An Etymological Dictionary of Astronomy and Astrophysics
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The Southern Crown. A small, faint southern → constellation, also called Corona Austrini. Abbreviation: CrA, genitive: Coronae Australis.

→ corona; L. australis "southern."

The Northern Crown. A small but prominent northern → constellation that lies east of → Arcturus, between → Boötes and → Hercules, and comprises a distinctive arc formed by the stars seven stars. Abbreviation: CrB; genitive: Coronae Borealis.

→ corona; L. borealis "northern."

An instrument which, when used in a telescope, produces an artificial eclipse, permitting the study of the → solar corona without a total eclipse of the Sun. It was invented in 1930 by the French astronomer Bernard Lyot (1897-1952).

From → corona + → -graph.

Of, or relating to → coronagraphy.

→ coronagraphy; → -ic..

A technique used to observe a relatively dim object (like an → exoplanet) lying close to an outshining bright source (such as star). This is done by blocking the bright object, in the same way the → solar corona is observed using a → coronagraph.

→ coronagraph + suffix -y.

Of or relating to a → corona.

Adj., from → corona + → -al.

A part of the → solar corona where the gas density and the temperature are higher than in its vicinity. The coronal condensations are visible on the solar limb, above → sunspot groups. Images in X-rays and those supplied by → coronagraphs in white light reveal that such condensations consist of structures in the form of nodes, underlining the corona magnetic field (M.S.: SDE).

→ coronal; → condensation.

Several phenomena occurring in the Sun's corona, such as a → helmet streamer, → polar plume, → coronal loop, → coronal hole.

→ coronal; → feature.

A component of the → interstellar medium in the Galaxy which appears as pockets of gas at temperatures of over one million degrees, but extremely low densities of 10⁴ atoms per cubic centimeter. The hot coronal gas is believed to be material blown out of violent supernova explosions. It is called "coronal gas", after a similarity with the hot gas in → solar corona.

→ coronal; → gas.

An area in the → solar corona which appears dark in X-rays and ultraviolet light. The gas density in these areas are very low, about 100 times less than that of coronal → active regions. The magnetic field lines in a coronal hole extend out into → interplanetary space rather than returning to the Sun's surface, as they do in other parts of the Sun (→ open magnetic field line). Ionized hot gas can escape easily along such a path, and this brings about high speed → solar wind streams.

→ coronal; → hole.

An → emission line in the spectrum of the → solar corona caused by highly ionized metal ions; especially those of iron, such as the red and green lines at 6375 Å and 5303 Å [Fe X] (Fe⁹⁺ ion) and [Fe XIV] (Fe¹³⁺ ion), respectively. From their discovery in 1870 until 1939, it was believed that these → forbidden lines would be due to an unknown element, called → coronium. Ultraviolet and X-ray coronal lines are also detectable in stars.

→ coronal; → line.

An arc-like structure in the Sun's → corona that is found around → sunspots and in → active regions. These structures are associated with the closed magnetic field lines that connect magnetic regions on the solar surface. The loops are sometimes as high as 10,000 km with their two ends situated in photosphere regions of opposite magnetic polarity. This implies that the coronal loops are tubes of magnetic flux filled with hot plasma. They last for days or weeks but most change quite rapidly.

→ coronal; → loop.

A huge eruption of material from regions of the solar corona in which the magnetic field is closed, but which suffer an extremely energetic disruption. Over the course of several hours up to 10,000 billion kg of this material is ejected into → interplanetary space with a a speed of as high as 3000 km/s. CMEs are most spectacularly observed by a white light coronagraph located outside Earth's atmosphere. Such observations from Skylab in the early 1970's were the first to reveal this phenomenon. CME's disrupt the flow of the → solar wind and can produce intense electromagnetic disturbances that can severely damage satellites and disrupt power grids on Earth. When these ejections reach the Earth, they give rise to → geomagnetic storms. The frequency varies with the → solar cycle; during solar minimum they come at a rate of about one per week, and during maximum there is an average of about two or three per day. See also → interplanetary coronal mass ejections (ICME).

→ coronal; → mass; → ejection.

Material that condenses in the Sun's corona and appears to rain down into the chromosphere, as observed at the solar limb above strong sunspots.

→ coronal; → rain.

A bright, narrow stream of particles traveling through the Sun's corona, visible in images taken with a coronagraph or during a total solar eclipse. Coronal streamers represent the most outwardly extended structures in the solar corona and result from the interaction between the solar slow wind and the large-scale magnetic field. → helmet streamer.

→ coronal; → streamer.

A hypothetical chemical element once thought to be at the origin of the → coronal lines seen during a solar → total eclipse

From coron-, from → corona + → -ium

A spiral-shaped density enhancement formed around a star when fast stellar winds collide with slower material. This large-scale wind structure can extend from the stellar surface to possibly several tens of stellar radii. The CIRs can be produced by intensity irregularities at the stellar surface, such as dark and bright spots, magnetic loops and fields, or non-radial pulsations. The surface intensity variations alter the radiative wind acceleration locally, which creates streams of faster and slower wind material. CIRs are responsible for the → discrete absorption components seen in some ultraviolet → resonance lines of → hot stars (S. R. Cranmer & S. P. Owocki, 1996, ApJ 462, 469).

→ corotate; → interaction; → region.

The act of corotating.

Verbal noun of → corotate.

1) In the → X-wind model of → accretion, the distance from the star where the → centrifugal force on a particle corotating with the star balances the → gravitational attraction; in other words, where the → accretion disk rotates at the same → angular velocity as the star.
2) In a → spiral galaxy, the place where the spiral → pattern speed has the same velocity as the → rotation curve of the → galactic disk. In the frame rotating with the wave, particles inside this radius will appear to revolve in the direction of the frame rotation (prograde) while outside this corotation radius, they will be retrograde.

→ corotation; → radius.

That condition of a → galactic disk at an orbital radius in which the → angular velocity of the disk equals the → pattern speed. It is significant that the spiral wave pattern rotates as a rigid body (Ω_P = const), whereas the galactic disk rotates differentially (Ω is a function of galactocentric distance r). The distance r_C at which the two angular velocities coincide (Ω(r_C) = Ω_P) is referred to as the → corotation radius. The corotation resonance and its position within the galaxy is one of the fundamental properties of a spiral galaxy.

→ corotation; → resonance.