Development of a strong shock in the outer heliosphere

On October 16, 2001, Voyager 2 at ∼65 AU observed a strong shock with a speed jump of over 100 km s−1, the strongest shock observed since 1991. However, unlike for many large shocks observed in the outer heliosphere, we could not find a single solar event which is directly linked to this shock. Instead, a series of solar events in April 2001 is found to be responsible. A multi-fluid one-dimensional MHD model is used to show that the solar wind stream structures associated with the April solar events observed at Earth have merged in the distant heliosphere into the single strong interplanetary shock that was subsequently detected by Voyager 2 in October 2001. Our demonstration that large shocks can and do form from this merging mechanism may have important consequences for the formation of merged interaction regions and the triggering of the heliospheric radio emission.