Sunrise effect on 40 kHz signal amplitude and its characteristic variation with respect to geomagnetic storms

The sunrise effect is a characteristic feature of very low frequency and low frequency radio waves propagated over a large distance. The 40 kHz signal level, transmitted from Miyakoji station (37.4 degrees N, 140.9 degrees E), Japan, and received at Tripura University (23 degrees N, 91.4 degrees E), is found to be attenuated during sunrise with an enhancement before the decrease in the signal level. On the basis of the nature of attenuation of the observed records from 2005 to 2006, those are classified into four different types, namely, type A (three step attenuation), type B (two step attenuation), type C (one step attenuation), and type D (no attenuation). During geomagnetically active days, 84% of type D cases and 31% of type C cases are observed, whereas only 0.9% of type A cases and 7% of type B cases are observed during geomagnetically active days. The fade amplitude of type C fade is also found to maintain a good negative correlation of 77.3% with the geomagnetic A(p) indices over the period of 2 years. From the model calculation it is found that in the altitude range from 65 to 80 km, on average the electron density increases by a factor of 5.22 times during geomagnetically active days versus normal days.