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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?

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The point here is the "the solar spectrum" received from "the Sun" 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 "Wien tail" 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.

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The point here is, that the kink you're pointing out is not belonging to the BB-spectrum of the sun. This part of the solar spectrum is a classic example for a broad class of radiation, called nonthermal radiation. The physical origin of this is in general that, every accelerated particle will radiate part of its energy away, with the experienced acceleration a, radiated power P and their relation $P \sim a^2$ in a wavelength-range depending on the exact process.
The origins of those accelerations are usually thought to be strong magnetic interactions on the solar surface plasma, creating the famous sunspots and coronal loops. In those loops, electrons and protons are being shot into space, following the loop, and then return back onto the solar 'surface'. Upon return they release Bremsstrahlung, as the interactions between loop-particles and surface plasma create strong braking accelerations.

Spectrae for such processes are derived in every standard textbook about theoretical astrophysics and are given by roughly $\frac{dP}{dt} \sim exp(-\frac{h \nu}{k_BT})$, which should fit the UV-part in the spectrum you gave pretty well. This now also explains the variability of this part of the solar spectrum: As the number of loops and sunspots correlates with solar activity, it is clear that the UV-bremsstrahlung-production should too.
There is however much more to this, if you are interested. Given certain violent outbreaks that happen upon release of magnetic energy, there are modulations of the emitted radio flux (also indicated in your graph) up higher-energetic regions of x-rays.

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  • $\begingroup$ The UV and X-ray radiation from the corona and chromosphere is thermal radiation. $\endgroup$
    – ProfRob
    Nov 16 '20 at 11:11

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