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:
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.
- 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.
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.