The Sun is traveling through a small interstellar (IS) cloud, called with a lot of inventiveness the "LOCAL CLOUD". The relative motion between the Sun and the interstellar medium creates a flow of interstellar gas and dust around our star, the so-called interstellar wind, mainly composed of H and He plasma (Figure below). Approaching the Sun, the neutral and ionized part of the flow behave differently. IS ions are deccelerated, heated and deviated to flow around the heliopause, the contact discontinuity between the confined solar wind and the IS plasma. Neutrals, on the other hand, are flowing freely through the heliopause and into the inner heliosphere and are increasingly affected by the solar wind and the solar radiation fluxes.

SWAN does not point directly the Sun. It looks at the solar wind imprint on the Interstellar neutral H flow. The observation principle is called solar wind (or respectively the IS neutral H) Ly-α method.