sentences:  The perturbation is zero at a distance from the double layer in every direction.If an incident charged particle, such as a precipitating auroral electron, encounters such a static or quasistatic structure in the magnetosphere, provided that the particle energy exceeds half the electric potential difference within the double layer, it will pass through without any net change in energy.Incident particles with less energy than this will also experience no net change in energy but will undergo more overall deflection.