Differential Rotation of the Sun

[Terra Australis]

It is known that the poles of the Sun rotate more slowly than the equator. This cannot be explained solely due to the lesser distance polar areas have to travel.
I don't believe it is correct to consider the core of the Sun as a solid object in this instance. Convince me otherwise.
I would appreciate any thoughts, discussion or inputs.
Regards TA.

 

[Catastrophe]

Hi TA,


mgur: The magic of the Internet

https://www.nasa.gov/mission_pages/sunearth/science/solar-rotation.html
HiSolar Rotation Varies by Latitude - NASA
https://www.nasa.gov › sunearth › science › solar-rotation

22 Jan 2013 — The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots. The Sun's rotation ...

Solar rotation - Wikipedia
https://en.wikipedia.org › wiki › Solar_rotation

Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods.
‎Sidereal rotation · ‎Using sunspots to measure... · ‎Internal solar rotation


Cat

 

[billslugg]

From .7 solar radii inward there is no convection, heat transfer is solely radiative. The particles are so dense there is no density difference between hotter and cooler packets. No motive force for convection. This portion of the Sun rotates as one. It is not solid but acts as if it were.
The outer portion of the Sun, from .7R outward has convection. There is a complex system of Hadley cells resembling conveyor belts that move heat from the interior outwards. This fluid is a neutral plasma, highly conductive to electrical currents. Conductive plasma moving through the Sun's magnetic field creates electrical currents which make magnetic fields. It is known as a dynamo, similar to how the currents in the rotor and stator of an automotive alternator support one another.
The result of this complex interplay in the Sun is the transfer of angular momentum from the poles to the equator making the equatorial regions rotate faster.
Consider a parcel of plasma located deep under the equator and moving upwards. Since the Sun has a dipolar magnetic field this parcel will be cutting across field lines. The conductive fluid is comprised of protons and electrons. Since the electrons have the same charge as the protons but only 1/1000 the mass they will travel fastest. They have much higher "mobility". Since the north pole of a magnet will swing towards the north pole of the Sun this means, just like on Earth, the magnetic pole at the north is a "south magnetic pole". Using the left hand rule for electrons one makes mutually perpendicular lines with thumb, forefinger and middle finger. Remember "F-B-I" The thumb indicates which direction the force will be, the "B", represented by the pointing finger, points towards the north magnetic pole. (In this case, the south pole of the Sun). The upward flow of electrons is represented by the middle finger which points away from the center of the Sun.
The net result is the thumb is pointing eastward at the equator which is the area that is experiencing enhance flow.

 

[Terra Australis]

From .7 solar radii inward there is no convection, heat transfer is solely radiative. The particles are so dense there is no density difference between hotter and cooler packets. No motive force for convection. This portion of the Sun rotates as one. It is not solid but acts as if it were.
The outer portion of the Sun, from .7R outward has convection. There is a complex system of Hadley cells resembling conveyor belts that move heat from the interior outwards. This fluid is a neutral plasma, highly conductive to electrical currents. Conductive plasma moving through the Sun's magnetic field creates electrical currents which make magnetic fields. It is known as a dynamo, similar to how the currents in the rotor and stator of an automotive alternator support one another.
The result of this complex interplay in the Sun is the transfer of angular momentum from the poles to the equator making the equatorial regions rotate faster.
Consider a parcel of plasma located deep under the equator and moving upwards. Since the Sun has a dipolar magnetic field this parcel will be cutting across field lines. The conductive fluid is comprised of protons and electrons. Since th electrons have the same charge as the protons but only 1/1000 the mass they will travel fastest. They have much higher "mobility". Since the north pole of a magnet will swing towards the north pole of the Sun this means, just like on Earth, the magnetic pole at the north is a "south magnetic pole". Using the left hand rule for electrons one makes mutually perpendicular lines with thumb, forefinger and middle finger. Remember "F-B-I" The thumb indicates which direction the force will be, the "B", represented by the pointing finger, points towards the north magnetic pole. (In this case, the south pole of the Sun). The upward flow of electrons is represented by the middle finger which points away from the center of the Sun.
The net result is the thumb is pointing eastward at the equator which is the area that is experiencing enhance flow.

Thank you billslugg,

This is a very clear description of the solar dynamo.
I have read where the core of the Sun is rotating nearly four times faster than the surface of the Sun. Do you have any thoughts on why this may be so?

 

[billslugg]

I imagine that the Sun, as it was formed, was rotating very fast. The inner core, being a "solid" does not have shear forces at work dissipating rotational energy thus it preserves its original angular momentum. The outer 30% of the Sun has intense convection thus lots of shearing. Shearing dissipates energy and thus slows down the outer envelope. I am making this up as I go along but it sounds good.

 

[Catastrophe]

From .7 solar radii inward there is no convection,

The outer 30% of the Sun has intense convection thus lots of shearing.

billslugg, I accept your expertise in this area, but I am just interested to know what happens at 0.7 solar radii, where "no convection" meets "intense convection!. I assume there must be a very steep convection gradient?

Cat

 

[Terra Australis]

I imagine that the Sun, as it was formed, was rotating very fast. The inner core, being a "solid" does not have shear forces at work dissipating rotational energy thus it preserves its original angular momentum. The outer 30% of the Sun has intense convection thus lots of shearing. Shearing dissipates energy and thus slows down the outer envelope. I am making this up as I go along but it sounds good.

billslugg: Thank you for your reply

I have read where the Sun has lost angular momentum due to solar wind flow. I have also read that photons produced in the core are responsible for loss of angular momentum as they pass from the photosphere where they act as a braking force to slow the rotation of the Sun.

billslugg, I accept your expertise in this area, but I am just interested to know what happens at 0.7 solar radii, where "no convection" meets "intense convection!. I assume there must be a very steep convection gradient?

Cat: I am glad you brought this up.
I was pondering this very thing a few days ago.


T.A.

 

[billslugg]

I am not sure why there is no convection deeper than .7R. In order for convection to occur there must be a lower density of hotter fluid parcels. Since the hotter parcels are not expanded I suppose the atoms must be slam together and unable to expand.
I also read that the solar wind drains angular momentum from the Sun's surface. Picture a particle deep in the Sun, it has little angular momentum. Raise it to the surface and it gains lots. Lose it to space and it take that with it.

 

[Catastrophe]

Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy.[1] The rate of surface rotation is observed to be the fastest at the equator (latitude φ = 0°) and to decrease as latitude increases. The solar rotation period is 24.47 days at the equator and almost 38 days at the poles. The average rotation is 28 days.

Wikipdia: Solar Rotation


Cat

 

[Catastrophe]

Solar Rotation Varies by Latitude | NASA

Imgur: The magic of the Internet

Imgur: The magic of the Internet

Cat

 

[Terra Australis]

I am not sure why there is no convection deeper than .7R. In order for convection to occur there must be a lower density of hotter fluid parcels. Since the hotter parcels are not expanded I suppose the atoms must be slam together and unable to expand.
I also read that the solar wind drains angular momentum from the Sun's surface. Picture a particle deep in the Sun, it has little angular momentum. Raise it to the surface and it gains lots. Lose it to space and it take that with it.

Thanks again billslugg,

I am particularly interested in what is happening at this interface between the radiative zone and the convective zone.

I was pondering also at what stage does gravity kick in to stop the Sun blowing itself apart. I see there is a point where internal forces pushing outward are counteracted by the force of gravity pulling inward.
Any thoughts?

 

[billslugg]

The force that pushes a star apart is the heat generated by fusion in its interior. The rate that fusion happens is very sensitive to pressure which is a function of how compact the star is. As the heat pushes the star out, the pressure decreases and the fusion slows down. The star contracts and the fusion speeds up. It is a self controlling balance of forces known as hydrostatic equilibrium.

At the interface between the "solid" radiative core and the convective envelope it is conceivable there may be convective features in the flow patterns mimicing dust devils, waterspouts or tornadoes on Earth. The same geometry (a solid hot surface overlaid by a convective fluid) exists thus simimlar phenomena might be expected. Note that on Earth, the heat flow rate is on the order of a kilowatt per square meter but at the interface of the radiative and convective zones on the Sun the heat flow rate is about 125 megawatts per square meter. The bigger the heat flow the bigger the feature!

 

[Terra Australis]

The force that pushes a star apart is the heat generated by fusion in its interior. The rate that fusion happens is very sensitive to pressure which is a function of how compact the star is. As the heat pushes the star out, the pressure decreases and the fusion slows down. The star contracts and the fusion speeds up. It is a self controlling balance of forces known as hydrostatic equilibrium.

Thanks again bill.
Are you suggesting gravity doesn't play a role in this?
T. A.

 

[billslugg]

Yes, the pressure inside a star is caused by gravity and the more compact the star is, the higher the force of gravity will be. Heat pushes out and gravity pushes in.

 

[Terra Australis]

Yes, the pressure inside a star is caused by gravity and the more compact the star is, the higher the force of gravity will be. Heat pushes out and gravity pushes in.

Great, thanks for clarifying.
Also thanks for the edit as well.
T.A.

 

[Helio]

billslugg, I accept your expertise in this area, but I am just interested to know what happens at 0.7 solar radii, where "no convection" meets "intense convection!. I assume there must be a very steep convection gradient?

Yes, this is an important region. It's known as the tachocline. I think I've read that many think it is an origin point for sunspots, which are magnetic storms. [I was going to make an analogy joke about trailer parks and tornados, but I won't. ]

The convective zone's density and temperatures allow for "boiling", where Texas-sized buoyancy regions form and these "bubbles" float upward. The powerful stuff Bill presented must also be taken into account.

Some solar physicists have been concerned that if the magnetic field strength keeps dropping during a solar minimum, then we could see an extended minimum time period where the sunspots are incapable of developing. This might have some dramatic climate effect for us, or might not. The science is interesting and pretty clear, but unclear as to just how much affect it would have.

For instance, cosmic particles are known to help cloud formation. A weak Sun (few sunspots) correlates to a weak solar field strength extended to and past Earth. Thus, the cosmic rays (protons) will more easily impact Earth. More clouds form increasing the albedo, possibly lowering the temperature on Earth. But some clouds can also hold heat so it's not a simple story, though I think the net makes it cooler for cosmic ray cloud creation. [The alliteration alone is worth the claim. ]

 

[Helio]

The force that pushes a star apart is the heat generated by fusion in its interior. The rate that fusion happens is very sensitive to pressure which is a function of how compact the star is. As the heat pushes the star out, the pressure decreases and the fusion slows down. The star contracts and the fusion speeds up. It is a self controlling balance of forces known as hydrostatic equilibrium.

I have the impression that the bigger story is with temperature since KE of the protons are so important, perhaps. Is this likely?

Stars have self-regulation of core temperatures, like our home thermostats. Any increase in the fusion rate would cause expansion (due to pressure) and lower temperatures. So I'm still trying to get a better picture of how the variables work with one another. I understand, fortunately, that the ideal gas law seems to apply.

 

[Helio]

It may even be worth mentioning that red dwarfs don't have radiative zones, only the core and convective zones. This is why the are so fisty with their flares.

 

[billslugg]

Yes, some smaller stars have no radiative zone, only convective.
Some larger stars have only radiative zone, no convective zone.

The radiative zone is characterized by the inability of atomic nuclei to hold onto any electrons, thus it is a transparent medium, easy for photons to move around.

At the tachocline it becomes cool enough for some atoms to form, larger nuclei taking on an electron or two. This renders the medium opaque. At the tachocline the opacity suddenly raises by a factor of 100.

Below the tachocline, in the radiative zone, heat flows by radiation, heat travels freely upwards heating the upper layers. The lower edge of the radiative zone, at .2R, is 7 million Kelvin, dropping to 2 million Kelvin at the top of the zone at .7R. The temperature gradient is less than the adiabatic lapse rate. Adiabatic lapse rate is how much a parcel cools off by expansion as it rises a certain distance (assuming no heat transfer into or out of [thus adiabatic]). Take a parcel, raise it up, let it cool and you find it is cooler than the medium around it, not warmer. Basically the same as a temperature inversion on Earth puts a lid on a cumulus cloud.

I'll keep reading up on it.

 

[Terra Australis]

I have the impression that the bigger story is with temperature since KE of the protons are so important, perhaps. Is this likely?

Stars have self-regulation of core temperatures, like our home thermostats. Any increase in the fusion rate would cause expansion (due to pressure) and lower temperatures. So I'm still trying to get a better picture of how the variables work with one another. I understand, fortunately, that the ideal gas law seems to apply.

To All,..
... thanks for this discussion.
I have read that the Sun rings like a bell. How much mechanical kinetic energy is transferred via soundwaves I wonder?
T.A.

 

[Catastrophe]

The entire Sun vibrates from a complex pattern of acoustical waves, much like a bell. If your eyes were sharp enough, you could see a bell's surface jiggle in complex patterns as the waves bounced around within it.

The Singing Sun - Stanford Solar Center


Cat

 

[Pogo]

I had read some years ago in Sky&Telescope that the Sun is kinda divided into concentric cylinders by latitude, so the region at, let's say, 45° N is connected to 45° S as they are in the same cylinder. That explains that frequently a northern sunspot has a southern sister so to speak.

Either the same article or another in a different issue spoke that yes, the sun has sound waves of various frequencies, one of the fundamental waves has a period of about 5 minutes, various regions raise and lower in tune with these periods, and there are spots where the amplitude is high (additive) and there are dead spots (subtractive). I imagine this has an effect on local temperatures as it would get hot when it experiences compression and vice versa.

 

[Terra Australis]

I had read some years ago in Sky&Telescope that the Sun is kinda divided into concentric cylinders by latitude, so the region at, let's say, 45° N is connected to 45° S as they are in the same cylinder. That explains that frequently a northern sunspot has a southern sister so to speak.

Hi Pogo ... interesting!
I also considered slicing the Sun into rings, like you might slice a pineapple. For every slice the rate of rotation at the core would be the same, however each slice would have a different rotational speed at the surface, Equator -> N & S Poles

Is there is a correlation between sunspots appearing N and S at the same time? There is some evidence that sunspots seem to appear at certain Carrington longitudes more than others.
The characteristic butterfly diagrams show the N - S feature of Sunspots during a cycle.
However the area where sun spots are formed is quite close to the surface of the Sun.

I am interested in how the rotational rate slows down once you move out of the core into the radiative zone, transtion zone and then the convective zone... and nwhether it is a sudden or gradual drop.
Also, at what point does fusion cease?

Either the same article or another in a different issue spoke that yes, the sun has sound waves of various frequencies, one of the fundamental waves has a period of about 5 minutes, various regions raise and lower in tune with these periods, and there are spots where the amplitude is high (additive) and there are dead spots (subtractive). I imagine this has an effect on local temperatures as it would get hot when it experiences compression and vice versa.

Helioseismology utilises these sound waves that propagate throughout the Sun to measure its invisible internal structure and dynamics. There are millions of distinct, resonating, sound waves, seen by the doppler shifting of light emitted at the Sun's surface. That's how we get far side maps indicating possible active regions.
How much energy is transferred into sound waves I wonder?

Thanks Pogo,
T.A.

 

[Terra Australis]

I have the impression that the bigger story is with temperature since KE of the protons are so important, perhaps. Is this likely?

Stars have self-regulation of core temperatures, like our home thermostats. Any increase in the fusion rate would cause expansion (due to pressure) and lower temperatures. So I'm still trying to get a better picture of how the variables work with one another. I understand, fortunately, that the ideal gas law seems to apply.

Hello Helio,
I picture photons shooting off in all directions, electrons whizzing around all over the place and soundwaves bouncing off the walls. Energy flows out of the system causing temperature drop, pressure drop and fusion to slow.
What do you think?
There is still so much more to understand.
T.A.

 

[billslugg]

Yes, core of the Sun contains nuclei of various sizes, electrons and EM waves, mostly x-ray and gamma rays. Heat moves out, core shrinks, pressure increases, more heat develops, core expands, less heat develops, a balance is struck.