Japan-US Multi-Instrument Study of Ionospheric Sporadic-E

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Intense echoes from field-aiegned irregularities are often observed by VHF radars in the mid-latitude ionospheric E-region. In the iregulaties we have found "quasi-periodic (QP)" structures that show intermittent appearance of echoes with periods of 5-10 minutes and with spatial wavelengths of 10-20 km. An observation campaign SEEK (Sporadic-E Exper iment over Kyushu) was conducted successfully for an extensive study of the QP structure conducted in Kyushu, Japan in August 1996. Two sounding rockets were launched from Uchinoura while QP structure was observed by the frequency agile radar (FAR) in Tane-gashima. The sounding rockets observed electric field fluctuations, electron density profiles and fluctuations. Neutral winds were also measured by a TMA (Trimethyl Aluminate) trail. Several ground-based observations were conducted concurrently with the rocket launching, including FAR,MU and Chung-Li/Taiwan VHF radars, MF radar, ionosondes, multicolor imager and Fabry Perot Doppler interferometer. From the TMA experiment, a very large vertical shear in the neutral wind was found near the Es layr. The in-situ electric field strengths exceeded 10 mV/m with large fluctuations at altitudes around 100-120 km. FAR received the most intense echoes around the altitude of the Eslayr, where fluctuations in the electric fields were the maximum. It seems that the irregularities were generated at steep density gradients on both sides of the Es layr.