The solar corona is the most thoroughly explored example of a hot, dilute, extended stellar atmosphere. Because we are so near the Sun, we can resolve and observe individual structures. Using atomic physics, we can develop and test spectroscopic methods and apply them to construct coronal models that accurately represent the run of temperature, density, and velocity.
The solar corona turns out to be a fascinating astrophysical environment. Beyond its own intrinsic interest, it is also important because it is typical of many other stellar atmospheres. X-ray detectors aboard the HEAO-2 satellite (‘Einstein’) have revealed soft X-ray emission from stars of every spectral type and luminosity class. Thus, the diagnostic techniques developed with solar observations may help us to understand the nature of stellar coronae in general.
Despite our advantage of being near the Sun, we have as yet very incomplete ideas about some of the basic coronal processes. For example, we are not yet certain how the corona is heated, or how it replenishes the material it loses to the solar wind. Better observations and improved diagnostic techniques will help us to answer these large questions.
In this chapter, I will focus on the spectroscopic diagnostic techniques that have been devised for the study of the solar corona (and which are applicable, in principle, to other coronae) and mention some of the results that have come to light. After briefly discussing monochromatic imaging of the corona, I will consider how coronal temperature density, velocity and magnetic field have been measured by spectroscopic techniques.