Date of Award
2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Space Science
Committee Chair
Gary P. Zank
Committee Member
Jakobus Le Roux
Committee Member
Laxman Adhikari
Committee Member
Lingling Zhao
Committee Member
Alexandher Pitna
Research Advisor
Gary P. Zank
Subject(s)
Solar cycle, Solar wind, Plasma turbulence, Space plasmas
Abstract
This thesis studies the effect of the solar cycle and interplanetary shocks on turbulent fluctuations at 1 AU. We examine the solar cycle dependence of turbulence cascade rates corresponding to the total turbulence energy using the methodology developed by Adhikari et al. that utilizes a Kolmogorov phenomenology. Turbulence cascade rates are derived using 25 years (1995–2020) of Wind spacecraft plasma and magnetic-field observations. We find that the total turbulence energy cascade rate changes with the solar cycle, such that the cascade rate is largest during solar maximum and smallest during solar minimum. It is positively correlated with the solar wind speed and temperature. The cascade rate also shows a clear dependence on the angle between the mean magnetic field and velocity field (θ_UB); it is largest near θ_UB ∼ 90◦ and smallest near 0◦ or 180◦. We then decompose the fluctuations into 2D and slab components and find that the 2D turbulence heating rate is higher than the slab heating rate. To explore how the solar cycle modulates individual magnetohydrodynamic (MHD) modes, the Linear Mode Decomposition technique of Zank et al. is applied to Wind data. This technique decomposes the overall turbulent fluctuations into various propagating MHD modes: Alfvén (forward and backward), fast (forward and backward), and slow (forward and backward) modes, as well as non-propagating modes: entropy and magnetic island modes, from solar wind intervals during different solar cycle phases. The results show that the amplitudes and energies of all modes are ∼1.5–4.5 times larger during solar maximum. The entropy mode dominates density fluctuations, the magnetic island mode dominates magnetic field fluctuations, and the Alfvénic modes dominate velocity fluctuations, providing the first observational evidence for solar cycle influence on linear MHD mode amplitudes. However, the inertial-range spectral slope of various modes does not change notably over the solar cycle. Finally, the LMD technique is applied to the upstream and downstream regions of 84 fast-forward quasi-perpendicular shocks to examine the transmission of various turbulent modes across the shocks. A superposed-epoch analysis demonstrates that the intensities of various MHD modes increase by ∼4–14 times from upstream to downstream, with the entropy, magnetic-island, and Alfvén modes being the principal contributors to density, magnetic-field, and velocity fluctuations, respectively. The downstream spectral slope of various modes closely follows the Kolmogorov-like slope, i.e., f^−5/3, but the upstream spectrum is flatter than that downstream, suggesting increased energy dissipation or stronger turbulent cascade downstream of the shock. This result confirms quite spectacularly that the shock-turbulence interaction does indeed generate and amplify MHD modes during the transmission process.
Recommended Citation
Gautam, Sujan Prasad, "Influence of the solar cycle and interplanetary shocks on turbulence at 1 AU" (2026). Dissertations. 479.
https://louis.uah.edu/uah-dissertations/479