Recent results from Cluster shed some light on the mechanism that brings particles from the solar wind into the Earth's magnetosphere, creating the Aurora and radiation belts. The basic mechanism is vortices generated in the sheared flow region between the magnetosphere and the solar wind. The mechanism behind the vortices is called the Kelvin-Helmholtz instability, and it's fairly generic to low velocity sheared flows, as the discussed in the article.

The same mechanism will affect any craft powered by mini-magnetospheric plasma propulsion (M2P2), but the particle transport will be the other way - from inside the magnetic bubble to outside (since the inner particle density will be higher than the solar wind particle density, at least in the tail region). This will cause loss of ions from the bubble, and may turn out to be the limiting factor for M2P2.

There is a nice picture of Kelvin-Helmholtz waves in the Earth's atmosphere here.

I wonder what an M2P2 drive would look like with this effect causing it's plasma sheath to stream off...or even if the 'tail' would be visible. Probably not, but I bet it'd be pretty if we could see it. Man-made comets:-).

Would this only affect plasma sails (M2P2) within a relatively short range of a planet? If so, then I don't really see a problem--the real utility of a plasma sail is in interplanetary (or further) flight, moreso than tooling around in low orbit.

Would this only affect plasma sails (M2P2) within a relatively short range of a planet?

Nope - it would affect them anywhere there is solar wind. The M2P2 concept works the same way as the earth's magnetosphere - all the physics is the same. The reason that earth isn't pushed around by the solar wind is that it's *way* too heavy. Still, over geologic time you could probably detect an effect. Swap out the earth for a spacecraft and you get useful propulsion. It's still a small effect, but over long times it adds up.

So, what's the bottom line from a layman's pov? Does it mean less delta-v for a plasma sail? More instability? More fuel? I'd like to think the last wouldn't be too bad, since no fuel is intentionally expended (and unless it leaks like a sieve, it'd still beat any system involving intentional expenditure of fuel).

For that matter... I wonder if there's a way to "shape" the sail, by using stronger magnetic fields in certain places on the engine, or by injecting different gasses into the bubble. Could something like that alter the bubble's shape enough to disrupt the effect, or improve handling or efficiency?

Bottom line is that the rate at which you pump plasma into the magnetic bubble goes up. At some point so much mass and energy is needed to compensate for the losses that you might as well just fire it out of a thruster. The jury is out until reasonable tests can be conducted, which will require tests outside the earth's magnetosphere.

As far as shaping goes, I think you can, to a limited extent. You can certainly adjust the orientation of the dipole field to get net thrust at an angle to the solar wind, for example.

So you don't think there's much chance of using this effect to grow an M2P2 magnetosphere?

Picking up propellent from the solar wind as you cruise around the solar system would definitely be 'kind of neat'.

So you don't think there's much chance of using this effect to grow an M2P2 magnetosphere?

No, you need to inflate the magnetosphere in order to have an effective 'sail' and relying on diffusion across the magnetopause will compress the magnetic bubble rather than expand it. M2P2 is a fascinating idea, but it really needs a deep space test, or much better a deep space test series. I think it could be done with a series of vehicles about the size of MOST (maybe a factor of 2 larger). You'd only get maybe 72 hours of data per craft, but that ought to be more than enough to prove out the concept if it was done over maybe 3-5 vehicles focused on different aspects of the problem.

Given my druthers (and I've always wanted a druther or two) I'd build a single large (~ 1 ton) mother ship with all the power supplies on board and a series of small superconducting rings with a small onboard gas source. The rings would be ejected in succession, and the gas ionized and heated by beaming from the mother ship. Tracking would be by ground based radar (obviously the rings would need reflectors to give a decent radar cross section).