Diffuse auroral emissions occur when plasma sheet particles are scattered from the loss cone and precipitate into the auroral zone without additional acceleration along geomagnetic field lines. Diffus
e auroral precipitation, which is usually sub-visual, is a continuous process that expands equatorward when geomagnetic activity increases.
Absorption features in the spectrum of stars identified in the ultraviolet, visible, and infrared regions. They have an interstellar origin, but despite extensive efforts, their carrier(s) have not ye
t been clearly identified.
Absorption features in the spectrum of stars identified in the ultraviolet, visible, and infrared regions. They have an interstellar origin, but despite extensive efforts, their carrier(s) have not ye
t been clearly identified.
An interstellar cloud in which hydrogen is completely dissociated and which is less dense and dusty than molecular clouds. In diffuse interstellar clouds photoabsorption of the background ultraviolet
(UV) radiation field is an important dissociating and ionizing process. Typical densities and temperatures of diffuse clouds are 10^2 to 10^3 cm^-3 and 20 to 100 K respectively. Because of modest extinctions (≤ 1 mag), photodissociation processes are important in diffuse clouds preventing the formation of larger molecules.
A type of molecular cloud in which the interstellar radiation field is sufficiently attenuated, so that the local fraction of molecular hydrogen becomes substantial (> 0.1). However, enough interstell
ar radiation is still present to photoionize any atomic carbon, or to photodissociate carbon monoxide (CO) such that carbon is predominantly still in the form of C+ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross diffuse atomic gas.