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This observation is difficult to perform with Planeterrella I, easier with Planeterrella II and since November 2012, very easy with Planeterrella III. In the first case, one must indeed use a strong power supply, tens of milli-amperes (warning ! One must be very careful with such intensities). In the second case, it suffices to approach the spheres as they are now mobile. We can even see – maybe for the first time in the world – the birth of a magnetosphere and a bow shock! To make this observation, the magnets in the spheres are positioned opposite to each other, magnetic north is up in the star (big sphere), and down in the planet (small sphere).

Electrons are driven along a magnetic field line that connects opposite poles: north of the star to south of the planet, or south of the star to the north of the planet. This creates a common zone between the 2 spheres where the electrons are concentrate enough so that the light emitted by gas excited through collisions becomes visible.

This zone recalls the one solarward to the Earth, 30,000 and 40,000 kilometers far called the “Bow shock”. However in the Earth case, there is not enough gas to emit light at this altitude. Between the bow shock and the star, the space is controlled by the solar wind. Earthward, one enters the magnetosphere where electrons are driven toward the radiation belts and the auroral ovals. In the magnetosphere and outside the radiation belts and the auroral regions, the environment is little aggressive for spacecrafts and astronauts. The boundary between the magnetosphere and the solar wind (therefore immediately below the bow shock) is called “magnetopause”.

The more the Sun is active, the more the magnetopause moves toward the Earth. Interestingly, geostationary spacecraft fly at 36,000 kilometers. When the Sun is quiet, they lie within the magnetosphere, in a relatively unaggressive environment. But when the Sun is active, the solar wind pressure pushes the magnetopause toward the Earth down to about 30,000 kilometers height. Satellites are then outside of the magnetosphere, directly exposed to energetic solar particles

 Artist view of the magnetopause Artist view of the magnetopause (credit NASA)

Institut de Planétologie et d'Astrophysique de Grenoble (IPAG)