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

Most of the low-latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP-Clemson-INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°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 São Luís, 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.