A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Latin root of the word corona means crown.

The high temperature of the corona gives it unusual spectral features, which led some to suggest, in the 19th century, that it contained a previously unknown element, "coronium". These spectral features have since been traced to highly ionized Iron (Fe(XIV)) which indicates a plasma temperature in excess of 10⁶ kelvin.

The corona is divided into three parts. The K-corona (K for continuum) interfaces directly with the chromosphere and is created by sunlight scattering off electrons. The E-corona (E for emission) contains abundant calcium and iron. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles.

Physical features

The Sun's corona is much hotter (by a factor of nearly 200) than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvin compared to the corona's one to three million kelvin. The corona is 10⁻¹² as dense as the photosphere, however, and so produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and sonic pressure waves from below (the latter being less probable now that coronae are known to be present in early-type, highly magnetic stars). The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating the solar wind.

Coronal Loops

main: Coronal loop

Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE, they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the LOS). In-situ measurements are required before a definitive answer can be arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time-being).