For the Teacher
(Reprinted from http://pwg.gsfc.nasa.gov/istp/nicky/cme-chase.html)
Coronal Mass Ejections, Flares, and the Solar Wind
Some of the most dramatic space weather effects occur in association with eruptions of
material from the solar atmosphere into interplanetary space. These eruptions are known
as coronal mass ejections, or CMEs. A large CME can contain 10.0E16 grams (a billion
tons) of matter that can be accelerated to several million miles per hour in a spectacular
explosion. Solar material streaks out through the interplanetary medium, impacting any
planets or spacecraft in its path. The coronal image below shows the release of a CME at
the Sun.
The event occurring here is on the side of the Sun –
or the limb – which means that it will not affect us
here on Earth. Sometimes, however, CMEs occur
on the front side of the Sun in a location directly in
front of Earth. These events appear to be very
different when viewed from Earth. Instead of
looking like a "bubble" of plasma, they form a
circle of bright light around the Sun. This light is
much dimmer than the Sun itself which is why you
need to put a disk in front of the disk of the Sun in
order to see what goes on around it. An example of
such a "halo" event is shown in the picture below.
Near solar activity maximum, the
sun produces about 3 CMEs every
day, whereas near solar minimum
it produces only about 1 CME
every 5 days. The faster CMEs
have outward speeds of up to 2000
kilometers per second,
considerably greater than the
normal solar wind speeds of about
400 kilometers per second. These
produce large shock waves in the
solar wind as they plow through it.
CMEs are sometimes associated with short periods of explosive energy release, known as
solar flares. These flares frequently occur in active regions during the period around solar
maximum. An example of a flare associated with an Earthward-directed CME is shown
below. Flares have lifetimes ranging from hours for large gradual events down to tens of
seconds for the most impulsive events. During a very strong flare, the solar ultraviolet
and x-ray emissions can increase by as much as 100 times above even active-region
levels. During solar maximum, approximately one such flare is observed every week.
Flares heat the solar gas to tens of millions of degrees. The heated gas then radiates
strongly across the whole electromagnetic spectrum from radio to gamma rays. The
largest of these explosions are so bright that they can even be seen from Earth in visible
light. The picture below shows the flare associated with an Earthward-directed CME
event which occurred on May 12, 1997. The January 6, 1997 CME shown above did not
have a flare associated with it.
Flares can accelerate protons and electrons that travel to Earth directly from the Sun
along the interplanetary magnetic field (which "channels" the charged particles). These
contribute to the high-energy particle environment in the vicinity of the magnetosphere if
Earth's location is magnetically connected to the flaring region by the interplanetary
magnetic field.
As noted above, solar flares have
associated X-ray emissions – the stronger
the flare, the larger the increase in the Xray levels. The plot on the left shows a 3
day plot of 5-minute solar X-ray emission
values (for the May 12, Earth-directed
CME shown above) measured on the
GOES 8 and 9 satellites which orbit the
Earth. The letters on the right-hand y-axis
are the flare classifications – i.e. an X-class
flare is much more intense than the C-class
flare shown here.
Major flares can be accompanied by energetic protons, which can reach Earth within 30
minutes of the flare's peak. During such an event, Earth is showered with highly energetic
solar protons released from the flare site. Some of these particles spiral down Earth's
magnetic field lines, reaching the upper layers of our atmosphere. These particles show
up as tiny white spots in the images taken by spacecraft cameras.
The area between the Sun and the planets is called the interplanetary medium. It is often
described as a vacuum, but this is not true. It is actually a turbulent area dominated by the
solar wind, which flows at velocities of approximately 250-1000 km/s (about 600,000 to
2,000,000 miles per hour). Other characteristics of the solar wind (density, composition,
and magnetic field strength, among others) vary with changing conditions on the Sun. In
general, disturbances in the solar wind arrive at Earth 2-4 days after leaving the Sun - the
CME on January 6-7, 1997 did not arrive at Earth until January 10, 1997. This CME
belongs to a particular subset of CMEs, termed magnetic clouds, which usually have a
greater effect on the Earth. The interplanetary space signature of a magnetic cloud is very
distinct. The most easily recognized characteristics are strong magnetic fields and a large
and smooth rotation of the magnetic field direction.
The figure to the left shows the magnetic
field signatures for the January 6, 1997
CME. The red line shows the total
magnetic field strength – and it is clear
that there was a definite increase in the
magnetic field strength on January 10
when the
cloud arrived at Earth. The black line shows the north-south direction of the field, and
there is a very obvious smooth rotation in the direction of magnetic field.
When these disturbances arrive at Earth, they do not always have the same effect. The
factor in determining how much the Earth will be effected by a CME is the direction of
the magnetic field – in particular, the north-south direction, or ‘z’ component. When the z
component is positive, this corresponds to a northward field, which has little or no effect
on the Earth. When the z component is negative, however, this corresponds to a
southward field. When the interplanetary magnetic field is southward, it opposes the
direction of the Earth’s magnetic field. In the same way that the different poles of a bar
magnet attract (in contrast to like poles repelling), an interaction between the two
magnetic fields will occur, allowing the energy from the solar wind to enter the Earth’s
protective shield – the magnetosphere. It is clear from the above plot of the interplanetary
magnetic field for January 9-11, 1997, that the z-component was first negative,
southward, and slowly turned northward throughout the event. This means that it had an
effect on the Earth – it was "geoeffective".
These solar wind disturbances can trigger global changes in Earth's magnetic field and
particle populations, called magnetic storms. A magnetic storm is a period when the
magnetic field measured on Earth is highly disturbed and auroras are produced. They
generally last several hours to several days. The Dst – Disturbed Storm Time – Index is a
measure of the magnetic field measured at Earth. An example of the Dst index associated
with a storm is shown below.
This is the famous storm of March 13, 1989 which caused a transformer failure on one of
the main power transmission lines in the HydroQuebec system precipitated a catastrophic
collapse of the entire power grid. 6 million people lost electrical power for 9 or more
hours.
A visual manifestation of a magnetic storm is the aurora. Auroras begin between 60 and
80 degrees latitude – this region is known as the auroral oval. Intensifications of auroral
light are caused by increases in the numbers of electrons and ions raining or
"precipitating" from the magnetosphere into the upper atmosphere in the auroral oval. As
a storm intensifies, the auroras spread toward the equator. During an unusually large
storm in 1909, an aurora was visible at Singapore, at the equator. The further the aurora
moves towards the equator, and the brighter the emissions, the more the Earth is being
effected by the CME event.
The image to the left shows
an extremely large aurora
that occurred March 14,
1989 at 01:51 over the
southern polar regions. The
picture to the right is a remapping of that image into
northern hemisphere to give
an idea of what the auroral
oval would have looked like
if the spacecraft had been
over the northern
hemisphere at this time.
From this image you can see
that you would have been
able to see the aurora in
Texas and Florida.<