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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..
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 electronsEvents are : Sunspots are places where very intense magnetic lines of force break through the Suns 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 suns surface). The surrounding penumbra, which is lighter, where the magnetic field lines are more inclined.
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.
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 Suns 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) Hales polarity law and the 22-year magnetic cycle, 4) Joys 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 itsaxis at the angular velocity Omega Asmall magnetic field is applied in thevertical direction which induces acurrent in the disk from the axistowards the edge of the conductingdisk.Then this current flows in a conductingloop which if orientated in anappropriate way, induced a magneticfield orientated in the same direction asthe initial one.Thus an infinitesimal magnetic fieldcan 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 Suns hot, ionized gases. We observe a variety of flows on the Suns surface and within its interior . All of these flows may contribute in one way or another to the production of the Suns 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 Suns rotation. This is called the alpha- effect after the Greek letter that looks like a twisted loop. length.
The Alpha Omega dynamo schemeCombining the 2 effects lead to the Alpha Omega Dynamo Omega effect Bpoloidal Btoroidal Alpha effect
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 ...