Abstracts submitted by Roland Grappin

Is the chromospheric transition region stable?

R. Grappin [1]; J. Leorat [1]; R. Pinto [1]; Y.-M. Wang [2]

[1] Observatoire de Paris; [2] NRL

The chromospheric transition region plays the role of a (partially) reflecting boundary. In fact, it has its own dynamics, which is essential to understand coronal sismology, and more generally coronal heating and wind. To progress, numerical modeling of the transition region must include the transition region not as a numerical boundary, but well within the simulation domain.
Published numerical simulations on the coronal dynamics including the transition region belong to two classes: either the transition region is given (or computed), but its oscillations are limited (relaxation models); or, in the rare self-consistent simulations in which the corona is heated by either viscous or ohmic heating (Gudiksen and Nordlund 2005; Suzuki and Inutsuka 2005), the transition region is free, and in the latter case, it is observed to be unstationary.
The present work explores as a preliminary step the issue in the 1D, hydrodynamic case (with some MHD cases), including chromosphere, corona and solar wind; we demonstrate that using relaxation models for the transition region leads to artificial stability, since when when this constraint is relaxed, the transition region shows unstable behaviour, by accumulating the energy of oscillations propagating upwards from the photospheric basis.
Spectral anisotropy of MHD turbulence with large mean field: recent numerical results

R.Grappin [1]; W.-C. Mueller [2]; J.Leorat [1]

[1] Observatoire de Paris, LUTH; [2] Max-Planck Institute, Garching

A well-known property of plasmas with a strong magnetic field is that small scales form preferentially with wave vectors perpendicular to the mean field, the weakness of small scale generation along the mean field being due to Alfvén waves decorrelation.
Dimensional arguments have been proposed, which lead to definite relations between the parallel and perpendicular slopes (Goldreich Shridhar, 1997).
To check such relations via direct simulations is not so simple. An important difficulty in this respect is that, while high resolutions such as 1024^3 are sufficient for an inertial range (hence also a slope) to show up in the perpendicular direction, this is not the case for the parallel direction, where the effective Reynolds number remains miserable.
One expects on the one hand that the so-called extended similarity, which holds in isotropic MHD turbulence (with zero mean field), also holds for the case of a strong mean field, which would lead to a definite relation between the perpendicular and parallel spectra valid over the whole spectral range, not limited to the inertial range. On the other hand, one also expects isotropy to hold in the dissipative range.
One answers here this dilemna by analysing results of 512 x 1024 x 1024 direct simulations of incompressible MHD. The implications for the coronal and solar wind plasmas are discussed.