Unseasonal development of post-sunset F-region irregularities over Southeast Asia on 28 July 2014: 2. Forcing from below?

K K Ajith,R G Caton,K Groves,S Sripathi,S Gurubaran,K Hozumi,T Yokoyama,M Yamamoto,S Sathish Kumar,S Tulasi Ram,B A Carter

doi:10.1186/s40645-018-0218-1

Abstract

An unseasonal development of Equatorial Plasma Bubble (EPB)/Equatorial Spread-F (ESF) activity in a wide longitudinal sector over India and Southeast Asia during the post-sunset hours of 28 July 2014 has been investigated in detail using the multiple independent observations in this paper and a companion paper by (Prog Earth Planet Sci 5:10, 2018). This post-sunset ESF/EPB event was preceded by a substantial elevation of the equatorial F-layer due to strong pre-reversal enhancement (PRE) during the season which is climatologically unfavorable for strong PRE and post-sunset EPBs. In this paper, it is found that a strong equatorward meridional neutral wind at off-equatorial E-region and enhanced longitudinal gradient of equatorial electrojet appear to be the responsible candidates for the enhanced PRE and EPB development on 28 July 2014. These changes in the local wind system and PRE are found to have an apparent connection with the planetary wave forces of lower atmospheric origin. Both the post-sunset height rise (PSSR) and lower thermospheric meridional neutral wind consistently exhibit quasi-2-day planetary wave-like oscillations during 20–31 July 2014 with maximum amplitudes on 28 July 2014. This study emphasizes the role of planetary wave forces from the lower atmosphere that can alter the local neutral wind system and E-region conductivities that may lead to the development of strong PRE and EPB activity which were otherwise climatologically unfavorable for post-sunset EPB/ESF development.

Highlights

The development of Equatorial Spread-F (ESF)/Equatorial Plasma Bubble (EPB) irregularities in the nighttime equatorial ionosphere has been studied for several decades since it was first reported by Booker and Wells (1938)
From the fan sector backscatter maps constructed from the 16 beam observations of EAR (Ajith et al 2015), it is further confirmed that the observed post-sunset EPBs on 28 July 2014 are freshly developed over EAR longitudes, and the small post-midnight EPB around 1830 UT is drifting into the field of view (FoV) of EAR from the western longitudes
This study indicates the possible role of planetary wave forces of lower atmospheric origin that can significantly modify the neutral wind at mesosphere and lower thermospheric (MLT) region and pre-reversal enhancement (PRE) system leading to the development of large post-sunset height rise (PSSR) and unseasonal large scale EPB/ESF activity

Summary

Introduction

The development of Equatorial Spread-F (ESF)/Equatorial Plasma Bubble (EPB) irregularities in the nighttime equatorial ionosphere has been studied for several decades since it was first reported by Booker and Wells (1938). The anomalous day-to-day variability in the occurrence of EPBs still remains to puzzle the researchers over the years. Because of their potential impact on trans-ionospheric radio communications and satellite-based navigation systems, EPBs have drawn wide attention and become one of the important topics of space weather interest today (Kelley et al 2011; Woodman 2009). The ESF/EPB irregularities generally initiate at the bottom side F-layer through the Rayleigh-Taylor (RT) instability (Haerendel 1973) and grow nonlinearly to the topside ionosphere via polarization electric fields within them. The spatial large-scale wave structure (LSWS) at the base of F-layer is believed to be the potential seed for the development of EPBs and

Results

Discussion

Conclusion