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In magnetized, stratified astrophysical environments such as the Sun's coronaand solar wind, Alfv\'enic fluctuations ''reflect'' from background gradients,enabling nonlinear interactions and thus dissipation of their energy into heat.This process, termed ''reflection-driven turbulence,'' is thought to play acrucial role in coronal heating and solar-wind acceleration, explaining a rangeof observational correlations and constraints. Building on previous worksfocused on the inner heliosphere, here we study the basic physics ofreflection-driven turbulence using reduced magnetohydrodynamics in an expandingbox -- the simplest model that can capture the local turbulent plasma dynamicsin the super-Alfv\'enic solar wind. Although idealized, our high-resolutionsimulations and simple theory reveal a rich phenomenology that is consistentwith a diverse range of observations. Outwards-propagating fluctuations, whichinitially have high imbalance, decay nonlinearly to heat the plasma, becomingmore balanced and magnetically dominated. Despite the high imbalance, theturbulence is strong because Els\"asser collisions are suppressed byreflection, leading to ''anomalous coherence'' between the two Els\"asserfields. This coherence, together with linear effects, causes the turbulence toanomalously grow the ''anastrophy'' (squared magnetic potential) as it decays,forcing the energy to rush to larger scales and forming a ''$1/f$-range''energy spectrum as it does so. At late times, the expansion overcomes thenonlinear and Alfv\'enic physics, forming isolated, magnetically dominated''Alfv\'en vortex'' structures that minimize their nonlinear dissipation. Theseresults can plausibly explain the observed radial and wind-speed dependence ofturbulence imbalance, residual energy, plasma heating, and fluctuation spectra,as well as making testable predictions for future observations.

Reflection-driven turbulence in the super-Alfv\'enic solar wind