Instabilites de jets resultant de fluctuations thermiques dans la couronne solaire

Spacecraft records of the solar wind show temporal variations in the velocity, magnetic field, temperature, density, with a fully developed spectrum. It has been generally accepted since Coleman (1967) that the origin of the large-scale spectrum lies in shear instabilities of adjacent streams. This idea is based on the fact that nothing prevents the solar wind plasma to behave as a homogeneous fluid.
We have shown however that
a) the inhomogeneity of the solar wind (due mainly to the plasma expansion associated with the advection by the main radial flow) is strong enough to prevent "normal" turbulent processes to develop
b) in particular, streams are stable in the supersonic solar wind, at least in the inner heliosphere.
c) both the onset and development of turbulence should take place in the subsonic part of the solar wind
d) coronal thermal fluctuations must play a basic role (see figures below) in the generation of unstable streams, hence indirectly in the onset of turbulence


[v59]
[v59cur] Legend of the figures
These two figures show the temperature of the plasma as found from integrating the Navier-Stokes equations with resolution 540*180, assuming axial symmetry. The (distance, latitude) plane is represented in the top figure, real coordinates are shown in the left figure. The distance range is from 5 to 20 solar radii, and latitudes are 22.5 degrees above and below the solar equator. Cold regions are black, warm regions are white. The temperature is conserved during advection, and there is no magnetic field present. The temperature profile at the inner boundary is fixed , i.e., it does not vary with time. It determines (after some transients) the mean radial velocity of the flow: a hot region generates a fast stream, a cold region a slow stream. One sees the development of von Karman streets at the boundary between cold and hot streams, and subsequent strong mixing. The flow strongly accelerates in the middle of the domain, as can be seen from the stretching of the patterns. The strongest instabilities are seen to appear where the (transverse) thermal gradients are the strongest.
The movie (53428 bytes) shows that the flow varies with time in an unpredictable way although the temperature is fixed at the inner boundary: it consists in seven successive steps of the evolution of the flow, the time interval between two successive pictures corresponding to about the time necessary for the mean flow to travel through the domain
We are presently working to include the magnetic effects in the simulations, in order to understand how the thermal and magnetic effects compete in the process of destabilization.