Electron Density Structure in the Mid-Latitude Ionosphere observed from the Impedance Probe on-board the Recent Japanese Sounding Rockets

We will present observations of the ionospheric electron density by the impedance probe on-board two sounding rockets, S-520-23 and S-310-38. The S-520-23 sounding rocket was launched as a part of WIND campaign from Uchinoura Space Center (southern part of Japan) on 2nd September, 2007. The lithium gas was released three times to measure the neutral wind velocity. The lithium release disturbed frequency spectrums of the probe impedance and made it difficult to determine the electron density from the UHR frequency. We therefore attempted to estimate the electron density from the capacitance of an ion sheath surrounding the probe. It should be noted that the variation of the sheath capacitance indicated sharp enhancements of the electron density about 1-2 order of magnitude after the each lithium release. The result indicates that the photoionization of lithium caused a growth of the electron density. We also analyzed the impedance probe data on-board the S-310-38 sounding rocket. This rocket experiment was carried out on 6th February, 2008, in order to clarify the 3-dimensional structure of the E-region ionosphere. A height profile of the electron density along the rocket trajectory was successfully deduced from the UHR frequency in the altitude range of 90-157 km. The sporadic-E layer was observed both in up-leg and down-leg around the 100 km altitude. On the other hand, an unexpected depression of the electron density was found in the altitude range of 110-120 km. In this altitude range, the electron density was less than 1000 /cc, which was lower than the threshold of the observable density of the impedance probe. A neutral wind shear probably played an important role to produce such an unusual low density region.