The radius of the sun is 0.7 million km. What percentage of the radius is taken up by the chromosphere?

Answer 1

The gases of the Sun extend far beyond the photosphere, which may be considered the lowest level of the solar atmosphere. The region immediately above the photosphere is called the chromosphere.

Observation of the Chromosphere:
The chromosphere is 2000-3000 km thick. It glows faintly relative to the photosphere and can only be seen easily in a total solar eclipse. When it can be seen it is reddish in color (because of strong Balmer H-alpha emission). This color is the origin of its name (chromos meaning ``color'').

The faint flow of the chromosphere is due to an emission spectrum from hot, low density gases emitting at discrete wavelengths. The discovery of helium noted earlier was from emission lines seen in the chromosphere during an eclipse in 1868. This new element was only found on the Earth in 1895.

Spicules:
The chromosphere contains spikes of gas called spicules that rise through it. The adjacent image shows spicules on the limb of the Sun as imaged by the Big Bear Solar Observatory (Ref). This image is a superposition of 11 limb images taken at different wavelengths (click on the image for a larger version that shows the spicules more clearly). In this image the longest spicules rise about 7000 km. Spicules are short-lived phenomena, corresponding to rising jets of gas that move upward at about 30km/sec and last only about 10 minutes.