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Features of the Sun in Hydrogen Alpha
By observing the sun with a narrow bandpass filter tuned to 6562.8Å, we can observe
the
behavior
of
the
Sun's
Chromosphere. The chromosphere
is like a shell of gas around the Sun's
photosphere, always moving and
changing.
The
chromosphere's
structure behaves differently in
active regions than quiet areas,
where magnetic field lines are
stronger. Thought to be tied to the photosphere, the chromosphere is governed by
magnetic forces and, yet it still has its own IntraNetwork (IN) of material oscillating
every 5 minutes.
On the limb, even a rather wide fil-
ter of 1Å or more will show
promi-
nences, a detail of the chromos-
phere projected against the dark
black contrast of space. To observe
the details of chromosphere on the
face of the sun, we need a narrow-
er filter to eliminate more off-band
light of the photosphere and continuum. We need a filter less than 1.0Å. The nar-
rower the filter's bandpass, the more contrast we will see - down to 0.4Å, where
prominence structure is reduced due to high velocity and subsequent wing shift.
Spicules
dominate the chromos-
phere in non-active regions and
have been studied exhaustively.
They are barely visible, last only
about 15 minutes, and resemble a
"burning prarie". Some jets can be
seen shooting 10,000 km up from
the Sun's limb at velocities of about
30km/sec. Studied exhaustively, they present a number of observing challenges, as
they are too small to resolve and move so quickly as to present wing-shift challenges.
Field Transition Arches (FTA's)
connect P and F spots - elements of opposite polarity.
Inside an active region, where
sunspots are originally linked by a
FTA, a shear boundary forms. Field
Transition Arches are different from
filaments in that they are thin and
not very dark. The FTA usually has
plage or granular structure under-
neath.
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Filaments
appear as large, dark eyebrows
across the surface of the Sun. With a
brightness of about 10% of the disk due to
scattering, they appear dark on the sur-
face, but on the limb, show as a promi-
nence. Active Region Filaments (ARF) dif-
fer from Quiescent Region Filaments (QRF). ARF are darker, smaller and have more coherent
fibril structure along their axis. A sheared magnetic field runs parallel to this axis, permitting
a sizeable flare. QRF may produce a big
Coronal Mass Ejection (CME). An ARF may
erupt and reform several times.
Plages:
Most of the active region area is
occupied by plage. Considerable atmos-
pheric heating takes place in the plage. It
is bright in everything from Halpha to the Calcium H and K lines. This heating is thought to
account for an absence of spicule. While
absent over plage, spicule are prominent
around its edges.
Elerman Bomb:
A remarkable feature of
Emerging Flux Regions is the Ellerman
bomb. Bright points with very broad H-alpha wings (±5Å) that are low in the atmosphere so
they are not visible on H alpha centerline. Called 'moustaches' for their appearance on spec-
trograph, they appear spectroscopically like wide moustaches with a gap in the middle. This
strange and tiny feature typically occurs at the center of the EFR or in the edges of spots -
where the field is breaking the surface.
Solar Flares
are intense, abrupt releases of
energy which occur in areas where the
magnetic field is changing by flux emer-
gence or sunspot motion. Stresses in lines
of force build up slowly and are released
in flares. They occur most frequently at
neutral lines where a filament is supported by horizontal sheared field lines. This event can
only take place along a magnetic inversion line. When many lines of force are involved, two
ribbons of emission appear, brightening simultaneously.
Emerging Flux Regions:
An area on the Sun where a magnetic dipole, or "flux tube" is surfac-
ing on the disk, eventually producing a bipolar sunspot group. Each pole of an EFR is often
marked by pores or small developing sunspots. Surges or even small solar flares can some-
times occur in EFRs. An EFR emerges with
small bright H region with little surges,
then weak arch filaments (AFS) over bright
plage connect small spots on each dipole.
Growth is rapid, forming in just a few
hours.
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