Comment by japhyr | Hacker News Reader

Comment by japhyr | Hacker News Reader

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Assuming you're talking about the overall sharp drop-off and not the bounce-backs, this was my favorite way to explain it to students:

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.

That makes sense as far as explaining another situation where you would see a similar pattern, but it doesn't really explain why. What's the equivalent of the land surrounding the cove here? The sun's gravity well? But that's a gradual drop-off, are we looking at the distance where the gravitational pull on particles is canceled out by some other force?

Hydraulic Jump on interstellar scale.

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.

But again, why is it a sudden cut-off and not a gradual one?

Have look at your sink [1], it is called termination shock. [2]

[1] https://en.wikipedia.org/wiki/Heliosphere#/media/File:Helios...

[2] https://en.wikipedia.org/wiki/Heliosphere#Termination_shock

Interesting! What is the interstellar medium? Is it entirely the combined stellar winds of all the other stars, or are there other components?

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 :)

AIUI, the solar wind does attenuate gradually; the relatively sharp drop-off is the transition from the "I feel the solar wind more than the interstellar medium" region to the "I feel the ISM more than solar wind" region.

This is a terrible explanation. It's about a completely different thing, it's fundamentally wrong, and it doesn't even make sense! Space is not shaped like a bay and Voyager is measuring a decrease in particles not an increase.

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