Opening thoughts

"Astronomy is the supreme example of the observational, as opposed to experimental, sciences in which the wit of the observer has to be pitted against the difficulties, and in particular the ambiguities, imposed by the need to interpret remotely sensed data. The remote observer finds himself in a situation, akin to that of a spectator at a magic show, where he is presented with a limited set of more or less remarkable data emanating from a source, the nature of which he is fascinated to discover but which he is not permitted to handle directly. In the magic show, the basic mechanism of the trick known only to the magician, is convoluted through the un-revealed process of his presentation, before appearing in strongly modified form to the spectator. In astronomy, the unknown basic physics of the observed source is convoluted through the source structure and emission processes (also unknown) before arriving at the observer's instrument. Mathematically this process can be described in terms of an integral operator relationship between the required source distribution and the observable data. The very essence of interpretation of astronomical observations in many cases, therefore, reduces essentially to the analysis (or `inversion') of data which have been subject to integral convolution processes."

-- I. J. D. Craig and J. C. Brown, in "Inverse problems in Astronomy" a guide to inversion strategies for remotely sensed data.

We will:

• Consider the basic (almost Philosophical) issue of how to extract information on emitting (or absorbing) plasmas from their emitted (or absorbed) spectra.
• Look at different philosophical approaches: ``forward'' versus ``inverse'' methods.
• Study the ways photons are emitted by atoms and atomic ions.
• Use the SUN's ATMOSPHERE as a place to test our ideas: EUV image

  EXERCISE: Think of one thing in astrophysics that you believe to be true, beyond a shadow of a doubt (and I don't mean the laws of physics), for example, that the Sun has an outer atmosphere (``corona'') that is much hotter than the surface layers, and make sure you can explain why you believe this ``fact'' is really on solid footing. Then think of another thing that you are not so sure of, but can also argue about it. Write both down, and I will compile them (or have one of you do it). I'll pick some good ones and we will briefly discuss them, and then we'll re-visit them at the end of the class.