The interaction of long and short waves in dispersive media

TL;DR

This paper critically examines the coupled NLS-KdV system for modeling long and short wave interactions, highlighting derivation inconsistencies and proposing alternative models for better accuracy.

Contribution

It identifies gaps in the derivation of the coupled NLS-KdV system and introduces alternative models to improve the understanding of long and short wave interactions.

Findings

The coupled NLS-KdV system is often assumed but not rigorously derived.

Inconsistencies exist in the time scale assumptions of the equations.

Alternative systems are proposed to better model wave interactions.

Abstract

The KdV equation models the propagation of long waves in dispersive media, while the NLS equation models the dynamics of narrow-bandwidth wave packets consisting of short dispersive waves. A system that couples the two equations to model the interaction of long and short waves seems attractive and such a system has been studied over the last decades. We evaluate the validity of this system, discussing two main problems. First, only the system coupling the linear Schr\"odinger equation with KdV has been derived in the literature. Second, the time variables appearing in the equations are of a different order. It appears that in the manuscripts that study the coupled NLS-KdV system, an assumption has been made that the coupled system can be derived, justifying its mathematical study. In fact, this is true even for the papers where the asymptotic derivation with the problems described above is presented. In addition to discussing these inconsistencies, we present some alternative systems describing the interaction of long and short waves.