Solar Images Archive and the Identification of Sunspots

2008 08 22 K
A sunspot, or not a sunspot?

2nd UPDATE Sep 5 – Image issues fixed. Or so I believe…

1st UPDATE Sep 5 – There are issues with the OAR server and some images are not showing properly. I will try again to source those images, or switch to MLSO’s.

Who would have thought that counting sunspots were such an opinion-based activity, near a minimum in solar activity?

After the back-and-forth revelations about August 2008 being spotless for the whole of humanity apart from one observer in Catania, Italy, whose keen eye has made both SIDC and NOAA change their mind, a friend pointed me in the direction of two web-based PSPT Solar Images Archives, one from 1996, beyond the usual SOHO stuff.

And an analysis of those archives reveals not only how different can the Sun appear at different wavelengths (a trivial concept, after all) but also how what looks like a sunspot in one image may be pretty much invisible in another.

But if some sunspots appear to be much easier to identify if the “correct” filters are used during observation, wouldn’t a more precise definition of “sunspot” be needed? For example “it appears in at least 3 filters” or “it can be seen in the SOHO MDI channel” or whatever else.

Otherwise when the Sun will wake up later this year or next, the count of sunspots is going to be truly enormous, and comparisons with the past impossible.

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What is the PSPT?

The Precision Solar Photometric Telescope (PSPT) [by the National Science Foundation (NSF)] produces seeing-limited full-disk digital (2048×2048) images in the blue continuum (409.4nm, FWHM 0.3nm), red continuum (607.1nm, FWHM 0.5nm), CaII K (393.4nm, FWHM 0.3nm) […] with an unprecedented 0.1% pixel-to-pixel relative photometric precision. […]
The National Solar Observatory (NSO) designed and built three PSPT units, a prototype which is currently in operation at the Osservatorio Astronomico di Roma (OAR), and two primaries for installation at NSO Sac Peak and Mauna Loa Solar Observatory (MLSO).

OAR provides “full disc images of the Sun from 1996” to today. The corresponding MLSO query page is hosted by the University of Colorado and starts in 2005.

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A few words on the available filters:

1. K (393.3nm)

Light at this wavelength is extremely faint but when the brilliant photospheric emissions are filtered out, the Ca II K line provides important information on the large-scale magnetic field structure in the chromosphere.

Because the blue Calcium K Line (393.3 nm) is sensitive to magnetic fields, magnetically active structures show up in high contrast against the surrounding chromosphere. Places where moderate magnetic fields exist show up bright whereas images of high magnetic fields are dark.

2. B (409.4nm “blue continuum”)
3. R (607.2nm “red continuum”)

The difference between those images can be used to identify sunspots.

4. C (430.6nm, the so-called G-band)

Many observations in the G-band are done in order to investigate the photospheric bright points and the connected magnetic elements, making use of the increased contrast of these structures

In the G-band the photospheric faculae show high contrast even near the solar disk center

5. G (535.7nm)

Used because free of Fraunhofer lines, this filter sits at the top of solar irradiance. It is the band used for measurements of the Sun’s diameter from the ground

 

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Let’s look now at the Aug 21-22 situation, when the only activity of the month has been reported. Well, there is something going on, but it’s only apparent in the K filter.

Remember, the K filter takes very faint light. Humans looking in the normal visible bands (B, R and most of all G) cannot see anything at all.

2008 08 21 K
2008 08 21 K
2008 08 22 K
2008 08 22 K
2008 08 21 B
2008 08 21 B
2008 08 22 B
2008 08 22 B
2008 08 21 R
2008 08 21 R
2008 08 22 C
2008 08 22 C
2008 08 21 C
2008 08 21 C
2008 08 21 G
2008 08 21 G
2008 08 22 G
2008 08 22 G

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Following are the pictures from June 20, when last numbered spot #1000 has been seen or maybe not.

Now there is something to look at in the B, R and G filters, but once again it’s only K that reveals the thing whole.

2008 06 20 K
2008 06 20 K
2008 06 20 B
2008 06 20 B
2008 06 20 R
2008 06 20 R
2008 06 20 C
2008 06 20 C
2008 06 20 G
2008 06 20 G

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As a way of comparison, look at the pictures from Jul 14, 2005, when sunspots were aplenty, in all filters.

2005 07 14 K
2005 07 14 K
2005 07 14 B
2005 07 14 B
2005 07 14 R
2005 07 14 R