2. Overview of lecture 10
• In this lecture we will focus on the solar activity cycle in terms of sunspots
and the Sun’s magnetic field:
– Observations of sunspots
– Proxies for solar activity and sunspot number
– The behaviour of the solar magnetic field
– Magnetograms
– Butterfly diagrams
• Finally, we will go through very recent work undertaken to predict the
sunspot number for the current solar cycle 25 and compare with
predictions made using conventional approaches
Space Environment – Space Weather
7. Sunspot number Space Environment – Space Weather
Direct observations of the Sun, including counts of sunspots, have been made
for nearly four centuries
8. Sunspot number Space Environment – Space Weather
We can also use proxies, such as the concentration of the cosmogenic isotopes
14C in tree rings or 10Be in ice cores, to reconstruct historical solar activity:
http://cc.oulu.fi/~usoskin/personal/nature02995.pdf
Atmospheric radiocarbon level
(% deviation from 1950 level)
Reconstructed sunspot number and
uncertainty (red curve corresponds to
telescopic observations since 1610)
9. Sunspot number Space Environment – Space Weather
Current levels of solar activity are higher than at any time in the last 9000 years
http://cc.oulu.fi/~usoskin/personal/nature02995.pdf
Reconstructed sunspot number Observed sunspot number since 1950
10. Sunspot number Space Environment – Space Weather
Current levels of solar activity are higher than at any time in the last 9000 years
Will it last?
http://cc.oulu.fi/~usoskin/personal/nature02995.pdf
The probability of the
total duration of a state of
high activity (SN level
exceeding 50)
The probability that the high
activity continues for another 5
decades for a total duration of 115
years is only 8%
11. Sunspot number Space Environment – Space Weather
Solar cycle length
Cumulative distribution function
of the solar cycle length:
• Fraction of cycles with a length
above a given value
• The 11-year cycle is actually
more like 10.5 years with an
interquartile range of 10 to 12
years
Source: Prof. Matthew Owens (Dept. of
Meteorology, University of Reading)
12. Sunspot number Space Environment – Space Weather
https://www.swpc.noaa.gov/products/solar-cycle-progression
“NOAA has released a new interactive tool to explore the solar cycle. It lets you
scroll back through time, comparing sunspot counts now to peaks and valleys of
the past.”
13. Beyond sunspots Space Environment – Space Weather
Magnetograms
• Magnetic maps covering the solar
surface
• Stacking longitudinal averages of
full-surface synoptic magnetic
maps (a full map summed up in
one pixel column) builds up the
so-called magnetic butterfly
diagram
https://en.wikipedia.org/wiki/File:Evolution_of_Magnetism_on_the_Sun.ogv
Magnetic field map of the entire solar surface June–
July 2011 (around the first peak of solar cycle 24)
https://oxfordre.com/physics/view/10.1093/acrefore/9780190871994.001.0001/acrefore-9780190871994-e-9
See also:
14. Beyond sunspots Space Environment – Space Weather
Butterfly diagram
• Each (pixel) column is a full-Sun
synoptic map (like the one shown
in previous slide), the sum of
magnetic fields along a given
(pixel) latitude.
• Each half of the butterfly wings
shows the polarity of the leading
spots in the corresponding
hemisphere (Hale’s law)
• The polar fields are the strongest
at the end of each cycle when the
strongest magnetic fields (spots)
are close to the solar equator
https://en.wikipedia.org/wiki/File:Evolution_of_Magnetism_on_the_Sun.ogv
Magnetic butterfly diagram of the last four solar cycles 20–24
(courtesy D. Hathaway)
https://oxfordre.com/physics/view/10.1093/acrefore/9780190871994.001.0001/acrefore-9780190871994-e-9
See also:
15. Beyond sunspots Space Environment – Space Weather
The magnetic solar cycle
• Each 11-year solar cycle is actually only
half of a solar magnetic “Hale” cycle
• The configuration of the Sun’s large-
scale magnetic field takes ~22 years to
repeat.
• At the start of a new solar cycle, sunspots
emerge at mid-latitude regions
• As the cycle progresses sunspots emerge at
lower latitudes
• Around solar maximum, the polar field
polarity reverses, but the sunspot
orientation remains the same, leading to a
build-up of polar field strength that peaks
at the start of the next cycle https://oxfordre.com/physics/view/10.1093/acrefore/9780190871994.001.0001/acrefore-9780190871994-e-9
Source: Prof. Matthew Owens (Dept. of Meteorology, University of Reading)
16. Beyond sunspots Space Environment – Space Weather
The magnetic solar cycle (continued)
• By the end of the cycle, the Sun has
oppositely oriented polar fields to those it
started with
• It will take the next cycle to get back to
the original poloidal field orientation.
• This is why the magnetic or Hale solar
cycle length is 22 years
• The polar fields serve as seed fields for the
succeeding solar cycle,
• The strength of polar fields are linked to
the amplitude of the succeeding cycle
https://oxfordre.com/physics/view/10.1093/acrefore/9780190871994.001.0001/acrefore-9780190871994-e-9
Source: Prof. Matthew Owens (Dept. of Meteorology, University of Reading)
17. Beyond sunspots Space Environment – Space Weather
A new (and alternative) view:
• “Scientists have struggled to predict both the length and the
strength of sunspot cycles because we lack a fundamental
understanding of the mechanism that drives the cycle.”
• Scott McIntosh, National Center for Atmospheric Research
1. Coronal bright points move from the Sun's high latitudes to
the equator over about 20 years. These bright points
correspond to the motion of magnetic field bands, which
wrap around the Sun
2. As they cross the mid-latitudes the bright points coincide
with the emergence of sunspot activity
3. When the bands from the northern and southern
hemispheres (which have oppositely charged magnetic
fields) meet at the equator, they mutually annihilate one
another leading to a "terminator" event
https://news.ucar.edu/132771/new-sunspot-cycle-could-be-one-strongest-record
18. Beyond sunspots Space Environment – Space Weather
A new (and alternative) view:
4. Terminators mark the end of a magnetic cycle, along with
its corresponding sunspot cycle, and act as a trigger for the
following magnetic cycle to begin
5. The magnetic field bands appear at high latitudes at a
relatively consistent rate (~ every 11 years) but they
sometimes slow as they cross the mid-latitudes
6. The slow-down extends the current solar cycle by delaying
the terminator event
7. This delay has the effect of inhibiting the sunspot
productivity of the next cycle
8. The longer the time between terminators, the weaker the
next cycle
9. The shorter the time between terminators, the stronger the
next cycle
https://news.ucar.edu/132771/new-sunspot-cycle-could-be-one-strongest-record
19. Beyond sunspots Space Environment – Space Weather
https://news.ucar.edu/132771/new-sunspot-cycle-could-be-one-strongest-record
A new (and alternative) view:
LEFT: Oppositely charged
magnetic bands, represented in
red and blue, march toward the
equator over a 22-year period.
When they meet at the equator,
they annihilate one another.
RIGHT: The top animation
shows the total sunspot
number (black) and the
contributions from the north
(red) and south (blue)
hemispheres. The bottom
shows the location of the spots.
Terminator separation (∆)
20. Beyond sunspots Space Environment – Space Weather
https://news.ucar.edu/132771/new-sunspot-cycle-could-be-one-strongest-record
A new prediction for cycle 25:
McIntosh et al prediction of maximum
sunspot number (best estimate = 233)
https://www.spaceweatherlive.com/en/news/view/400/20191210-official-solar-cycle-25-forecast-update.html
21. Recap of lecture 10
• In this lecture we focused on the solar activity cycle in terms of sunspots
and the Sun’s magnetic field:
– Observations of sunspots
– Proxies for solar activity and sunspot number
– The behaviour of the solar magnetic field
– Magnetograms
– Butterfly diagrams
• Finally, we saw very recent research on “terminators”, which have enabled
a new prediction of the maximum sunspot number for cycle 25 to be made
that seems to contradict the prevailing consensus view
Space Environment – Space Weather
22. Activities
• The paper “Overlapping Magnetic
Activity Cycles and the Sunspot
Number” by Scott McIntosh et al. is
available on the Blackboard site in the
“Course Content” folder for you to read
• You can also find two other articles,
covering the solar cycle and the use of
Carbon-14 proxies, in the “Course
Content” folder on Blackboard
Space Environment – Space Weather
