From Sunspots to solar

dynamo model

A. Bayadroun

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

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..

General view

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

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.

The sunspot cycle 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.

Dalton

Minimun

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.

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

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)

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

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 ?

.

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.

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.

The Alpha Omega dynamo scheme

Bpoloidal

Alpha effect

Omega effect

Btoroidal

Combining the 2 effects lead to the Alpha Omega Dynamo

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 ...

Questions ?

Thank you for your

attention

Bibliographies & web link

http://www.iki.rssi.ru/mirrors/stern/earthmag/dynamos_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_Augier_2.pdf

http://www.emse.fr/~bouchardon

http://baudolino.free.fr

http://www.asc-csa.gc.ca

http://en.wikipedia.org/wiki/Sun