Indian Journal of Radio & Space Physics Vol. 38, August 2010, pp 194-202 Diurnal and seasonal variation of fading rates of E- and F- region echoes during IGY and IQSY at the equatorial station of Ibadan E O Somoye Department of Physics, Lagos State University, Lagos, Nigeria E-mail: femi2000somoye@yahoo.com Received 20 November 2009; revised 8 March 2010; re-revised received and accepted 14 May 2010 Fading rates of E- and F-region echoes observed at Ibadan (7.4°N, 3.9°E, 6°S dip) during the International Geophysical Year (IGY) 1958 and the International Quiet Year of the Sun (IQSY) 1964 are investigated for diurnal and seasonal variations. The fading rates of E- and F-region echoes are found to be of the same order of magnitude during the IQSY. However, the same is not true during IGY indicating that fading of F-region echoes is dependent on solar activity while that of E-region echoes is not as fading rate of E-region echoes are about the same during both epochs. The fading rate is found to be maximum at noon and minimum at sunrise and sunset for E-region echoes during both epochs and for F-region echoes during IQSY. During IGY, fading rate is maximum at noon; and minimum at sunrise and 2000 hrs LT (i.e. post sunset) at both epochs and for the two regions about same times of reversal of F-region East-West (EW) and E×B vertical drift during solar maximum. The maximum at noon appears to be due to: (i) continuous daytime rise in hmF2 which enhances irregularities and (ii) spread F irregularities generated by sudden increase in E×B plasma uplift not seen by the close-spaced antenna technique used in the present study. F-region fading rate is maximum during December solstice while E-region fading rates show no seasonal variation. Keywords: Ionospheric irregularities, Radio echoes, Fading rate, Diurnal variation, Seasonal variation, Vertical drift, East- West drift PACS Nos: 94.20.Vv; 94.20.dg; 94.20.dj 1 Introduction The knowledge of the fading of radio signals is important for HF radio propagation 1-3 . The three types of fading are due to: (i) interference which results in multipath fading; (ii) ordinary and extraordinary waves called polarisation fading; and (iii) inhomogeneities in electron concentration of the ionosphere commonly referred to as ionospheric irregularities. The last one, which is the most difficult to avoid, is investigated in the present study for: (i) diurnal variation; (ii) seasonal influence; and (iii) similarities or differences during high and low sunspot periods for both E- and F- region echoes. The multipath fading due to multiple transmission paths caused by reflections between ionosphere and receiver 4-5 can be avoided since it is frequency selective 6 . By the careful selection of transmitting and receiving aerials, polarization fading caused by ordinary (O) and extraordinary (E) waves are avoidable. The size, elongation and drift velocities of the irregularities causing the fading of radio echoes are found to have higher values in the F-region than in the E-region 7 . The present study aims to find out the trend of fading of radio echoes from E-and F-region. 2 Materials and Method Amplitude fading records of E- and F- region echoes are obtained using closed spaced antenna technique. The equipment consists of a pulse transmitter, an electronics switch, transmitting and receiving aerials, delayed pulse generators, a receiver and display units. The transmitter is similar to the Mark II ionosonde used in obtaining hf curves (ionograms). It has coverage of 0.7 - 25 MHz of frequency and send out pulses of width varying between 50 and 500 sec at a repetition rate of 50 per second. The receiver, which is connected to the receiving aerials via the electronics switch, is a convectional heterodyne type. It operates in such a way that a pulse is received from each of the three receiving aerials in turn while the operation of the electronic switch is controlled from a synchronising pulse from a delayed pulse generator. Thus, the change over from one receiving aerial to another takes place roughly half way between pulses.