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It is evident from eclipse photographs that gas-magnetic field interactions are important in determining the structure and dynamical properties of the solar corona and interplanetary medium. Close to the Sun in regions of strong field, the coronal gas can be contained within closed loop structures. However, since the field in these regions decreases outward rapidly, the pressure and inertial forces of the solar wind eventually dominate and distend the field outward into interplanetary space. The complete geometrical and dynamical state is determined by a complex interplay of inertial, pressure, gravitational, and magnetic forces. The present paper is oriented toward the understanding of this interaction. The 'helmet' streamer type configuration with its associated neutral point and sheet currents is of central importance in this problem and is, therefore, considered in some detail.Integration of the relevant partial differential equations is made tractable by an iterative technique consisting of three basic stages, which are described at length. A sample solution obtained by this method is presented and its physical properties discussed.

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Three independent observations by rocket, Skylab, and OSO-8 have all indicated the presence of steady downftows of the order of a few kilometers per second in the solar transition region overlying the chromospheric network. Using density estimates at these heights from traditional transition region models, we find that the downward mass fluxes associated with these velocities are comparable with the estimated upward mass flux in spicules, originating in the same regions. Since both observations and theoretical calculations show that the solar wind can accept only a small fraction of the upward spicule flux, we suggest that the downflow represents spicular material returning to the chromosphere after being heated to coronal temperatures. In this context, the differential velocity measurement of Cushman and Rense is interpreted as indicating a difference in downflow speeds rather than a difference in expansion speeds.Moreover, the enthalpy flux associated with the downflow of coronal material into these regions is shown by various estimates to exceed the inward heat flow expected by thermal conduction and it may constitute the dominant energy source for the transition region. Simplified analytical models are used to explore the nature of the transition region overlying the supergranulation boundaries, under the assumption that the thermal structure results from a balance of the downward convection of enthalpy and radiative losses. Models based upon these considerations are shown to be consistent with the observed emission measures.

Abstract. The magnetic reconnection theory for two-ribbon flares and flare loops hypothesizes that the gradual energy release during the decay phase is a direct result of dissipative relaxation of the open coronal magnetic configuration created by an eruptive prominence/coronal transient precursor. This scenario is here developed quantitatively to the point where a realistic comparison with observational data can be attempted. Our major refinements are (i) to use an analytical description of the reconnecting field geometry specifically chosen to characterize the spatial scale of the active region where a flare occurs, and (ii) to take account of the fact that the volume occupied by X-ray-emitting plasma (hot loops) generally increases with time during the flare.As a test of the modified theory we have undertaken a new representation of the Skylab observations of the large two-ribbon flare of 29 July, 1973, It is hereby found that the simultaneous inclusion of the above two factors yields an excellent agreement between the theoretical prediction and observations of the time variation of flare-plasma energy density; such was not possible within the framework of previous work along these lines. The agreement extends, moreover, to rather early times in the flare history, where the shortcomings of earlier studies became especially noticeable. This result constitutes strong evidence to support previous speculations that magnetic reconnection may provide the sole energy source throughout nearly the entire flare lifetime.

The Trident laser was used to induce shock waves in samples of solid elements, with atomic numbers ranging from Be to Au, using pulses of 527 nm light around 1 ns long with irradiances of the order of 0.1 to 10 PW/m(2). States induced by the resulting ablation process were investigated using laser Doppler velocimetry to measure the velocity history of the opposite surface. By varying the energy in the laser pulse, relations were inferred between the irradiance and the induced pressure. For samples in vacuo, an irradiance constant in time does not produce a constant pressure. Radiation hydrodynamics simulations were used to investigate the relationship between the precise pulse shape and the pressure history. In this regime of time and irradiance, it was possible to reproduce the experimental data to within their uncertainty by including conductivity-dependent deposition of laser energy, heat conduction, gray radiation diffusion, and three temperature hydrodynamics in the treatment of the plasma, with ionizations calculated using the Thomas-Fermi equation. States induced in the solid sample were fairly insensitive to the details of modeling in the plasma, so Hugoniot points may be estimated from experiments of this type given a reasonable model of the plasma. More useful applications include the generation of dynamic loading to investigate compressive strength and phase transitions, and for sample recovery.

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Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 Å of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding 5×1013 W/cm2 are needed for assured stability beyond 2 ns; (3) longer foams at lower densities are needed for effective mitigation with shorter wavelength light; (4) the gold layer hastens conversion of the structured foam to a uniform plasma.