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describe how astronomers observe the Sun at different wavelengths
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demonstrate an understanding of the appearance of the Sun at different
wavelengths of the electromagnetic spectrum, including visible, H-alpha,
X-ray
The emissions in each area of the electromagnetic spectrum are related to the
temperature of the object. By observing the Sun at different wavelengths, a clearer
view of events taking place can be seen across the whole spectrum. We see the
surface of the Sun at 5,800K.
At this temperature, the majority of the
electromagnetic spectrum is released as visible light.
Other parts of the
electromagnetic spectrum should be weak emissions. However, the Sun is a
powerful source of X-rays, which can only be produced at very high temperatures.
Using telescopes to detect X-rays has allowed astronomers to understand what has
produced the emissions and therefore given astronomers a better understanding of
the Sun.
Telescopes on Earth and in space have analysed emissions from the Sun over many
years. Radio receivers like ‘WAVES’ on the WIND satellite detect storms on the
surface of the Sun (Coronal Mass Ejections CMEs). Infrared telescopes show cooler
areas of the Sun as dark regions in images taken. The very successful EIT (Extreme
ultra-violet Imaging Telescope) has taken amazing images of the chromosphere and
low corona regions, when intense activity has been occuring. EIT forms part of
SOHO (the SOlar and Heliospheric Observatory), the main observatory for viewing the
Sun at different wavelengths. The observatory was launched on 2nd December 1995
and contains a collection of 12 experiments to observe the activity of the Sun.
Visible light observations allow details about sunspots, the solar wind and the
corona to be studied.
The biggest land solar telescope
is the Dunn Solar Telescope at
Sacramento Peak, New Mexico
(seen on the left). The telescope
is over 41 metres high above
ground, but extends well into the
ground (a further 67 metres).
The telescope is on the leading
edge of visible observations with
two adaptive optics units, which
in effect take out the ‘wobble’
caused by the atmosphere. This
gives much clearer imaging. The
whole of the column of the
telescope has a vacuum inside,
to prevent distortion of the
image due to convection currents
which would be created by the
extreme heat from the Sun.
Picture credit : NSO/AURA/NSF
One day, a 4m Advanced Technology
Solar Telescope will be built in Hawaii.
Activity in the Solar Atmosphere (seen in visible light)
 SOLAR FLARES are violent eruptions, which occur when magnetic energy stored
up in the complex magnetic field near a sunspot is suddenly released in a
massive explosion. The flares will emit strong X-rays, ultraviolet light, visible
light and solar wind. The amount of energy released is the equivalent of
millions of 100 megaton hydrogen bombs exploding at the same time! The
flares can last up to an hour and can affect events on the Earth if, as usually
happens, the flare is associated with a coronal mass ejection. The biggest
solar flares occur at, or near to, the solar maximum. Also, the extra activity
can lead to the aurorae being seen in Britain. The next solar maximum will be
around 2012, so we are beginning to see a rise in the Sun’s activity again.
Picture credit : SOHO/ESA & NASA
A coronal mass ejection (CME) linked with a huge solar flare
- seen in visible light
 PROMINENCES are eruptions of gas into the chromosphere and corona. They
are cooler than their surroundings, being between 10,000 K to 20,000 K.
Usually, prominences are about a few times the size of the Earth.
There are 2 types of prominences:-
1). Quiescent Prominences (Loop prominences)
Loops of gas are trapped in the magnetic field around a pair of sunspots.
These can erupt up to 50,000 km from the Sun’s photosphere and last a few weeks.
Picture credit : SOHO/ESA & NASA
2). Eruptive Prominences
Similar to a ‘very small’ flare, throwing material 500,000 km from the Sun.
Picture credit : SOHO/ESA & NASA
HYDROGEN ALPHA (Hα) filters are commonly used to safely view the Sun. Viewing
just the HYDROGEN ALPHA (Hα) wavelength of 656 nm and ignoring all the other
wavelengths of light allows good views of solar flares, prominences, filaments
(cooler strands over the disc of the Sun caused by prominences) and extra details of
sunspot activity to be seen.
Picture credit : Venturescope www telescopesales.co.uk
The view of the Sun seen in HYDROGEN ALPHA (Hα).
The surface features of the Sun are seen incredibly clearly.
Picture credit : © Pedro Ré
Different appearances of the Sun with different filters
As well as telescopes for individual astronomers and large observatories on Earth,
satellites are in space, containing a range of instruments to study the Sun in
different regions of the electromagnetic spectrum. SOHO has performed for much
longer than expected. Hinode (meaning sunrise), is the more recent Japanese solar
observatory.
Picture credit : NAOJ
The range of telescopes can be seen on board the Hinode spacecraft. Hinode will
use the X-ray telescope (XRT) to study the life cycle of solar magnetic field events.
Telescopes studying X-rays have discovered details about the corona –
the atmosphere of the Sun.
X-ray image of the Sun
showing a solar flare in
the corona and other
regions of intense activity.
Picture credit : NASA/GSFC and SOHO