Basics of Vacuum-UV Spectroscopy

VUV spectroscopy is the key tool to determine remotely the thermodynamic state and dynamics of astrophysical plasmas. Fundamental differences exist between spectroscopic data and filtergraph imagery:
  • Spectroscopy reveals details of the local plasma dynamics in the remotely sensed area, in contrast to imaging which only permits to infer unresolved apparent motions in that region.
  • Spectra provide information on the line of sight component of resolved bulk flows (via line Doppler shifts) and unresolved micro- and macro-turbulence (via line broadening). It complements ideally with the information on motions on the plane of sky derived by imaging.
  • The radiation of a distinguished emission line is spectrally clean and follows the strict rules of atomic physics.
Hires VUV spectroscopy is still limited to slit spectrographs that need to step the slit in successive exposures to cover a given field of view. This leads to the unavoidable trade-off between valuable information available in spectra and the loss of spatial and temporal simultaneity across the FOV. Thus, spectroscopy is ideally and naturally complemented by imaging and a wise combination of spectra and images is the most powerful tool to study the solar atmosphere.

As an example we show a macrospicule observed in the light of the O V line at 62.9 nm, here shown as radiance image (right) and as Doppler image (left). The Doppler flows are scaled from +30 km/s (red) to -30 km/s (blue). The Doppler image clearly shows that the spicule swirls like a tornado along the magnetic field, a motion that would not be so obviously detectable having only images.