Coherence Scale and Directivity of Nighttime Equatorial Plasma Irregularities: Results From Swarm Formation Flight

Since the launch in 2013, the three satellites of the Swarm constellation have been conducting multipoint observations of ionospheric plasma density. The variety of their flight formations is advantageous for investigating (a) coherence scale and (b) directivity of nighttime Equatorial Plasma Irregularities (EPIs). In this study, we address the two topics statistically using in situ plasma density measured at 2 Hz rates by the Swarm constellation from 2013 to 2021. Maximum cross-correlation coefficients between two Swarm density profiles decrease as longitude differences between the observation pair increase. The coefficient is larger than 0.6 only when two satellites are within about 0.1° in geographic longitude (GLON), which approximately corresponds to 10 km. When two density profiles are considerably correlated, we can determine preferred bearings of the EPI structure. A majority of EPIs conform to the backward-C shapes astride the dip equator. The preference for backward-C is more conspicuous later at night than in the early evening. Different GLON sectors exhibit slightly different directions of EPI structures, but the behavior is not well organized with the geomagnetic declinations of the respective sectors. EPI directions do not display monotonic dependence on Ap or F10.7, but further studies during the coming solar maximum are necessary to better represent high solar/geomagnetic activity.