Magnetic braking is a theory explaining the loss of stellar angular momentum due to material getting captured by the stellar magnetic field and thrown out at great distance from the surface of the star. It plays an important role in the evolution of binary star systems.
L
Thus the rotation rate must be braked during the first 100,000 years of the star's life to avoid this scenario. One possible explanation for the braking is the interaction of the protostar's magnetic field with the stellar wind. In the case of the Solar System, when the planets' angular momenta are compared to the Sun's own, the Sun has less than 1% of its supposed angular momentum. In other words, the Sun has slowed down its spin while the planets have not.
Ionized material captured by the magnetic field lines will rotate with the Sun as if it were a solid body. As material escapes from the Sun due to the solar wind, the highly ionized material will be captured by the field lines and rotate with the same angular velocity as the Sun, even though it is carried far away from the Sun's surface, until it eventually escapes. This effect of carrying mass far from the centre of the Sun and throwing it away slows down the spin of the Sun.[1] [2] The same effect is used in slowing the spin of a rotating satellite; here two wires spool out weights to a distance slowing the satellites spin, then the wires are cut, letting the weights escape into space and permanently robbing the spacecraft of its angular momentum.
As ionized material follows the Sun's magnetic field lines, due to the effect of the field lines being frozen in the plasma, the charged particles feel a force
F
F=qv x B
q
v
B
PB
| P | ||||
|
Since magnetic field strength decreases with the cube of the distance there will be a place where the kinetic gas pressure
Pg
Pg=nmv2
Due to the high conductivity of the stellar wind, the magnetic field outside the sun declines with radius like the mass density of the wind, i.e. decline as an inverse square law.[3] The magnetic field is therefore given by
B(r)=Bs
| R2 | |
| r2 |
where
Bs
R
PB=Pg ⇒
| |||||||
| 2\mu0 |
=nmv2 ⇒
| ||||||||||||
|
=nmv2
If the solar mass loss is omni-directional then the mass loss
nm=
| dM/dt | |
| 4\pir2v |
rc=R\left(
| |||||||||
|
\right)1
Currently it is estimated that:
| M |
=dM/dt=2 ⋅ 109\rmkg/s
v=5 ⋅ 105\rmm/s
Bs ≈ 10-4\rmT
R=7 ⋅ 105\rmkm
This leads to a critical radius
rc=15R\odot
The amount of solar mass needed to be thrown out along the field lines to make the Sun completely stop rotating can then be calculated using the specific angular momentum:
| j\odot | =\left( | |
| jc |
| R\odot | |
| rc |
\right)2 ≈ 0.5\%
In 2016 scientists at Carnegie Observatories published a research suggesting that stars at a similar stage of life as the Sun were spinning faster than magnetic braking theories predicted.[4] To calculate this they pinpointed the dark spots on the surface of stars and tracked them as they moved with the stars' spin. While this method has been successful for measuring the spin of younger stars, the "weakened" magnetic braking in older stars proved harder to confirm, as the latter notoriously have fewer star spots. In a study published in Nature Astronomy in 2021, researchers at the University of Birmingham used a different approach, namely asteroseismology, to confirm that older stars do appear to rotate faster than expected.[5]