Download Sunspots Today: A Cheshire Cat

Sunspots Today: A Cheshire Cat
W. Livingston, National Solar Observatory, 950 N. Cherry Ave, Tucson AZ 85718; Email [email protected];
M. Penn, National Solar Observatory, Tucson AZ; E-mail [email protected]
Physical conditions in the infrared at 1.5 microns, including maximum magnetic
field strength and temperature, have been observed spectroscopically in 1391
sunspots 1990 to 2009 (1). We emphasize the quantitative difference between our IR
sunspot measurements and the visible light results from most solar magnetographs
employed world-wide. The latter are compromised by scattered light and measure
flux, not field strength. A lower limit of ~1800 Gauss is required to form spot umbra.
The umbral maximum field strength has declined over the above interval, perhaps
because spots have on average diminished in size. The present condition of solar
activity minimum has more spotless days than since the 1910s (2). The Cheshire Cat
behavior is related to magnetic surface fields often appearing without accompanying
dark spots.
Sunspots recently are behaving like a Cheshire Cat: the smile is there (magnetic fields)
but the body is missing (no dark markings). We are unsure about past cycles but at
present sunspots, with their usual umbrae and penumbrae, are failing to materialize.
For hundreds of years the Sun has shown an approximately periodic 11-year alteration in
its activity where the number of sunspots increases and then decreases. Sunspots are dark
regions on the solar disk with magnetic field strengths greater than 1500-1800 Gauss.
The last sunspot maximum occurred in 2001. Magnetically active sunspots at that time
(Figure 1) produced powerful flares, caused large geomagnetic disturbances, and
disrupted some space-based technology. At present, presumably leaving a deep solar
minimum, nothing more than tiny spots, or “pores”, have been seen for some time (again
Figure 1). In the current solar minimum the number of spotless days has not been
equaled since 1914 (2), see Figure 2. Some look at this figure and feel reassured; this has
happened before. Others sense abnormality.
Why is a lack of sunspot activity interesting? During a period from 1645 to 1715 the Sun
entered an extended period of low activity known as the Maunder Minimum. For a time
equivalent to several sunspot cycles the Sun displayed few sunspots. Models of the Sun’s
irradiance suggest that the solar energy input to the Earth decreased during that epoch,
and that this lull in solar activity may explain the low temperatures recorded in Europe
during the Little Ice Age (3).
In 1990, working with S. Solanki, we began exploratory measurements at the McMathPierce telescope of the infrared magnetic field strength, temperature, and brightness in
dark sunspot umbrae. These observations use the most sensitive probe of sunspot
magnetic fields: Zeeman splitting of the infrared spectral line of Fe I at 1565 nm. This
splitting yields total field strength not flux (see below). Because the splitting is always
complete in sunspot umbrae the measurement is independent of atmospheric blurring, or
seeing (providing the line is visible). Temperature was deduced from the depth of nearby
molecular OH lines. Higher temperature meant brighter continuum intensity and weaker
OH. Starting in 2000 this work became systematic, where each spot was measured only
once at the darkest position in its umbra. The resulting data set of 1391 observations
represents the longest time-sequence of total field strengths in sunspots. Figure 3 is a plot
of these observations. We believe most of the scatter is real; the errors are likely to be in
intensity and not field strength. Sky transparency and image quality, or seeing, are of
course somewhat variable and this affects intensity. Data with obvious clouds were
discarded. Two conclusions: 1) there is not a unique relation between sunspot brightness
and magnetic field and 2) the lower limit to the magnetic field to produce a dark marking
is around 1500-1800 Gauss. This lower limit is uncertain because of noise in intensity
(brightness) signals.
It was also found that the magnetic field strengths in umbrae were on average decreasing
with time independent of the sunspot cycle. Or it may be that spots are simply getting
smaller (4). OH has practically disappeared today. A simple linear extrapolation of our
magnetic data suggests that sunspots might largely vanish by 2015, assuming the 1800
Gauss lower limit, see Figure 4.
The brightness and magnetic fields of large sunspots had earlier been discovered to
change in-sync with the solar cycle as seen by ground-based telescopes (5). Automated
solar magnetographs (e.g. Mt Wilson, Kitt Peak, SOHO) measure surface magnetic flux
using spectral polarization signals from the Zeeman effect. Flux measurements are
subject to scattered light; the fields they deduce in sunspot umbrae are much less, often
by a factor of two, than the field strength given by the Fe 1564 nm splitting (6). The latter
does not involve polarization sensing. Magnetograph instruments, however, are in wide
use both in space and ground-based –with a time span going back over 50 years. They do
record non-sunspot magnetic flux (which the simple non-polarized Fe 1564 nm splitting
cannot do) and have detected the onset of the next solar cycle active regions. This
deduction is based on the expected high solar latitude hemispheric magnetic polarity
reversal, the “Hale cycle”. Yet all new cycle number 24 spots that we have observed
have been tiny “pores” without penumbrae (e.g. Figure 1). Nearly all of these features are
seen only on magnetograms and are difficult or impossible to see on white-light images.
Thus the analogy to the Cheshire Cat [Roberts, 2009].
Physical explanations of this deep minimum are at present speculative. Modelers invoke
flux transport, meridional flows, and other subsurface mechanisms. Whether this
diminished vigor in sunspots is indicative of another Maunder Minimum, remains to be
seen. We should mention, too, that the solar wind is reported to be in a lower energy state
than found since space measurements began nearly 40 years ago (7). Will the Cheshire
Cat Effect persist?
References:
1. Penn, M.J. and Livingston, W., Temporal Changes in Sunspot Umbral Magnetic
Fields and Temperatures, Astrophys. Jour., 649, L45-L48, (2006).
2. Janssens, J., Spotless days website, (2009)
(http://users.telenet.be/j.janssens/Spotless/Spotless.html
3. Lean, J., A. Skumanich, and O. White, Estimating the Sun’s Radiative Output
During the Maunder Minimum, Geophys. Res. Lett., 19(15), 1591–1594 (1992).
4. Schad, T.A., and Penn, M.J. (2008), Solar Cycle Dependence of Umbral
Magneto-Induced Line Broadening, EOS Trans. AGU 89(23), Jt. Assem. Suppl. Abstract
SP41B-06 (2008).
5. Albregtsen, F. and Maltby, P., Solar Cycle Variation of Sunspot Intensity, Solar
Physics, 71, 269-283 (1981).
6. Private communication from J. Harvey, (2009).
7. Fisk, L.A., and Zhao, L., The Heliospheric Magnetic Field and the Solar Wind
During the Solar Cycle, in Universal Heliophysical Processes, Proceedings of the
International Astronomical Union, IAU Symposium, Volume 257, pp 109-120 (2009).
Acknowledgement:
Roberts, Harry, Sydney Observatory, private communication re. Cheshire Cat (2009).
Figures:
Figure 1a. (yellow) An image of a sunspot from near the maximum of the last solar
cycle, cycle 23, taken at the McMath-Pierce telescope on 24 October 2003. The sunspots
clearly show a dark central umbra surrounded by a brighter, filamentary penumbra. The
magnetic fields seen here range from 1797 to 3422 Gauss. Figure 1b. (orange) An image
of a pore – a tiny sunspot with no penumbral structure – taken from the MDI instrument
on the SOHO spacecraft, 11 January 2009; this is an example of what we observe today
at solar minimum. The larger pore had a magnetic field of 1969 Gauss. Presently, the
solar surface is mostly devoid of spots. Both images have the same spatial scale, and are
roughly 360 Mm horizontally.
Figure 2. Number of spotless days at cycle minima in the past.
Figure 3. Maximum magnetic field vs. continuum brightness for all data 1990-2009.
Figure 4. The maximum sunspot total field strength is plotted versus time, during the
period from 1992 to Feb 2009; a 12 point running mean is shown, and a linear fit to the
data is made. Apart from a few measurements, the linear trend has been seen to continue
throughout the present solar minimum.