The absolute and convective instability of the magnetospheric flanks

Despite the existence of flank waveguide modes which are Kelvin-Helmholtz u nstable, the flanks of the terrestrial magnetosphere are observed to be rem arkably stable and free of nonlinear disturbances. We suggest the explanati on may be found in a more detailed stability analysis which shows that loca lized disturbances are convectively unstable in the Earth's rest frame. Thi s means that as a wave packet grows and broadens, it also propagates at a s ufficiently high speed so it is convected away leaving ultimately no distur bance at any fixed point in space (as t --> infinity). We estimate that the magnetopause surface wave has an e-folding length of the order of an Earth radius and soon becomes nonlinear, resulting in a magnetopause boundary la yer [e.g., Manuel and Samson, 1993]. In contrast, the waveguide modes (whic h penetrate deep into the body of the magnetosphere) should grow by no more than a factor of about e as they propagate around the flanks to the tail. This also explains why theorists have had such success at modeling basic UL F waveguide processes with linear theory and why nonlinear waves in, or dis ruptions to, the body of the magnetospheric flanks are not observed: Wavepa ckets may grow by only a small amount as they propagate into the tail. Ulti mately, they leave the flank undisturbed and with the appearance of stabili ty, although they are actually convectively unstable.