Dark Solar Wind.

We study the solar emission of light dark sector particles that self-interact strongly enough to self-thermalize. The resulting outflow behaves like a fluid which accelerates under its own thermal pressure to highly relativistic bulk velocities in the solar system. Compared to the ordinary noninteracting scenario, the local outflow has at least ∼10^{3} higher number density and correspondingly at least ∼10^{3} lower average energy per particle. We show how this generic phenomenon arises in a dark sector composed of millicharged particles strongly self-interacting via a dark photon. The millicharged plasma wind emerging in this model has novel yet predictive signatures that encourages new experimental directions. This phenomenon demonstrates how a small step away from the simplest models can lead to radically different outcomes and thus motivates a broader search for dark sector particles.