Collisionless Plasma Physics below the Ion Scales: Instabilities, Reconnection, and Turbulence
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Liu-zhuol-phd-nsecse-2026-thesis.pdf
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Author(s)
Liu, Zhuo
Advisor(s)
Loureiro, Nuno F. G.
Date Issued
February 2026
Publisher
Massachusetts Institute of Technology
Abstract
Weakly collisional plasmas dissipate and reorganize energy primarily at and below ion kinetic scales, where wave-particle interactions, magnetic reconnection, and turbulence are intricately coupled. This thesis advances a first-principles understanding of several fundamental processes via numerical simulations and analytical theories.
The first part of this thesis reexamines the long-standing hypothesis that ion-acoustic turbulence (IAT) provides the anomalous resistivity potentially responsible for fast, collisionless magnetic reconnection.
Using first-principles Vlasov-Poisson simulations, we show that IAT-driven resistivity is transient and weak -- insufficient to sustain reconnection electric fields or regulate outflows -- thereby ruling out IAI as a possible source of anomalous resistivity for reconnection events.
The remainder of the thesis focuses on reconnection-mediated decaying turbulence below the ion scale. To understand magnetic reconnection in this regime, we develop an analytical theory of "electron-only" reconnection, which predicts reconnection rates that increase with decreasing structure size. These predictions are verified through gyrofluid and fully kinetic particle-in-cell (PIC) simulations across a range of values of plasma beta and system sizes, establishing the first self-consistent theoretical framework for reconnection below ion scales.
Building on this foundation, we investigate the interplay between magnetic reconnection and turbulence in strongly magnetized, low-beta plasmas. We show that sub-ion-scale reconnection-mediated turbulence differs fundamentally from its magnetohydrodynamic (MHD) counterpart. A new, time-dependent scaling theory of the sub-ion energy cascade is derived and validated through gyrokinetic-reduced simulations, revealing that small-scale magnetic structures can undergo successive coalescence, enabling magnetic energy to transfer inversely toward system-size scales.
Finally, we extend this analysis to the high-beta, weakly magnetized regime relevant to magnetogenesis problem. PIC simulations initialized with magnetic fields relevant to a Weibel-saturated state demonstrate that pressure-anisotropy-driven microinstabilities, particularly the firehose instability, remove magnetic tension, suppress reconnection-mediated coalescence, and ultimately arrest the inverse cascade. This identifies a microphysical mechanism that limits the growth of large-scale magnetic fields from small-scale seeds and delineates the plasma conditions under which inverse transfer is either inhibited or permitted.
Together, these studies elucidate how kinetic turbulence, microinstabilities, and electron-scale reconnection collectively determine energy dissipation pathways and magnetic-field evolution in weakly collisional plasmas. The results have broad implications for heliospheric, magnetospheric, and astrophysical systems, including solar wind turbulence, planetary magnetotail dynamics, intracluster-medium heating, and the origin of cosmic magnetic fields.
MIT Department
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
Massachusetts Institute of Technology. Center for Computational Science and Engineering
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