The irradiance of the sun is primarily dependent on the wavelength of the radiant energy it emits. This is because different wavelengths correspond to different energy levels and spectral features of solar radiation. The intensity of the emitted radiation varies across different wavelengths, influencing the overall irradiance.
While other factors like the number of sunspots and solar wind particles can affect solar output and conditions in space, they do not directly determine the irradiance in the same way that the emission spectrum does.