The direct illumination of the solar disk is an "unwanted" component of the light that UVCS measures. Thus, many of the UVCS mechanisms are devoted to blocking out this light while simultaneously letting in the light from the much dimmer extended corona.
Below is a portion of the ultraviolet spectrum of the solar disk as measured by the SUMER instrument on SOHO:
By far, the brightest emission line in the ultraviolet is the Lyman alpha transition of neutral hydrogen at a wavelength of 1216 Angstroms.
Note that, in the ultraviolet, the solar spectrum is composed mainly of emission lines, which have an excess of intensity at the wavelengths of discrete electron transitions in atoms. This is the opposite of the solar spectrum at visible wavelengths, which is composed mainly of absorption lines (i.e. dark lines on a bright "continuum").
The difference between the two regions arises because of the radial variation of temperature T in the solar atmosphere:
Why is this important? Spectral lines are narrow regions of higher "opacity" compared to the surrounding wavelengths. That is, the interactions between photons and the gas is more efficient at the line wavelengths, and thus when we look at the solar disk, our line of sight is "stopped" by the plasma much sooner for photons at the line wavelengths, while photons at non-line wavelengths penetrate much deeper. Thus, in the UV, the line photons see a hotter (brighter) gas than the non-line photons - giving rise to lines that are brighter than the light at surrounding wavelengths (i.e., an "emission spectrum"). Conversely, in visible wavelengths, the line photons see a cooler (less bright) gas - thus giving rise to lines that are darker than the light at surrounding wavelengths (i.e., an "absorption spectrum").
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