'Hollow' sunspots
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Quia
I've seen these 'hollow' spots a few times, are they actual real magnetic features, or is it an over saturation effect?
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Mjtbarrett
Good Question, I've wondered about the doughnuts as well... There's also a very grey area in the centre; does this show a magnetically "neutral" zone? The context view shows a huge sunspot below this one. I think that that may have been responsible for an X 10 flare!
See: http://www.solarmonitor.org/full_disk.php?date=20031030&type=shmi_maglc®ion=Posted
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pahiggins
To the second question: yes it was! and note the very strong 'double' polarity separation line in that region (10486). The region above shows a double PSL, but also emerged incredibly quickly- for such a large complex feature, rapid emergence is a pretty good sign that there could be some fireworks...
To the first question, the hollow effect is in fact due to a saturation effect. The problem is that for MDI, much of the data was processed on-board the spacecraft. Because of the way that the processing was designed from the beginning, the model used to calculate the magnetic fields broke down for very large fields (>2000 gauss), resulting in the saturation. Unfortunately, the effect isn't even linear, so even at lower fields (1k-1.5k G) you start to see problems. Also, the non-linearity makes it hard to correct for.
Here is a paper on the effect: http://adsabs.harvard.edu/abs/2006SPD....37.0715L
I will try to write a blog post on this in the near future explaining how magnetograms are made, and touch on the saturation problem.Good catch!
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Mjtbarrett
@pahiggins thanks again! I only seem to be able to view the abstract in the link above. I looked for an arxiv pre-print or free version but couldn't seem to track one down. Any suggestions on how to view the paper for an impovrished non-academic please? 😃
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pahiggins
Whoops, I sent the wrong link. Try this: http://adsabs.harvard.edu/abs/2007SoPh..241..185L
Let me know if it isn't accessible.Posted
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Mjtbarrett
Sorry, the link leads to a paywall - I could probably see it for £29.95! No problem though, I'll take your word for it 😃 saturation it is!
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Quia
Ah the joys of paywalls. Thank you for confirming that this is a saturation effect!
You can read the introduction of that paper here http://link.springer.com/article/10.1007%2Fs11207-007-0296-5?LI=true if you click 'look inside,' but that's the best I've been able to find.
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