On Stability of Thomson’s Vortex N-gon in the Geostrophic Model of the Point Bessel Vortices

A stability analysis of the stationary rotation of a system of N identical point Bessel vortices lying uniformly on a circle of radius R is presented. The vortices have identical intensity Γ and length scale γ −1 > 0. The stability of the stationary motion is interpreted as equilibrium stability of a reduced system. The quadratic part of the Hamiltonian and eigenvalues of the linearization matrix are studied. The cases for N = 2,..., 6 are studied sequentially. The case of odd N = 2 ℓ +1 ≥ 7 vortices and the case of even N = 2 n ≥ 8 vortices are considered separately. It is shown that the (2 ℓ + 1)-gon is exponentially unstable for 0 < γ R < R * ( N ). However, this (2 ℓ + 1)-gon is stable for γ R ≥ R * ( N ) in the case of the linearized problem (the eigenvalues of the linearization matrix lie on the imaginary axis). The even N = 2 n ≥ 8 vortex 2 n -gon is exponentially unstable for R > 0.