The extreme ultraviolet (EUV) solar spectrum is considered in this review to cover a decade in wavelength from about 300 Å to about 3000 Å. The lower end is close to the practical limit of normal-incidence optics, and the upper end is the approximate limit of visibility from Earth’s surface. The solar plasma that gives rise to emission within this interval is very complex and covers a huge range of physical conditions. Temperatures range from about 4 × 103 K (the temperature minimum in the low chromosphere, observed in the 1600 Å continuum) to about 4 × 106 K. (corresponding to emission of the FeXVI doublet at 335 and 361 Å). The density of the emitting plasma ranges from 1015 cm-3 in the upper photosphere to 108 cm-3 in the quiet corona. Major types of energy transport and deposition within the plasma include not only radiation, but also acoustic waves, magnetohydrodynamic waves, thermal conduction, and convective flow. Magnetic energy often completely controls detailed structure and energy balances within the plasma. Inhomogeneities are not a small perturbation on the overall structure of the plasma, but rather may completely dominate that structure. Extreme departures from local thermodynamic equilibrium (LTE) are the rule rather than the exception. (It is interesting to contrast this complex situation with that seen in the next wavelength decade, from 3000 Å to 3 μ, which includes the visible spectrum. With the exception of a few strong chromospheric lines, this radiation emerges from a comparatively isothermal (5000 K<T<6000 K), horizontally homogeneous, atmosphere in hydrostatic and radiative equilibrium, in which LTE is the rule rather than the exception.)
A new solar spectrum in the extreme ultraviolet.