On the Relationship Between the Rate of Change of Total Electron Content Index (ROTI), Irregularity Strength ( C k L ), and the Scintillation Index ( S 4 )

We present a new quantitative theory for the rate of change of total electron content index (ROTI) by noting its straightforward relationship to the phase structure function of ionospheric turbulence. The theory provides the dependence of ROTI on the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of the irregularities. We also present useful approximations to the full theory that elucidate the principal dependencies. We show, for example, that ROTI varies with the effective scan velocity ( V eff ) to the power ν − 1/2, where 2ν + 1 is the spectral index of the 3‐D irregularities. This dependence on V eff persists in the ratio of ROTI to the intensity scintillation index ( S 4 ), which thus depends on the particular viewing geometry for each satellite. While irregularity strength cancels when forming this ratio, the dependence on Fresnel scale remains. We validate the theory by comparing predictions of irregularity strength ( C k L ) and S 4 from 1‐Hz ROTI measurements with calculations derived from 20‐Hz intensity samples collected at Ascension Island. The theory explains the observations well, except when the scan is directed nearly along the field‐aligned irregularities (within 20° of meridional). For cross‐field scans the standard deviations of the error in predicting S 4 from 1‐Hz ROTI were 0.05, 0.11, and 0.14 for weak, moderate, and strong scintillation, respectively. These encouraging results suggest the possibility of using dense networks of inexpensive total electron content monitors to provide quantitative diagnostics of ionospheric scintillation and the irregularities that cause them.