The Seasonal Dynamics of Equatorial Spread-F Over Puerto Maldonado - South America

Abstract

Equatorial spread-F (ESF) phenomenon refers to intense post-sunset ionospheric irregularity that occurs around the geomagnetic equator due to perturbations in the ambient plasma. It has remained a very active area of ionospheric research for over eight decades now, owing to its adverse effects on trans-ionospheric radio signals. For many years, experiments have shown that spread-F can occur shortly after dusk and much later in the night, especially in the versatile equatorial ionosphere. Both pre-midnight and post-midnight ESF occurrences have been reported over the Indian and the South American sectors. Other studies have also endeavoured to explain the possible causes of these irregularities. However, not much work has been done to discover their persistence and seasonality over the equatorial region where their effect on radio signals is most severe. Globally, this situation has been aggravated by poorly distributed ionospheric measurements along the equatorial region. Therefore, using a recently installed ionospheric radio sounder, this study not only aims at establishing the seasonality and persistence patterns of both pre-MN and post-MN ESF, but also intends to characterize the diurnal and seasonal influence of the vertical E x B drift and electron density on ESF morphology over Puerto Maldonado (12.6°S, 69.2°W, dip lat. O.l°N), an equatorial trough site that has in the past received very little attention. It further aims at revealing the unexplored effect of solar flux on ESF intensity. To achieve this, daily ionograms from the ionosonde have been analyzed on a 15 minute cadence, recording critical frequencies (foF2) and evaluating the vertical E x B drifts from virtual height (h'F2) measurements over the solar maximum period between December 2012 and November 2013. We further evaluated ESF percentage occurrence and measured the daily and monthly average spread ranges. The results revealed that the monthly occurrence trends were practically the same with both pre-midnight and post-midnight events having high occurrences of about 80-100% during summer months, and a minimum of about 25% during winter. This high summer prevalence is attributed to the effects of seasonal alignment of ITCZ with the magnetic field leading to the enhancement of gravity waves, thus increasing ESF occurrence. Pre-midnight ESF events lasted for ~4 hours longer in summer compared to post-midnight events which lasted 1-2 hours longer in winter and generally commenced earlier. Seasonal variations in strength of PRE . drift and appearance of sporadic-E layer causes changes in meridional polarization and is thought to affect ESF occurrence duration. Summer and equinoctial months also recorded high foF2 and experienced high prevalence of ESF. Both daytime and the nighttime plasma drifts exhibited significant diurnal and seasonal dependence and portrayed a similar trend to that of the occurrence and development of ESF. The post-sunset PRE drift generally attained a threshold value of about 30mls coinciding with the general start time of ESF when the daytime electric field orientation reverses with the shifting solar terminator. Solar flux indices showed a weak but significant direct correlation with ESF prevalence, but correlated inversely to the spread ranges a situation thought to be caused by the confinement of PRE drift peak and F-layer peak altitude to a narrow height range. The results of this study can be used to predict the ionosphere above Puerto Maldonado and further used to enhance the reliability of HF radio navigation and communication systems. These results will provide a guide to selecting the right frequency ranges to be used in trans-ionospheric communication.