DayStar Filters SolaREDi Odyssey Manuel d'utilisation - Page 6

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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

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.

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.