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We explain the fast-moving, ripple-like features in the edge-on debris diskorbiting the young M dwarf AU Mic. The bright features are clouds of sub-microndust repelled by the host star's wind. The clouds are produced by avalanches:radial outflows of dust that gain exponentially more mass as they shatterbackground disk particles in collisional chain reactions. The avalanches aretriggered from a region a few AU across -- the "avalanche zone" -- located onAU Mic's primary "birth" ring, at a true distance of $\sim$35 AU from the starbut at a projected distance more than a factor of 10 smaller: the avalanchezone sits directly along the line of sight to the star, on the side of the ringnearest Earth, launching clouds that disk rotation sends wholly to thesoutheast, as observed. The avalanche zone marks where the primary ringintersects a secondary ring of debris left by the catastrophic disruption of aprogenitor up to Varuna in size, less than tens of thousands of years ago. Onlywhere the rings intersect are particle collisions sufficiently violent to spawnthe sub-micron dust needed to seed the avalanches. We show that this pictureworks quantitatively, reproducing the masses, sizes, and velocities of theobserved escaping clouds. The Lorentz force exerted by the wind's magneticfield, whose polarity reverses periodically according to the stellar magneticcycle, promises to explain the observed vertical undulations. The timescalebetween avalanches, about 10 yr, might be set by time variability of the windmass-loss rate or, more speculatively, by some self-regulating limit cycle.

Stellar Winds and Dust Avalanches in the AU Mic Debris Disk