Jeffrey A. Tessein

SPECTRAL INDICES FOR MULTI-DIMENSIONAL INTERPLANETARY TURBULENCE AT 1 AU

We examine Advanced Composition Explorer and Helios 1 data in search of evidence for an anisotropic spectrum of interplanetary magnetic and velocity field fluctuations. Specifically, we focus on the power-law indices of the fluctuation spectra and associated second-order structure functions and ask whether the index varies systematically with the angle between the mean magnetic field and the wind velocity. We extend previous results to show convincingly that it does not. Several popular theories for magnetohydrodynamic turbulence predict a significant variation as part of the turbulent cascade dynamic. We offer some observations on why the predicted anisotropy is not present.

ASSOCIATION OF SUPRATHERMAL PARTICLES WITH COHERENT STRUCTURES AND SHOCKS

Various mechanisms have been proposed to explain observed suprathermal particle populations in the solar wind, including direct acceleration at flares, stochastic acceleration, shock acceleration, and acceleration by random compression or reconnection sites. Using magnetic field and suprathermal particle data from the Advanced Composition Explorer (ACE), we identify coherent structures and interplanetary shocks, and analyze the temporal association of energetic particle fluxes with these coherent structures. Coherent structures having a range of intensities are identified using the magnetic Partial Variance of Increments statistic, essentially a normalized vector increment. A stronger association of energetic particle flux in the 0.047-4.75 MeV range is found with intense magnetic discontinuities than is found with shocks. Nevertheless, the average profile of suprathermals near shocks is quite consistent with standard models of diffusive shock acceleration, while a significant amount of the energetic particles measured and strong discontinuities are found by ACE within six hours of a shock. This evidence supports the view that multiple mechanisms contribute to the acceleration and transport of interplanetary suprathermal particles.

EFFECT OF COHERENT STRUCTURES ON ENERGETIC PARTICLE INTENSITY IN THE SOLAR WIND AT 1 AU

Solar energetic particles in the solar wind are accelerated in both solar flares and shocks assocated with fast coronal mass ejections. They follow the interplanetary magnetic field and, upon reaching Earth, have implications for space weather. Space weather affects astronaut health and orbiting equipment through radiation hazard and electrical infrastructure on the ground with ground induced currents. Economic impacts include disruption of GPS and redirection of commercial polar flights due to a dangerous radiation environment over the poles. By studying how these particles interact with the magnetic fields we can better predict onset times and diffusion of these events. We find, using superposed epoch analysis and conditional statisitics from spacecraft observations that there is a strong association between energetic particles in the solar wind and magnetic discontinuities. This may be related to turbulent dissipation mechanisms in which coherent structures in the solar wind seem to be preferred sites of heating, plasma instabilites and dissipation. In the case of energetic particles, magnetic reconnection and transport in flux tubes are likely to play a role. Though we focus on data away from large shocks, trapping can occur in the downstream region of shocks due to the preponderance of compressive turbulence in these areas. This thesis lays the ground work for the results described above with an introduction to solar wind and heliospheric physics in Chapter 1. Chapter 2 is an introduction to the acceleration mechanisms that give rise to observed energetic particle events. Chapter 3 describes various data analysis techniques and statistics that are bread and butter when analyzing spacecraft data for turbulence and energetic particle studies. Chapter 4 is a digression that covers preliminary studies that were done on the side; scale dependent kurtosis, ergodic studies and initial conditions for simulations. Chapter 5 contains that central published results of this thesis, that there is a strong

Systematic averaging interval effects on solar wind statistics

The choice of interval of averaging in computing statistics of solar wind fluctuations is known to be a sensitive issue in which the need for adequate sampling statistics must be balanced with the complications of establishing an ensemble, given that the solar wind admits inhomogeneity, structure, and variability at its sources. Here we examine the quantitative dependence of interval of averaging (sample size) on estimates of basic statistics such as means, variances, and anisotropies of the measured interplanetary magnetic field.

Turbulence spectrum of interplanetary magnetic fluctuations and the rate of energy cascade

There is growing evidence that a turbulent cascade of energy from large to small scales accounts for the dissipation of fluid energy (magnetic and velocity fluctuations) that heats the background plasma. However, much remains to be done to understand the dynamics of that cascade. We apply a structure function formalism originally derived for hydrodynamic turbulence and recently extended to include magnetohydrodynamics (MHD) to map the cascade of energy in the inertial range at 1 AU. We also examine the anisotropies associated with inertial range magnetic fluctuations in the hope of better understanding inertial‐ and dissipation‐range dynamics.

GENERATING SYNTHETIC MAGNETIC FIELD INTERMITTENCY USING A MINIMAL MULTISCALE LAGRANGIAN MAPPING APPROACH

The Minimal Multiscale Lagrangian Mapping procedure developed in the context of neutral fluid turbulence is a simple method for generating synthetic vector fields. Using a sequence of low-pass filtered fields, fluid particles are displaced at their rms speed for some scale-dependent time interval, and then interpolated back to a regular grid. Fields produced in this way are seen to possess certain properties of real turbulence. This paper extends the technique to plasmas by taking into account the coupling between the velocity and magnetic fields. We examine several possible applications to plasma systems. One use is as initial conditions for simulations, wherein these synthetic fields may efficiently produce a strongly intermittent cascade. The intermittency properties of the synthetic fields are also compared with those of the solar wind. Finally, studies of cosmic ray transport and modulation in the test particle approximation may benefit from improved realism in synthetic fields produced in this way.

Preliminary Results from SEP and ESP Studies

We are in the process of building a data base of energetic particle and magnetic field measurements in the vicinity of interplanetary shocks in order to test several existing and emerging theories for particle acceleration and wave excitation in these environs. At present we have over 40 such intervals analyzed and the set is growing. We show here that there exist clear examples of shock acceleration to ∼200 keV while in the presence of an intense energetic proton (SEP) population where the seed ions for shock acceleration come from the cold background protons that make up the thermal plasma. This happens in spite of the existence of a potential seed population made up of pre‐energized ions. We also show that at higher energies the seed ions are formed from the pre‐energized background, indicating that both sources are possible.