PDA

View Full Version : Forces on heliosphere



PetTastic
2018-Aug-03, 03:09 PM
As a star or the Sun travels through the ISM are the forces on the surface of the heliosphere/termination shock transmitted down onto the star or Sun?

Ken G
2018-Aug-04, 05:55 AM
Not forces on the outer heliosphere, because the solar wind is both supersonic and super-Alfvenic, meaning that signals (either pressure or magnetic field perturbations) cannot propagate backward toward the Sun. However, neutrals in the ISM can propagate through all that and get right down to the actual surface of the Sun, so there can be some direct forces that do get transmitted. But they are very small in comparison to forces already in the Sun, and would be neglected in any interior model-- there are much worse idealizations already going on!

Also, since the scale of the forces increases as you go deeper into a star, there would be some depth in the star where you could neglect surface forces to whatever accuracy you are working. Some stars have very extended surface regions where the force scale gets very weak, so the surface layers could be affected. Perhaps you are wondering if a star would ever look skewed by virtue of surface forces, and I would tend to think the force scale would already make this pretty negligible by the time you get to layers that you can actually see.

chornedsnorkack
2018-Aug-05, 05:10 AM
So, the whole forces of interstellar wind are expended on entraining the solar wind shed by Sun and dragging it into the heliotail, except the tiny amount of neutrals that pass through and hit Sun?

How asymmetric is solar wind? How much propulsive force is Sun generating?

Ken G
2018-Aug-05, 08:24 AM
The force on the Sun from wind asymmetry must be tiny-- the wind can be asymmetric at any given moment, but over its lifetime that would very strongly average out. There are net flows in the ISM on the order of tens of km/s, but those too should presumably average out over time. Perhaps there is some residual drag on the Sun's orbit, I've never seen that calculated but it must be very tiny compared to the Sun's large orbital momentum.

PetTastic
2018-Aug-05, 09:37 AM
I was thinking in terms of momentum being transferred to the ISM as the Sum moves through it. Leaving gravitational drag out for now as it gets complicated by Dark Matter.
Modelling the solar wind as particles there is only momentum transfer from solar wind to the ISM.

Is there any evidence of the Sun's magnetic field being distorted by flow related forces from the ISM?
Would I be right in thinking that assuming equilibrium, all of those forces would be transmitted to the Sun?

Can a magnetic field inflated by the solar wind be treated in a similar manner ( assuming inflating forces equal in all directions) ?

This question comes from Gaia observations finding no evidence for a dark matter disk. Therefore, what forces counter random star interaction thickening the galaxies disk?

selden
2018-Aug-05, 10:54 AM
A popularization of recent research into the shape of the heliosphere is available at https://www.nasa.gov/feature/goddard/2017/nasa-s-cassini-voyager-missions-suggest-new-picture-of-sun-s-interaction-with-galaxy

The prevalent picture of the heliosphere was one of comet-shaped structure, with a rounded head and an extended tail. But new data covering an entire 11-year solar activity cycle show that may not be the case: the heliosphere may be rounded on both ends, making its shape almost spherical. A paper on these results was published in Nature Astronomy on April 24, 2017. (https://www.nature.com/articles/s41550-017-0115)

“Instead of a prolonged, comet-like tail, this rough bubble-shape of the heliosphere is due to the strong interstellar magnetic field — much stronger than what was anticipated in the past — combined with the fact that the ratio between particle pressure and magnetic pressure inside the heliosheath is high,” said Kostas Dialynas

chornedsnorkack
2018-Aug-05, 07:53 PM
The mass shed by Sun as solar wind must join interstellar matter.

Where does it end up?

Hornblower
2018-Aug-05, 09:16 PM
The mass shed by Sun as solar wind must join interstellar matter.

Where does it end up?

It disperses far and wide and eventually diffuses into the interstellar matter. I would say that the heliosphere boundary is a transient structure.

chornedsnorkack
2018-Aug-07, 04:27 PM
As Sun-originating matter is slowed down by and mixed with upstream interstellar matter, it must reach a point where any continued expansion is slower than downstream drift relative to Sun. Correct?

PetTastic
2018-Aug-09, 09:44 AM
Under the conditions of the local bubble, it seem very difficult to produce any forces related to the solar wind that could effect the Sun's path around the galaxy.
What proportion of their time do star spend in denser ISM?

Magnetic forces look more interesting, with the Sun's magnetic field moving relative to an electrical conductor like the ISM.
However, estimating how far induced currents and secondary dynamo type effect travel out into the ISM is anybody's guess.

The Sun's magnetic field rotating also complicates issues because the forces generated are not parallel to the direction of travel. Correct?

tusenfem
2018-Aug-09, 04:44 PM
here is a science news article about new horizons and the solar system's edge
https://www.sciencenews.org/article/new-horizons-may-have-seen-glow-solar-systems-edge

PetTastic
2018-Aug-10, 06:34 AM
At the other end of events, if any magnetic forces are being transferred down to the Sun, the NASA Parker Solar Probe might detect something in the Sun's magnetic field.

Could forces on the outer edges of the Sun's magnetic field be a explanation for coronal heating?

chornedsnorkack
2018-Aug-10, 08:13 AM
With respect to magnetic fields, Sun is a magnetic dipole, not a magnetic monopole. So the interstellar magnetic fields should not exert a force to move Sun - they should exert a torque to turn Sun around its centre.

The solar wind swept to downstream heliotail should be exerting gravitational attraction on Sun.

PetTastic
2018-Aug-10, 10:12 AM
With respect to magnetic fields, Sun is a magnetic dipole, not a magnetic monopole. So the interstellar magnetic fields should not exert a force to move Sun - they should exert a torque to turn Sun around its centre.

The solar wind swept to downstream heliotail should be exerting gravitational attraction on Sun.

Not sure what you mean here.
Sun just acts like a normal magnet. Move an electrical conductor near a magnet and a force is generated on the magnet.

chornedsnorkack
2018-Aug-10, 01:53 PM
Not sure what you mean here.
Sun just acts like a normal magnet. Move an electrical conductor near a magnet and a force is generated on the magnet.

An uniform magnetic fiekl exerts no force on a magnetic dipole, only torque.
A nonuniform magnetic field does exert force on a magnetic dipole.

PetTastic
2018-Aug-10, 02:39 PM
An uniform magnetic fiekl exerts no force on a magnetic dipole, only torque.
A nonuniform magnetic field does exert force on a magnetic dipole.

Try dropping a magnet down a copper pipe.
https://www.youtube.com/watch?v=uh0bbW6S3BY