The High Altitude Observatory has a very detailed solar education page. Full sun images can be assembled over the course of an hour by taking 36 exposures centered at different positions on the sun. Here is an example of a full sun mosaic taken October 10 1998. Clicking on the image will download the full 7.5MB gif image. The plasma temperature is about 1.0 million degrees.
Bright regions have higher density of coronal gas than dark regions.