54 September 2014 sky & telescopeOBSERVING
Exploring the Solar SystemThe Sun in calcium-K (Ca-K) light off ers the most amaz-
ing display of solar details and activity that you may never
see in an eyepiece. You read that right: the Ca-K band at
393.3 nanometers lies so close to the ultraviolet that many
observers see little more than a dim violet disk through a
Ca-K fi lter. This is why you’ll rarely see observers look-
ing through Ca-K telescopes alongside the more common
hydrogen-alpha scopes. But the details are there, ranging
from sunspots and the bright plages that outline spots’
surrounding magnetic fi elds, to solar fl ares, explosive
magnetic reconnection events, and even the brighter solar
prominences. These details can change in a short time
frame and reveal a dynamic Sun that — with the aid of a
video camera and monitor — is in fact visible to everybody.
The region of the Sun seen in Ca-K resides in the
chromosphere, about 500 to 2,000 kilometers (300 to
1,200 miles) above the photosphere, which we often call
the Sun’s “visible surface” and is the layer we see in white
light. Sunspots in Ca-K appear very similar to how they
look in white light, displaying details of the umbral and
penumbral regions. These spots are surrounded by brightplages, emission regions that coincide with (and closely
resemble) the bright photospheric faculae usually seen
foreshortened near the solar limb in white light. But these
plages are much brighter in Ca-K compared to faculae,
and you can see them clearly anywhere on the solar disk
at this narrow wavelength.
The plages are emission regions tied to strong magnetic
fi elds and are nearly identical to the shape of the magnetic
regions visible on solar magnetograms. When several spot
groups, or active regions, lie very close together you might
have diffi culty in white light telling which spots are in
which regions. In Ca-K, the plages may make it easier to
determine to which active region the spots belong.
Although all sunspots reside on the photosphere
beneath a plage, many plages develop without any asso-
ciated spots. Plages form before spots become visible
and persist long after the spots have faded away, if the
spots form at all. You can identify some plages several
solar rotations after their spots have disappeared. Even
when there are very few spots on the Sun, it’s exciting
to observe bright plages as they burst into view, expand,
and mature over days to weeks, and then slowly spread
apart and diff use into the bright, web-like network of the
chromosphere. Sometimes new plages emerge within an
old fading plage, making it diffi cult to tell where the old
plage ends and the new one begins.
Explosive solar activity frequently appears as bright
speckles within plages, caused by catastrophic recon-
nection of the Sun’s intense magnetic fi elds. Sometimes
called mini-fl ares, these speckles may extend for hun-
dreds or thousands of kilometers. You can follow the evo-
lution of this activity over several minutes, and sequential
images may display these rapidly evolving changes. Occa-
sionally, large regions of the plages and spots brighten
quickly into powerful solar fl ares many times larger
than Earth. These are exciting to watch as they brighten,
expand, and then migrate and fade over a period of tens
of minutes. Sometimes a ripple eff ect seems to trigger
fl ares and reconnection events in adjacent active regions
as well. In Ca-K light, it truly becomes possible to observe
and follow the action within these active regions.
Evolution of solar activity is also visible on the solar
limb in the form of plasma structures known as promi-
nences that hover, quiver, and occasionally erupt into
space. In 1891 George Ellery Hale used the calcium-H andThe Violet Sun
A K-line fi lter can reveal solar activity normally hidden from view.
Although diffi cult for some observers to see, the Sun in one of
the emission lines of ionized calcium (Ca-K) off ers the best of
both white-light and Hα views: dark sunspots, bright plages, and
rapidly evolving prominences along the solar limb.HOWARD ESKILDSENETSS_layout.indd 54 6/23/14 12:17 PM