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

Abstract

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)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(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.

New Index to Characterize Ionospheric Irregularity Distribution

“…Multiple previous studies have concluded that post-midnight bubbles are the extension of bubbles generated during pre-midnight local time sector (Huang et al, 2014). Other studies, using ground based passive VHF (Yizengaw et al, 2013) and active backscatter (Patra et al, 2009;Rodrigues et al, 2019) radars, even observed post-midnight strong scintillation and field-aligned irregularity structures, respectively, during the nights when no pre-midnight irregularity pattern observed. All this confirms that the DGRI index can be used as a proxy to characterize ionospheric bubbles/irregularities and identify "all clear" zone at all local times.…”