For the first time, vertical rise velocities of postmidnight field‐aligned irregularities (FAIs) at low geomagnetic latitudes have been examined near the June solstice by using two‐dimensional maps of F region FAI echoes observed with the Equatorial Atmosphere Radar in Indonesia for 3 years starting in May 2010. We found 15 freshly growing FAIs at postmidnight between May and August during the 3 years. The rise velocities of FAIs are smaller at postmidnight than at postsunset, and most postmidnight FAIs do not exceed an altitude of 450 km. Based on the rise velocities, a lower limit for the creation time of the postmidnight FAIs is estimated to be between 21:30 LT and 02:00 LT for 14 of the 15 events, indicating that this class of FAIs is distinct from the postsunset FAIs.
We investigated a postmidnight field‐aligned irregularity (FAI) event observed with the Equatorial Atmosphere Radar at Kototabang (0.2°S, 100.3°E, dip latitude 10.4°S) in Indonesia on the night of 9 July 2010 using a comprehensive data set of both neutral and plasma parameters. We examined the rate of total electron content change index (ROTI) obtained from GPS receivers in Southeast Asia, airglow images detected by an all‐sky imager, and thermospheric neutral winds and temperatures obtained by a Fabry‐Perot interferometer at Kototabang. Altitudes of the F layer (h′F) observed by ionosondes at Kototabang, Chiang Mai, and Chumphon were also surveyed. We found that the postmidnight FAIs occurred within plasma bubbles and coincided with kilometer‐scale plasma density irregularities. We also observed an enhancement of the magnetically equatorward thermospheric neutral wind at the same time as the increase of h′F at low‐latitude stations, but h′F at a station near the magnetic equator remained invariant. Simultaneously, a magnetically equatorward gradient of thermospheric temperature was identified at Kototabang. The convergence of equatorward neutral winds from the Northern and Southern Hemispheres could be associated with a midnight temperature maximum occurring around the magnetic equator. Equatorward neutral winds can uplift the F layer at low latitudes and increase the growth rate of Rayleigh‐Taylor instabilities, causing more rapid extension of plasma bubbles. The equatorward winds in both hemispheres also intensify the eastward Pedersen current, so a large polarization electric field generated in the plasma bubble might play an important role in the generation of postmidnight FAIs.
Predicting the daily variability of Equatorial Plasma Bubbles (EPBs) is an ongoing scientific challenge. Various methods for predicting EPBs have been developed, however, the research community is yet to scrutinize the methods for evaluating and comparing these prediction models/techniques. In this study, 12 months of co-located GPS and UHF scintillation observations spanning South America, Atlantic/Western Africa, Southeast Asia, and Pacific sectors are used to evaluate the Generalized Rayleigh-Taylor (R-T) growth rates calculated from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). Various assessment metrics are explored, including the use of significance testing on skill scores for threshold selection. The sensitivity of these skill scores to data set type (i.e., GPS versus UHF) and data set size (30, 50, 60, and 90 days/events) is also investigated. It is shown that between 50 and 90 days is required to achieve a statistically significant skill score. Methods for conducting model-model comparisons are also explored, including the use of model "sufficiency." However, it is shown that the results of model-model comparisons must be carefully interpreted and can be heavily dependent on the data set used. It is also demonstrated that the observation data set must exhibit an appropriate level of daily EPB variability in order to assess the true strength of a given model/technique. Other limitations and considerations on assessment metrics and future challenges for EPB prediction studies are also discussed. ESF/EPB research has generally been motivated by the curious aspects of the equatorial ionosphere as a natural plasma physics laboratory. However, the now heavy reliance of global society on satellite communications and navigation signals, and the adverse impacts that EPBs have on such signals, has strongly motivated research aimed at the prediction of these space weather events in recent times (e.g.
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