Sunspots
•Measuring the Period
of Solar Rotation
•Solar Activity and
Terrestrial Climate
fig 1
NOAA #0759 Gary Palmer, May 18, ’05 Los Angeles, CA
What Is a Sunspot?
• Sunspots are dark regions on the solar
surface.
– They appear dark because they are generally
between 1,500 and 2,000 K cooler than their
surroundings.1
» It is generally accepted that sunspots manifest as a
result of interactions between the Sun’s magnetic
field and convection currents.
Caprino Bergamasco of Italy penciled this
Image of sunspot group 0756 in May.
Courtesy of spaceweather.com
fig 2
Why do they form?
• Modern theory suggests that twisted,
convoluted magnetic field lines disrupt
normal convection flows, which prevents
cooler regions from sinking.
– Sunspots can last anywhere from a period of
a few hours to a few months and groups can
be as large as the planet Jupiter.2
fig 3
This close-up digital photo
image of 0756, taken using a
coolpix camera and 8” reflector
telescope, compares with
Bergamasco’s masterpiece.
Observation
• We made our first
observation on April
26th 2005.
8” Orion Dobsonian
mounted Newtonian
reflector telesope,
and the sunspotter
projection device.
Observation Summary
• The first 10 days of
observing met with
cooperative skies.
• Observing conditions
worsened thereafter.
May 3rd 2005 image captured
With Nikon coolpix and 8” Orion
Reflector.
NOAA 07563
Complications with
Observation
• Weather was a complicating factor.
• Determining the latitude of the sunspots.
• Although there were complications, having any
sunspots at all was fortunate, due to the
proximity to solar minimum.
– This illustrates the Sun’s unpredictable behavior!
• If we were aiming for extremely accurate data,
we would have to account for torsional
oscillations and meridional flows.4
Sunspotter Images
0756 near the
Eastern Limb,
April 26th
Image captured April 26th at 2:20 PM at TESC
Image was captured Tuesday, May 1st, at 11 AM with digital camera.
Image from May 6th, about 24 hours before NOAA sunspot group 0756
traveled beyond the Sun’s Western limb. It was cloudy on the 7th.
Following in the footsteps of great astronomers, May 16th, at Noon
Calculations
• The period of rotation is roughly 25 days at the
equator and 35 near the poles, thus the rotation
is dependent on heliographic latitude.
• W=14.37 -3.1sin2x , where x is the latitude in degrees, gives
the daily progress of a sunspot at a certain latitude in
degrees per day.5
• So, 360°/W= the number of days for sidereal
rotation.
• Synodic rotation period: 360º+x(1°/day)= wx,
where x=days and w=sunspot angular velocity.6
• 360/(w-1)=x gives the synodic rotation period.
The Sun’s Magnetism
• Magnetism is caused by the interaction of
highly ionized particles.7
• The Sun has a strong magnetic field.
– The magnetic field around sunspots is roughly
2,500 times as powerful as Earth’s.8
• Distortion of the magnetic field is caused
by the peculiar phenomenon of solar
differential rotation.
Differential Rotation
• The convective envelope does not rotate
according to the laws of a fixed body.9
• Equatorial regions travel much faster than
the polar regions.
– They actually complete full rotation in less
time even though they have much further to
travel.
– The difference is as much as ten days!
Differential Rotation and
Magnetism
Fig. 4
Solar activity and magnetism are directly related.
Why does the Sun exhibit
differential rotation?
• The jury is still out on differential rotation.
• Solar differential rotation defies Kepler’s
third law.
• Rob’s Theory: Since there is so much
more mass rotating around the equatorial
region, it has to spin faster to keep from
collapsing! (Are there some problems with
Rob’s theory?) DOH!
Sunspots, Magnetism and
Differential Rotation
• Differential rotation distorts the magnetic
field.
– The distorted magnetic field lines interfere
with convection.
• Sunspots result from disrupted convection
currents.10
• Distortion of the magnetic field causes the
11 and 22 year solar cycle.
– This culminates in magnetic pole reversal.11
Sunspot Evolution
all upper right solar
images courtesy of
sohowww.nascom.nasa.gov
• Generally, sunspots appear at midlatitudes at times of low solar activity.
• Sunspots progressively develop
(generally) nearer the equator toward solar
maximum.
Sunspots travel in
groups which have
opposing polarities,
with the preceding
member having the
same polarity as that
hemisphere’s pole.12
fig 5
Helioseismology
• The magnetic opposition of polarity should
cause sunspots to dissipate rapidly, but in many
cases it does not.
– SOHO enabled helioseismology is helping scientists
to discover why.13
• A vortex, or whirlpool, beneath sunspots
maintains them and continuously pulls in
magnetic field lines and plasma.14
• This explains the Wilson depression, why
sunspots don’t dissipate, and possibly the
increase in solar luminosity.15
Sunspots in History
• Historians have evidence that
Theophrastus of Athens observed
sunspots in 350 B.C.E.16
• Records indicate Chinese astronomers
have observed sunspots since 28 B.C.E.17
• The first telescopic observations of record
were made by Fabricus of Germany and
Galileo of Italy in 1611.18
Modern History
• In 1858 the use of helio-photography
greatly enhanced the study of sunspots.
– This enabled a much higher degree of
accuracy in measuring rotation.19
• Doppler imaging of the red and blue shifts
at the limbs of the solar disk is used to
measure rotation (including differential) to
a higher degree of accuracy.20
Cosmology
• Sunspots have been interpreted with a wide
array of ideas as to their origin.
• There is little mention of sunspots in mythology
due to the difficulty in observation, because the
sun is very bright.
• Pictures of Egyptian Sun-God’s solar disk with
spots.21
• Aztec creation myths have a Sun-God with a
pockmarked face.22
How do sunspots affect
climate on Earth?
• Sunspots are indicative of solar activity.
• There is a wide array of opinion and study
on how solar activity affects climate on
Earth.
– Some say there is little to no affect on Earth’s
climate from variance in solar activity
– Some say there is overwhelmingly dominant
influence on Earth climate from solar activity.
– Reality is somewhere in between these
extremes.
fig 6
Arguments for great influence:
• The Maunder minimum occurred between about
1645 and 1715. This was a period of virtually no
sunspot (hence low solar) activity. It was also
associated with a “little ice age” during which the
river Thames in London froze.23
• Javaraiah, “On long time scales from decades to
millenia, the solar luminosity variations
associated with solar activity are known to cause
significant variations in terrestrial climate on
global scales.”24
• There are also certain industries which favor a
solar influence view of climate change.
Arguments Against
fig 8
• The lack of high quality observational data
makes the correlation of solar activity and Earth
climate change tenuous at best.
• Solar radiation levels vary on the order of +/0.1% between solar minima and maxima.25
• Earth’s climate doesn’t always respond to solar
activity in predicted ways. Sometimes the
predicted result is magnified beyond what would
be expected. It may even be the reverse of what
would be expected.26
Conclusion
• Earth’s climate is a complex system of
interactions and feed-back mechanisms
which humanity does not fully understand.
This makes an evaluation of the Sun’s
impact quite difficult.
• Accurate measurement the period of solar
rotation is possible using sunspots.
Endnotes
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
1.
http://brahms.phy.vanderbilt.edu/a102/web_sunspots.shtml
2.
http://www.meadows3.demon.co.uk/html/glossary.html
3.
http://www.ngdc.noaa.gov/stp/SOLAR/SSN/ssn.html
4.
Javaraiah J, Gokhale M.H. (2002), The Sun’s Rotation, New York: Nova Science Publications pp 53-61
5.
Javaraiah, p. 3 ibid.
6.
Equation developed by Bryan
7.
http://www.es.ucl.ac.uk/research/planetaryweb/undergraduate/dom/magrev/sunmag.htm
8.
http://www.crh.noaa.gov/fsd/astro/sunspots.htm
9.
http://solarphysics.livingreviews.org/Articles/lrsp-2005-1/index.html
10.
http://en.wikipedia.org/wiki/Sunspot
11.
Freeman, (2005) Universe
12.
Freeman, ibid.
13.
Britt, R. R. (2001, Nov. 6). Inside sunspots: New view solves old puzzle. Space.com. Retrieved May 1, 2005, from
http://www.space.com/news/sunspot_inside_011106.html
14.
http://www.space.com/news/sunspot_inside_011106.html ibid
15.
http://www.space.com/news/sunspot_inside_011106.html ibid
16.
Benestad (2002), Solar Activity and Earth’s Climate, Chichester, UK: Praxis Publishing page 12.
17.
Benestad, ibid. pg 12
18.
Benestad, ibid pg 13
19.
Benestad, ibid pg 15
20.
http://cat.middlebury.edu/~PHManual/doppler.html
21.
http://www.eclipse-chasers.com/egypt1.htm
22.
http://www.exploratorium.edu/sunspots/history.html
23.
Hoyt and Schatten, (1997), The Role of the Sun in Climate Change, Oxford: Oxford University Press pg. 197
24.
Javaraiah J. and Gokhale M.H. pg 21
25.
Hoyt and Schatten, pg. 223.
26.
Hoyt and Schatten, pg. 223.
Image and figures reference:
1.
2.
3.
4.
5.
6.
7.
8.
www.spaceweather.com
www.spaceweather.com
Rob Whitlock, digital camera
Freedman, Universe, pg 398
Science.nasa.gov/ssl/pad/solar/sunspots.htm
Images.google.com search for solar wind
http://web.dmi.dk/fsweb/solarterrestrial/sunclimate/welcome.shtml
http://www.exploratorium.edu/sunspots/research5.html