SEEK-2 (Sporadic-E Experiment over Kyushu 2) – Project Outline, and Significance

SEEK-2 (Sporadic-E Experiment over Kyushu 2) is an observation campaign to study the spatial structure of the field-aligned irregularity (FAI) and sporadic-E(Es)-layer by means of two sounding rockets and a ground-based observation network with radars and optical instruments. The experiment was successfully conducted on 3 August 2002, with successive launches of two sounding rockets from the Uchinoura Space Center (USC) of the Japan Aerospace Exploration Agency (JAXA). The timing of the experiment was carefully selected, while intense quasi-periodic (QP) echoes were observed with two radars in Tanegashima. The main Es-layer, with its double-layered structure, was observed at altitudes of 103–105 km, the presence of which was well accounted for by the ion accumulation due to neutral-wind shear. Several minor peaks were detected in the electron density profiles at altitudes of up to 130 km. The intensity of the electric field was 5–10 mV/m and showed intense fluctuations below 110 km. Wave-like variation of the electric field was seen above 110 km. From radar experiments, we found that QP echoes appeared around 105 km, which agreed well with the main Es-layer height. The QP echoes propagated to the west-northwest, with frontal structures elongated from north-northeast to south-southwest. Radar observations conduced throughout the SEEK-2 period, on the other hand, showed that frontal structures of the QP echoes were most frequently propagated to the southeast. This result was consistent with the direction of gravity-wave propagation observed with the OH imager during the same period. The rocket beacon experiment with the Es-layers revealed the spatial structure of the plasma densities. On the basis of these results and those from SEEK-1 in 1996, we examined the structures of the nighttime mid-latitude E-region. We concluded that the QP echoes reflect the horizontal structures of the main Es-layers. The source of the structures was not clearly determined from the experiments, but the candidates are gravity waves and the Kelvin-Helmholtz instability. The azimuth-dependent Es-instability may have contributed to enhance structures of the QP echoes, although this instability may not be a major source of the QP structure in SEEK-2. Polarization electric fields were induced from the Es-layer with QP echoes, mapped upward along the geomagnetic field, and played an important role in determining the structures of the whole ionospheric E-region.