In this article, we present an electron density profile retrieved from total electron density estimated from the difference in phase path excess between GPS frequencies L1 and L2 measured by the FORMOSAT-3/COSMIC satellite, in which the radio occultation inversion technique is employed for retrieval. Except for a regular F layer peak located at a height of about 290 km and a minor peak centered at a height of 140 km, a pronounced sporadic E layer was observed at a height of about 105 kin. This intense electron density layer with thickness of about 10 km has very sharp boundaries on the top and bottom sides with scale lengths of -22 and 13 km, respectively. At the time when COSMIC GPS radio occultation took place in the vicinity of Taiwan, the Chung-Li 30 MHz coherent radar detected strong backscatter from 5-meter plasma irregularities. The peak radar backscatter is situated at a height of about 110 km in the topside of the Es layer with a very steep electron density gradient. Interferometry measurement made by the four separate and independent receiving channels of the Chung-Li 30 MHz radar indicates that the configuration of the large scale plasma structure constituted by 5-meter scale field-aligned irregularities is patch-like, and a 2-minute oscillation in zonal displacement of the plasma structure was found. From the temporal displacement of the echo patterns from the plasma irregularities in the bottom side of the layer, the plasma structure in the bottom side of the Es layer was found to move westward at a trace velocity of about 6.2 in s(-1). The exceedingly small drift velocity combined with the relatively large scale length of the electron density gradient seem to suggest that the 5-meter plasma irregularities are very unlikely generated through the non-linear cascade process of the large plasma structure at kilometer scale induced by gradient drift instability. Moreover, in light of the fact that both the observed drift velocity (less than 15 in s(-1)) of the kilometer-meter scale plasma wave and the measured Doppler velocity (about 50 m s(-1)) of the 5-m plasma irregularities are much smaller than the 280 in s(-1) that is required to directly excite plasma waves through gradient drift instability, it suggests that the 5-m plasma irregularities observed by the Chung-Li 30 MHz radar are very unlikely the result of direct excitation through gradient drift instability.
