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Magnetic Structure
The sunspots which blemish the solar surface are areas of high magnetic flux, and can be classified according to their appearance, complexity and magnetic structure. They are cooler than the surrounding photosphere so appear dark on images taken in the visible continuum. A record of the monthly total number of sunspots led to the discovery of the well known eleven-year active-cycle of the Sun. Sunspots are part of a group of larger areas known as active regions. Active regions comprise all phenomena associated with emergent flux and many active regions exist without producing sunspots. Sunspots and active regions generally occur at low latitudes, whereas shorter lived, smaller scale, magnetic areas known as ephemeral active regions are evident over a higher latitude spread. The organisational quality of the supergranular flow tends to concentrate the magnetic field into the cell boundaries. But the finite efficiency of this process means there is some remnant field. This left over field eventually reaches an equilibrium with the convective and turbulent motions and this creates a third scale of magnetic field known as the intranetwork or internetwork, studies of which can only be achieved with deep magnetograms which have high spatial resolution and long exposure times. Unfortunately the small structures also evolve over short timespans so further progress in this field will require achieving the desired spatial resolution without trading off temporal resolution.
Images in the cores of weak lines, especially the G-band, show small features associated with increased magnetic field. A pattern of strings of small bright points can be found strung along intergranular lanes. These filigree can be found in the quiet Sun at supergranular boundaries, and in active regions as faculae. Each bright point can be spatially associated with a magnetic element of nearly the same size. Magnetic tubes of around 1000 G are concentrated by compression of converging granules in a diameter of around 400 km. This results in a large flux of 1017 Mx. Because of this strong interaction with energetic granules, filigree are unstable with a mean lifetime of around five minutes, and usually appearing in a dark junction of several granules, concentrating the field further. The gas inside the filigree is heated by the gas outside, due to its higher pressure. A combination of this along with the fact that deeper layers are viewed creates the brightness. The strong correlation with magnetic elements means filigree can be used to track chaotic flux. Magnetic waves, transverse to the field, are generated by this chaotic motion at the top of the convection zone and move up through to the corona with an exponential growth of amplitude. Dissipation of energy by these magnetohydrodynamic waves is suggested as a possible coronal heating mechanism.
Figure:
Imaging throughout the solar atmosphere. All images were taken by the Dunn Solar Telescope, Sacramento Peak and are of the same region at disk centre with a field of view of 120" by 120". Images in the core of the H line are chromospheric. However moving away from disk centre, images are increasingly photospheric. At 1Å into the blue wing, granulation dominates the images, similar to images taken in the continuum
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Next: Chromospheric Features
Up: Photospheric Features
Previous: Granulation
James McAteer
2004-01-14