Corona

1. A set of one or more colored rings of small angular radii concentrically surrounding the the sun, moon, or other light source when veiled by a thin cloud.
   The corona can be distinguished from the halo of 22° due to its much smaller angular radius, which is often only a few degrees, and by its color sequence, which is from bluish white on the inside to reddish on the outside, the reverse of that in the 22° halo. Further, the color sequence of the corona can be repeated. Fraunhofer diffraction theory is often used to provide an approximate description of the corona. This theory predicts that the center of the corona is essentially white and that the radius of a ring of a particular color is approximately inversely proportional to the drop radius. Consequences of this are that the rings are most discernible and have the purest colors when the droplets in a particular portion of a cloud are nearly uniform in size (mono disperse); the rings are most nearly circular if the droplets that produce different portions of the corona are nearly the same size (spatial homogeneity); rings are caused by droplets with a radius of less than about 15 μm (the rings from larger droplets are washed out by the angular width of the sun). When there is a broad range of droplet sizes, the rings are distinct, the colors faint, and the phenomenon is often called an aureole. Although it is possible for colorful coronas to be produced by ice crystals, usually the broad range of sizes and shapes of crystals precludes this. Similarly, coronas are rarely produced by dust due to the broad range of particle sizes normally present, but rare observations, such as Bishop's rings, have been reported.
   
2. (Also called solar corona.) The outermost layer of the solar atmosphere, characterized by low density (<10⁹ cm⁻³) and high temperature (>10⁶ K). The very high temperatures in the corona are still unexplained. The white light emission observed at solar eclipse or with a coronagraph arises from the scattering of photospheric radiation from free electrons in the corona. The shape of the corona varies during the solar cycle (sunspot cycle).
   
3. 
   See aurora.
   
4. 
   See corona discharge.
   

Term edited 13 August 2018.

With respect to aviation weather observations, same as collective.