• PHYSICAL PROBLEM: How can we explain the very high speed of the solar wind and its temporal variations with latitude and distance?

 
• OBSERVATIONS: the high atmosphere of the Sun (corona) is not static: it evaporates and gives rise to a supersonic wind after about 5 to 10 solar radii. The magnetic field plays an important role.

 
• EQUATIONS: Magnetohydrodynamics, polytrope (conducting fluids = Navier-Stokes + Laplace force + induction equations for the magnetic field, polytropic index unity)

 
• NUMERICAL EXPERIMENTS: a) weak magnetic field b) strong magnetic field c) fluctuating magnetic field (Alfvén waves)

 
• CONCLUSION: Alfvén waves with LARGE enough amplitude have several effets: they acclerate the wind up tp and above the required 800 km/s (this was expected), but they also contribute to the low plasma density and to the low level of compressible fluctuations observed when far from the heliospheric current sheet

 
• REFERENCE: (previous work on small-amplitude waves) R. Grappin, J. Léorat, A. Buttighoffer, "Alfvén wave propagation in the high solar corona", Astronomy and Astrophysics, vol 362, p.342, 2000.

run A: weak magnetic field	run B: strong magnetic field	C: Strong varying magnetic field (FILM:MOV 0,8Mb, GIF 1,3Mb)	Zoom of run C:
		<== Alfvén waves   <== Neutral sheet   <== Alfvén waves
Wind is SLOW (= 400 km/s) at low latitudes The neutral sheet (location of the polarity reversal) is THIN	Wind is always SLOW The Neutral sheet is very thick	Matter is expelled by waves => the neutral sheet is thinner Wind is much faster (conversion of transverse into radial movement)	The white zone is a dense region where matter is at rest, emprisoned by magnetic arches. On the contrary, the black channels north and south are rarefied, because matter has been expelled by the waves.