DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

In planetary physics, a corona is a large structure
of combined volcanic and tectonic origin.
Most coronae are found on Venus, although the
term is also used to describe tectonic features
on the Uranian moon of Miranda. The Venusian
coronae typically consist of an inner circular
plateau surrounded first by a raised ridge and
then an annulus of troughs. Most of the interior
features of the corona are typical volcanic
structures, including calderas, small shield volcanos,
and lava flows. Coronae tend to be very
large structures, often 300 km or more in diameter.
Planetary scientists believe they form
when a large blob of hot magma from Venus’s
interior rises close to the surface, causing the
crust to bulge and crack. The magma then sinks
back into the interior, causing the dome to collapse
and leaving the ring. Rising and collapsing
diapirs of material have also been proposed to
explain the coronae on Miranda.
coronagraph A telescope designed to observe
the outer portions of the solar atmosphere.
The bright emission of the solar disk is blocked
out in coronagraphs by means of an occulting
disk, bringing the faint outer corona into view.
Coronagraphs typically view the corona in white
light, though filters can be used to achieve specific
wavelength observations. Because of the
need for an occulting disk, they only observe
the corona above the solar limb, projected onto
the plane of the sky. Modern coronagraphs, such
as the one on the SOHO spacecraft, can observe
the corona between 1.1 and 30 solar radii. Coronagraphs
can also be operated from the ground
as long as the air column above the coronagraph
is thin enough to reduce atmospheric scattering
sufficiently. The first coronagraph was operated
by B. Lyot from the Pic du Midi in the Pyrenees
at an altitude above 2900 m. Coronagraphs provide
the most startling observations of coronal
mass ejections, helmet streamers, and prominences.
Coronal Diagnostic Spectrometer (CDS) A
Wolter II grazing incidence telescope equipped
with both a normal incidence and a grazing incidence
spectrometer flown on board the SOHO
spacecraft. This instrument is designed to measure
absolute and relative intensities of selected
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coronal hole
EUV lines (150 to 800 Å) to determine temperatures
and densities of various coronal structures.
coronal dimming During an eruptive event
such as coronal mass ejection or a long duration
flare, a large mass of plasma is ejected from
the solar corona. When observed in soft X-ray
wavelengths, the expulsion of million degree
plasma is called coronal dimming. This coronal
dimming relates the removal of hot material
from the low corona to the higher, cooler material
commonly associated with a coronal mass
ejection, as seen in white light.
coronal heating The temperature of the
solar atmosphere increases dramatically from
the photosphere, through the chromosphere and
transition region, to the corona with temperatures
in the corona varying from 2 to 3 million
degrees Kelvin in the quiet diffuse corona
to as much as 5 to 6 million degrees in active
regions. The reason why the corona is so hot remains
a mystery although it is now clear that the
sun’s magnetic field plays a crucial role in the
transport and dissipation of the energy required
to heat the corona. The total energy losses in
the corona by radiation, conduction, and advection
are approximately 3 × 10 21 J or about
500Wm −2 . Balancing these losses requires
only about 1 part in 100,000 of the sun’s total
energy output.
coronal hole A low density extended region
of the corona associated with unipolar magnetic
field regions in the photosphere, appearing dark
at X-ray and ultraviolet wavelengths. The magnetic
field lines in a coronal hole extend high
into the corona, where they couple to the solar
wind and are advected into space. The corona,
the outermost gravitationally bound layer of the
solar atmosphere, is a very hot plasma (temperatures
in the range of 1 to 2×10 6 K). The largescale
structure of the coronal gas consists of relatively
dense regions whose magnetic field lines
are “closed” (anchored at two points in the photosphere)
and lower-density regions (the coronal
holes), whose magnetic field lines are “open”
(anchored at a single point in the photosphere
and extending outward indefinitely). The solar
wind emerges along these open field lines.