Diurnal and Seasonal Variation of Critical Frequency in Malaysia from 2005 to 2007

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.