STEREO space craft:
Small prominences in
coronal magnetic cavities
Anaglyphs by John Wattie from STEREO mission images
Images in extreme ultraviolet are taken by the SOHO and STEREO satellites at discrete wavelengths, determined by the emission line of various ionised elements in the coronal plasma.
Since the electron quantum jumps responsible for the line emissions are only possible when the temperature is hot enough, the wavelength is a guide to coronal temperature.
The corona gets hotter the higher it is above the photosphere and so the temperature-dependant wavelength is a rough guide to height in the corona. This layering means emission line images have similar properties to a medical x-ray tomogram (which shows just one layer in the human body).
The colours of EIT images are artificial and assigned by the SOHO engineers, since ultraviolet is invisible to humans. STEREO follows the SOHO conventions, thank goodness.
|
EIT wavelength bands (SOHO)
| Wavelength (Å) |
Dominant ion |
Characteristic temperature |
Main Image Features |
| 284 Å (Yellow) |
Fe XV |
2x10 6 K
Hotter, higher |
High coronal loops - shows coronal holes.
(x-ray, e.g. Hinode, is hotter still) |
| 195 Å (Green) |
Fe XII |
1.3x10 6 K |
Corona - spreading magnetic field |
| 171 Å (Blue) |
Fe IX, X |
900x10 3 K |
Corona - bright above active regions |
| 304 Å (Red) |
He II |
80x10 3 K
Cooler, lower |
Hot chromosphere -> Transition zone |
| 6563 Å |
H alpha |
10x10 3 K |
Cool chromosphere: visible red light |
8x10 4 K means 8 times 10 raised to the 4th power, but not all internet browsers follow style sheet conventions and show superscripts. 8x10 4 K is 80,000 degrees Kelvin (which is virtually the same as 80,000 degrees Celsius or Centigrade)
STEREO image at 171 Å. Quiet sun, April 2007.
Note the dark coronal holes, which are nearly always present at the poles, especially at solar minimum, but can also show up on the solar face. Coronal holes emit the fast solar wind and if this is directed at earth, a geomagnetic storm is possible.
On the solar limb, the loop structures (9 o'clock) are plasma confined to looped magnetic field lines. These are most prominent around active regions (usually sun-spots).
The straight lines at the poles are magnetic field lines heading out into space (and carrying the fast solar wind). Eventually these lines will curve back to enter the opposite solar pole.
At 11 o'clock and 1 o'clock there is an oval cavity in the magnetic field. It is easy to see the bottom and sides of the cavities but the roofs are vague. During a solar eclipse the huge helmet streamers in the corona show such a cavity and often a filament runs along in the cavity, which is really a magnetic tunnel. When seen on-end as here, the tunnel looks like a cavity.
According to H. Morgan and S. R. Habba of Hawaii (A&A 464, 357–365 (2007)), filaments are markers to the bases of the huge coronal streamers. Streamers stretch far out from the sun as seen on LASCO, which is the coronagraph flown on SOHO. The paired coronagraphs on STEREO should be able to confirm this 3 dimensional intepretation, but so far I have not been able to access STEREO coronagraph images. Coronal streamers split at the base and the two limbs straddle a streamer, as we begin to see on these 3D images.
 Use Red/Cyan goggles to see the images in 3D
|
The 171 Ångstrom image shows "sticks
" at the start and end of the 11 o'clock filament. However, the 2 o'clock filaments do not have "sticks."
304 Å image, is lower in temperature and so lower in the corona (in the transition zone).
Prominences show best in H-alpha images and on the disc they are called filaments. Since they are cooler than the photosphere, filaments in H alpha look dark because red, H alpha, photosphere light is absorbed by them. It is much like the opaque shadows seen on medical x-ray images. When the same structures are seen on the edge of the disc as prominences, they look bright against the dim light of the corona.
304 Å images show the cool prominences, which are hard to see on the higher temperature ultraviolet images. SOHO images in 2D also showed edge prominences well, but it was difficult to trace them onto the solar disc. Stereoscopic viewing of 304Å STEREO images allows disc prominences (called filaments) to be picked out because they are higher in the corona, although it gets more difficult towards the middle of the disc.
Stereoscopic 304Å images show the filaments both on the disc and at the edge, confirming they are the same structures.
Roll-over image
Run your mouse over and see that the prominences at 1 o'clock are sitting in an oval cavity in the magnetic field. This hole lies at the boundary between North and South magnetic fields, at the base of a helmet streamer. The magnetic field of the cavity acts as a "hammock" to support the prominence. The full extent of the helmet streamer needs a solar eclipse or an x-ray image to reveal it and currently I cannot find Hinode image taken at this time to confirm my suspicion that we are looking at the base of a helmet streamer.
Prominences (and filaments, which are the same thing when seen on the solar disc) are stable for long periods. A coronal cavity which supports a filament can be a pre-cursor to a Coronal Mass Ejection. (See here) A CME has a central hole, which is the greatly expanded cavity. The solar filament which lay in the cavity forms part of the CME as it erupts and expands to fill part of the large hole in the CME. A CME is often a huge helmet streamer which has become unstable and carries the twisted magnetic field with it. In fact, a CME is essentially a method for the sun to rid itself of magnetic helicity. Differential solar rotation winds up the magnetic field and the tension in the helical flux tubes has to be got rid of somehow. |
Mouse roll-over image:
Run your mouse on and off the image to see it change.
Sticks are discussed further here.
NEXT: Top left, magnified view of the sun, to show the cavity in detail.
Next Day: The Prominence has erupted out of its magnetic cavity
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