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Correlation of geomagnetic field changes with ionospheric motions determined by a doppler technique Lewis, Trevor John

Abstract

Standard measurements of the geomagnetic field at the earth's surface are correlated with doppler measurements. The doppler shift in the frequency of a radio wave travelling in the ionosphere measures the change in refractive index with respect to time, integrated along the wave's path. For a vertical path the integrated change in electron density is measured, and the ionosphere's vertical motion can be deduced, using simplifying assumptions. A definite correlation between short period (less than four minutes) geomagnetic field changes and the doppler shift has been found. For longer period changes, evidence for a correlation is not so definite. The magnitude of the ratio of the doppler shift (in a frequency of 4 or 5 Mc.) to the observed geomagnetic field change for short period events is between 0.1 and 0.4 cps/ɣ and for longer events, 0.05 to 0.15 cps/ɣ. Sometimes the doppler shift is proportional to ΔH or ΔD. However no relation that would hold in general to predict the doppler shift from the observed geomagnetic changes was found. Apart from the diurnal variation in the doppler shift, three types of uncorrelated events were found. The first was a continuous variation occurring during the daytime which has been observed by many workers. The second was a large travelling ionospheric disturbance which followed a sudden change in the geomagnetic field, and thus might be considered indirectly to be correlated. The last type was a train of large amplitude irregular oscillations with periods from about 3 to 0.5 minutes, the longer periods appearing first. The problem of determining ionospheric motions from the doppler shift is very complex, and in order to make any progress, certain simplifying assumptions must be made which cannot be completely justified. By assuming also that the observed long period geomagnetic field changes are caused by overhead currents, and by using average ionospheric conductivities, the electric field in the ionosphere is calculated, the values ranging between 10⁻⁷ and 10⁻⁶ e.s.u. For shorter period geomagnetic field changes, the doppler shift is larger (approximately by a factor of 3) than the value which has previously been calculated by assuming that overhead currents cause the geomagnetic changes.

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