Chromosphere

. The density of the Sun's chromosphere decreases exponentially with distance from the center of the Sun by a factor of roughly 10 million, from about at the chromosphere's inner boundary to under at the outer boundary. The temperature initially decreases from the inner boundary at about but then increasing to upwards of Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere. The chromosphere's spectrum is dominated by emission lines when observed at the solar limb. In particular, one of its strongest lines is the Hα at a wavelength of ; this line is emitted by a hydrogen atom whenever its electron makes a transition from the n=3 to the n=2 energy level. A wavelength of is in the red part of the spectrum, which causes the chromosphere to have a characteristic reddish colour. == Phenomena ==

Many different phenomena can be observed in chromospheres. Plage A plage is a particularly bright region within stellar chromospheres, which are often associated with magnetic activity. Oscillations Since the first observations with the instrument SUMER on board SOHO, periodic oscillations in the solar chromosphere have been found with a frequency from to , corresponding to a characteristic periodic time of three minutes. Oscillations of the radial component of the plasma velocity are typical of the high chromosphere. The photospheric granulation pattern usually has no oscillations above ; however, higher frequency waves (, or a period) were detected in the solar atmosphere (at temperatures typical of the transition region and corona) by TRACE. Loops Plasma loops can be seen at the border of the solar disk in the chromosphere. They are different from solar prominences because they are concentric arches with maximum temperature of the order (too low to be considered coronal features). These cool-temperature loops show an intense variability: they appear and disappear in some UV lines in a time less than an hour, or they rapidly expand in 10–20 minutes. Foukal studied these cool loops in detail from the observations taken with the EUV spectrometer on Skylab in 1976. When the plasma temperature of these loops becomes coronal (above ), these features appear more stable and evolve over longer times. == Network ==

Images taken in typical chromospheric lines show the presence of brighter cells, usually referred to as the network, while the surrounding darker regions are named internetwork. They look similar to granules commonly observed on the photosphere due to the heat convection. == On other stars ==

Chromospheres are present on almost all luminous stars other than white dwarfs. They are most prominent and magnetically active on lower-main sequence stars, on brown dwarfs of F and later spectral types, and on giant and subgiant stars. A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson S-index. == See also ==