The Chromosphere

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 about 10,000 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" derives from the Greek word for "color."

Temperature
As illustrated in the top right figure, the temperature in the lower chromosphere is similar to that of the upper photosphere, but there is a rapid rise in temperature beginning in the upper chromosphere and the transition region to the corona. (Note that the horizontal axis is a logarithmic scale, so this increase is enormous, leading from temperatures of about 6000 K in the photosphere to temperatures approaching a million degrees in the corona only a few thousand kilometers higher.) This rise in temperature is not well understood but we shall see when we discuss the corona that it is connected with transfer of energy by the Sun's magnetic field.

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. This image is a superposition of six limb images taken at different wavelengths. 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 30 km/sec and last only about ten minutes. They are cooler than the surrounding region and therefore darker. They tend to be concentrated on the boundaries of supergranules and may represent conduits of energy between the solar surface and the corona.

Information from the Flash Spectrum

We have noted that the presence of helium in the Sun is inferred from the flash spectrum. In addition, from a detailed analysis of the chromospheric flash spectrum as the eclipse proceeds and different parts of the chromosphere are covered and uncovered, it is possible to study both the temperature and density profile of the lower portion of the Sun's atmosphere.

The Flash Spectrum
The light of the chromosphere is faint compared with the photosphere, so normally the spectrum of the chromosphere is drowned in the light of the photosphere. However, in a total solar eclipse the chromosphere and its emission spectrum flash into view briefly as the light of the photosphere is completely blocked. For this reason, the spectrum taken of the chromosphere in totality is called a flash spectrum. The flash spectrum emission lines are generally the same as the absorption lines of the photosphere, with the exception of new lines associated with neutral helium (see the right panel).