Edit: disregard, I think it's probably still measurable, just not as well.
https://en.wikipedia.org/wiki/Pioneer_10#Pioneer_10_in_popul...
Photo to illustrate: https://en.wikipedia.org/wiki/Heliosphere#/media/File:Helios...
We live near the ocean, and we have a rocky shoreline. We have a couple coves nearby. One cove is about the a half-mile across, but the opening to the larger bay nearby is just a couple hundred feet. On most days, the cove is really calm and the bay has roughly two foot waves.
So, you can go out to the cove, pick up the biggest rock you can lift, and heave it into the water. You'll make a giant splash that amazes young kids, and then you can watch the ripples fan out over the bay. But you also see those ripples stop as soon as they reach the bay, where the larger waves absorb the smaller ripples from the rock. The rock represents the sun, the ripples represent solar wind, and the waves on the bay represents interstellar space.
I believe that's a reasonable way of explaining it; if I was wrong after all this time I'd love to know it.
Heliopause. The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance.
My initial intuition was to wonder why the vectors of all the other stellar winds wouldn't be expected to nearly cancel each other out, but then it seems like the ones that would be pushing in the same direction as the sun's would have been blocked on the other side of the sphere, so it does seem to make sense that the net direction would be to point inward. But then I realized that I have no idea if any of that reflects an accurate mental model of what's going on :)
[1] https://en.wikipedia.org/wiki/Heliosphere#/media/File:Helios...
[2] https://en.wikipedia.org/wiki/Heliosphere#Termination_shock
How can sound travel so fast in the interstellar medium?
The catch is that you can only transmit very low frequency sounds - ti can be thought of like the variations in travel time for any individual particle drown out any high frequency signal.
It's hot, so the speed of sound is high.
Given the impedance mismatch[0], even the parts of the solar system outside Kármán lines where the interplanetary medium can support pressure levels equivalent to normal speaking (including low Earth orbit), I'm told human ears can't respond to that pressure change properly.
Sensors can be built to pick it up, but that may not be in scope for your question as we can also do that for acoustic waves in the CMB.
[0] https://en.wikipedia.org/wiki/Impedance_matching#Acoustics
Not sure if it would actually be useful, because I'm sure radio waves would be much more practical, but sound in space is a fascinating idea.
So although solar wind sounds hardcore, at that distance its pressure about matches that of the nothingness that makes up most of the galaxy. Interesting!
BUT- The graph says 2-3 particles/sec hitting the detector, which in sub-atomic terms is like 2 drops of water in an ocean's worth of volume. How much meaningful particle interaction is happening when everything is so close to a true vaccuum? Is this another one of those weird quantum-field-theory things? (Asking as a layman not a physicist obviously)
A solar eruption may impose 10~ AU of continued heliosphere at this distance.
This is not correct. The particles are not in orbit about the Sun, they're coming from the Sun--they're the solar wind. The heliopause is where the solar wind particles are stopped by the pressure of the surrounding interstellar medium. When Voyager passed that point (the heliopause), the number of particles hitting it dropped drastically.
The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance. The crossing of the heliopause should be signaled by a sharp drop in the temperature of solar wind-charged particles,[30] a change in the direction of the magnetic field, and an increase in the number of galactic cosmic rays.[34]
The particles hitting Voyager aren't orbiting the Sun; they're from the Sun, the solar wind. The heliopause is the point where they are stopped by the interstellar medium. That point is what Voyager passed as shown in the graph.
If they had greater than escape velocity, they’d be escaping and we’d not see the graph we see.
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.
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.
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.
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.
I suspect you got your numbers reversed.
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:
https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1024G/abstra...
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.
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.
You might want to rethink your claim.
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.
https://web.archive.org/web/20130913162459/http://news.natio...