Melvyn Bragg and guests discuss the phenomenon behind the auroras at Earth's poles, the stream of charged particles spreading out from the Sun to the border of the solar system.

Melvyn Bragg and guests discuss the flow of particles from the outer region of the Sun which we observe in the Northern and Southern Lights, interacting with Earth's magnetosphere, and in comet tails that stream away from the Sun regardless of their own direction. One way of defining the boundary of the solar system is where the pressure from the solar wind is balanced by that from the region between the stars, the interstellar medium. Its existence was suggested from the C19th and Eugene Parker developed the theory of it in the 1950s and it has been examined and tested by a series of probes in C20th up to today, with more planned.

With

Andrew Coates

Professor of Physics and Deputy Director in charge of the Solar System at the Mullard Space Science Laboratory, University College London

Helen Mason OBE

Reader in Solar Physics at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Fellow at St Edmund's College

And

Tim Horbury

Professor of Physics at Imperial College London

Producer: Simon Tillotson