Further investigations on 150-km echoing riddle using simultaneous observations of 150-km and E region echoes from off-electrojet location Gadanki

Abstract

In this paper we present a detailed study of the off-electrojet daytime 150-km echoes based on simultaneous observations of daytime 150-km and E region echoes made on a few days per month during July 2005 to August 2006 using the Gadanki radar. Daytime 150-km echoes are found to occur in all seasons over Gadanki. Although echo occurrence is found to be maximum in equinoxes and minimum in summer, no clear seasonal preference over Gadanki has been observed. In contrast, E region echo occurrence made simultaneously shows a strong seasonal dependence with remarkably low occurrence in winter as compared to other seasons. Interestingly, the 150-km echoes show a distinctly different morphology in winter as compared to other seasons. Echoes are observed in two distinct height regions in winter as compared to one broad region in other seasons. Also the strongest echo and the lowest altitude of echo occurrence are found to be associated with winter. Further, a significant share of Doppler velocities is found to be downward in winter as compared to predominantly upward velocities observed in other seasons. No seasonal dependence in the spectral width of 150-km echoes, however, has been observed. Seasonally averaged Doppler velocities are quite similar to those of the F region drifts reported from Jicamarca. Again seasonally averaged drifts are found to be downward in winter afternoon as compared to upward velocities observed in other seasons. Notwithstanding the seasonal differences observed in the 150-km echoes, echo occurrence is found to have a common feature of strong forenoon-afternoon asymmetry with forenoon occurrence being greater. These observations are discussed in the light of present understanding on the daytime 150-km echoing phenomenon.