Date of Award

Summer 2024

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

Physical Sciences

Committee Chair

Kshitija Deshpande

Committee Co-Chair

Matthew Zettergren

Committee Advisor

Kshitija Deshpande

First Committee Member

Matthew Zettergren

Second Committee Member

Jonathan B. Snively

Third Committee Member

Seebany Datta-Barua

College Dean

Peter Hoffman

Abstract

A radio wave propagating through a structured or turbulent ionosphere undergoes multiple effects, such as refraction, diffraction, etc., that distort the incident radio wave by inducing phase and amplitude fluctuations. These fluctuations are called ionospheric scintillation. Scintillation effects can be detrimental to Global Navigation Satellite Systems (GNSS) such as Global Positioning System (GPS), but the observed effects can be used as a tool to study the underlying plasma process that causes scintillation. Scintillation is commonly seen in equatorial and high-latitude regions. This study centers around the scintillation and its causative plasma processes that dominantly happen in the high-latitude ionosphere over the auroral regions (or the auroral ionosphere where we see northern and southern lights). The aim of this dissertation is to investigate the impact of ionospheric density irregularities on radio wave propagation by studying their characteristic features and ionosphere conditions during radio scintillation. We employ a modeling approach using a radio wave 3D propagation model: Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere (SIGMA), interfaced with various plasma density models, including spectral models, namely Hybrid, Shkarofsky, stochastic model, Configuration Space Model (CSM), and plasma-based density model Geospace Environment Model of Ion-Neutral Interactions (GEMINI), to simulate the radio wave scintillation through ionospheric density irregularities. SIGMA simulates a GPS signal propagated through a phase screen from a moving satellite to the ground. The phase screen is the spatial electron number density distribution characterized using one of the density models that affect the phase of the forward propagating radio signal.

We have examined and compared the irregularity morphology of E- vs. F-region density irregularities using SIGMA coupled with spectral models (Hybrid and Shkarofsky) and inputs from auxiliary measurements, such as Incoherent Scatter Radar (ISR) and scintillation measurements from Scintillation Auroral GPS Array (SAGA). We perform an inverse method to derive the parameters describing the ionospheric irregularities by fitting the power spectral density (PSD) of the observed phase with the simulated phase. We found that the E-region density irregularities have rod-like shapes, which are elongated more in the magnetic field direction. In contrast, F-region irregularities are wing-/sheet-like structures with irregularity extension along the magnetic field and across the field lines. Notably, the spectral slopes for E- and F-region irregularities are found to be different.

We have also utilized the plasma-density model GEMINI to investigate how energetic electron precipitation generates density irregularities leading to radio scintillation. We focus on modeling the density structures arising from impact ionization, which is controlled by total energy flux (Q) and the electron characteristic energies (E0). We use camera data that helps us characterize the auroral waveform by revealing its arc width, motion, and edge gradient scales. We found that the scintillation is stronger when the energy flux is higher, with the arc forms moving faster. Small-scale precipitation is proven to be the major source of diffraction effects that contribute to amplitude fluctuations.

We further investigated the phase and amplitude scintillation events to determine the ionospheric conditions and plasma structuring that lead to the diffraction and refraction effects. We utilized the Rytov method, a well-known analytical model for estimating irregularity parameters by analyzing observed log amplitude and phase spectrum. We examine the phase and amplitude fluctuations observed over Poker Flat recorded by SAGA receivers. We run SIGMA over 4-D grid space using the inputs obtained from a) Rytov analysis and b) a Rytov-independent Inverse analysis, extract the simulated PSD that best fits the observed PSD to find the optimal values of irregularity parameters which describe the ionospheric conditions. In addition to the modeling study, we investigate the spectrum of amplitude and phase fluctuations to estimate different irregularity scale sizes responsible for these fluctuations (energy cascading).

We finally compare the scintillation signatures caused by density irregularities generated by different density models, such as gradient drift instability (GDI)- generated structures vs. precipitation-generated structures vs. spectrum of density structures generated using spectral and stochastic models. This dissertation comprehensively analyzes the characteristic features of auroral density irregularities, their structuring, and the underlying plasma processes that help us study the effects of ionospheric density irregularities on radio wave propagation.

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