IS THE EARTHS MAGNETOTAIL BALLOON UNSTABLE

We consider the ideal MHD stability of two-dimensional equilibrium models of the geomagnetic tail, where the magnetic field is in the xz plane and nothing depends on the coordinate y. Ballooning-type perturbations are considered for which \k(y)\L much greater than 1 where L is the scale length for variations in the xz plane. We employ the energy principle to test the stability of each flux tube of the plasma sheet for a ''hard'' ionospheric boundary condition (v = 0) and present a physical argument suggesting that it is the most appropriate simple boundary condition for the Earth's plasma sheet. Numerical results are presented for compressible ballooning modes that am symmetric about the center of the current sheet. For such a hard boundary condition, we found no reasonable magnetotail configuration that was unstable to compressible symmetric ballooning but stable against interchange. Therefore compressible symmetric ideal-MHD ballooning seems incapable of substantially altering the large-scale configuration of the magnetotail.