Does Magnetic Braking Theory hold up?

[Gfellow]

Magnetic braking is a theory explains the loss of stellar angular momentum and is used extensively to describe the given rotation stars.
However, on a smaller and more directly observable level, when we consider Jupiter's rapid rotation rate of less than ten hours and its accompanying powerful magnetic field in conjunction with its four sizable Jovian moons, we have a situation in which there should be a considerable braking rate. Should we not be seeing a planet with a rotation rate that has all but ground to a standstill?

Any thoughts?

 

[Drakkith]

However, on a smaller and more directly observable level, when we consider Jupiter's rapid rotation rate of less than ten hours and its accompanying powerful magnetic field in conjunction with its four sizable Jovian moons, we have a situation in which there should be a considerable braking rate.

How considerable?

 

[Gfellow]

Well, compared to the Sun? Depending who you ask, they formed pretty much at the same time. Sun=28 days, Jupiter=10 hours.
...And to scale, Jupiter has a hell of a lot more magnetic braking going on, no?

 

[Drakkith]

...And to scale, Jupiter has a hell of a lot more magnetic braking going on, no?

How so? As far as I know the current interplanetary medium is very sparse compared to the protostellar disk present during the formation of the Sun. This presents far less material to transfer angular momentum to. In addition, from the little reading I've done on the subject, magnetic braking appears to mainly prevent the forming protostar from spinning up in the first place, not slow it down from a high speed (I don't know if the latter is an additional effect, but it seems plausible).

 

[Gfellow]

...As far as I know the current interplanetary medium is very sparse compared to the protostellar disk present during the formation of the Sun...

By chance, this article appeared a few days ago and may be of interest on this topic:
"...Jupiter is the oldest planet of the solar system, and its solid core formed well before the solar nebula gas dissipated, consistent with the core accretion model for giant planet formation..."

Being Significantly smaller than the Sun, Jupiter did not achieve nuclear fusion, so it would seem likely that within its own gravitational and magnetic sphere of influence, asteroid objects, gas and dust would have been retained after solar fusion, making a very favorable environment for magnetic braking. Add to that, billions of years of time, combined with four proportionally sizable moons, it seems to me that its present rotational velocity in comparison to the Sun is grossly disproportional.
Certainly enough to give pause to reliably applying magnetic braking as a solid application to stellar theory?

 

[Drakkith]

Certainly enough to give pause to reliably applying magnetic braking as a solid application to stellar theory?

I don't see how. The formation of the Sun and the formation of the planets was very different and took place under very different circumstances. Are you aware of any of the details of magnetic braking? How it works, over what time and distance scales it functions, the required conditions, etc? Do you have any references linking magnetic braking to planetary formation? Unless we get into some of the details we aren't going to get anywhere.

 

[Gfellow]

Unless we get into some of the details we aren't going to get anywhere.

Agreed. Actual data on OBSERVED stellar magnetic breaking is rather sparse, so let's end it here.

 

[stefan r]

Does magnetic braking effect only ions? The sun ionizes a lot more gas than Jupiter.

 

[Gfellow]

OK, let's approach this from another angle. If magnetic breaking is so effective in slowing down star rotation, and considering the immense age that we can attribute to stars, how come the Kepler Space Telescope has not observed a single star that has ground to a complete halt?

 

[Drakkith]

OK, let's approach this from another angle. If magnetic breaking is so effective in slowing down star rotation, and considering the immense age that we can attribute to stars, how come the Kepler Space Telescope has not observed a single star that has ground to a complete halt?

Like I said before, appeared to me that magnetic braking was most effective during the initial formation of the protostar and far less effective once formation had ceased. I believe there were several possible causes being studied, including a large mass loss by the rapidly rotating protostar, where the magnetic field keeps the expelled material rotating at the same angular velocity (requiring that its angular momentum increase, removing it from the star) until the material passed some distance beyond which is behaved more freely. I believe there was also a paper regarding the formation of zones in the accretion disk where the infalling matter is temporarily halted, caused by the magnetic field of the protostar transferring angular momentum and energy to the material to make up for the momentum and energy lost as it accretes.

These conditions are very different from the conditions in and surrounding mature stars.

Unfortunately I don't have the specific papers I looked at before and I don't have the time to look for them at the moment. But if you do a google search for "stellar magnetic braking" or "protostar magnetic braking" I think you'll find plenty of papers going over all of the possibilities.

 

[stefan r]

This goes both ways right? Incoming ions will accelerate the star if they are passing on the prograde side?