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- Title
Power Spectral Characteristics of In‐Situ Irregularities and Topside GPS Signal Intensity at Low Latitudes Using High‐Sample‐Rate Swarm Echo (e‐POP) Measurements.
- Authors
Mohandesi, Ali; Knudsen, David J.; Skone, Susan; Langley, Richard B.; Yau, Andrew W.
- Abstract
Ionospheric density structures at low latitudes range in size from thousands of kilometers down to a few meters. Radio frequency (RF) signals, such as those from global navigation satellite systems, that propagate through irregularities suffer from rapid fluctuations in phase and intensity, known as scintillations. In this study, we use the high‐sample‐rate measurements of the Swarm Echo (CASSIOPE/e‐POP) satellite's GPS Occultation (GAP‐O) receiver taken after its antenna was re‐oriented to vertical‐pointing, simultaneously with e‐POP Ion Mass Spectrometer surface current observations as a proxy for plasma density, to obtain the spectral characteristics of GPS signal intensity and in‐situ irregularities at altitudes from 350 to 1,280 km. We show that the power spectra of both measurements can generally be characterized by a power law. In the case of density irregularities, the spectral index with the highest occurrence rate is around 1.7, which is consistent with previous studies. Also, all the power spectra of GPS signal intensity in this study show a single spectral index near 2. Moreover, roll‐off frequencies estimated in this work range from 0.4 to 2.5 Hz, which is significantly higher than Fresnel frequencies calculated from ground GPS receivers at low latitudes (between 0.2 and 0.45 Hz). Part of this increase is due to the 8 km/s orbital velocity of Swarm Echo near perigee. Another key difference is that variations in the GPS signals in this study are dominated by the topside ionosphere, whereas GPS signals received from ground are affected mostly by the relatively dense F‐region plasma in the 250–350 km altitudinal range. Plain Language Summary: Variations in ionospheric electron density, called irregularities, cause fluctuations in radio frequency signals traversing the ionosphere. The Swarm Echo satellite carries a high‐sample‐rate GPS receiver with an antenna normally pointing in the horizontal direction. In this study, the satellite was reoriented while it was passing the low latitude region so that the receiver antenna would point vertically in order to probe irregularities above the satellite at high resolution. Our spectral analysis of GPS signal intensity and plasma density variations measured in situ (on board the spacecraft) revealed power law behavior in both measurements, a well‐established feature of ionospheric turbulence. More specifically, our results show that the spectral index (which is the negative of the power law slope) of in‐situ density irregularities and GPS signal intensities are 1.7 and 2, respectively. The roll‐off frequencies in the GPS signal intensity results are considerably higher than ground‐based estimates. This difference is attributable to the satellite's high velocity (8 km/s) and the fact that, comparing to the ground estimates which are dominated by the dense plasma density between 250 and 350 km altitude, Swarm Echo observations are mostly affected by the upper reaches of the ionosphere. Key Points: The most frequently occurring spectral index of the in‐situ irregularity power spectra is 1.7The power spectra of GPS signal intensity show a single spectral index near 2, which is smaller than ground‐based weak scintillation valuesThe roll‐off frequencies in the signal intensity power spectra are 0.4–2.5 Hz, significantly exceeding ground‐based Fresnel frequency values
- Subjects
IONOSPHERIC electron density; GLOBAL Positioning System; PLASMA turbulence; LATITUDE; GPS receivers; ORBITAL velocity; POWER spectra
- Publication
Radio Science, 2024, Vol 59, Issue 5, p1
- ISSN
0048-6604
- Publication type
Article
- DOI
10.1029/2023RS007885