Traits of sub-kilometre F-region irregularities as seen with the Swarm satellites

<p class="rtejustify">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\&nbsp;2014 to October\&nbsp;2018 when the 16 Hz electron density data were available. For comparison, both the absolute (<em>d</em><em>N</em>_e) and relative (<em>d</em><em>N</em>_e/<em>N</em>_e) 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 (<em>d</em><em>N</em>_e) showed a symmetrical distribution about the magnetic equator, while <em>d</em><em>N</em>_e/<em>N</em>_e showed a magnetic-equator-centred Gaussian distribution. High values of <em>d</em><em>N</em>_e and <em>d</em><em>N</em>_e/<em>N</em>_e 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 <em>d</em><em>N</em>_e/<em>N</em>_e on solar and magnetic activity showed little distinction in the correlation between equatorial and off-equatorial latitudes, whereas <em>d</em><em>N</em>_e showed significant differences. With regard to seasonal and longitudinal distribution, high <em>d</em><em>N</em>_e and <em>d</em><em>N</em>_e/<em>N</em>_e values were often found during quiet magnetic periods compared to magnetically disturbed periods. The <em>d</em><em>N</em>_e 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.</p>