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We present refinements of a method of ionospheric D-region sounding thatmakes opportunistic use of powerful (10<sup>9</sup>–10<sup>11</sup> W) broadbandlightning radio emissions in the low-frequency (LF; 30–300 kHz) band. Suchemissions are from "Narrow Bipolar Event" (NBE) lightning, and they arecharacterized by a narrow (10-μs), simple emission waveform. Thesepulses can be used to perform time-delay reflectometry (or "sounding") ofthe D-region underside, at an effective LF radiated power exceeding byorders-of-magnitude that from man-made sounders. We use this opportunisticsounder to retrieve instantaneous LF ionospheric-reflection height whenevera suitable lightning radio pulse from a located NBE is recorded. We show howto correct for three sources of "regular" variability, namely solar zenithangle, radio-propagation range, and radio-propagation azimuth. The residualmedian magnitude of the noise in reflection height, after applying theregression corrections for the three regular variabilities, is on the orderof 1 km. This noise level allows us to retrieve theD-region-reflector-height variation with solar X-ray flux density forintensity levels at and above an M-1 flare. The instantaneous time responseis limited by the occurrence rate of NBEs, and the noise level in the heightdetermination is typically in the range ±1 km

Low-frequency ionospheric sounding with Narrow Bipolar Event lightning radio emissions: regular variabilities and solar-X-ray responses