UVCS Tutorial: Atoms and Ions in the Extended Corona

The photons that we want to measure with UVCS do not come directly from the solar disk, but are instead emitted in the extended corona. Coronal photons are emitted because the corona is not a vacuum, but instead contains a tenuous ionized gas, or plasma (comprised of some neutral atoms, but mostly positive ions and the free electrons that got stripped off the ions).

UV spectral-line photons emitted in the corona arise from two primary phenomena (both roughly illustrated in this cartoon):

• Resonance scattering of UV photons from the bright solar disk. Incoming photons from the photosphere and chromosphere can impact bound atoms and ions in the corona, causing their electrons to jump into excited states. A fraction of a second later, these electrons "decay" back down to their ground level, and the atom re-emits a photon at the wavelength corresponding to the energy-difference between the two levels. Only incoming photons at the proper wavelength (i.e., in "resonance" with the atomic transition of interest) can have this effect. Unlike the incoming photons, which came from a well-defined spatial source, the outgoing photons are emitted in a random direction. Of course, we then see only those photons that are emitted in the direction of the SOHO spacecraft.

• Collisions between atoms and free electrons. In the corona, free electrons have temperatures similar to the temperatures of the other atoms and ions. Thus they have similar kinetic energies, i.e.,
  

  An electron, though, is about 1840 times lighter than a proton (thus much lighter than any atom or ion), so in order to have a similar kinetic energy as an atom or ion, it must be moving, on average, at least 43 times faster. These tiny "bullets" often ricochet off atoms, losing some of their kinetic energy in the process. Like in the scattering of photons, above, this new "input" of energy to the atom is used to temporarily raise up the energy-level of one or more of its bound electrons, and eventually this energy is re-emitted as a photon.

Visible photons emitted in the corona arise primarily from the following phenomenon:

• Thomson scattering of visible photons from the bright solar disk. Photons from the solar disk can also be scattered by free electrons; this is known in the relativistic limit as Compton scattering, and in the non-relativistic limit as Thomson scattering. In the non-relativistic limit, the scattering process does not depend on wavelength. Thus, the best wavelengths to observe this effect are those where the Sun emits the majority of its photons - this is the visible part of the spectrum. Thomson scattering is the cause of a non-zero polarization of the observed coronal radiation, and is what the UVCS White Light Channel (WLC) measures to deduce the electron density in the corona.

In the frame of reference of an atom or ion, UV spectral-line photons are emitted at nearly the exact wavelengths corresponding to the energy difference between the relevant electron energy levels. However, these emitting atoms are not sitting still in the corona. They are flowing out with a "bulk" outflow speed in the solar wind, and they also have a "random" component of their velocity that corresponds to their temperature. The photons that we see get Doppler shifted because they come from moving sources. This is what makes the spectral lines have a definite shape, and not look like simple "delta functions" at the atomic transition wavelength. By measuring the shapes and total numbers of photons, we can measure the densities, outflow speeds, and temperatures of the different atoms and ions.

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