Application of spherical cap harmonic analysis to plasma convection mapping at high latitudes
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The primary goal of this work is to develop, validate, and apply a new technique for mapping the high-latitude ionospheric plasma flow (convection pattern) from velocity measurements routinely performed by the Super Dual Auroral Radar (SuperDARN) network of high frequency (HF) radars. The currently employed FIT technique relies heavily on assumptions that are not always justifiable. A spherical cap harmonic analysis (SCHA) technique, traditionally used in handling geomagnetic field data, is introduced for mapping the high-latitude ionospheric convection pattern based on SuperDARN velocity measurements. The SCHA technique does not require contributions from a statistical model which is dependent on the magnitude and orientation of the interplanetary magnetic field (IMF), and does not confine the high-latitude flows to a specific region based on magnetic latitude. Several steps are taken to validate the SCHA convection mapping technique. First, it is demonstrated that the SCHA technique can reproduce an arbitrary pattern based on simulated data modified by a random noise component. SCHA maps of the global scale plasma flow pattern for various IMF conditions are next shown to be consistent with expectations for patterns reported in the literature. SCHA maps are compared to ion drifts measured by the Defense Meteorological Satellite Program (DMSP) satellites and with convection vectors inferred by merging SuperDARN measurements at beam crossings. The SCHA technique is shown to perform comparably to the FIT technique over regions of good data coverage. The SCHA technique provides a better representation of the ionospheric convection pattern for regions with limited data coverage and over regions of highly variable flow, particularly near the equatorward edge of the mapping region. SCHA analysis of SuperDARN data to create convection maps is expanded to include magnetometer measurements of the perturbation magnetic field. Plasma flow is determined from magnetometer data by combining the equivalent current determined from the external component of the perturbation magnetic field with a model of the ionospheric conductivity. The SCHA technique is used to investigate the reconfiguration of the convection pattern and changes to the cross polar-cap potential (CPCP) associated with a sudden transition in the vertical component of the IMF from stable positive to stable negative values. For such events, the FIT technique might misrepresent the convection pattern if the fitting is dominated by the a-priori statistical convection model. Both magnetometer and SuperDARN data sets are examined. The IMF transition wavefront impinges upon the magnetosphere near the 10 MLT sector; perturbations are clearly seen on the dayside with a ~10 minute delay on the nightside. This translates into a dayside-to-nightside progression of the ionospheric response observed in the magnetic perturbations and SuperDARN velocities, contrary to what was reported for a number of other events in the literature. The foci of the new dawn and duskside convection cells are shown to steadily shift toward the dayside (over a period of 10-12 minutes, beginning 4-6 minutes after the onset of the ionospheric response) and do not 'snap' to their final position. Once the convection foci reach a final location, the overall convection pattern enhances for a period of ~25 minutes. These results support the idea that the ionospheric convection response to a southward turning of the IMF is a two-stage process; (1) an initial dayside-to-nightside progression of the observed ionospheric response and a reconfiguration of the convection pattern, and (2) the subsequent intensification of the convection pattern. An additional investigation is performed to determine whether the polar cap north (PCN) magnetic index is satisfactory for estimating the CPCP and average cross polar cap flow velocity (CPCV). A roughly linear increase of both the CPCP and CPCV with PCN is found for 0 ≤ PCN ≤ 4, with a tendency for saturation for PCN > 4. The CPCP calculated using the SCHA-technique is found to be larger than that calculated using the FIT-technique. PCN is concluded to be a good proxy for the CPCV and CPCP for 0 ≤ PCN ≤ 4.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentPhysics and Engineering Physics
SupervisorKoustov, Alexander V.; Boteler, David H.
CommitteeHussey, Glenn H.; McWilliams, Kathryn A.; Morozov, Igor; Smolyakov, Andrei
Copyright DateDecember 2011
spherical cap harmonic analysis