Critical Thresholds in Euler-poisson Equations Shlomo Engelberg, Hailiang Liu, and Eitan Tadmor Dedicated with Appreciation to Ciprian Foias and Roger

We present a preliminary study of a new phenomena associated with the Euler-Poisson equations — the so called critical threshold phenomena, where the answer to questions of global smoothness vs. finite time breakdown depends on whether the initial configuration crosses an intrinsic, O(1) critical threshold. We investigate a class of Euler-Poisson equations, ranging from one-dimensional problem with or without various forcing mechanisms to multi-dimensional isotropic models with geometrical symmetry. These models are shown to admit a critical threshold which is reminiscent of the conditional breakdown of waves on the beach; only waves above certain initial critical threshold experience finite-time breakdown, but otherwise they propagate smoothly. At the same time, the asymptotic long time behavior of the solutions remains the same, independent of crossing these initial thresholds. A case in point is the simple one-dimensional problem where the unforced inviscid Burgers’ solution always forms a shock discontinuity except for the non-generic case of increasing initial profile, u′0 ≥ 0. In contrast, we show that the corresponding one dimensional Euler-Poisson equation with zero background has global smooth solutions as long as its initial (ρ0, u0)configuration satisfies u ′ 0 ≥ − √ 2kρ0 – see (2.11) below, allowing a finite, critical negative velocity gradient. As is typical for such nonlinear convection problems one is led to a Ricatti equation which is balanced here by a forcing acting as a ’nonlinear resonance’, and which in turn is responsible for this critical threshold phenomena. AMS subject classification: Primary 35Q35; Secondary 35B30.