Equatorial ionosphere observation network and its first results
Abstract
Due to the horizontal geomagnetic field line, equatorial ionosphere has a number of unique phenomena. Instabilities in the F layer have been particularly studied theoretically and experimentally. Evening enhanced eastward electric field rise up the post sunset equatorial F region sometimes leads to Rayleigh-Tailor instability. Disturbed area extends to low latitude ionosphere and is called equatorial spread-F (ESF). Inside the disturbed area, decreased ionization goes upward rapidly. Radio wave propagates through ESF suffers phase and amplitude modification even in microwave band. It is required to estimate ESF activity that causes satellite communication link and navigation system disturbances.
Eastward electric field, zonal and meridional wind condition, geomagnetic activity, atmospheric gravity waves, and flux tube integrated conductivities contributes ESF generation [J. Atmos. Solar-Terr. Phys. 63, 869 (2001)]. Incoherent scatter radar observation at Jicamarka found a vortex structure in the evening ionosphere prior to ESF onset [J. Geophys. Res. 104, 28163 (1999)]. Maruyama, et al. [J. Atmos. Solar-Terr. Phys. 64, 1557 (2002)], using Cebu (10.27N, 123.87E) and Manila (14.7N, 121.1E) ionosphere data, pointed out that such a vortex structure causes time delay in the ionospheric vertical drift motion at two points spaced several degrees meridionally. For further understanding of ESF, it is needed the ionosphere observation chains deployed one along the geomagnetic equator and another along some meridian.
ESF depends on solar activity. Sunspot number reached to cycle 23 maximum in 2000 and now declining toward solar minimum supposed around 2006. Evening enhancement appeared firmly around 19 LT in Cebu through 1998−2000. The F layer often showed the spread condition at the peak of uplift. However, 2003 observation shows declined ESF activity. Evening enhancement often disappeared but the F layer sometimes rose up in midnight and then followed the spread F condition.
Ionosphere above Chumporn shows similar variation to Cebu. F layer height variation during the evening enhancement at Chumporn is smaller than that of Cebu. Long term observation is needed to evaluate the magnetic declination control of ESF [J. Geophys. Res 86, 11443 (1981)]. Time difference between Cebu and Chumporn is steady to 1.5 hour for evening enhancement but varies for the midnight enhancement. What causes the midnight enhancement and spread F is left for future study.
