Unseasonal development of post-sunset F-region irregularities over Southeast Asia on 28 July 2014: 1. Forcing from above?

Brett A Carter,Kefei Zhang,Julie Currie,Endawoke Yizengaw,Keith Groves,John Retterer,Ronald Caton,Tatsuhiro Yokoyama,Robert Norman,Rezy Pradipta,Michael Terkildsen,S Tulasi Ram

doi:10.1186/s40645-018-0164-y

Abstract

This contribution is the first of a two-part investigation into an unseasonal post-sunset equatorial F-region irregularity (EFI) event over the Southeast Asian region on the evening of 28 July 2014. Ground-based GPS scintillation data, space-based GPS radio occultation (RO) data, and ionosonde data show the existence of EFIs shortly after sunset over a region spanning 30° in longitude and 40° in latitude, centered on the geomagnetic equator. This post-sunset EFI event was observed during a time of the year when post-sunset equatorial plasma bubbles (EPBs) are very infrequent in the Southeast Asian longitude sector. GPS RO data shows that the EFI event over Southeast Asia coincided with the suppression of peak-season EPBs in the African and Pacific longitude sectors. Ionosonde data shows the presence of a strong pre-reversal enhancement (PRE) in the upward plasma drift over Southeast Asia prior to the detection of EFIs. Further, it is reported that this PRE was significantly stronger than on any other day of July 2014. An analysis of the geophysical conditions during this event reveals that this enhanced PRE was not caused by disturbed geomagnetic activity. Therefore, it is hypothesized that forcing from lower altitudes, perhaps tidal/planetary waves, was the potential cause of this strong PRE, and the subsequent EPB/EFI activity, on this day over the Southeast Asian sector.

Highlights

Equatorial plasma bubbles (EPBs) are common nighttime features in the low-latitude ionosphere
The physical mechanism responsible for the growth of post-sunset EPBs is understood to be the generalized R-T plasma instability, in which the vertical density gradient in the bottomside F layer combines with the pre-reversal enhancement (PRE) in the upward plasma drift to destabilize the plasma after sunset (Sultan 1996 and references therein)
The analysis of GPS scintillation data is augmented by the two Scintillation Network Decision Aid (SCINDA) VHF scintillation stations: Kwajalein Atoll (KWA) in the Marshall Islands in the Pacific sector (167.47 °E, 9.40 °N, MLAT: 4.32 °N) and Bahir Dah (BHD) in Ethiopia in the African sector (37.40 °E, 11.57 °N, MLAT: 2.61 °N)

Summary

Introduction

Equatorial plasma bubbles (EPBs) are common nighttime features in the low-latitude ionosphere. The bottom of the F layer becomes unstable to the generalized Rayleigh-Taylor (R-T) plasma instability, and low-density plasma bubbles rise up into the high-density ionospheric plasma at higher altitudes (e.g., Kelley et al 2011). These EPBs generate a wide spectrum of plasma waves/irregularities, called “equatorial F-region irregularities” (EFIs), that interfere with radio signals that propagate through them, such as those used in satellite communications and Global Navigation Satellite Systems (e.g., the Global Positioning System (GPS)). EPB occurrence variability can be split into two categories: (1)

Results

Discussion

Conclusion