If you have ever watched the movie Solar Attack, then you would
have heard the terms solar flare and coronal mass ejection. You may even have
wondered "What are they?"
To begin with, solar flares and coronal mass ejections are not the
same. A coronal mass ejection or CME is a massive release of solar matter
consisting of plasma (keV protons and electrons from the solar surface) and
magnetic fields, which rises above the corona and is released into space. A solar
flare is a much smaller, localized release of energy from the sun. While CMEs
tend to occur from similar sections of the sun as solar flares, there is no
proof that CME's are caused by these flares.
To observe CMEs and solar flares, astronomers attach a Coronagraph
to the telescope which blocks out the direct light of the sun, allowing the
dimmer radiation from the corona to be observed. There are different types of
coronagraph which can be used depending on the application.
Coronal Mass Ejection:
Many ejections originate from active regions on the surface of the sun and are especially common near sunspots. The frequency and number of CME's depends on the solar cycle which has a period of approximately 11 years and is measured by observing visible sunspots. At solar maxima, when the magnetic field lines are distorted to the greatest degree, there are on average 3 CME's per day. This number drops significantly over the next 5 years to an average of 1 CME every 5 days during the solar minima.
So how exactly do they occur?
Good Question! Not even astronomers know exactly how these events
occur, although they do have some ideas...
Originally it was believed that CMEs were caused by the extreme
heat released by solar flares. This theory went out the window however when CMEs were observed before any flare had occurred. ;)
Recent observations and computer models suggest that these
phenomena are caused by rearrangements of magnetic field lines of opposing
direction. However, current models based on this idea are small scale. Will
they hold when they are applied to models scaled up to solar dimensions?
What danger do they pose?
CME's and solar flares can cause havoc with electrical devices,
disrupting radio transmissions and damaging satellites and power-distribution
stations. This is due to the geomagnetic storm they create as they pass the
earth, compressing the magnetic field on the sunward side and extending it on
the night side. The return of the magnetic field to its normal position
releases terawatts of power into the Earth's atmosphere leading to these
disruptions.
The increased amount of radiation contained in the solar wind impacts with the atmosphere resulting in much more extensive and intense aurorae. This intense burst of radiation can also be dangerous for astronauts and cosmonauts as it can lead to a higher chance of radiation-associated illnesses.
So can we predict these stars to prevent this damage?
Unfortunately the solar processes which lead to CME and solar
flare development are unknown and still too complex for accurate, long-term prediction systems to have been produced. There are numerous models which allow
for some short term predictions, similar to weather predictions on Earth. For
instance sigmoids, "S" shaped structures of coronal matter; appear to
be common just before an eruption and so can provide some warning of these
events.
Currently NASA is developing and testing a software package which
measures the free magnetic energy of a solar region from a magnetogram and
provides a forecast of
solar weather.
Sources:
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