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The approximation of spherical symmetry of electron density is the main error source in the inversion of ionospheric radio occultation (IRO). The study in this paper shows that only even terms of the asymmetry contribute to the inversion error. An asymmetry factor for IRO is defined, and it is approximately equal to the relative error of the inversion result. The characteristics of the asymmetry factor are studied and are shown in the following: (1) the absolute value of asymmetry factor (AVAF) of a zonal occultation is much smaller than that of the meridian one; (2) higher solar activity corresponds to larger AVAF; (3) AVAF is largest in winter, second largest in equinox season, and smallest in summer; and (4) generally, AVAF at middle geomagnetic latitudes is smaller than at other latitudes, and it is large at the magnetic equator, the crest region, and the middle‐latitude trough. The asymmetry factor is applied to correct the retrieved electron density, and the inversion error is greatly reduced.
Wu et al. (Sun,) studied this question.