Could Past Sunspot Variations Lead to the Current “Blank Sun”?

The Sun as seen on Aug. 6th 2008. Still no sunspots. It's like watching paint dry... (SOHO/MDI)

Wow, what an unremarkable few months the Sun is having. Yes it is going through its solar minimum and yes that means it’s going to be fairly quiet, but the total (and I mean total) lack of sunspots is beginning to get a little boring. Sometimes the Sun does this, it does something unpredicted, like generating historic X-ray flares after solar maximum (like in 2003) or being unseasonably quiet (like now). This is the big issue with solar physics; although we can study our nearest star in great depth, we still do not appreciate what drives the inner workings of the Sun. We don’t fully understand why its atmosphere (corona) is so hot, let alone the nature of the 11-year solar cycle.

So, when asked “what are your views on the current lack of sunspots?”, I have to remain vague and point out that any form of solar forecasting is not possible at this stage, and more work needs to be done when working out the nature of sunspot activity. But now, with the help of a fellow blogger, a paper has been brought to my attention that actually predicted there will be no sunspots by 2015. What makes this enthralling is that this dual-author paper was written in 2006… back when the Sun was winding down from a pretty ferocious Solar Cycle 23. Could their prediction be coming true?

Firstly, I’d like to thank Dr. Bruce Cordell over at 21st Century Waves for telling me about an unpublished paper entitled “Sunspots may vanish by 2015,” by William Livingston and Matthew Penn, National Solar Observatory at Kitt Peak. According to Bruce, the paper was submitted to Nature, but promptly turned down after a review.

So what’s this paper all about? Basically the two researchers have analysed spectroscopic data from sunspot observations over a 15-year period from 1990 to 2005. In total, over 1000 sunspots had been measured, noting their umbral brightness, temperature and magnetic field strength. The sunspot “umbra” is the innermost and darkest region of a sunspot. Surrounding the umbra is the “penumbra,” a highly structured, warmer region. Generally, one would expect the umbra to have a peak radiation at a temperature of 2200K, the penumbra warmer at 3000K (the surrounding photosphere is approximately 6000K). The umbra appears dark as it is highly contrasting with the radiation from the hotter photosphere.

The fine detail of a sunspot. The umbra (centre) and penumbra (filaments) are obvious. By the Swedish 1-meter Solar Telescope on La Palma in the Canary Islands. Credit: Courtesy Royal Swedish Academy of Sciences

Sunspots are very good indicators for the activity of the Sun. During solar maximum, when the solar magnetic field is at its most stressed, magnetic flux is pushed to the solar surface, filling with solar plasma as it does so. These tubes of flux filled with plasma undergo heating (possibly from wave-plasma interactions and/or nanoflare activity), producing beautiful, dynamic arcs called coronal loops. At the base of many of these loops, sunspots can be found. The more sunspots there are, the more coronal loops and therefore the more active the Sun is. During periods of calm, like solar minimum, the solar magnetic field is at its least stressed state; minimal magnetic flux, low coronal loop population, minimal sunspot number.

The key point that needs to be made here is that sunspots are magnetically dominated structures. The umbra is formed by the upper layers of the Sun being pushed aside by vertical magnetic flux, exposing the cooler, inner Sun (or the uppermost layer of the convection zone). So, the Kitt Peak astronomers Livingston and Penn used their solar observatory to take data from 1000 sunspots, including spectroscopic measurements (intensity of various emission lines) and magnetogram measurements (magnetic field strength inside the spot).

Sample sunspot spectra from the data set. The dashed line is from a sunspot observed in June 1991, and the solid line was observed in January 2002. These provide examples of the trends seen in the data, where the OH molecular lines decrease in strength over time, and the magnetic splitting of the Fe line decreases over time. A magnetic splitting pattern for the January 2002 Fe line of 2466 Gauss is shown, while the June 1991 spectrum shows splitting from a 3183 Gauss field. (Livingston & Penn, 2006)

Firstly, the spectroscopic data appears to show a decrease in intensity from 1991 to 2002. The two critical features in the figure above shows a large decrease in intensity for the molecular OH lines (dashed line is from 1991 data, solid line is from 2002 data) and Fe lines. It is worth noting that these data are from two single sunspots deemed “typical” from the 1991 and 2002 observations.

A linear fit to observed magnetic fields extrapolated to the minimum value observed for umbral magnetic fields; below a field strength of 1500G as measured with the Fe I 1564.8nm line no photospheric darkening is observed. By 2015, no sunspots should be observed (Livingston & Penn, 2006)

The second oddity comes from the magnetic data for the whole 1990-2005 period (figure left); the umbral magnetic field appears to be decreasing rapidly from nearly 3000 Gauss in the late 1990′s to nearly 2000 Gauss in 2005. If this trend is decreasing linearly, by 2015, the umbral magnetic field strength will hit 1500 Gauss. At this point, the sunspot structure will not be maintained, no umbral darkening will be observed. Therefore there can be no sunspots!

Supporting these data, Livingston and Penn have derived quite a large increase in sunspot temperature as seen in the spectroscopic analysis. In 1990, the average umbral temperature was 4670K; in 2005 it had increased toward more photospheric temperatures, 5350K. This 680K temperature increase also means that if the trend continues, familiar dark sunspots will not be seen as they will blend in with the rest of the photosphere.

The magnetic results are however the driving factor behind this research. If the solar magnetic field continues to decrease, the Sun will continue to become less active and the 11-year solar cycle will effectively be “put on hold” until the mystery mechanism driving this phenomenon decides to start up again.

Sunspot intensity seems to decrease steadily regardless of the solar cycle (Livingston & Penn, 2006)

Although this is compelling research, after all (as of 2006) there had been very little attention on the analysis of individual sunspots, we need to be cautious about what interpretations we can make of Livingston and Penn’s results. The biggest issue for me is the small amount of data. We are only looking at 15 years worth of sunspots, that’s only just a little more than one cycle. If this was a trend stretching back over several solar cycles, I’d be more inclined to support their 2015 prediction.

There is however a nagging thought in the back of my mind. Although the Sun can often be unpredictable, what if Livingston and Penn’s prediction is correct (albeit after analysis of a small dataset)? What if the Sun really is “winding down” and we are facing the reality of a featureless Sun? Well, I’ll leave that debate for another couple of months; if the Sun is still “blank” we can start making some more predictions. Perhaps by then this sunspot research will have been accepted for publication in Nature…

Full paper: Livingston & Penn, 2006