MELISSA: System description and spectral features of pre- and post-midnight F-region echoes

摘要

Most of the low-latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (similar to 75 degrees W). The deployment of the 30 MHz FAPESP-Clemson-INPE (FCI) coherent backscatter radar in the magnetic equatorial site of Sao Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (similar to 45 degrees W). However, despite being operational for several years (2001-2012), FCI only made observations during daytime and pre-midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full-night F-region observations. This radar is referred to as Measurements of Equatorial and Low-latitude Ionospheric irregularities over Sao Luis, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre- and post-midnight F-region echoes with focus on the spectral features of post-midnight echoes and how they compare to spectra of echoes observed in the post-sunset sector. The radar observations indicate that post-midnight F-region irregularities were generated locally and were not a result of "fossil" structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field-of-view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F-region apparent uplifts prior to post-midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F-region conditions leading to ESF development.