Application of the MHD energy principle to magnetostatic atmospheres

The MHD energy principle is applied to the stability of a magnetized atmosphere which is bounded below by much denser fluid, as is the solar corona. The two fluids are treated as ideal; the approximation is consistent with the energy principle, and the dynamical conditions that must hold at a fluid-fluid interface are used to show that if vertical displacements of the lower boundary are premitted, then the lower atmosphere must be perturbed as well. However, displacements which do not perturb the coronal boundary can be properly treated as isolated perturbations of the corona alone

Environments for Magnetic Field Amplification by Cosmic Rays

We consider a recently discovered class of instabilities, driven by cosmic ray streaming, in a variety of environments. We show that although these instabilities have been discussed primarily in the context of supernova driven interstellar shocks, they can also operate in the intergalactic medium and in galaxies with weak magnetic fields, where, as a strong source of helical magnetic fluctuations, they could contribute to the overall evolution of the magnetic field. Within the Milky Way, these instabilities are strongest in warm ionized gas, and appear to be weak in hot, low density gas unless the injection efficiency of cosmic rays is very high.Comment: 9 pages, 8 figures; Accepted to Ap

Kelvin-Helmholtz Instability in a Weakly Ionized Medium

Ambient interstellar material may become entrained in outflows from massive stars as a result of shear flow instabilities. We study the linear theory of the Kelvin - Helmholtz instability, the simplest example of shear flow instability, in a partially ionized medium. We model the interaction as a two fluid system (charged and neutral) in a planar geometry. Our principal result is that for much of the relevant parameter space, neutrals and ions are sufficiently decoupled that the neutrals are unstable while the ions are held in place by the magnetic field. Thus, we predict that there should be a detectably narrower line profile in ionized species tracing the outflow compared with neutral species since ionized species are not participating in the turbulent interface with the ambient ISM. Since the magnetic field is frozen to the plasma, it is not tangled by the turbulence in the boundary layer.Comment: 21 pages, 4 figure

Magnetic Merging in Colliding Flux Tubes

We develop an analytical theory of reconnection between colliding, twisted magnetic flux tubes. Our analysis is restricted to direct collisions between parallel tubes and is based on the collision dynamics worked out by Bogdan (1984). We show that there is a range of collision velocities for which neutral point reconnection of the Parker-Sweet type can occur, and a smaller range for which reconnection leads to coalescence. Mean velocities within the solar convection zone are probably significantly greater than the upper limit for coalescence. This suggests that the majority of flux tube collisions do not result in merging, unless the frictional coupling of the tubes to the background flow is extremely strong.Comment: uuencoded-compressed-tarred PostScript (un-tars to 2 postscript files), 15pp text + 3 figures. Accepted for publication in The Astrophysical Journal. POP-00

1D Modeling for Temperature-Dependent Upflow in the Dimming Region Observed by Hinode/EIS

We have previously found a temperature-dependent upflow in the dimming region following a coronal mass ejection (CME) observed by the {\it Hinode} EUV Imaging Spectrometer (EIS). In this paper, we reanalyzed the observations along with previous work on this event, and provided boundary conditions for modeling. We found that the intensity in the dimming region dramatically drops within 30 minutes from the flare onset, and the dimming region reaches the equilibrium stage after $\sim$1 hour later. The temperature-dependent upflows were observed during the equilibrium stage by EIS. The cross sectional area of the fluxtube in the dimming region does not appear to expand significantly. From the observational constraints, we reconstructed the temperature-dependent upflow by using a new method which considers the mass and momentum conservation law, and demonstrated the height variation of plasma conditions in the dimming region. We found that a super radial expansion of the cross sectional area is required to satisfy the mass conservation and momentum equations. There is a steep temperature and velocity gradient of around 7 Mm from the solar surface. This result may suggest that the strong heating occurred above 7 Mm from the solar surface in the dimming region. We also showed that the ionization equilibrium assumption in the dimming region is violated especially in the higher temperature range.Comment: accepted for publication in The Astrophysical Journa