The Neupert effect refers to an empirical tendency for high-energy ('hard') X-ray emission to coincide temporally with the rate of rise of lower-energy ('soft') X-ray emission of a solar flare.^[1] Here 'hard' and 'soft' mean above and below an energy of about 10 keV to solar physicists, though in non-solar X-ray astronomy one typically sets this boundary at a lower energy.

This effect gets its name from NASA solar physicist and spectroscopist Werner Neupert, who first documented a related correlation (the integral form) between microwave (gyrosynchrotron) and soft X-ray emissions in 1968.^[2] The standard interpretation is that the accumulated energy injection associated with the acceleration of non-thermal electrons (which produce the hard X-rays via non-thermal bremsstrahlung) release energy in the lower solar atmosphere (the chromosphere); this energy then leads to thermal (soft X-ray) emission as the chromospheric plasma heats and expands into the corona.^[1] The effect is very common, but does not represent an exact relationship and is not observed in all solar flares.^[3]