Study of Coulomb collisions and magneto-ionic propagation effects on incoherent scatter radar measurements at Jicamarca.

摘要

In this dissertation, Coulomb collisions and magneto-ionic propagation effects on the incoherent scatter radar measurements have been studied and analyzed in detail. The present study aims at modeling radar observations of the equatorial ionosphere carried out at the Jicamarca Radio Observatory (Lima, Peru) using antenna beams pointed perpendicular to the Earth's magnetic field B.;A Monte Carlo procedure based on the simulation of charged particle trajectories in a magnetized plasma (with suppressed collective interactions) was developed to account for the effects of Coulomb collisions on the shape of the incoherent scatter spectrum. Statistics of simulated electron and ion trajectories, single-particle ACF's, and associated Gordeyev integrals are utilized in the general framework of incoherent scatter spectrum models [e.g., Kudeki and Milla, 2006] to produce theoretical spectra for different plasma configurations. Our simulation method effectively extends the procedure of Sulzer and Gonzalez [1999] into three dimensions and is valid for all magnetic aspect angles including the direction perpendicular to B: The 3D trajectories, randomized by Coulomb collisions, are described by a generalized Langevin equation with velocity-dependent friction and diffusion coefficients taken from the standard Fokker-Planck collision model of Rosenbluth et al. [1957]. A statistical analysis of the simulated trajectories shows that the ion motion is well modeled as a Brownian-motion process with Gaussian displacement distributions (and constant friction and diffusion coefficients), in which case, an analytical expression for the single-ion ACF can be obtained [e.g., Woodman, 1967]. However, the simulated electron motions do not fit a Brownian model because the electron displacement distributions in the direction parallel to B are sharper than a Gaussian. To account for these effects on our incoherent scatter spectrum model, a numerical library of electron statistics in an oxygen plasma (single-electron ACF's) had to be developed. The library spans a set of densities, temperatures, and magnetic fields as needed for Jicamarca F-region applications.;The antenna beams used in perpendicular-to-B radar observations at Jicamarca have angular widths of the order of a degree. Within this range of small magnetic aspect angles, different modes of magneto-ionic wave propagation are excited. These characteristic modes vary from linearly polarized in the direction perpendicular to B (Cotton-Mouton regime) to circularly polarized at aspect angles greater than 0.5° (Faraday rotation regime). In order to model the magneto-ionic propagation effects on incoherent scatter radar measurements, a computer algorithm based on the Appleton-Hartree equation for electromagnetic wave propagation in a magnetized plasma was developed. Simulation studies show that magneto-ionic propagation effectively modifies the shapes of the radar beams and does have an impact on the incoherent scatter radar measurements because the polarization of the incident and backscattered fields vary as they propagate through the ionosphere.;A soft-target radar equation, which incorporates our collisional incoherent scatter spectrum and magneto-ionic propagation models, is formulated to model the radar measurements collected at Jicamarca. Voltages detected by the radar antenna are represented as the beam-weighted sum of ionospheric backscattered signals corresponding to the range of magnetic aspect angle directions illuminated by the antenna beam. This integration is carried out numerically using a finite-element-like integration method that takes advantage of the slow variation of physical parameters in the direction transverse to the geomagnetic field. The resultant radar model is utilized in the inversion of ionospheric parameters in a three-beam radar experiment conducted at Jicamarca. The experiment interleaves radar observations with perpendicular-to-B and off-perpendicular antenna beams. The data model matches very closely the different features of the measured data; for instance, it predicts the enhancement of the measured power in the direction perpendicular-to-B at ionospheric altitudes where the electron temperature is greater than the ion temperature. F-region electron density and temperature ratio (Te/T i) estimates were obtained using a least-squares inversion algorithm. The inversion results show a good agreement with ionosonde data, validating our model for incoherent scatter radar measurements.