Combined Differential Faraday and Differential Absorption Measurements at 40 Megahertz

Abstract

Differential Faraday rotation measurements of the 40-and 41-MHz beacons of satellite 1964-64A/BE-B were made at London, Ontario (43øN, 81øW) through the use of a continuous output polarimeter of the phase comparison type, and simultaneous differential absorption measurements were made by use of the ordinary and extraordinary mode amplitudes at 40 MHz. Sufficient measurement accuracy was obtained to employ the second-order expansion of the magnetoionic refractive and absorption index equations to extract total electron contents, the ratio of the density squared integral to the total electron content, a mean ionospheric electron collision frequency, and a parameter sensitive to the height of an electron density irregularity, from the data. Two passes, orbit 17239, March 17, 1968, and orbit 17280, March 20, 1968, were selected for analysis. Peak electron densities, mean plasma scale heights, and mean electron temperatures were computed from the reduced data by the assumption of an electron density height profile; they were found to correspond to within experimental error with comparable published measurements. During both passes examined, F region electron density profile broadenings were detected that were, for March 20, attributed to adiabatic expansions of the F region electron gas, which was driven by localized neutral gas motions, and for March 17 the broadenings were attributed to thermal expansions of the F region plasma. On both occasions, the fractional errors in the data were sufficiently small over some portions of the pass to permit a determination of the dominant ionospheric absorption mechanism. the estimated (maximum) error in the measured Faraday rotation angle is -+0.4%.The two-frequency analysis method of Yeh and Swenson [1961] can thus be used to compute the 627 628 LIVINGSTONE AND LYON total electron content (corrected for second-order effects), and the second-order analysis method of Ross [1965] can be employed to extract •N•'dh/ J'Ndh. An examination of receiver stability and amplitude calibration reveals that signal amplitude variations can be measured to within ---0.04 db.