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**Session 24 - General Interstellar Medium.**

*Oral session, Monday, June 10*

*Historical Society, *

## [24.02] Reduced Four-Field System for Compressible Magnetohydrodynamic Turbulence in the Solar Wind and the Interstellar Medium

*A. Bhattacharjee, C. S. Ng, S. Spangler (University of Iowa)*
Turbulent plasmas in the solar wind and the interstellar medium often
contain a large directed magnetic field, and the plasma beta (\beta)
is frequently of order unity. To describe the dynamics of these
plasmas, we derive a system of reduced equations from the fully
compressible MHD equations using the Alfven Mach number as a small
parameter [G. P. Zank and W. H. Matthaeus, Phys. Fluids A 5,
257 (1993)]. These reduced equations constitute a closed system
to leading order and involve four field variables: the magnetic
flux, vorticity, pressure and parallel flow. The reduction of the
fully compressible MHD equations to a four-field system offers
significant computational advantages. If \beta << 1, these
equations reduced to the well-known Rosenbluth-Strauss equations
(otherwise known as RMHD equations). However, for \beta \sim 1,
if the background equilibria have non-vanishing pressure gradients,
the effect of compressibility is shown to enter at leading order.
In such cases, our equations differ significantly from those of
Zank and Matthaeus who claim that ``compressible effects ride
parasitically on the back of the 2D incompressible flow field.''
We present numerical results involving current sheet formation,
magnetic reconnection and intermittent turbulence that show
clearly the dynamical differences between RMHD and four-field
MHD. Implications for observations on pressure-balanced
structures will be discussed.
This work is supported by NSF and AFOSR.

**Program
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