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Ann. Geophys., 23, 2295-2305, 2005
www.ann-geophys.net/23/2295/2005/
© European Geosciences Union 2005
SEEK-2 (Sporadic-E Experiment over Kyushu 2) − Project Outline, and Significance
M. Yamamoto1, S. Fukao1, R. T. Tsunoda2, R. Pfaff3, and H. Hayakawa4
1Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto, Japan
2Center for Geospace Studies, SRI International, Menlo Park, CA, USA
3NASA/Goddard Space Flight Center, Greenbelt, MD, USA
4Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan
Abstract. 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.
Keywords. Mid-latitude ionosphere – Ionospheric irregularities
– Ionosphere-atmosphere interactions
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