Statistical Analysis of Equatorial Plasma Irregularities Retrieved From Swarm 2013–2019 Observations

Abstract

In this study, we present a statistical analysis of equatorial plasma irregularities (EPIs) by using in situ plasma density measurements of the Swarm constellation from December 2013 to December 2019. The occurrence patterns for both postsunset and postmidnight EPIs with respect to longitude, season, local time, latitude, solar activity, and geomagnetic activity level are investigated. The main findings are as follows: (1) The postsunset/postmidnight EPIs occurrence rates exhibit different longitudinal and seasonal dependence: The postsunset EPIs have the maximum occurrence rate over the American-Atlantic sectors during the December solstice and equinoxes, and the postmidnight EPIs have the maximum occurrence rate during the June solstice, especially over the African sector. (2) The postsunset EPIs occurrence rates have a positive correlation with solar activity, while the postmidnight EPIs are negatively correlated with it.(3) The latitudinal distribution of EPIs exhibits a double-peak structure around ±5 • magnetic latitude with a more significant peak in the summer hemisphere. (4) The EPIs occurrence rate increases with increasing geomagnetic activity level. (5) The main controlling factors for the distribution of postsunset EPIs are the magnetic declination angle, equatorial vertical E × B drift, and thermospheric zonal wind. For the postmidnight EPIs, the main controlling factors are likely to be atmospheric gravity waves and equatorward thermospheric meridional wind associated with midnight temperature maximum.

Ionospheric S4 Scintillations from GNSS Radio Occultation (RO) at Slant Path

“…As a further evidence of the ESF/EPBs-induced scintillations, Figure 6 shows a seasonal variation of the RO S4 from ht = 30 km as a function of month and longitude in five selected local time bins. The patterns in Figure 6 match the seasonal variations of ESF/EPBs occurrence from the in-situ measurements at various satellite altitudes [29,[45][46][47] as well as those on the ground [43]. The RO S4 variations appear to agree better with the vertical ion velocity (Vz) than the EPB occurrence frequency derived from plasma density anomalies in logarithmic scale [45], showing the seasonal asymmetry with a slightly higher mean in February-March than November-December.…”

“…It was also found with the satellite in-situ measurements that the S4 amplitudes are correlated better with density-based EPB detection (i.e., ΔN) than with log(density)-based detection (i.e., ∆N/N0) [43,46]. The ∆N/N0-based detection appears to be more sensitive to the post-midnight EPB events than the N-based method [46,47]. In summary, the RO S4 results at ht = 30 km support the conclusion on the better correlation between S4 and the ΔN-based EPB detection.…”

The Occurrence Feature of Plasma Bubbles in the Equatorial to Midlatitude Ionosphere During Geomagnetic Storms Using Long‐Term GNSS‐TEC Data

“…Additionally, they can be generated in the dawn sector due to the eastward electric fields and suppressed in the dusk sector due to the westward electric fields by an over‐shielding penetration and/or a disturbance dynamo during the recovery phase (Abdu, 2012; de La Beaujardière et al., 2009; J. Li et al., 2012). However, most of these studies were based on case studies, and the K p index was mainly used as an indicator of the geomagnetic activity level for statistical analyses of the occurrence features of plasma bubbles (Aa et al., 2020; G. Li et al., 2009; Stolle et al., 2006). Because the K p index represents the magnitude of geomagnetic field variation every 3 h, we cannot identify the main and recovery phases of geomagnetic storms from the K p index.…”

“…Although some statistical studies have investigated the geomagnetic activity dependence of plasma bubble occurrence and spatial and temporal distributions of storm‐time plasma bubbles (Aa et al., 2020; G. Li et al., 2009; Stolle et al., 2006), most studies used the K p index to define the level of geomagnetic activity. However, since the K p index represents only the magnitude of the geomagnetic field variation every 3 h, the main and recovery phases of geomagnetic storms cannot be identified only by the K p index.…”

“…Although some statistical studies have investigated the geomagnetic activity dependence of plasma bubble occurrence and spatial and temporal distributions of storm-time plasma bubbles (Aa et al, 2020;G. Li et al, 2009;Stolle et al, 2006), most studies used the K p index to define the level of geomagnetic activity.…”