Cuesta, Manuel Enrique2023-08-212023-08-212023https://udspace.udel.edu/handle/19716/33120The solar wind is a space plasma that defines the volume of the heliosphere beyond the solar corona, the outermost atmospheric layer of the Sun. It is mainly composed of ionized hydrogen and other solar material. Once this material is heated enough to overcome the Sun’s gravitational pull, it generally flows radially away from the Sun at supersonic speeds. This outward propagation of charged particles, with the interplanetary magnetic field, is the solar wind, which has anisotropic properties. Once solar wind plasma crosses a planetary bow shock or the termination shock, it ceases to be a part of the solar wind. To accurately quantify properties of a space plasma, in-situ measurements are significantly important. In-situ measurements play a vital role in describing physical systems and provide ground truth for confirming experimental observations and numerical simulations of the same systems. When it comes to improving the understanding of physical processes in natural systems, in-situ measurements are the highest priority. However, simulations and experiments are extremely helpful when those environments are difficult to access and measure directly. ☐ Fortunately, the solar wind is one of the few space plasmas for which in-situ measurements are available thanks to many spacecraft missions. Other space plasmas that have been sampled in-situ include several planetary magnetospheres and the interstellar medium. The Voyager mission led to the only two spacecraft, Voyagers 1 & 2, that surveyed the solar wind from 1 Astronomical Unit (au) out to the interstellar medium, which surrounds the heliosphere. Other missions, such as the Magnetospheric Multiscale Mission (MMS) and Cluster, are active multi-spacecraft that have probed the near-Earth solar wind as well as magnetospheric plasma. More recently, the Parker Solar Probe (PSP) spacecraft, launched in 2018, is the first spacecraft to probe the solar wind at heliocentric distances as close as 13 Solar Radii from the Sun. ☐ This dissertation will focus on turbulence properties, including intermittency, anisotropy, and compressibility, developing from the inner-heliosphere out to radial distances as large as 10 au, as observed by several single-spacecraft. A related topic to intermittency is its scaling behavior with heliocentric distance, spatial or temporal lag, and Reynolds number. Well-known in the field of hydrodynamics are these scaling relations due to the abundant availability of data from experiments involving wind tunnels. Applying the same concepts to the solar wind, we compare for the first time the scaling relation of the kurtosis, a measure of intermittency, and Reynolds number between two vastly different physical systems, the solar wind plasma and wind tunnel experiments. ☐ Data analysis of in-situ measurements in the solar wind makes it possible to study its dynamics as it expands with increasing heliocentric distance. The data used in this dissertation comes from an array of spacecraft launched by the National Aeronautics and Space Administration (NASA), more specifically Voyager 1, Advanced Composition Explorer (ACE), WIND, Helios 1, and PSP spacecraft. Parameters measured in the solar wind that pertain to results presented in this dissertation include the magnetic field, solar wind velocity, proton density, and proton thermal velocity. ☐ Turbulence properties examined include the radial evolution of physical scales such as the correlation and ion inertial scales, the turbulent system size of the energy cascade or effective Reynolds number, and intermittency of inertial range coherent turbulent structures. The correlation and ion inertial scales are often approximated as the outer and inner scales, which are the scales that define the range of inertial scales. The outer scale separates the energy-containing scales from the inertial scales, and the inner scale separates the inertial scales from the dissipation range. I find that the inner and outer scales are observed to scale as nearly di ∼ R and λC ∼ √ R, respectively, where R is the heliocentric distance. These scales are then used to define the effective Reynolds number (Re), which is seen to scale nearly as Re ∼ R −2/3. ☐ We also examine the isotropization and evolution of the outer scale anisotropy as a function of heliocentric distance. Anisotropy refers to the preference of system dynamics relative to the direction of the interplanetary background magnetic field, in the case of the solar wind. This results in the elongation or shortening of certain physical scales, as well as energy transfer and storage preferences, along the direction of wavevectors perpendicular and/or parallel to the background magnetic field. I find that as one moves outward from 0.08 au to 0.40 au, the parallel and perpendicular correlation lengths (λ∥C and λ⊥C , respectively) isotropize from an initial anisotropy favoring longer λ⊥C . A deviation from isotropy occurs once the solar wind reaches 1 au, favoring longer λ∥C . This supports the heuristic picture that the solar wind is becoming more fully-developed within 1 au, at which point other system dynamics may begin to dominate over, such as the decreasing bandwidth of the inertial range, as shown by a systematic decrease in Re. ☐ To investigate turbulence intermittency, we compute the second-order and fourth-order structure functions, scale-dependent kurtosis, and intermittency parameters. The scale-dependent kurtosis is written as an explicit function on Re and spatial lag ℓ, allowing a systematic study of intermittency while varying Re or radial distance and keeping the spatial lag fixed to constant multiple of the inner scale. I find that at any particular scale fixed to the inner scale, the kurtosis is decreasing with increasing heliocentric distance, signaling weaker intermittency at the given scale at larger radial distances. Significantly, the kurtosis at a fixed inner scale is correlated with Re. Comparison of this relationship between the solar wind and wind tunnel experiments are presented for the first time, showing a striking similarity in both the corresponding values of the two systems as well as their scaling behaviors. ☐ Another related topic explored involves the relationship between density fluctuations and turbulent Mach number (Mt), which is important for studies of turbulent compressibility. Theories of the scaling relationship between density fluctuations and Mt are discussed, which predict a M2t scaling for a nearly-incompressible (NI) homogenous medium at large plasma beta (ratio between the thermal and magnetic pressure) or a Mt scaling based on linear wave theory as well as NI theory for an inhomogenous medium. PSP data reveals a nearly linear scaling, with its linear dependence affected by values of cross-helicity and plasma beta, reflecting the same effects as seen in 3D MHD simulations.AnisotropyCompressibilityIntermittencyReynolds numberSolar windTurbulenceThesis1398227464https://doi.org/10.58088/gjsx-yz482023-06-26en