Spread F echoes variability along solar flux and seasonality conditions over the 50-MHz radar on Christmas Island/ Espalhe a variabilidade dos ecos de F ao longo do fluxo solar e das condições de sazonalidade no radar de 50 MHz na Ilha Christmas

Abstract

Perturbations in the amplitude (or phase) of radio signals are caused by irregularities in the ionospheric electron density. Ionospheric structures can disrupt the propagation of radio waves, and are commonly observed, for instance, by ionospheric vertical radio sounders (ionosondes). For historical reasons, the signatures of ionospheric irregularities in different types observations (optical, radio, in-stu ) are referred to as equatorial spread F (ESF). Previous studies show that the occurrence rate can be greatly affected by solar and magnetic conditions. Observations during the period of 2003 to 2012 of nighttime echoes made from the 50-MHz radar on Christmas Island (2.0 o N, 157.4 o W) have revealed time and altitude pattern distribution along the descending phase of solar cycle 23 and the recent extended solar minimum phase. We present the study of spread F echoes as a function of solar flux conditions and seasonality in order to quantify their variability. Under higher solar flux conditions, echoes reach higher altitudes but decay earlier. Only during solar minimum conditions the echoes exist throughout the whole night, since the post?reversal anti?zonal background electric field is weaker. Thus, irregularities during solar maximum will be dominated by dynamics near the time of the PRE. Since the population of irregularities during solar minimum exists throughout the whole night, post?reversal ionospheric conditions may play a role in the morphology of plasma irregularities, especially with the coincidentally weaker PRE during solar minimum. Peak time occurrence of echoes along the current period show a well defined pattern, meanwhile the peak altitude occurrence of echoes show a slight regular pattern.