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Open Access Publications from the University of California

Observational Signatures of Nonlinear Interactions in the Solar Wind

  • Author(s): Bowen, Trevor
  • Advisor(s): Bale, Stuart D
  • et al.

Spacecraft observations from the interplanetary medium of our solar system reveal the presence of a magnetized super-sonic flow emanating from the sun, commonly known as the solar wind. Empirically, in-situ measurements from spacecraft suggest that the solar wind is in a turbulent state frequently occurring fluid-like systems. Though theories of non-magnetized hydrodynamic turbulence have been successfully adapted to account for plasma dynamics relevant to the solar wind (e.g. strong magnetization, multi-particle composition, non-viscous dissipation, and weak collisionality), there is lacking consensus regarding the physical processes responsible for empirically observed phenomena: e.g. compressible fluctuations, intermittent coherent features, injection of energy at large scales, and particle heating. Interpreting in-situ spacecraft measurements is often complicated by limitations associated with single point me which most often consist of a single point (or at best a few points) located near Earth. At the largest physical scales, processes associated with solar wind generation and evolution consist of temporal variation over the 11 year solar cycle, with spatial gradients extending over the large scale heliosphere, ~200 AU. At the smallest scales, heating and dissipation process can occur on electron kinetic scales corresponding to ~ kHz frequencies and centimeter length scales in the inner heliosphere. Even in observing fluid-like magnetohydrodynamic (MHD) fluctuations of the solar wind, ``easily'' measurable by spacecraft at 1 AU, significant ambiguity exists in distinguishing effects associated with plasma transport from the processes related to the generation (heating and acceleration) of the solar wind in the inner-heliosphere.

The source of the solar wind is the corona, a hot magnetized upper-atmosphere of our sun with ambient temperatures ranging from 10^5-10^6 Kelvin: orders of magnitude larger than the solar photospheric surface at 5800 Kelvin. Even the roughest estimation of the coronal energy budgets suggest that the magnetic field must be responsible for heating the corona to these temperatures. However, the specific processes which drive coronal heating, and subsequently accelerate the solar wind, are yet unknown; though many models of coronal heating exist, little empirical evidence is currently available to distinguish between theories.

The NASA Parker Solar Probe (PSP) mission, launched in August 2018, recently became the closest human-made object to orbit the sun. During its closest perihelion approach, PSP will reach an altitude of 9.8 solar radii (0.045 AU), well within the expected boundary between the solar wind corona, known as the Alfven point. By measuring the local plasma environment, PSP will provide an empirical understanding of the processes responsible for coronal heating and solar wind acceleration which cannot be observed using remote sensing techniques. In addition, through studying the turbulent environment present in the inner heliosphere, PSP will inevitably make significant contribution to our understanding of magnetized turbulence and the role it plays in shaping astrophysical systems.

This dissertation highlights the development of observational techniques and instrumentation used in studying nonlinear dynamic processes, e.g. turbulence and plasma instabilities, in astrophysical plasmas. Part 1 consists of a discussion of incompressible magnetohydrodynamic turbulence in the solar wind and the observed coupling with compressible fluctuations. Chapter 1 contains an overview of the historical and mathematical development of MHD turbulence based on both empirical observations from spacecraft and theory of hydrodynamic turbulence. Chapter 2 contains original research on the effect of intermittency on the observational signatures of MHD turbulence. Chapter 3 discusses the the nature of compressible fluctuations in the solar wind based on the mathematical and observational techniques developed in Chapter 2. Chapter 4 describes an observational study which examines the existence of parametric mode coupling in the solar wind which could drive compressible fluctuations as well as initiate non-linear turbulent interactions in the heliosphere.

Part 2 surveys the calibration and operation of the PSP/FIELDS magnetometer suite. Chapter 5 highlights the operation and calibration of the PSP/FIELDS DC fluxgate magnetometer (MAG). Chapter 6 consists of an overview of the PSP/FIELDS search coil magnetometer (SCM) and an in depth discussion of instrument calibration through the framework of linear time invariant filter design. Chapter 7 describes a merged fluxgate and search coil data product for PSP created using optimal filter design techniques.

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