Aimee A. Norton
W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA

Even though Solar Cycle 24 is weak, it has still produced some large sunspots. HMI has recorded full-disk, vector magnetic field data at a 12-minute cadence from May 2010 till the present¹. For that time period, every sunspot can be studied in detail as it crosses the solar disk, limb-to-limb. The two movies linked below show an example of a large group observed by HMI in May 2012. Note the interesting vortical flow beginning May 10 12:00 UT for the small bipole region.

NOAA AR 111476 Continuum Movie (above) and B_r (below)|
SHARP cylindrical equal area projection data products² are shown for NOAA active region 11476 from 8-15 May 2012. Above (below) are the continuum and B_r data. B_r is the radial field strength (Gauss) calculated from the vector magnetic field data assuming a filling fraction of unity.

What is a “large” sunspot? For simplicity, let’s consider a sunspot to be large if it’s visible to the unaided human eye (using a proper filter to avoid eye damage). A conservative criteria for a naked-eye sunspot is that it must have a combined penumbral and umbral radius greater than 30”. For perspective, to meet this criteria, a sunspot needs to be about the size of Venus observed in transit. A Mercury transit is not visible to the naked eye at roughly 10”.

Sunspot areas can be separated into umbral and penumbral areas but are most often recorded as the sum of both. This area may consist of single or multiple magnetic polarities and spots. The umbra is usually small compared to the penumbra, which is 5-6 times larger³.

Only a few percent of sunspots produced in any cycle are naked-eye spots. A table of naked-eye sunspots for Cycle 24 are listed below on the day of their maximum area. The area is recorded in the NOAA/USAF active region summary. I chose 600 μHem for the minimum area to be naked-eye visible. This value can be quibbled with a bit, but is a fair value as to what could be seen by the unaided eye. 1 μHem equals 10^-6 the area of a solar hemisphere which corresponds roughly to 3 million km². A perfectly round 600 μHem sunspot represents a sunspot whose penumbral radius is 34” across.

The extreme specimens of any sample are interesting as they provide clues about the formation processes. How big and strong can sunspots get? We do not yet know. The largest sunspot groups ever photographed were in 1946 and 1947 during Cycle 18. They reached more than 4000 μHem and survived for three months. Livingston and Harvey⁴ found 55 sunspots from 1917-2004 with field strengths of 4000-6100 Gauss.

How dark and cool do sunspots get? The brightness ratio is a measure of how dark a sunspot is: it’s the continuum value in the darkest part of the umbra normalized by the surrounding quiet-Sun intensity. NOAA #11899 in November 2013 has a 5.4% brightness ratio uncorrected for scattered light. This is further reduced to 2.7 – 2.8% after correcting for scattered light using two independently developed HMI point-spread functions as developed by Yeo et al.⁵ and Norton et al. (in preparation). Using the Planck function and a quiet-Sun temperature of 5777 K, the corrected brightness ratio implies an umbral temperature of 3060 K. That’s cool!

Problematic aspects of HMI vector field data of which scientific users should be aware are as follows. 1) In strong field regions and at high spacecraft velocity, the spectral line is shifted outside of the optimum spectral range to which the filters are tuned. This results in a lowering of measured field strength in a patch in the strongest part of the umbra. Because the spacecraft-Sun orbital velocity reaches a maximum every 12 hours, one can see the effect when the velocity is highest (roughly 0 and 12 UT). 2) For certain pixels, the fit to the data does not converge properly so the returned field strength values are the maximum allowed at 5000 Gauss. These spatially-scattered bad pixels can usually be spotted near the penumbral/umbral boundary. These are not correlated with spacecraft velocity.

I’ll end on a light note with a joke referencing the classic Red Riding Hood tale.
     Earth: “Grandmother Sun, what big umbrae you have.”
     Sun: “All the better to CME you with, my dear!”
While sunspot size is not correlated with coronal mass ejection (CME) frequency, NOAA AR#11476 was associated with a halo CME on May 17, 2012.

References

[1] Hoeksema, J.T., et al., 2014, Sol Phys, accepted, in press.
[2] Bobra, M.G., Sun, X., Hoeksema, J.T., Turmon, M., Liu, Y., Hayashi, K., Barnes, G., Leka, K.D., 2014, Sol Phys, submitted, in press.
[3] Hathaway, D.H., 2013, Sol Phys, 286, 347.
[4] Livingston, W. , Harvey, J.W., Malanushenko, O.V., and Webster, L., 2006, Sol Phys., 239, 41.
[5] Yeo, K.L., Feller, A., Solanki, S.K., Couvidat, S., Danilovic, S., and Krivova, N.A., 2014, A&A, 561, 22.