High frequency Faraday rotation observations of the solar corona.

摘要

The million degree solar corona generates the solar wind that in turn controls the Earth's "space weather". The solar coronal magnetic field within 0.25AU (60 solar radii) plays a critical role in the acceleration but is largely "invisible", and can presently only be measured by the Faraday rotation of high frequency electromagnetic radiation. Faraday rotation is the observed rotation in the plane of polarization of an EM wave as it traverses a magnetized medium. The amount of observed Faraday rotation is the integration along the propagation direction of the product of the component of the magnetic field parallel to the propagation vector and the electron density.;Faraday rotation is clearly useful for measuring the solar coronal magnetic field. As a remote observation, Faraday rotation measurements require careful consideration of the medium in the analysis. This thesis details the theory of Faraday rotation, previous experiments observing Faraday rotation using the carrier signal from a spacecraft in superior conjunction, the equipment used for the Cassini Faraday rotation observations, the signal analysis and steps taken to acquire a Faraday rotation observation from radio frequency data, the model used to fit the observations, all ancillary data required for these steps, and all the code created for this purpose. The data and code are provided in the attached DVD media.;All previous Faraday rotation experiments observed coronal mass ejections (CMEs) producing either 'W' or sigmoid features. These observations are reproduced herein using a Taylor-state flux-rope model crossing the line of sight at different sizes, twist, and orientations, showing that Faraday rotation can be used to measure the magnetic field of CMEs.;Using a forward model to fit Faraday rotation and columnar electron density observations, a first order investigation into force balance in the solar corona was conducted. From these fits, the gradients in the magnetic and thermal pressure and the gravitational force per volume were calculated. For the solar wind to escape the gravitational force of the Sun, the magnetic and thermal pressure gradients must dominate. We show from the fits on 2002 June 20 that small adjustments to the PFSS model can provide the necessary magnetic field strengths to supply the needed pressure for solar wind flow; however, the fits from June 21st cannot. The closest approach of the June 21st measurements were all below the source surface of 2.5 solar radii indicating a problem in the use of the PFSS model to determine the structure of the coronal magnetic field below the source surface.;Large amplitude 4 minute period Alfven waves have been observed in Helios and Cassini Faraday rotation observations. Using a simple open-ended box model through which magnetohydrodynamic waves can propagate, it is demonstrated that the combination of Faraday rotation and columnar electron density observations can distinguish Alfven waves due to their lack of fluid perturbation. It is also shown that the 2nd harmonic in the Faraday rotation observations is the result of the electron density fluctuation in the magnetosonic (fast and slow) modes. This demonstrates that previous Helios observations producing the 2nd harmonic were MHD magnetosonic waves. Cassini's observation of an Alfven wave is modeled to determine the amplitude of the magnetic perturbation. If we assume that these waves are continuously generated in all directions then the wave energy flux is 1.6 x 1019W; for comparison, the kinetic energy flux of the solar wind at 1AU is 1.7 x 1020W.;With better technology and the maturity of 3D tomography, the solar radioscience community is experiencing a resurgence of interest in the phenomenon of Faraday rotation. This thesis demonstrates that Faraday rotation can be used to determine the magnetic structure of CMEs, the solar wind, and MHD waves propagating from the solar corona. These observations enable us to predict the geoeffectiveness of a CME, study force balance in the solar wind, and measure magnetic energy flux in important regions such as the solar wind acceleration region.