Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsa14a..06o&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SA14A-06
Physics
[2415] Ionosphere / Equatorial Ionosphere, [2439] Ionosphere / Ionospheric Irregularities, [6929] Radio Science / Ionospheric Physics, [7944] Space Weather / Ionospheric Effects On Radio Waves
Scientific paper
A VHF radar with operating frequency of 30.8 MHz and peak power of 20 kW has been operated at Kototabang (0.20S, 100.32E; dip latitude 10.4S), Indonesia since February 2006. From the continuous observation from February 2006 to August 2011, we find that FAIs appeared frequently at pre-midnight at equinoxes and at post-midnight between May and August. The pre-midnight FAIs coincided well with GPS scintillation observed at the same site. Seasonal and local time variations of the pre-midnight FAI occurrence are consistent with those of equatorial plasma bubbles reported in previous studies. These results indicate that the pre-midnight FAIs could be associated with the equatorial plasma bubbles. On the other hand, seasonal variation of the post-midnight FAIs were inconsistent with that of the pre-midnight FAIs. We have analyzed ionosonde data obtained at Chumphon, Thailand, which is located near the magnetic equator at the almost same longitude as Kototabang. Comparing seasonal and local time variations of the F-region FAIs with those of the F layer peak altitude over Chumphon, we find that the post-midnight FAIs coincide with uplift of the F layer at the magnetic equator. The uplift of the F layer can be considered to trigger plasma bubbles which could be generated by the Rayleigh-Taylor (R-T) instability. In order to investigate an effect of the F layer uplift on the R-T instability, we evaluate the linear growth rate of the R-T instability. The growth rate is proportional to eastward effective electric field driven by eastward electric field (E) and gxB drift (g/ν, where ν is ion-neutral collision frequency). During nighttime, E is westward, so that the growth rate is negative or small. On the other hand, g/ν becomes larger during nighttime for the solar minimum condition because nu is proportional to the neutral density, which is smaller during nighttime than daytime and decreases with decreasing the solar activity. g/ν is also larger at higher altitude because the neutral density decreases with altitude exponentially. Therefore, the uplift of the F layer makes the growth rate larger. Seasonal average of ExB drift velocity (E/B) is approximately 10 m/s during nighttime for the solar minimum condition. Since |E/B| is smaller than g/ν, the growth rate is positive. Consequently, around the midnight in the solar minimum period, the R-T instability can operate and plasma bubbles may be generated. Because electric field during nighttime is westward on average, the uplift of the F layer around midnight could not be caused by electric field. Further study is needed to reveal the cause of the F layer uplift around the midnight.
Nagatsuma Tadao
Nishioka Masaya
Otsuka Yoichi
Perwitasari S.
Shiokawa Kazuhiko
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