 signatures have been much studied in the mesosphere-lower thermosphere region, relatively little is known about how WN4 impacts the ionosphere above the F-peak. We present an analysis of WN4 in the topside ionosphere at low latitudes as measured by the Defense Meteorological Satellite Program (DMSP) spacecraft, using monthly averages of total ion densities binned by latitude and longitude. We found that WN4 is strongly present throughout the year, especially during equinox. In May-August, ion densities near 180-270 GLON are enhanced, and the WN4 pattern moves 5-10 degrees north; in November-February, ion densities are reduced in this region, and WN4 moves 5-10 degrees south. No solar cycle effects were found in dN/N, latitude or longitude position of the peaks, or relative intensity of the peaks. The WN4 pattern changes rapidly near dawn, but is very constant throughout the afternoon and evening in terms of dN/N and latitude position of the peaks.