Dynamics of the Magnetized Wake andthe Acceleration of the Slow solar Wind

¹Università di Pisa

F. Rappazzo¹, M. Velli², G. Einaudi¹, R. B. Dahlburg³

²Università di Firenze

³Laboratory for Computational Physics & Fluid Dynamics, NRL, Washington, DC

Acceleration of the slow solar wind above helmet streamers

The solar wind has two distinct components: the fast wind originates in the polar coronal holes, and the slow wind originates from the region above helmet streamers.

Observations of plasmoids detaching above helmet streamers by the LASCO coronagraph onboard SOHO

The formation and outward movement of a coronal density enhancement on 1996 May 24, as seen in running differenceimages obtained with the C2 (left) and C3 (right) coronagraphs.

From Sheeley N.R. Jr. et al., Astrophys. J., 484, 472, 1997

We model the region above the cusp of a helmet streamer as a current sheet embedded in a broader wake flow

In our simulations we use compressible, dissipative MHD equations

xAxB y tanh0 xhAxB z sec0

xhxu y sec10

1 21

MT

Equilibrium fields

1sc

uM5.1

u

cA A5

B

V

a

a

Both Kelvin-Helmoltz and tearinginstabilities have varicose modes, that have the same spatial parity.

Expanding Box Model

Melon seed force

Time evolution of magnetic islands

Density enhancement of magnetic islands

Fast wind

Fast wind

Neutral line

Mass density lowers because of radial expansion.

The melon seed force results in the initial rapid acceleration,while fluid instability shapes the global acceleration.

• Resistive instability allows the development of a

Kelvin-Helmoltz instability in the nonlinear regime.

Conclusions

• Moving plasmoids accelerating outward are observed.

• The fast wind accelerates and modulates the slow wind.