solar dynamo presentation from Abdesselam

1. From Sunspots to solar
dynamo model
A. Bayadroun

2. Characteristics
 Core : nuclear fusion extend
from center to ~ 0.2R
 Radiative zone : from 0.2R to
0.7R, heat transfert is only
thermal radiation.
 Convective zone : from 0.7R to
the sun ‘s surface du soleil, heat
ltransfert is only thermal
convection
 Photosphere : is the visible surface
of the sun ~ 400 km width ,
 Solar atmosphere includes : the
chromosphere , the coronna and
the heliosphere

3. The solar atmosphere
 Chromosphere : low T from
4000 K to100000 T , width
~ 2000 km
 Coronna : High T 1 billion
K , chaotic mouvement , can
be seen as halo during a total
solar eclipse
 Heliosphere : is the region
from 20R to above..

4. General view

5. Solar magnetic field : key element to
understand the sun
 Magnetism, or magnetic field, is produced on the Sun by the
flow of electrically charged ions and electrons
Events are :
 Sunspots are places where very intense magnetic lines of force
break through the Sun's surface.
The sunspot cycle results from the recycling of magnetic fields
by the flow of material in the interior.
 The prominences seen floating above the surface of the Sun are
supported, and threaded through, with magnetic fields. The
streamers and loops seen in the corona are shaped by magnetic
fields

6. Sunspots history
 Early observations have been recorded by Asian
astronomers (-28 BC) and in Europe in the ‘moyen
Age’ then in 1611 in details par Christoph Scheiner
and Galileo in 1612 with his astronomic telescope.
 The astonomic Observatory of Zurich continue to
observe and records these sunspots.
 The sunspot itself can be divided into two parts:
The central umbra, which is the darkest part, where
the magnetic field is approximately vertical (normal to
the sun's surface).
The surrounding penumbra, which is lighter, where
the magnetic field lines are more inclined.

7. The sunspot cycle Dalton
Minimun
 In 1843 the astronomer Heinrich Schwabe after he
records these sunspots remarks a cycle of about 11
years
In 1849 the Swiss astronomer Johan Rudolf Wolf
establish a formula to calculate the solar activity since
year 1761
The sunspot number = number of spot’s group x
number of each group
 Also the Royal Greenwich Observatory records these
sunspots which contains not only the number but also
size and position. In the beginning of the cycle
thesesunspots appears at high altitude in the 2
hemispheres (~ at 40 ).During the cycle the sunspots
will then move near the equator until the next cycle .
This gives us the the butterfly diagram ---->
 From these datas the astromers finds that the velocity
rotation at the equator is higher than in the poles.

8. Relation between sunspots and
magnetic field
 Hale in 1908 was the first who tougth
that a high magnetic fields is located in
the sunspots ( until 4kG) .these are flux
tube coming to or from.They are
organised by doublet aligned
East/West direction.
 Hale-Nicholson Law:
1/the polarity of these doublet are
always opposite (bipolar)
2/the order of the polarity is inversed
from one hemisphere to the other
3/the polarity of the bipolar are
inversed from one cycle of 11 years to
the other ---> the cycle of the
magnetic field is de 22 years.

9. The solar dynamo
 The fact that the Sun's magnetic field changes dramatically over the course of
just a few years, and the fact that it changes in a cyclical manner indicates that
the magnetic field continues to be generated within the Sun ---->dynamo
 A successful model for the solar dynamo must explain several observations:
1) the 11-year period of the sunspot cycle,
2) the equator-ward drift of the active latitude as seen in the butterfly diagram,
3) Hale's polarity law and the 22-year magnetic cycle,
4) Joy's law for the observed tilt of sunspot groups and,
5) the reversal of the polar magnetic fields near the time of cycle maximum as
seen in the magnetic butterfly diagram

10. The dynamo disk
 In the years of 1831–1832, Michael Faraday
discovered the operating principle of
electromagnetic generators
Faraday disk, the first electric generator
When the disk was turns , this induced a
radial electric current. The current flows out
through the contacts , through the external
circuit, and come back into the center of the
disk through the axis.
 But is it possible that the current generated
by this process generates also the magnetic
field that the system need ? This is the
mecanism proposed by Larmor in 1919 to
explain the solar magnetic field and this can
be illustrated by the Bullard dynamo (1949)

11. The Bullard dynamo disc (1949)
A conducting disk rotates around its
axis at the angular velocity Omega A
small magnetic field is applied in the
vertical direction which induces a
current in the disk from the axis
towards the edge of the conducting
disk.
Then this current flows in a conducting
loop which if orientated in an
appropriate way, induced a magnetic
field orientated in the same direction as
the initial one.
Thus an infinitesimal magnetic field
can be amplified by this mechanism,
leading to a dynamo

12. The 2 effects
 Magnetic fields are produced by electric currents.
These currents are generated within the Sun by
the flow of the Sun's hot, ionized gases. We
observe a variety of flows on the Sun's surface
and within its interior . All of these flows may
contribute in one way or another to the
production of the Sun's magnetic field.
 Magnetic fields are a little like rubber bands.
They consist of continuous loops of lines of
force that have both tension and pressure. Like
rubber bands, magnetic fields can be
strengthened by stretching them, twisting them,
and folding them back on themselves. This
stretching, twisting, and folding is done by the
fluid flows within the Sun.
 2 effects ( Alpha and Omega ) have been
identified for the solar dynamo :
 What is these 2 effects ?
.

13. The Omega Effect
 Magnetic fields within the
Sun are stretched out and
wound around the Sun by
differential rotation - the
change in rotation rate as a
function of latitude and
radius within the Sun. This is
called the omega-effect after
the Greek letter used to
represent rotation.

14. The Alpha Effect
 Twisting of the magnetic field lines
is caused by the effects of the Sun's
rotation. This is called the alpha-
effect after the Greek letter that
looks like a twisted loop. length.

15. The Alpha Omega dynamo scheme
Combining the 2 effects lead to the Alpha Omega Dynamo
Omega effect
Bpoloidal
Btoroidal
Alpha effect

16. Conclusion
 The study of the sunspots
has began long time ago but
we still have some questions
about predictions of the
magnitude of the sunspots ,
the depth and how the
energy is transported to the
surface of the sun .
 A lot of models have been
proposed but none of them
is accepted by all
astronomers .
 These small sunspots have
still long story ...

17. Questions ?
 Thank you for your
attention

18. Bibliographies & web link
 http://www.iki.rssi.ru/mirrors/stern/earthmag/dynam
os_fr.html
 http://mpimichelet.free.fr/faraday.html
 http://solarscience.msfc.nasa.gov/dynamo.shtml
 http://www.ikonet.com
 http://enslyon.free.fr/rapports/physique/Pierre_Augie
r_2.pdf
 http://www.emse.fr/~bouchardon
 http://baudolino.free.fr
 http://www.asc-csa.gc.ca
 http://en.wikipedia.org/wiki/Sun