A coronal mass ejection is a massive
burst of gas and magnetic field arising
from the solar corona and being released
into the solar wind, as observed in a
coronagraph.
Coronal mass ejections are often
associated with other forms of solar
activity, most notably solar flares or
filament eruptions, but a broadly
accepted theoretical understanding of
these relationships has not been
established. CMEs most often originate
from active regions on the Sun's
surface, such as groupings of sunspots
associated with frequent flares. Near
solar maxima, the Sun produces about
three CMEs every day, whereas near solar
minima, there is about one CME every
five days.
Description
Coronal mass ejections release huge
quantities of matter and electromagnetic
radiation into space above the sun's
surface, either near the corona, or
farther into the planet system, or
beyond. The ejected material is a plasma
consisting primarily of electrons and
protons. While solar flares are very
fast, CMEs are relatively slow.
Coronal mass ejections are associated
with enormous changes and disturbances
in the coronal magnetic field. They are
usually observed with a white-light
coronagraph.
Cause
Recent scientific research has shown
that the phenomenon of magnetic
reconnection is closely associated with
CMEs and solar flares. Magnetic
reconnection is the name given, within
magnetohydrodynamic theory, to the
rearrangement of magnetic field lines
when two oppositely directed magnetic
fields are brought together. This
rearrangement is accompanied with a
sudden release of energy stored in the
original stressed magnetic fields.
On the sun, magnetic reconnection may
happen on solar arcades—a series of
closely occurring loops of magnetic
lines of force. These lines of force
quickly reconnect into a low arcade of
loops, leaving a helix of magnetic field
unconnected to the rest of the arcade.
The sudden release of energy during this
process causes the solar flare and
ejects the CME. The helical magnetic
field and the material that it contains
may violently expand outwards forming a
CME. This also explains why CMEs and
solar flares typically erupt from what
are known as the active regions on the
sun where magnetic fields are much
stronger on average.
Impact on Earth
When the ejection is directed towards
Earth and reaches it as an
interplanetary CME, the shock wave of
the traveling mass of solar energetic
particles causes a geomagnetic storm
that may disrupt Earth's magnetosphere,
compressing it on the day side and
extending the night-side magnetic tail.
When the magnetosphere reconnects on the
nightside, it releases power on the
order of terawatt scale, which is
directed back toward Earth's upper
atmosphere.
Solar energetic particles can cause
particularly strong aurorae in large
regions around Earth's magnetic poles.
These are also known as the Northern
Lights in the northern hemisphere, and
the Southern Lights in the southern
hemisphere. Coronal mass ejections,
along with solar flares of other origin,
can disrupt radio transmissions and
cause damage to satellites and
electrical transmission line facilities,
resulting in potentially massive and
long-lasting power outages.
Humans at high altitudes, as in
airplanes or space stations, risk
exposure to relatively intense cosmic
rays. Cosmic rays are potentially lethal
in high quantities. The energy absorbed
by astronauts is not reduced by a
typical spacecraft shield design and, if
any protection is provided, it would
result from changes in the microscopic
inhomogeneity of the energy absorption
events.
Physical properties
A typical coronal mass ejection may have
any or all of three distinctive
features: a cavity of low electron
density, a dense core, and a bright
leading edge.
Most ejections originate from active
regions on the Sun's surface, such as
groupings of sunspots associated with
frequent flares. These regions have
closed magnetic field lines, in which
the magnetic field strength is large
enough to contain the plasma. These
field lines must be broken or weakened
for the ejection to escape from the sun.
However, CMEs may also be initiated in
quiet surface regions, although in many
cases the quiet region was recently
active. During solar minimum, CMEs form
primarily in the coronal streamer belt
near the solar magnetic equator. During
solar maximum, they originate from
active regions whose latitudinal
distribution is more homogeneous.
Coronal mass ejections reach velocities
between 20 to 3,200 km/s with an average
speed of 489 km/s, based on SOHO/LASCO
measurements between 1996 and 2003.
These speeds correspond to transit times
from the sun out to the mean radius of
Earth's orbit of about 86 days to 13
hours and 3.5 days, respectively. The
average mass ejected is
7012160000000000000♠1.6×1012 kg. The
mass values are only lower limits,
because coronagraph measurements provide
only two-dimensional data analysis. The
frequency of ejections depends on the
phase of the solar cycle: from about one
every fifth day near the solar minimum
to 3.5 per day near the solar maximum.
These values are also lower limits
because ejections propagating away from
Earth can usually not be detected by
coronagraphs.
Current knowledge of coronal mass
ejection kinematics indicates that the
ejection starts with an initial
pre-acceleration phase characterized by
a slow rising motion, followed by a
period of rapid acceleration away from
the Sun until a near-constant velocity
is reached. Some balloon CMEs, usually
the slowest ones, lack this three-stage
evolution, instead accelerating slowly
and continuously throughout their
flight. Even for CMEs with a
well-defined acceleration stage, the
pre-acceleration stage is often absent,
or perhaps unobservable.
Association with other solar phenomena
Coronal mass ejections are often
associated with other forms of solar
activity, most notably:
Solar flares
Eruptive prominence and X-ray sigmoids
Coronal dimming
Moreton waves
Coronal waves
Post-eruptive arcades
The association of a CME with some of
those phenomena is common but not fully
understood. For example, CMEs and flares
are normally closely related, but there
was confusion about this point caused by
the events originating beyond the limb.
For such events no flare could be
detected. Most weak flares do not have
associated CMEs; most powerful ones do.
Some CMEs occur without any flare-like
manifestation, but these are the weaker
and slower ones. It is now thought that
CMEs and associated flares are caused by
a common event. In general, all of these
events are thought to be the result of a
large-scale restructuring of the
magnetic field; the presence or absence
of a CME during one of these
restructures would reflect the coronal
environment of the process.
Theoretical models
It was first postulated that CMEs might
be driven by the heat of an explosive
flare. However, it soon became apparent
that many CMEs were not associated with
flares, and that even those that were
often started before the flare. Because
CMEs are initiated in the solar corona,
their energy source must be magnetic.
Because the energy of CMEs is so high,
it is unlikely that their energy could
be directly driven by emerging magnetic
fields in the photosphere. Therefore,
most models of CMEs assume that the
energy is stored up in the coronal
magnetic field over a long period of
time and then suddenly released by some
instability or a loss of equilibrium in
the field. There is still no consensus
on which of these release mechanisms is
correct, and observations are not
currently able to constrain these models
very well. These same considerations
apply equally well to solar flares, but
the observable signatures of these
phenomena differ.
Interplanetary CMEs
CMEs typically reach Earth one to five
days after leaving the Sun. During their
propagation, CMEs interact with the
solar wind and the interplanetary
magnetic field. As a consequence, slow
CMEs are accelerated toward the speed of
the solar wind and fast CMEs are
decelerated toward the speed of the
solar wind. CMEs faster than about 500
km/s eventually drive a shock wave. This
happens when the speed of the CME in the
frame of reference moving with the solar
wind is faster than the local fast
magnetosonic speed. Such shocks have
been observed directly by coronagraphs
in the corona, and are related to type
II radio bursts. They are thought to
form sometimes as low as 2 Rs. They are
also closely linked with the
acceleration of solar energetic
particles.
Related solar observation missions
= NASA mission Wind=
On 1 November 1994, NASA launched the
WIND spacecraft as a solar wind monitor
to orbit Earth's L1 Lagrange point as
the interplanetary component of the
Global Geospace Science Program within
the International Solar Terrestrial
Physics program. The spacecraft is a
spin axis-stabilized satellite that
carries eight instruments measuring
solar wind particles from thermal to
>MeV energies, electromagnetic radiation
from DC to 13 MHz radio waves, and
gamma-rays. Though the WIND spacecraft
is nearly two decades old, it still
provides the highest time, angular, and
energy resolution of any of the solar
wind monitors. It continues to produce
relevant research as its data has
contributed to over 150 publications
since 2008 alone.
= NASA mission STEREO=
On 25 October 2006, NASA launched
STEREO, two near-identical spacecraft
which from widely separated points in
their orbits are able to produce the
first stereoscopic images of CMEs and
other solar activity measurements. The
spacecraft orbit the Sun at distances
similar to that of Earth, with one
slightly ahead of Earth and the other
trailing. Their separation gradually
increased so that after four years they
were almost diametrically opposite each
other in orbit.
History
= First traces=
The largest recorded geomagnetic
perturbation, resulting presumably from
a CME, coincided with the first-observed
solar flare on 1 September 1859, and is
now referred to as the Carrington Event,
or the solar storm of 1859. The flare
and the associated sunspots were visible
to the naked eye, and the flare was
independently observed by English
astronomers R. C. Carrington and R.
Hodgson. The geomagnetic storm was
observed with the recording magnetograph
at Kew Gardens. The same instrument
recorded a crochet, an instantaneous
perturbation of Earth's ionosphere by
ionizing soft X-rays. This could not
easily be understood at the time because
it predated the discovery of X-rays by
Röntgen and the recognition of the
ionosphere by Kennelly and Heaviside.
The storm took down parts of the
recently created US telegraph network,
starting fires and shocking some
telegraph operators.
Historical records were collected and
new observations recorded in annual
summaries by the Astronomical Society of
the Pacific between 1953 and 1960.
= First clear detections=
The first detection of a CME as such was
made on 14 December 1971, by R. Tousey
of the Naval Research Laboratory using
the seventh Orbiting Solar Observatory.
The discovery image was collected on a
Secondary Electron Conduction vidicon
tube, transferred to the instrument
computer after being digitized to 7
bits. Then it was compressed using a
simple run-length encoding scheme and
sent down to the ground at 200 bit/s. A
full, uncompressed image would take 44
minutes to send down to the ground. The
telemetry was sent to ground support
equipment which built up the image onto
Polaroid print. David Roberts, an
electronics technician working for NRL
who had been responsible for the testing
of the SEC-vidicon camera, was in charge
of day-to-day operations. He thought
that his camera had failed because
certain areas of the image were much
brighter than normal. But on the next
image the bright area had moved away
from the Sun and he immediately
recognized this as being unusual and
took it to his supervisor, Dr. Guenter
Brueckner, and then to the solar physics
branch head, Dr. Tousey. Earlier
observations of coronal transients or
even phenomena observed visually during
solar eclipses are now understood as
essentially the same thing.
= Recent events=
On 1 August 2010, during solar cycle 24,
scientists at the Harvard-Smithsonian
Center for Astrophysics observed a
series of four large CMEs emanating from
the Earth-facing hemisphere of the Sun.
The initial CME was generated by an
eruption on 1 August that was associated
with NOAA Active Region 1092, which was
large enough to be seen without the aid
of a solar telescope. The event produced
significant aurorae on Earth three days
later.
On 23 July 2012, a massive, and
potentially damaging, Solar Superstorm
barely missed Earth, according to NASA.
There is an estimated 12% chance of a
similar event hitting Earth between 2012
and 2022.
On 31 August 2012 a CME connected with
Earth's magnetic environment, or
magnetosphere, with a glancing blow
causing aurora to appear on the night of
3 September. Geomagnetic storming
reached the G2 level on NOAA's Space
Weather Prediction Center scale of
geomagnetic disturbances.
See also
References
Further reading
Books
Gopalswamy, Natchimuthukonar; Mewaldt,
Richard A; Torsti, Jarmo, eds.. Solar
Eruptions and Energetic Particles.
Geophys. Monograph Series 165. Am.
Geophys. Union. doi:10.1029/GM165. ISBN
0-87590-430-0. 
Internet articles
Bell, Trudy E; Phillips, Tony. "A Super
Solar Flare". Science@NASA. NASA.gov. 
Phillips, Tony. "Cartwheel Coronal Mass
Ejection". Science@NASA. NASA.gov. 
Odenwald, Sten F; Green, James L.
"Bracing the Satellite Infrastructure
for a Solar Superstorm". Scientific
American. 
Lavraud, Benoit; Masson, Arnaud.
"Cluster captures the impact of CMEs".
ESA Science & Technology. ESA.int. 
Morring Jr., Frank. "Major Solar Event
Could Devastate Power Grid". Aviation
Week & Space Technology. 
External links
NOAA/NWS Space Weather Prediction Center
Coronal Mass Ejection FAQ
STEREO and SOHO observed CME rate versus
the Sunspot number /
