Observations of optical lightning data with a photometer (PH4) at wavelengths of 599–900 nm on the International Space Station were conducted simultaneously with observations of low-frequency (LF) electromagnetic waves with a ground-based LF lightning locating system. The relationship between the PH4 light curve and electromagnetic waveforms in the LF band was examined for 11 lightning events. The PH4 sensor detected a small optical change even for weak light emitted from lightning discharges in clouds, including preliminary breakdown. Particularly, return strokes, including subsequent return strokes, showed a clear relationship between radiated LF waves and optical waveforms. For negative return strokes, we found a clear correlation between the absolute optical intensity and peak current. The slope of the regression line is 9.7 × 10−8 kA W−1 with an intercept of 9.9 kA.
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