10
Our star the Sun
8[8] Bright regions called plage and dark, snake-like filaments abound in this
hydrogen-alpha image.
[9] The Sun imaged through a Calcium-K telescope.
[10] Filaments rising from the surface of the Sun.9chromosphere, approximately 6000 miles deep. The fine "rice grain" texture
of the photosphere is replaced by a coarser pattern which resembles the
skin of an orange, due to the presence of "dark mottles".
H-alpha features are heavily influenced by the Sun’s magnetic field and
this is particularly evident around active sunspot regions. Ironically, the high-
contrast view of a sunspot under white light is lost in H-alpha, with significant
detail being seen in and around the group itself. Of particular note are bright
patches associated with active regions, known as plage.
H-alpha filters reveal some of the most dynamic features of our nearest
star including vast clouds of hydrogen held above the chromosphere. When
seen on-disc, these appear as dark snaking filaments. As the Sun rotates,
these features may eventually reach the Sun’s limb where they appear to
hang off the curved edge, bright against the black space beyond. In this state,
they are known as prominences.
H-alpha filters are also very good at revealing the violent outbursts from
the Sun’s surface known as flares. A powerful flare may hurl material off
the Sun completely, sending it through the Solar System as a coronal mass
ejection or CME.CaK Filters
CaK filters produce an image in a part of the spectrum which is difficult
to see visually. For this reason instruments with a CaK filter fitted tend to
be used more for imaging applications. They pick out regions of glowing
calcium atoms and reveal a region of the chromosphere which lies very
close to the photosphere. Here the view is similar to that seen visually
with the exception that the background granulation visible in white light
is replaced by a network of faint glowing lines which are known as the
chromospheric network.
The view here, apart from the more obvious chromospheric network, isn’t
dissimilar to that presented by a white-light filter, with the exception that
features such as plage which appear clearly towards the edge of the CaK
Sun are also clearly visible in the centre too. Bright prominences may also
be seen using a CaK filter, although they tend not to be as dramatic as those
seen through an H-alpha filter.