ISIS 2 orbits at an altitude of about 1400 km and sounds the ionosphere in the range from 0.1 MHz to 20 MHz. On analyzing a sequence of ionograms measured on April 26, 1971, covering a region from about −73° to 88° in geographic latitude along a trajectory that is mostly close to 80°W longitude, at about 1000 UT or about 5 a.m. local time, one finds large-scale structures in the spatial fluctuations of the electron density above the F peak. The structural features are analyzed by using isoelectronic contours, separating out trends in the data to obtain the spatial fluctuations in the altitude for various levels of electron density. The altitude variations for the various electron density levels strongly correlate, and spatial fluctuation waves are found on the order of 2000 km in scale size. Another example is shown for February 20, 1975, and is for an orbit that is in near conjunction with the Atmosphere Explorer C (AE-C) satellite. AE-C is at a much lower altitude, about 250 km, and measures, among many parameters, in situ ionization density. Comparison of the structures observed by ISIS 2 and AE-C in the electron density fluctuations relates features above and below the F peak; the same spatial wave system is found in the data of both satellites with scale sizes from about 600 km to more than 2000 km.
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