No, they are traveling at velocities far exceeding the gravitational escape velocity of the sun. There is no meaningful sense in which they are orbiting.
Except they aren’t, which is why they are there and there is a heliopause instead of them being in interstellar space and there not being a heliopause.
If they had greater than escape velocity, they’d be escaping and we’d not see the graph we see.
Orbit means going very fast around something in a circular motion. These particles are heading directly streaming out from the sun, not going round it.
As I understand it it’s where these particles reach equilibrium with the stellar medium. The sun is like a comet at a large enough scale, with a long tail of particles as it moves through the galaxy.
The graph is not a graph of interactions between solar wind particles and interstellar medium particles. It is a graph of solar wind particles detected by Voyager 1.
You might want to re-read that graph. And conservation of momentum means the particles leaving the sun don’t stop rotating when they leave the sun - the sun is rotating.
You might want to re-read the Wikipedia page. It explicitly says that Voyager 1 saw the density of plasma around it increase by a factor of 40 as it crossed the heliopause. (For Voyager 2, it was a factor of 20, as I have posted elsewhere in this discussion.) It also explicitly says that the solar wind is stopped at the heliopause due to the pressure of the interstellar medium, which, last I checked, means the interstellar medium is interacting with the solar wind.
> conservation of momentum means the particles leaving the sun don’t stop rotating when they leave the sun - the sun is rotating
Sure, with a period of about 27 days. Go do the math and compare the tangential velocity that equates to with the tangential velocity required to orbit the Sun just above the Sun's surface.
> If they had greater than escape velocity, they’d be escaping
Only if the space they were escaping into were vacuum. Which it isn't. What stops them is not the Sun's gravity but the plasma in the interstellar medium.
If the intersteller medium is not a vacuum, what is? Last I checked, it was literally billions of times lower density than the hardest vacuum we’ve been able to produce on earth.
> If the intersteller medium is not a vacuum, what is?
There is no threshold of low enough density at which there is suddenly "vacuum". If there are particles present, there are particles present, and they can have effects.
> Last I checked, it was literally billions of times lower density than the hardest vacuum we’ve been able to produce on earth.
[Edit--these numbers are off--see my post downthread]
And the solar wind is much, much less dense than that. Interstellar medium density is about a trillion particles per cubic meter. Solar wind density is about 5 thousand particles per cubic meter. So the interstellar medium is more than dense enough to stop the solar wind.
You are correct that the numbers I cited were off, because I had neglected to check specifically for numbers at the heliopause. Here is a better set of numbers:
The plasma density in the outer heliosphere is typically about 0.002 cm-3. The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm-3 ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause.
In other words, the density of the interstellar medium just outside the heliopause, as detected by Voyager 2, was about 20 times larger than the density of the plasma just inside the heliopause.
and yet, atmospheric particles that have a mean free path that doesn’t intersect with other atmospheric particles, or when they do the average velocity delta and direction reach escape velocity, escape and are lost.
You might want to rethink that. It’s a useful model in bulk in the lower atmosphere, but it’s far from true in the upper atmosphere.
First, while the upper atmosphere is much less dense than the lower, and the fluid approximation becomes less and less useful as you gain altitude, that still doesn't mean that "a bunch of particles in free-fall orbits" becomes a useful model. The average thermal velocity of a molecule in the upper atmosphere is still well short of orbital velocity at that altitude. Some molecules acquire sufficient velocity to escape, sure, but that doesn't mean the others are in orbit.
Second, the Earth's atmosphere is not a good analogy for what is happening at the heliopause anyway.
