Dispersive and polarimetric properties of waves in space plasma

Detta är en avhandling från Uppsala : Acta Universitatis Upsaliensis

Sammanfattning: Various aspects regarding dispersive and polarimetric properties of waves in space plasma are discussed from a theoretical viewpoint, which is extended to include numerical as well as experimental studies. From a more technical viewpoint, two scientific instruments are described. One of them is a three-axis digital radio receiver, aimed at measurg electromagnetic wave polarization from the ground or in space. The other is the dual Langmuir probe system on board the Astrid-2 micro-satellite, operational between January ll and July 24, 1999 which continuously measured the plasma density of the Earth's topside ionosphere. In more detail: The dispersive properties of small amplitude, high-frequency (HF) waves in relativistic, unmagnetized and magnetized plasm are investigated. The fully relativistic descriptn is shown to become important for electron temperatures in the 5-500 keV range and for wave frequencies about the gyro harmonics, even at lower temperatures. An exact expression for the relativistic plasma frequency is derived and shown to be proportional to the exact relativistic thermal velocity, thproportionality being the Debye length. An alternative derivation of the non-relativistic magnetized plasmpermittivity tensor is presented. Dispersion laws for electrostatic and electromagnetic waves in fully relativistic, unmagnetized Maxwell plasma are derived and compared with the corresponding dispersion laws obtained in the non-relativistic and the ultra-relativist approximations. A numerical solution of the fully relativistic dispersion equation is to this end used as a reference solution. To describe the polarimetric properties of electromagnetic waves from arbitrary directions, a generalization of the two-dimensional Stokes' parameters description of polarization to three dimensions is performed. The generalized formalism is then implemented in a three-axis digital radio receiver, primarily designed for determining the direction of arrival of electromagnetic waves. The wave normal of whistler waves, measured by the three-axis search coil magnetometer on board the Freja satellite, are analyzed. The wave normal is seen precess around the geomagnetic field B0 with an angle to B0 of 20°- 30°, indicating ducting by density structures. The Langmuir INterferometer and Density instrument for the Astrid-2 micro-atelli (LINDA) is discussed. LINDA data is used to map the earth's global plasma density at 1000h altitude. The density is shown to vary by at least four orders of magnitude, from below 10 cm-3 up to 100 000 cm-3. Seasonal variations in the auroral zone are reported and frequently occurring equatorial density enhancements are shown to be related to the Sun's rotation, and therefore to specific regions on the Sun's surface.

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