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Ionospheric layer plays an important role as the main medium for communication of wireless radio in the High Frequency (HF) band. Critical frequencies are important parameters of the ionosphere. Critical frequencies vary diurnally following the pattern of day and night due to the activities of the sun. The critical frequency for Parit Raja station is determined using an ionosonde at latitude 1° 52' N and longitude 103° 48' E, which are close to the geomagnetic equator at geographic latitude 5° N. This work analyses the dependence of the critical frequency during the solar minimum of Solar Cycle 23. From the observations, the critical frequency observed in 2005 was the highest, corresponding to a higher sunspot number of between 11 and 192. The general trend of the daily critical frequency was 6.8 MHz to 12.0 MHz from 00:00 to 17:30 Coordinated Universal Time (UTC) and decreasing to 7.0 MHz from 18:00 to 23:00 UTC. The average value of the critical frequency for 2005 to 2007 was 7.5 MHz, with a minimum of 1.8 MHz and a maximum of 11.0 MHz. The results showed a slow decrease from 2006 to 2007 for the critical frequencies in 2006 and 2007.

<p>This study attempts to develop a functional relationship between air temperature and urban green space using Landsat data. It also aims to estimate the contributions of various forcings, namely, urban green space, population density, urban area, and automobile densities on urban heat island. Subsequently, the impact of urban heat island on temperature humidity index will be assessed quantitatively, followed by surface energy budget analysis of Jabotabek area. Air temperature series are derived from Landsat data, including the NDVI which is used as the bases in generating urban green space of the study area. Principal Component Analysis is utilized in order to establish the relative importance of forcing variables on urban heat island; in order to simplify the structure of factor loadings a varimax rotation is carried out. It is found that air temperature and urban green space for the study area is best represented by a nonlinear equation when a maximum coefficient determination (R2adj) and a minimum standard deviation (S) are to be fulfilled. A 10 to 50% reduction in urban green space would bring air temperature to raise between 0,2 to 1,8 oC. It is interesting to note that this study reveals the same percentage increase in urban green space would only lower the temperature by 0,1 to 0,5 oC. Automobile density is found to be the most important forcing for urban heat island in Jakarta; on the other hand, urban green space is the most dominant forcing in Tangerang and Bekasi. Surface energy budget analysis indicated that the an increase of 1,0 oC in urban heat island would result in additional 80,3 and 69,0 Wm-2 sensible heat fluxes to submedium (G) and to air, respectively.</p>

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