DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

coronal lines
Except possibly for the periods of highest solar
activity, the largest coronal holes are located
at relatively high heliographic latitude, often
with irregularly shaped extensions to lower latitude,
sometimes into the opposite hemisphere.
During maximum activity periods, equatorial
coronal holes can appear and last for several solar
rotations.
coronal lines Forbidden spectral emission
lines emitted from highly ionized atomic species,
inahightemperature, dilutemediumwhere
collision between ions and electrons dominates
excitation and ionization, as in the solar corona.
In such plasma the temperature (1 to 2 ×10 6 Kin
the solar corona) and hence the kinetic energy
of ions and electrons is so high that collisions
have sufficient energy to ionize atoms. The first
coronal emission line was identified at 530.3 nm
duringthetotalsolareclipseof1869. Onlyinthe
1940s were most of the coronal lines identified
as forbidden transitions from elements such as
iron, nickel, and calcium in very high ionization
stages. Ratios of coronal line fluxes, similarly
to ratios of nebular lines, are used as diagnostics
of temperature and density. See forbidden lines,
nebular lines.
coronal loops The solar corona is comprised
primarilyofmagneticloop-likestructureswhich
are evident at all scales in the corona and are
thought to trace out the magnetic field. Loops
are seen at soft X-ray, EUV, and optical wavelengths.
Typical configurations of loops occur
in active regions, where many bright compact
loop structures are associated with strong surface
magnetic fields, and in arcades spanning
a magnetic neutral line and often overlaying a
filament or filament channel. The interaction
and reconfiguration of these structures often accompany
the dynamic eruptive phenomena on
small scales in solar flares and very large scales
in coronal mass ejections.
coronal mass ejection (CME) An ejection
of material from the sun into interplanetary
space, as a result of an eruption in the
lower corona. This material may sometimes
have higher speeds, densities, and magneticfield
strengths relative to the background solar wind
and may produce shocks in the plasma. The
© 2001 by CRC Press LLC
fastest CMEs can have speeds of 2000 km s −1
compared with normal solar wind speeds closer
to 400 km s −1 . CMEs are more common at solar
maximum, when three per day can be seen,
than solar minimum, when one may be seen in
five days. If the material is directed towards the
earth, then the CME may cause a disturbance to
the Earth’s geomagnetic field and ionosphere.
See solar wind.
coronal rain Cool plasma flowing down
along curved paths at the solar free-fall speed
of 50 to 100 km s −1 ; material condensing in the
corona and falling under gravity to the chromosphere.
Typically observed in Hα at the solar
limb above strong sunspots.
coronal transients A general term for shorttime-scale
changes in the corona but principally
used to describe outward-moving plasma
clouds. Erupting prominences are accompanied
by coronal transients, which represent outward
moving loops or clouds originating in the low
corona above the prominence. As many as one
coronal transient per day is observed to occur
during the declining phase of the solar cycle and
are most commonly associated with erupting filaments.
coronal trap The region of the corona in
which charged particles are trapped between
two areas of converging magnetic field, i.e., a
magnetic bottle. The converging field causes
a strengthening of the field and consequently
a strengthening of the Lorentz force felt by a
charge particle of velocity v. The particle’s
pitch-angle, θ = cos −1 (vz/v) where z is the direction
parallel to the field direction, increases
as the particle moves into the region of increasing
field strength until all of the particle’s momentum
is converted into transverse momentum
(θ = 90 ◦ ). This location is known as the mirroring
point because the particles cannot pass into
a region of greater field strength and therefore
become trapped. When collisions and waveparticle
interactions are ignored, the conditions
for a particle to be trapped are defined by the
equation sinθ/B = sinθ0/B0 where θ0 is the
particle’s initial pitch angle and B0 is the coronal
field. Note that for a prescribed field convergence
B/B0, particles with initial pitch angles