Broadband spectrum of Sun Broadband spectrum is one which is inclusive of all the observed wavelength ranges. In the case of the BB spectra of our Sun, there is a sharp (non-differentiable) kink in the curve in the UV-range (as shown above). What is the significance of this kink?
The point here is the &quot;the solar spectrum&quot; received from &quot;the Sun&quot; actually comes from different places depending on the frequency. The (stylised) smooth curve at optical and IR frequencies is pseudo-blackbody radiation from the photosphere. Pseudo-blackbody, because the radiation comes from different depths and temperatures in the photosphere, which gives rise to absorption features.
The photosphere is not hot enough to produce significant amounts of blackbody UV radiation, which accounts for the sharp &quot;Wien tail&quot; dying away into the UV.
The dashed lines in the UV and X-ray frequency range are emission that comes from the chromosphere and corona. i.e. They arise from a physically distinct region and thus there is no reason why they should form a continuous curve with the radiation from the photosphere.
The UV and X-ray emission is a combination of thermal radiation from hot plasma in the corona (at $\sim 10^6$ K), which produces continuum bremsstrahlung and recombination lines from highly ionised metal ions like Fe X or O VI, and plasma at $10^4$-$10^5$ K in the chromosphere and transition region that is dominated by discrete recombination lines produced by ions like Mg II and C IV.
The reason that a discontinuity can persist is that the photosphere, chromosphere and corona are not radiatively coupled. i.e. The underlying photosphere is not the source of heat for the chromosphere and corona. The heating agent is the solar magnetic field.