Abstract
Abstract. During the night, in the F-region, equatorial ionospheric irregularities manifest as plasma depletions observed by satellites, and they may cause radio signals to fluctuate. In this study, the distribution characteristics of ionospheric F-region irregularities in the low latitudes were investigated using 16 Hz electron density observations made by a faceplate which is a component of the electric field instrument (EFI) onboard Swarm satellites of the European Space Agency (ESA). The study covers the period from October 2014 to October 2018 when the 16 Hz electron density data were available. For comparison, both the absolute (dNe) and relative (dNe∕Ne) density perturbations were used to quantify the level of ionospheric irregularities. The two methods generally reproduced the local-time (LT), seasonal and longitudinal distribution of equatorial ionospheric irregularities as shown in earlier studies, demonstrating the ability of Swarm 16 Hz electron density data. A difference between the two methods was observed based on the latitudinal distribution of ionospheric irregularities where (dNe) showed a symmetrical distribution about the magnetic equator, while dNe∕Ne showed a magnetic-equator-centred Gaussian distribution. High values of dNe and dNe∕Ne were observed in spatial bins with steep gradients of electron density from a longitudinal and seasonal perspective. The response of ionospheric irregularities to geomagnetic and solar activities was also investigated using Kp index and solar radio flux index (F10.7), respectively. The reliance of dNe∕Ne on solar and magnetic activity showed little distinction in the correlation between equatorial and off-equatorial latitudes, whereas dNe showed significant differences. With regard to seasonal and longitudinal distribution, high dNe and dNe∕Ne values were often found during quiet magnetic periods compared to magnetically disturbed periods. The dNe increased approximately linearly from low to moderate solar activity. Using the high-resolution faceplate data, we were able to identify ionospheric irregularities on the scale of only a few hundred of metres.
Highlights
Noticeable features in the low-latitude ionosphere are plasma density irregularities which occur after sunset in the F region (Kil and Heelis, 1998)
They may be identified as density decrease along satellite passes referred to as equatorial plasma bubbles (EPBs) or range and frequency spread signatures on ionograms commonly called equatorial spread F (ESF) (Woodman and La Hoz, 1976; Burke et al, 2004)
Multi-peak electron density fluctuations were not observed in the results presented by Stolle et al (2006) due to the low sampling rate of the planar Langmuir probe (LP; PLP) measurements
Summary
Noticeable features in the low-latitude ionosphere are plasma density irregularities which occur after sunset in the F region (Kil and Heelis, 1998). Multiple studies have used high-resolution data sets when available to check the distribution characteristics of ionospheric irregularities and have been able to resolve plasma density structures to smaller scales along satellite tracks (e.g. Lühr et al, 2014; Huang et al, 2014, etc.). Buchert et al, 2015; Zakharenkova et al, 2016; Xiong et al, 2016b, c; Wan et al, 2018; Yizengaw and Groves, 2018; Jin et al, 2019; Kil et al, 2019, etc.) Most of these studies have used electron density measurements at a frequency of 2 Hz or 1 Hz made by the Langmuir probes onboard Swarm. We show that the electron density measurements of the Swarm faceplate can be applied to examine the characteristics of ionospheric equatorial irregularities at sub-kilometre scale lengths.
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