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## Plasma echoes near stable Penrose data

Echoes in a plasma are the excitement of new waves due to nonlinear interaction. The excitement may happen at an arbitrarily large time, which is the main source of difficulties in understanding Landau damping. For analytic data, the echoes are suppressed as the electric field is exponentially localized in time, and the nonlinear Landau damping holds for such data, as was first obtained by Mouhot and Villani in their celebrated work (Acta Math 2011; see also the extension to include Gevrey data). The nonlinear Landau damping remains largely elusive for less regular data (e.g., data with Sobolev regularity).

Recently, in a collaboration with E. Grenier (ENS Lyon) and I. Rodnianski (Princeton), we give an elementary proof of the known Landau damping results, which I also blogged it here, that were seen as a simple perturbation of the free transport dynamics, whose damping is direct (that is, the phase mixing). In the companion paper with E. Grenier and I. Rodnianski, we construct a class of echo solutions, which are arbitrarily large in any Sobolev spaces (in particular, they do not belong to the analytic or Gevrey classes studied by Mouhot and Villani), but nonetheless, the nonlinear Landau damping holds. In this blog post, I shall briefly discuss the plasma echo mechanism and our new results.

## Landau damping for analytic and Gevrey data

Landau damping is a classical subject in Plasma Physics, which studies decay of the electric field in a collisionless plasma in the large time. The damping was discovered and fully understood by Landau in the 40s for the linearized evolution near Maxwellians, and later extended by O. Penrose in the 60s for general spatially homogenous equilibria. The first mathematical proof of the nonlinear Landau damping was given by Mouhot and Villani for analytic data in their celebrated work (Acta Math, 2011). Their proof was then simplified, and the result was extended by Bedrossian, Masmoudi, and Mouhot to include data in certain Gevrey classes (Annals of PDEs, 2016).

Recently, in a collaboration with E. Grenier (ENS Lyon) and I. Rodnianski (Princeton), we give an elementary proof of these same results, which I shall give a sketch of it in this blog post. To avoid some tedious algebra, I mainly focus on the analytic case, which is precisely the case originally studied by Mouhot and Villani, leaving some remarks to the Gevrey cases at the very end of the post, where you’ll also find the slides of my recent lectures over Zoom on this topics.

## Bob Glassey

I am sadden to learn that Bob Glassey passed away this weekend after a long illness. Bob was a pioneer in the mathematical study of kinetic theory and nonlinear wave equations. He, together with Walter Strauss, was the first to initiate the mathematical study of Vlasov-Maxwell systems that describe the dynamics of a collisionless plasma. One of his fundamental theorems, known as Glassey-Strauss’ theorem (ARMA 1986), is to assert that solutions to the relativistic Vlasov-Maxwell system in the three dimensional space do not develop singularities as long as the velocity support remains bounded. The latter condition was subsequently verified by him and his former PhD student Jack Schaeffer for the case of low dimensions; namely, when particles are limited to one or two spatial domains. Their work has inspired several attempts from the mathematical community to tackle the full three dimensional case, which remains an outstanding open problem in the field.

Together with J. Schaeffer, Bob was also one of the first to initiate the mathematical study of Landau damping for Vlasov-Poisson systems in the presence of low frequency (or unconfined spatial domain). More precisely, for confined plasma (say, plasma on a torus), it was discovered and fully understood by Landau in the 40s that at the linearized level near a Gaussian, the electric field decays exponentially or polynomially depending on the regularity of initial data in the large time. The linear Landau damping remains to hold for more general spatially homogenous equilibria, known as Penrose stable equilibria. Later, Mouhot and Villani (Acta Math, 2011) verified this damping for data with analyticity for the nonlinear equations. In the unconfined case, Glassey and Schaeffer proved that the linear damping holds and is optimal at a much slower rate, which is surprisingly worse for Gaussians, due to the failure of the Penrose stability condition that holds in the confined case.

Bob also made fundamental and beautiful studies on the blowup issue for semilinear Heat, Wave, and Schr\”odinger (e.g., the Glassey’s trick), among other things. His book “The Cauchy problem in kinetic theory (SIAM 1996)” remains a fundamental textbook in the field.

Although Bob was already retired when I came to Indiana for my graduate study, he kindly participated and generously offered valuable guidances in a working seminar that I ran on the DiPerna-Lions theory for Boltzmann equations in the summer of 2008.

## Move to New Blog Address

I moved to this NEW blog address, hosted by Penn State University, more secure and no ads. Sorry for the inconvenience.

## Landau damping for screened Vlasov-Poisson on the whole space

In a recent joint work with Daniel Han-Kwan (CMLS, Ecole polytechnique) and Frédéric Rousset (Paris-Sud University), we give an alternative proof of the Landau damping for screened Vlasov-Poisson system near stable homogenous equilibria on the whole space, a result that was first established by Bedrossian, Masmoudi and Mouhot, for data with finite Sobolev regularity (they remarked that 36 derivatives were sufficient). In this work, via a Lagrangian approach, we prove the Landau damping for data with essentially Lipschitz regularity, yielding the time decay estimates that are essentially sharp, being the same as those for free transport, up to a logarithmic correction. Below, I shall briefly present the result and sketch the proof.

## Dafermos and Rodnianski’s r^p-weighted approach to decay for wave equations

Dafermos and Rodnianski introduced an ${r^p}$-weighted vector field method to obtain boundedness and decay of solutions to wave equations on a Lorentzian background, avoiding to use global vector field multipliers and commutators with weights in ${t}$ and thus proving to be more robust in dealing with black holes such as in Schwarzschild and Kerr background metrics. The approach has found numerous applications; see, for instance, Dafermos-Rodnianski, Moschidis, or Keir for many insightful discussions. In this blog post, I will give some basic details of this approach, based on lecture notes of S. Klainerman, focusing mostly on the flat Minkowski spacetime.

## Mourre’s theory and local decay estimates, with some applications to linear damping in fluids

In his famous 1981 paper, Mourre gave a sufficient condition for a self-adjoint operator ${H}$ to assure the absence of its singular continuous spectrum. More precisely, consider a self-adjoint operator ${H}$ on a Hilbert space ${\mathcal{H}}$ (e.g., ${L^2}$ with the usual norm), and assume that there is a self-adjoint operator ${A}$, called a conjugate operator of ${H}$ on an interval ${I\subset \mathbb{R}}$, so that

$\displaystyle P_I i[H,A] P_I \ge \theta_I P_I + P_I K P_I \ \ \ \ \ (1)$

for some positive constant ${\theta_I}$ and some compact operator ${K}$ on ${\mathcal{H}}$, where ${P_I}$ denotes the spectral projection of ${H}$ onto ${I}$, the commutator ${[H,A] = HA - AH}$, and the inequality is understood in the sense of self-adjoint operators. Then, Mourre’s main results are

• the point spectrum of ${H}$ in the interior of ${I}$ is finite.
• for any closed interval ${J \Subset I \cap \sigma_c(H)}$ and any ${z\in J}$, the operator ${\langle A \rangle^{-1} (H - z - i\epsilon)^{-1} \langle A\rangle^{-1}}$ is bounded on ${\mathcal{H}}$ uniformly as ${\epsilon \rightarrow 0}$, where ${\langle A \rangle = \sqrt{1+|A|^2}}$.

The Mourre’s theory is proven to be very useful in the study of spectral and scattering theory for Schrödinger operators and other dispersive PDEs. For instance, it yields the limiting absorbing principle, which in turn gives the Kato’s local smoothing estimate and the scattering RAGE’s theorem; for instance, see this blog post of T. Tao.

Below, I shall give a sketch of the proof of the Mourre’s beautiful theorem, then derive some local decay estimates on solutions to Schrödinger equations, and discuss some quick applications to linear damping in fluids.

## L-infinity instability of Prandtl layers

In 1904, Prandtl introduced his famous boundary layer theory in order to describe the behavior of solutions of incompressible Navier Stokes equations near a boundary as the viscosity goes to $0$. His Ansatz was that the solution of Navier Stokes equations can be described as a solution of Euler equations, plus a boundary layer corrector, plus a vanishing error term in $L^\infty$ in the inviscid limit. In this post, I briefly announce my recent work with E. Grenier (ENS Lyon) on the Prandtl’s boundary layer theory, where we prove

• the Prandtl’s Ansatz is false for shear profiles that are unstable to Rayleigh equations;
• the Prandtl’s asymptotic expansion is invalid for shear profiles that are monotone and stable to Rayleigh equations.

## Bardos-Degond’s solutions to Vlasov-Poisson

This is part of my lecture notes on Kinetic Theory of Gases, taught at Penn State last semester, Fall 2017. In this part, I’d like to introduce this nice Bardos-Degond 1985’s global solutions to the Vlasov-Poisson system. Of course, the global smooth solutions are already constructed, without any restriction on size of initial data (e.g., Pfaffelmoser, Schaeffer ’91; see also the previous lecture), however they give no information on their asymptotic behavior at large time. Now, for initial data that are sufficiently small near zero, Bardos and Degond were able to construct global smooth solutions that decay in large time. To my knowledge, this was the first result where dispersion is rigorously shown for kinetic equations (they appear to be motivated by similar results for nonlinear wave equations where dispersion was (still is) the key to deduce the global behavior at the large time; e.g., Klainerman, Ponce, Shatah, among others, in the early 80s).

## Stability of source defects in oscillatory media

Patterns are ubiquitous in nature, and understanding their formation and their dynamical behavior is always challenging and of great interest. Examples include patterns in fluids (e.g., Rayleigh-Benard convection between two flat plates, Taylor-Couette flow between rotating cylinders, surface waves in hydrothermal fluid flows,…), as well as in nonlinear optics, oscillatory chemical reactions and excitable biological media. Many of them arise from linear instabilities of an homogenous equilibrium, having space, time, or space-time periodic coherent structures such as wave trains (spatially periodic travelling waves). In presence of boundaries or defects, complex patterns form and thus break the symmetry or the periodic structures. Below, I shall briefly discuss some defect structures and my recent work on the subject.