Coherence Scale and Directivity of Nighttime Equatorial Plasma Irregularities: Results From Swarm Formation Flight. Issue 5 (27th April 2022)

• Record Type: Journal Article
• Authors: Park, Jaeheung
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-27
• Subjects: equatorial plasma irregularity -- Swarm -- ionospheric coherence scale -- backward‐C structure; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA030233
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: 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. Plain Language Summary: Equatorial Plasma Irregularities (EPIs) in the nightside ionosphere have profound effects on the contemporary radio communications and navigation systems. In spite of the long history of study since the 1930s, the coherence scale and directivity of EPIs have not yet been investigated with global‐scale multiyear data sets. In this study we address the two EPI characteristics by cross‐correlating in situ plasma density profiles from three satellites under formation flight in polar orbits: the Swarm constellation. As East‐West separation between satellites increases, the pair of plasma density profiles lose similarity. The cross‐correlation coefficients go down below 0.6 when the East‐West separation is larger than ∼0.1° in geographic longitude (GLON), which amounts to about 10 km. The satellite pairs allow us to further investigate the directional preference of EPI structures. A majority of the EPI structures constitute backward‐C shapes straddling the dip equator. The preference for backward‐C becomes more and more evident as the night progresses. The directions of the EPI structures are not well organized with geomagnetic declination angles of the GLON sector. Neither do the directions exhibit monotonic changes with geomagnetic or solar activity, but further studies during the coming solar maximum will give us more decisive conclusion. Key Points: Latest changes in the flight formation of the Swarm mission are beneficial for interspacecraft correlation analyses As zonal distance between two Swarm spacecraft increases, the two along‐track profiles of density irregularities become less coherent Most nighttime equatorial ionospheric irregularities, especially late at night, conform to backward‐C shapes straddling the dip equator