A Radial and Tangential Framework for Studying Transient Reactivity

Abstract: In modeling biological systems and other applications, an important recurring question is whether those systems maintain healthy regimes not just at dynamic attractors but also during transient excursions away from those attractors. For ODE models, these excursions are not due only to nonlinearities. Even in a linear, autonomous system with a global attractor at the origin, some trajectories will move transiently away from the origin before eventually being attracted asymptotically. Reactivity, defined by Neubert and Caswell in 1997, captures this idea of transient amplification of perturbations by measuring the maximum instantaneous rate of radial growth. In this paper, we introduce a novel framework for analyzing reactivity and transient dynamics in two-dimensional linear ODEs using a radial and tangential decomposition of the vector field. We establish how to view the eigen-structure of the system through this lens and introduce a matching structure of orthovectors and orthovalues that characterize where the radial velocity becomes positive. From this perspective, we gain geometric insight about the regions of state space where positive radial growth occurs and how solutions trajectories traverse through these regions. Additionally, we propose four standard matrix forms that characterize both transient and asymptotic behavior and which highlight reactivity features more directly. Finally, we apply our framework to explore the limits of reactivity and maximal amplification and to characterize how transient reactivity can accumulate in nonautonomous linear systems to result in asymptotically unstable behavior.