We may all know that sunspots are cooler, so they appear more black than the surroundings. But do we know why they are cooler than other regions? Lets see why they are cooler.
Convective Cells
There are some convective cells or granules on the surface of the sun. They are small areas, where the hot fluid comes up to the surface, loses heat, and then sinks into the surface of the sun just like water when it is being boiled. These cells also contribute to the formation of the sunspots.
Magnetic field of the Sun
You must be knowing how magnetic field lines emerge out of a magnet, like in the picture shown below:
This same thing happens in the sun. And these field lines are formed by the hot plasma that is present inside the sun, not a bar magnet piercing through top and bottom of the sun. As you know that sun moves faster at the center than at the poles, hot plasma at the center also moves faster. So, there is not a straight magnetic field line connecting the north and south pole of the sun. Its like straight from the north pole, then a pointing structure towards the east, then again straight to the south pole. This thing continues so long that the same line takes a whole round of the sun and comes back to touch itself like a snake touching its own tail. This process makes all field lines crowded in a single area. Now, if the part of the field line in the northern hemisphere is having the positive charge, the crowded lines there repel each other due to same charge. They do this by exerting pressure, which is known as magnetic pressure.
Now the convection process (as explained above), plays its role. As heat is transferred to the surface by convection, the fluid coming up pushes the field line above. This pushing causes the field line to poke through the surface from inside, forming a loop and taking all the plasma on the part of the line above, as shown below:
Where they poke through the surface, sunspots are formed there. But why is their temperature down? That's because of magnetic pressure. Now, forget about the poking of the field line through the surface. On the surface of the sun, gas pressure is equally distributed. Now this poking happens. Then, near the area where it poked, there are many more field lines with the same polarity as the field line that is above. So, they repel each other by magnetic pressure. So, near the poked surface, there are two types of pressures - magnetic and gas pressure. So, in these area, the pressure is more than the rest of the surface. So, any how, the pressure must be equally distributed. We can't get rid of the loop until those crowded field lines are given enough time to go away from each other. So, we should lower gas pressure. But how? By inhibiting the convection process. This allows lesser heat than before, so that pressure is equally maintained. So, less heat means less temperature for those areas. So, sunspots are formed.
But they are not that cold. Only 1000 K less than other regions! (5000 K on surface and 4000 K on sunspots).
Why sunspots are temporary?
Sunspots are of course temporary as when convection is inhibited, the field line slowly comes back inside again. And, those crowded lines also get enough time to repel each other. So, everything comes back to normal.
Now, after studying this, you can make out where sunspots are maximum in number. They will be maximum at the central region as magnetic lines get crowded there more than near the poles. But, sunspots also get formed near the poles, but less in number.
Article by - M.Santosh
________________________________________________________________________
Some sources that had been helpful for the making of this article and maybe helpful to you too -
- http://cse.ssl.berkeley.edu/segwayed/lessons/sunspots/
________________________________________________________________________
No comments:
Post a Comment