Multi-instrumented observations of the equatorial F-region during June solstice: large-scale wave structures and spread-F

Abstract

Typical equatorial spread-F events are often said to occur during post-sunset, equinox conditions in most longitude sectors. Recent studies, however, have found an unexpected high occurrence of ionospheric F-region irregularities during June solstice, when conditions are believed to be unfavorable for the development of plasma instabilities responsible for equatorial spread-F (ESF). This study reports new results of a multi-instrumented investigation with the objective to better specify the occurrence of these atypical June solstice ESF in the American sector and better understand the conditions prior to their development. We present the first observations of June solstice ESF events over the Jicamarca Radio Observatory (11.95°S, 76.87°W, ∼ 1 • dip latitude) made by a 14-panel version of the Advanced Modular Incoherent Scatter Radar system (AMISR-14). The observations were made between July 11 and August 4, 2016, under low solar flux conditions and in conjunction with dual-frequency GPS, airglow, and digisonde measurements. We found echoes occurring in the pre-, post-, and both pre-and post-midnight sectors. While at least some of these June solstice ESF events could have been attributed to disturbed electric fields, a few events also occurred during geomagnetically quiet conditions. The late appearance (22:00 LT or later) of three of the observed events, during clear-sky nights, provided a unique opportunity to investigate the equatorial bottomside F-region conditions, prior to ESF, using nighttime airglow measurements. We found that the airglow measurements (630 nm) made by a collocated all-sky camera show the occurrence of ionospheric bottomside F-region perturbations prior to the detection of ESF echoes in all three nights. The airglow fluctuations appear as early as 1 hour prior to radar echoes, grow in amplitude, and then coincide with ESF structures observed by AMISR-14 and GPS TEC measurements. They also show some of the features of the so-called large-scale wave structures (LSWS) that have been detected, previously, using other types of observations and have been suggested to be precursors of ESF. The bottomside fluctuations have zonal spacings between 300 and 500 km, are aligned with the magnetic meridian, and extend at least a few degrees in magnetic latitude.

Coordinated Ground‐Based and Space‐Based Observations of Equatorial Plasma Bubbles

“…One of the top research priorities in the global space weather community is to better understand the generation mechanisms and the dynamic features of EPBs, because they can severely disrupt the amplitude and phase of transionospheric radio waves so as to cause adverse effects on relevant communication and navigation systems. In addition, the altitudinal information of irregularities embedded within EPBs can be examined through observations of plume-like structures from coherent backscatter radar or incoherent scatter radar measurements (Ajith et al, 2015;Jin et al, 2018;Li et al, 2013;Rodrigues et al, 2018;Tulasi Ram et al, 2017;Yokoyama & Fukao, 2006). During quiet times, the prereversal enhancement (PRE) of the zonal electric field is responsible for the enhanced upward E × B drift after sunset, which elevates the ionospheric height and subsequently amplifies the growth rate of R-T The morphological features and spatial/temporal variability of EPBs have been widely investigated via case studies and statistical analysis using different observational methods.…”

On the Nature of the Intraseasonal Variability of Nighttime Ionospheric Irregularities Over Taiwan

“…Interestingly, the dominant periodicity of 25/28.5 day in vernal/autumnal equinox season was similar in ionospheric S4 and the geomagnetic parameters. Recent investigation based on continuous observations over successive days has indicated a close relationship between the ionospheric irregularities observed using the Jicamarca radar and the auroral electrojet (Rodrigues et al, 2018). Such a relationship is not limited to a few intense geomagnetic storms.…”

Simultaneous 6300 Å airglow and radar observations of ionospheric irregularities and dynamics at the geomagnetic equator

“…For this night we can directly observe that there are concurrent airglow depletions and small-scale irregularities, but recently Rodrigues et al (2018) showed that airglow signatures can occur before post-midnight irregularities. In the ASI images from this night, depletions in the entire field of view are only visible after 04:00 UT yet there are large plumes of irregularities visible in the RTI plots earlier.…”

“…We do not expect other undulations to move in the same way and have the same extent and shape in the field of view. Precursors to post-midnight ESF as described in Rodrigues et al (2018) show airglow structures that are not associated with ESF and are likely due to LSWS. Equatorial spread F structures tend to move with regular motion eastward and the precursors Rodrigues et al (2018) had very little motion, often staying in the same location for multiple images.…”

“…Precursors to post-midnight ESF as described in Rodrigues et al (2018) show airglow structures that are not associated with ESF and are likely due to LSWS. Equatorial spread F structures tend to move with regular motion eastward and the precursors Rodrigues et al (2018) had very little motion, often staying in the same location for multiple images. In the analysis here the structures are observed over multiple images and we wait until the structures are fully formed to measure their properties, so we do not think that ESF precursors impact our analysis.…”