Altitude Distribution of Large and Small‐Scale Equatorial Ionospheric Irregularities Sampled From an Elliptical Low‐Earth Orbit

Abstract

We provide insight into the vertical distribution of multi‐scale scintillation‐inducing irregularities in the low‐latitude ionosphere. In four sets of novel experiments, we sampled altitudes from 330 to 1,280 km in the 18–24 magnetic local time (MLT) sector using the Swarm Echo GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP‐O) GPS receiver with its antenna oriented toward zenith. In order to identify multi‐scale irregularities both above and at the satellite's position, we utilize high‐sample‐rate GAP‐O amplitude and phase measurements along with a measurement of net current on the surface of the imaging and rapid‐scanning ion mass spectrometer sensor on board, which serves as a proxy for density variations. By calculating the rate of change of total electron content index using two sets of GPS parameter choices, we are able to sample irregularities as small as 160 m, which is comparable to or smaller than the Fresnel scale responsible for scintillation‐inducing irregularities. During one campaign, we find that amplitude scintillations on the GPS signal coincide with strong in‐situ small‐scale density irregularities in 32% of cases, indicative of a broad irregularity region extending from the satellite's position to hundreds of kilometers above. Also, we show that large‐scale ionospheric disturbances (larger than 80 km) occur predominantly below 500 km, and down to the 330 km perigee of Swarm Echo in the 18–21 MLT sector. In contrast, small‐scale variations of total electron content are detected at all MLTs between 18 MLT and magnetic midnight and at all altitudes sampled in this experiment. However, they are more frequent in the 22–24 MLT range.

Regional Mapping of Small‐Scale Equatorial Ionospheric Irregularities Using Swarm Echo Satellite Measurements

“…Formation and characteristics of EPBs in low solar activity periods have been studied previously (e.g., Carter et al., 2013; Chou et al., 2020; Kepkar et al., 2020). Recently, we reported the occurrence and vertical distribution of multi‐scale irregularities in the low‐latitude post‐sunset sector using the same observations considered in the present study (Mohandesi et al., 2022). It is shown in that paper that, while large‐scale electron density variations, which are consistent with EPBs, form at altitudes close to the perigee of the Swarm Echo satellite during early post‐sunset hours, small‐scale irregularities occur in the entire examined altitudinal range (330–1,280 km) between 18 and 24 MLT.…”

“…The Imaging and Rapid-scanning Ion Mass Spectrometer (IRM) aboard Swarm Echo measures the net current incident onto its outer surface, which is reported at 100 Hz rate (Yau et al, 2015). We use the standard deviation of the IRM surface current (SC) calculated over 1-min time intervals as a proxy for relative in situ electron density variations (see Mohandesi et al (2022) for details).…”

“…We use the standard deviation of the IRM surface current (SC) calculated over 1‐min time intervals as a proxy for relative in situ electron density variations (see Mohandesi et al. (2022) for details).…”

Power Spectral Characteristics of In‐Situ Irregularities and Topside GPS Signal Intensity at Low Latitudes Using High‐Sample‐Rate Swarm Echo (e‐POP) Measurements

“…The sensor surface is held at the same potential as the satellite chassis, meaning the collected current has contributions from both ions and electrons, with a relation to plasma density that is highly sensitive to the spacecraft potential, and to the atmospheric or spacecraftgenerated photoelectrons and other energetic electrons that may be present. Nevertheless, the variations of the IRM surface current (SC) can be used as a relative measure of plasma density (see Mohandesi et al (2022) for more detail). We calculated the PSD of the relative variations of SC, ΔI, supposing that they are approximately proportional to in-situ electron density variations:…”

“…Nevertheless, the variations of the IRM surface current (SC) can be used as a relative measure of plasma density (see Mohandesi et al. (2022) for more detail). We calculated the PSD of the relative variations of SC, Δ I , supposing that they are approximately proportional to in‐situ electron density variations: $$${\Delta }I=\frac{I-\bar{I}}{\bar{I}}\approx \frac{n-\bar{n}}{\bar{n}}$$$ where n is the in‐situ electron density, and $$\bar{I}$$ and $$\bar{n}$$ are averages of SC and electron density for each 1‐min interval, respectively.…”