Why does the solar wind switch to flowing straight by the time it reaches Earth?

Between 02:01 and 02:29 the new NASA Goddard video 5 New Discoveries from NASA's Parker Solar Probe explains

finding # 4, A Breaking Point:

Before parker, scientists knew that the corona rotates with the visible surface below it. But they didn’t know how — or where — the solar wind switched to flowing straight by the time it reaches Earth.

Parker has finally spotted signs of this transition — and the changeover happens significantly farther out than expected.

Naively I'd apply conservation of angular momentum and assume that the particles are in individual ballistic orbits, but the solar wind contains charged particles as well as neutrals and so even though of low density and long mean free paths they can interact collectively.

But I don't understand why the solar wind would suddenly stop rotating beyond some radius from the Sun well inside Earth's orbit, rather than beyond the heliopause.

Why does this happen?

• What do you mean by flowing straight? Radially outwards as opposed to having an azimuthal velocity component? 'Straight' in outer space is not very well defined. – AtmosphericPrisonEscape Dec 5 '19 at 2:12
• @AtmosphericPrisonEscape apparently NASA Goddard and I both understand it to mean along a straight line away from the Sun. I've simply adopted the same term from the video, as shown in the quote. Neither of us is discussing flow far from the ecliptic, since Parker is in the ecliptic. – uhoh Dec 5 '19 at 2:29
• I'm happy that your understanding and NASA's align so well. Many of those coincidences these days, seemingly. Anyways, so you mean radially away from the sun, thanks, that's vocabulary that your fellow scientist can understand. – AtmosphericPrisonEscape Dec 5 '19 at 2:32
• And what about the fact that the Sun, Earth and Parker are all in the ecliptic at the same time? Too many coincidences for comfort! – uhoh Dec 5 '19 at 2:37

When plasma moves in a magnetic field then the charged particles follow helical paths around the field lines, due to the $$q\vec{v}\times \vec{B}$$ Lorentz force. If the radius of gyration is small, then the plasma is effectively tied (or frozen) to the field lines.
The magnetic energy density scales with magnetic field squared and the field drops further from the Sun (e.g. a dipole field falls as $$r^{-3}$$), whereas the plasma kinetic energy density (which depends on the plasma density and temperature) could remain relatively high (the solar wind density drops only as $$r^{-2}$$ and the temperature is roughly constant). There is a transition between the magnetic field dominating the plasma close to the Sun, to where the plasma dominates and effectively carries the field with it. This transition point is known as the Alfven radius and is usually at a few tenths of an au from the Sun.
• Oh I see. It's nothing like conserving angular momentum, the rotation mentioned is the Sun's rotation rate $\omega$ (circa 14 deg/day), which is definitely going to be unsustainable at some distance. This is much clearer now, thanks! – uhoh Dec 5 '19 at 0:33