On the QBO modulation in the frequency of occurrence of pre noon F3 layers over the dip equatorial location of Thiruvananthapuram

Using the ionosonde observations, over Thiruvananthapuram (8.5° N, 77oE, 0.5oN dip lat.), India, for a period of 20 years (1990-2009), the present study reveals the presence of quasi biennial oscillation (QBO) in the pre noon F3 layer occurrence percentage. The presence of a strong QBO modulation in the occurrence of pre noon F3 layers is observed during the period from 1992 to 2000 beyond which it is rather weak/nonexistent. The wavelet periodogram analysis indicates the fact that F3 occurrences are more during the eastward phase of MQBO and less during the westward phase of MQBO.