Persistent longitudinal features in the low-latitude ionosphere

Various longitudinal wave patterns exist in the low-latitude ionosphere, but the investigation has been limited to the wave number (WN) 3 and 4 patterns. This study extends the investigation to the wave patterns that have not yet been explored. The persistent ionospheric wave patterns are investigated by using the measurements of the ion density during March 1999-June 2004 by the first Republic of China satellite. The investigation is performed with data sets in the magnetic south (20°S-0°) and magnetic north (0°-20°N). The dominant wave features in plasma density are the WN1, WN2, WN3, and WN4 patterns. Among them, the WN1 pattern in the magnetic south during the June solstice is the most pronounced feature. Except for this component, the WN3 pattern is the most persistent and pronounced feature. Fundamental difference exists between the WN1 and WN2 patterns and the WN3 and WN4 patterns. First, the wave phases in the south and north are the same in the WN3 and WN4 patterns, but they are opposite in the WN1 and WN2 patterns. Second, the amplitudes of the WN3 and WN4 patterns show similar annual variation in the south and north, but those of the WN1 and WN2 patterns show opposite annual trends in the opposite hemispheres. These observations indicate that the WN1 and WN2 patterns and the WN3 and WN4 patterns are created by different mechanisms. We discuss the creation of the WN1 and WN2 patterns in association with the geomagnetic field configuration.