The Sunspot Cycle


pic src: Universe (Fifth Edition)


Formation of Sunspots

    Sunspots are observed to start forming at latitude 30oN and 30oS of the Sun's Equator. The regions in which sunspots are formed are known as active regions. The formation of sunspots is due to the differential rotation of the Sun. Differential rotation is a phenomenon whereby different parts of the Sun rotates at different speed. Between latitude 30oN and 30oS around the Sun's equator, (near the Poles). Due to this phenomenon, the magnetic field lines of the Sun, which run from the North Pole to the South Pole, are slowly twisted to be "wrapped" around the Sun. The originally vertical magnetic field lines is now parallel to the Sun's equator.
(View Animation)
  
click here for a bigger view
pic src: Universe (Fifth Edition)


    Due to the uprising convection current from the inner core of the Sun, the lines are pulled (or pushed) "up". Cyclonic motion of gases then twists the field lines. This twisted magnetic flux lines (or tubes) experiences external pressure by the gas around it and also internal pressure, which is the sum of the magnetic pressure and gas pressure inside. Since the temperature inside and outside these flux tubes is the same, so the internal gas pressure is lower than the external gas pressure. Therefore, the flux tubes float towards the photosphere. Near the photosphere, the external pressure decreases and the flux tubes bulges out above the atmosphere. Hence a pore is formed. Many (hundreds) of these flux tubes are held together and this concentration of magnetic field inhibits the convection current. This causes the region to be lower in temperature and hence darker than the surroundings. As a result, sunspots are formed. However, convection current can still "flow" through the spaces between the flux tubes. This explains the why the penumbra is brighter than the umbra.
(View Animation)

    The spots will travel either towards the west or east. The leading spot will move in a westerly direction and then towards the pole in a easterly direction; the following spots, on the other hand, will drift slightly eastwards and towards before coming to a stop.


The Cycle

    One sunspot cycle lasts for 11 years. Why 11 years? This is due to the fact that sunspots formation is related to differential rotation. Differential rotation causes the magnetic field lines to flow from east to west. This flowing motion of the magnetic field lines also causes the sunspots to move, in a general direction, from east towards west. It is through this movement that scientist (or Galileo) are able to detect that different parts of the Sun have different speeds of rotation. Eventually, the differential rotation "untangles" the magnetic field lines and the parallel magnetic field lines now flow vertically from the North Pole to the South Pole.

    Sunspots formation occurs when the magnetic field lines are flowing horizontally from east to west. Since sunspots are formed in active regions, sunspot maxima occurs when the Sun is very active. The number of sunspots is actually an indication of the Sun's activity. Sunspot minima is a period where no sunspots can be seen. This corresponds to the period when the magnetic field lines flow normally from the North Pole to the South Pole.

    This whole process of "tangling" and "untangling" the magnetic field lines takes 11 years to complete. Then the "tangling" process will start again, and another sunspot cycle is started. This process also indentifies the periods whereby the Sun is very active and inactive.


Resource books: "Universe (Fifth Edition)", "Guide to the Sun"   
Pictures of "Universe (Fifth Edition)"   

Page of GEM1506K, Group 2 (http://sg.geocities.com/apollonus02)   

1