Vidotto: ExoplanEts and thE stEllar wind

A&G • February 2013 • Vol. 54 1.25 The continuous flow of matter that escapes out of the solar gravitational well is known as the solar wind. As the material flows out of the Sun, it is accelerated to between 400 and 800 km s. Through this continuous flow of material, the Sun loses more than one million tonnes every second. But this is just a tiny fraction, corresponding to 2 × 10 of a solar mass that is lost every year. However, the solar wind does not only involve particles, it also carries the solar magnetic field lines. The magnetized solar wind permeates the entire solar system, having an effect on any body encountered on its way. The meeting of the solar wind and a solar system planet can result in complex interactions that depend on the characteristics of both the local solar wind and the planet. Factors such as whether the planet is magnetized or whether it has an atmosphere can play different roles in this interaction. Planets that are weakly magnetized or not magnetized, such as Mars, can have their atmospheres exposed to the impact of the solar wind. The solar wind may then remove the atmosphere of the planet by sputtering processes. Evidence suggests that Mars once had a thicker atmosphere, but most of it may have disappeared due to interaction with the young Sun’s wind, believed to have been more intense in the distant past. In the case of a magnetized planet such as the Earth, the planet’s field acts as a large obstacle for the solar wind: the flow of particles produced by the Sun cannot penetrate all the way to the surface of our planet, but ends up being deflected around the Earth’s magnetic field lines (see figure 1). Because of the speed of the flow, the impact of the solar wind in the magnetosphere of the Earth produces a bow shock that surrounds the dayside magnetosphere of our planet (the side towards the Sun). The formation of bow shocks is not the only signature of the interaction between the magnetized solar wind and a magnetized planet. Mediated by magnetic reconnection events, energetic electrons are released in the system. Some of these electrons spiral along planetary magnetic field lines, giving rise to cyclotron emission at radio wavelengths via a process called cyclotron maser instability. Because this kind of inter action is regulated by magnetic reconnections, it can only exist between magnetized bodies.