Latitudinal four-peak structure of the nighttime F region ionosphere: Possible contribution of the neutral wind

Abstract

In this study, we provide a detailed analysis of the latitudinal four-peak structure of the F region electron density observed by the Swarm B satellite during the night of January 31, 2017. Consisting of three satellites flying at different local times, Swarm provides an opportunity to investigate the temporal evolution of the nighttime latitudinal four-peak structure. For this event, Swarm A/C did not observe the two crests of equatorial ionization anomaly (EIA) at 17:55/18:01 LT, but Swarm B, which flew over the same longitudinal sector approximately 4.5 h later, observed a clear latitudinal four-peak structure. This provides direct evidence that the two inner peaks of the latitudinal four-peak structure are not remnants of the EIA crests from sunset. In addition, simultaneous measurements of the vertical plasma drift from the incoherent scatter radar at Jicamarca and neutral wind from the ground-based Fabry–Perot interferometer (FPI) at Arecibo were compared to reveal the possible driving mechanisms. The observed F region vertical plasma drift was generally downward from sunset throughout the local night of January 31, 2017, which reveals that an enhanced upward plasma drift is not necessary to cause the nighttime latitudinal four-peak structure. The neutral wind measurements from the FPI located at Arecibo showed enhanced eastward and southward components during the night when the latitudinal four-peak structure was observed by Swarm B, both with a difference of approximately 100 m/s compared with the other two days. This suggests that the neutral winds play an important role in the nighttime latitudinal four-peak structure. Further simulations using the SAMI2 (another model of the ionosphere) model support these observations. In general, eastward and equatorward winds from both hemispheres at night provided favorable conditions for the latitudinal four-peak structure. However, when the equatorward wind is too large, a strong hemispheric asymmetry of the background Ne can occur, which prevents the development of the latitudinal four-peak structure at night.

Keywords

• nighttime ionosphere
• equatorial ionization anomaly
• latitudinal four-peak structure
• swarm satellites
• neutral wind