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The characteristics and world-wide propogation of PI 2 micropulsations Rostoker, Gordon

Abstract

Over the years, the study of the aurora has drawn on many different aspects of physics and geophysics in an attempt to explain this common phenomenon. Two prominent magnetic effects which are associated with auroral displays are geomagnetic bays and Pi 2 micropulsations. Both these geomagnetic perturbations attain a maximum amplitude in the auroral zone, but may also be seen in mid and low latitude regions. A bay may be described as the magnetic field of a westward flowing current in the auroral zone coupled with an eastward flowing return current in the mid-latitude and polar cap regions. The means by which Pi 2 micropulsations propagate from the auroral zone centre of activity to mid and low latitude field points is not as yet known. To study the propagation characteristics of Pi 2 micropulsations, a set of five fluxgate magnetometer units was set up stretching across the continent from Victoria, B.C. to Montreal, P.Q. with all stations lying on approximately geomagnetic latitude 56°N. Continuous recordings of H, D and a filtered H component (bandpass 25 - 200 sec.) were made from June 1, 1965 till Aug. 15, 1965. Further information on pertinent disturbances was obtained from stations distributed over the continents of North America and Antarctica. A theoretical study of some of the possible modes of propagation was carried out and specific characteristics of each mode were established in order to permit a check with the experimental data obtained. The pulsations of interest were digitized and power spectra as well as Fourier spectra were computed for the H and D components. The position in longitude of the source of the disturbance was determined making use of pulsation amplitudes, bay amplitudes, and the direction of the disturbance vectors in the horizontal plane. An intensive investigation of the polarization of the pulsations in the horizontal plane was carried out, making use of the complete chain of several cycles but drawing each cycle separately for comparison purposes. This analysis disagrees with previous claims that Pi 2 pulsations are polarized anticlockwise after local midnight and clockwise before local midnight. On the basis of the analysis performed with the data, it was found that the mode of propagation most likely responsible for the transfer of Pi 2 activity from high to mid latitudes could be described by a plane electromagnetic wave propagating approximately normal to the earth's magnetic field lines through the lower E-region of the ionosphere. It is found that the maximum period in a Pi 2 event increases with increasing Kp index in the range Kp ~0 + to 2o. This complements the earlier observation that the dominant period in a Pi 2 event decreases as the Kp index increases; this fact is also verified in the analysis of the experimental data. It is found that the number of peaks in the frequency spectrum of a Pi 2 event increases approximately linearly with increasing Kp .

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