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Global study of Equatorial Plasma Bubbles based on GPS Radio Occultation from FormoSat-3/COSMIC

GPS radio occultation technique showing progression of tangential point for deriving vertical profiles from series of occultations.
Lupe

Ankur Kepkar (Sucessful finalization September 2018)

The study focuses on providing a climatology of equatorial scintillation phenomenon, so called plasma bubbles, occurring at low latitude region of the ionosphere. For this thesis work, a decade long GPS radio occultation measurements obtained from the constellation of FormoSat-3/COSMIC (2007-2016) is taken into account. The six FormoSat-3/COSMIC satellites provide in total about 2,000 radio occultation profiles per day on an average. About 5 million profiles were processed for this study, of which 0.2% contained strong disturbances in the ionospheric F-region.

For investigating disturbances, we analyse electron density data from the vertical GPS radio occultation profiles. This method is based on satellite-satellite tracking on high-low mode. The geometry of this technique requires low Earth orbiting satellite to track the L- band frequencies of GPS signal as it passes the Earth's atmosphere crossing Earth's limb. This weather independent technique provides global coverage, high accuracy and a high vertical resolution. The GPS signals contain information on prevailing ionospheric conditions due to the refraction of the GPS electromagnetic waves induced by electron density gradients at ionospheric altitudes.

Strong electron density gradient in the F-region of the dispersive ionosphere accounts for GNSS signals to refract the most. This refraction of GPS signals result in weakening of the signal intensity at reception. Therefore, variations in the Signal-to-Noise (SNR) can be associated with vertical changes in the electron density that mainly occur in line with irregularities in the Earth's ionosphere.

Since the GPS-RO technique is relatively new and emerging technique, additional validation with different techniques were done against encountered irregularities in the F-region. A comparative study was performed to ensure integrity with magnetospheric data obtained from ESA's Swarm satellite mission and ground based Ionosonde data. These validations were used to correlate the high scintillating events from the GPS-RO technique with flagged bubble and spread F events from Swarm and ionogram readings respectively.

From the analysis performed, it is observed that the F-region irregularity phenomenon is associated with plasma bubbles occurring mainly at night-time close to the Earth's magnetic equator. Distinctive seasonal variations as well as dependency on solar cycle can be seen from such investigations when analysed for different years of data. This phenomenon is traditionally explained as a consequence of plasma instability. In contrast to the other contemporary techniques, GPS radio occultation technique uniquely personifies altitude resolution of the plasma bubbles on a global scale.

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