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Explaining the acceleration of the fast solar wind

1er juin 2005

It has been more than four decades since the existence of the solar wind has been confirmed by the measurements of the Mariner 2 spacecraft. However, the solar wind’s acceleration at supersonic speeds of about 700-800 km/s still remains unexplained. Parker’s theory, based on thermal conduction, results into a very low speed ; this led most of the scientists to look for an additional form of energy in order to explain this acceleration. A team of astronomers working at LESIA at the Paris Observatory has proposed an alternative theory based on the role of electrons that are not in thermodynamic equilibrium ; these electrons would be the main driving force of the acceleration. This approach explains, for the first time, the fast solar wind without any assumption of additional energy.

The theoretical difficulty lies in the fact that the plasma is weakly collisional, a field of plasma physics that is still not well understood. The hydrodynamic models are based on local thermodynamic equilibrium, which is far from being true in the case of solar wind electrons. Indeed, the velocity distribution functions of the electrons measured in situ are not Maxwellian, but present an excess of suprathermal electrons (Figure 1).

Figure 1 :

These electrons play a key role in the wind acceleration in a theoretical approach recently proposed by Ioannis Zouganelis and collaborators. This study allowed to obtain, for the first time, high speed winds that match the actual measurements (Figure 2). It is about a kinetic model of a plasma out of thermodynamic equilibrium which makes possible to reproduce transonic speed profiles, without any ad hoc additional energy [Zouganelis et al., ApJ, 2004].

These results have just been confirmed by independent kinetic simulations which take into account Coulomb collisions between particles. This work also unifies previous theoretical approaches which were valid only for particular initial conditions [Zouganelis et al, ApJL, 2005, in press]. This generalization makes possible for the model to include phenomena other than the solar wind, for example solar-type stellar winds where the corona of the star would "explode" and the wind would already be supersonic at its base. This theory could only be confirmed by in situ measurements in the corona very close to the Sun, since the biggest part of the acceleration would take place at a distance lower than 10 solar radii (Figure 2). The existence of non Maxwellian distribution functions in the corona, as well as their origin, may be revealed by the future mission "Solar Probe," which will visit the corona with a perihelion of 4 solar radii (Figure 3). This mission is planned for 2015-2020 and French astronomers are taking an active part in its definition.

Figure 2 :

References

  • Acceleration of weakly collisional solar-type winds I. Zouganelis *, N. Meyer-Vernet *, S. Landi, M. Maksimovic * et F. Pantellini * The Astrophysical Journal Letters, 2005, in press astro-ph/0505324 A transonic collisionless model of the solar wind I. Zouganelis *, M. Maksimovic *, N. Meyer-Vernet *, H. Lamy et K. Issautier * The Astrophysical Journal, 2004, 606, 542 astro-ph/0402358 Radial evolution of the electron distribution functions in the fast solar wind between 0.3 and 1.5 AU M. Maksimovic *, I. Zouganelis *, J.-Y. Chaufray, K. Issautier *, E.E. Scime, J.E. Littleton, E. Marsch, D.J. McComas, C. Salem, R.P. Lin et H. Elliott Journal of Geophysical Research, 2005, in press (*) Paris Observatory (LESIA)

Contact

  • Ioannis Zouganelis
    Observatoire de Paris, LESIA
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