New Solution to Old Mystery: Why Doesn’t the Inside of the Solar System Spin Faster?

The key to solving a longstanding mystery about thin gas disks rotating around young stars: the motion of a tiny number of charged particles. This is according to a new study from the California Institute of Technology (Caltech).

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Tau.Neutrino said:

New Solution to Old Mystery: Why Doesn’t the Inside of the Solar System Spin Faster?

The key to solving a longstanding mystery about thin gas disks rotating around young stars: the motion of a tiny number of charged particles. This is according to a new study from the California Institute of Technology (Caltech).

more…

> The inward spiral motion of the accretion disk is analogous to a skater drawing their arms in—and as such, the inner part of the accretion disk should spin faster. Astronomical observations do indeed show that the inner part of an accretion disk does spin faster. Curiously, however, it does not spin as fast as predicted by the law of conservation of angular momentum.

Well, duh, gas drag causes a loss of energy. So do inelastic collisions. Both slow down the inner portions.

> According to the dominant current hypothesis, magnetic fields cause a phenomenon known as a “magnetorotational instability” that results in the production of magnetic turbulence and gas—effectively forming friction that slows down the rotational speed of inward spiralling gas. That concerned me. People always want to blame turbulence for phenomena they do not understand. There’s a big cottage industry right now arguing that turbulence accounts for getting rid of angular momentum in accretion disks.

Laughing myself to tears. This is absolutely spot on. Too true. This actually goes back to the days of Johannes Kepler in the early 1600s.

> A decade and a half ago, Bellan began investigating the question by analyzing the trajectories of individual atoms, electrons, and ions in the gas that constitutes an accretion disk

So did I. It was part of my work for CSIRO. Though for me it was about the accretion of chondrules that later coalesced to form asteroids.

> created a computer model of a spinning, super-thin, virtual accretion disk. The simulated disk contained around 40,000 neutral and about 1,000 charged particles that could collide with each other, and the model also factored in the effects of both gravity and a magnetic field.

Good. That actually takes some doing, to get the algorithm speed up to where it can accurately model that many particles.
They would have to be fake collisions, though, because that’s not nearly enough particles to get real collisions.

> The computer simulation showed collisions between neutral atoms and a much smaller number of charged particles would cause positively charged ions, or cations, to spiral inward toward the center of the disk, while negatively charged particles (electrons) spiral outward toward the edge. Neutral particles, meanwhile, lose angular momentum and, like the positively charged ions, spiral inward to the center.

That’s unexpected. In my simulations I only simulated neutral particles, but I worked with an assumed wind of ionised particles that dragged the neutral particles along with them. I can see why negatively charged particles would tend to spiral outwards, they are smaller (electrons) and therefore more mobile. The positively charged particles that spiral inwards don’t reach the Sun, they are ejected perpendicularly from the disk as the stellar jets that we can see, and drag neutral particles out of the accretion disk along with them.

This could be right.