The global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between ±35° geographic latitudes has been studied with the ROCSAT data during moderate to high solar activity years of 1999 to 2004. The result indicates that the global occurrence distribution of the intermediate‐scale (0.1 to 50 km) density irregularities can be grouped into two different populations, one in the equatorial region and the other in the middle‐to‐subauroral latitude region. The global seasonal/longitudinal (s/l) distribution of equatorial irregularities in the current report reproduces the result of McClure et al. (1998) obtained with the AE‐E observations of the mesoscale (50 to 1000 km) plasma bubble structures during high solar activity years of 1978 to 1980, two solar cycles ago. This implies that the density irregularities of different scales from multistage cascading process of the large‐scale (>1000 km) gravitational Rayleigh‐Taylor instability have manifested in same global s/l distribution pattern. Furthermore, global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. In addition, the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about ±30°. A different kind of midlatitude irregularity distribution starts to fill in from this dip latitude. In other words, the equatorial density irregularity inside a depleted flux tube can only rise, on statistical average, to an apex height of ∼2000 km. Different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes. The occurrence frequency of equatorial density irregularities increases with solar flux intensity; whereas the midlatitude density irregularity is more likely to occur during low solar activity period. The equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times. On the other hand, the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity. The local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight. Such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate‐scale density irregularities.

Contrasting longitudinal variation in equatorial spread‐F (ESF) density irregularity distribution has been observed by ROCSAT‐1 during high solar activity year of 2000 against moderate solar activity year of 2003. The monthly averaged solar flux intensity F10.7 was higher every month in 2000 than in 2003, but the irregularity occurrence rate did not always correlate with solar activity at every longitude in a season. For the equinox seasons, the occurrence rate was higher at every longitude in 2000 than in 2003. Higher occurrence rate was also observed in 2000 than in 2003 during a solstice season at longitudes of the so‐called ESF longitudes. On the other hand, higher occurrence rates were observed in 2003 at the so‐called low‐ESF longitudes from 230° to 10° during the June solstice season and from 90° to 260° during the December solstice season, where the magnetic flux tube makes a large‐angle alignment with the sunset terminator during the respective solstice season. Such anti‐solar activity correlation of ESF irregularity occurrences at these low‐ESF longitudes could be due to transequatorial meridional wind‐induced instability suppression agent which becomes more effective to offset the Rayleigh‐Taylor instability growth rate in a solstice season during a high solar activity period than during a low solar activity period.

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The effect of Mg doping on the electrical and optical properties of the p-GaN/AlGaN structures on a Si substrate grown by metal organic chemical vapor deposition was investigated. The Hall measurement showed that the activation efficiency of the sample with a 450 sccm Cp2Mg flow rate reached a maximum value of 2.22%. No reversion of the hole concentration was observed due to the existence of stress in the designed sample structures. This is attributed to the higher Mg-to-Ga incorporation rate resulting from the restriction of self-compensation under compressive strain. In addition, by using an AlN interlayer (IL) at the interface of p-GaN/AlGaN, the activation rate can be further improved after the doping concentration reaches saturation, and the diffusion of Mg atoms can also be effectively suppressed. A high hole concentration of about 1.3 × 1018 cm−3 can be achieved in the p-GaN/AlN-IL/AlGaN structure.

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