Daniel S. Spicer

The Dynamic Formation of Prominence Condensations

We present simulations of a model for the formation of a prominence condensation in a coronal loop. The key idea behind the model is that the spatial localization of loop heating near the chromosphere leads to a catastrophic cooling in the corona. Using a new adaptive grid code, we simulate the complete growth of a condensation and find that after ~5000 s it reaches a quasi-steady state. We show that the size and growth time of the condensation are in good agreement with data and discuss the implications of the model for coronal heating and for observations of prominences and the surrounding corona.

A Numerical Study of the Breakout Model for Coronal Mass Ejection Initiation

A leading theory for the initiation of coronal mass ejections (CMEs) is the breakout model, in which magnetic reconnection above a filament channel is responsible for disrupting the coronal magnetic field. We present the first simulations of the complete breakout process including the initiation, the plasmoid formation and ejection, and the eventual relaxation of the coronal field to a more potential state. These simulations were performed using a new numerical code that solves the numerical cavitation problems that prevented previous simulations from calculating a complete ejection. Furthermore, the position of the outer boundary in the new simulations is increased out to 30 R☉, which enables determination of the full structure and dynamics of the ejected plasmoid. Our results show that the ejection occurs at a speed on the order of the coronal Alfven speed and hence that the breakout model can produce fast CMEs. Another key result is that the ejection speed is not sensitive to the refinement level of the grid used in the calculations, which implies that, at least for the numerical resistivity of these simulations, the speed is not sensitive to the Lundquist number. We also calculate, in detail, the helicity of the system and show that the helicity is well conserved during the breakout process. Most of the helicity is ejected from the Sun with the escaping plasmoid, but a significant fraction (of order 10%) remains in the corona. The implications of these results for observation and prediction of CMEs and eruptive flares is discussed.

The Thermal Nonequilibrium of Prominences

We present numerical simulations and analytic theory for the thermal nonequilibrium of solar coronal flux tubes that have a stretched-out, dipped geometry, appropriate for a prominence/filament. Our simulations indicate that if the heating in such a flux tube is localized near the chromosphere, then condensations appear which undergo a continuous cycle of formation, motion, and destruction, even though the heating and all other imposed conditions on the loop are purely time independent. We show how this nonsteady evolution can be understood in terms of simple scaling-law theory. The implications of thermal nonequilibrium for observations of the solar corona are discussed. We argue that the model can explain both the formation of prominence condensations and recent observations of their dynamics.

The Neon-to-Magnesium Abundance Ratio as a Tracer of the Source Region of Prominence Material

A survey of all 1000 spectroheliograms in the Skylab spectroheliograph plate collection was made to identify prominences above the limb and prominence-like features. The Ne/Mg abundance ratios obtained from the measurements have been determined from relative intensities of Ne VI and Mg VI lines at 400 A of seven prominences and prominence-like features observed above the solar limb. The derived abundance ratios have values intermediate between the photosphere and corona, while none are as low as the ratio of 0.7 expected in the corona, which implies that the material found in prominences is photospheric in origin. The significance of these results for the formation of prominences is briefly discussed.

Magnetic structure of overexpanding coronal mass ejections: Numerical models

Numerical simulations are presented of the evolution of overexpanding coronal mass ejections (OCMEs), which are also magnetic clouds. The OCME is assumed to arise from the evolution of a magnetic flux rope with high plasma and magnetic pressure and high plasma density near the Sun in a high-speed solar wind. It is shown that the flux rope maintains its integrity from near the Sun to ≈ 5 AU, resisting hydrodynamic forces that tend to distort it. Thus OCMEs that are magnetic clouds at large heliocentric distances should have simply evolved from near-Sun flux ropes. It is shown that an initially circular flux rope is distorted into a hemispheric shape by its interaction with solar wind plasma flows. Forward and reverse shock pairs form with the forward shock being curved while the reverse shock is straight. The magnetic field properties at large distances are shown to depend on whether the initial flux rope undergoes overexpansion. A flux rope that is convected passively in the solar wind without overexpansion will ultimately have a magnetic field profile dominated by its toroidal component, so would not be observed as a magnetic cloud. The overexpanding flux ropes modeled here maintain an approximately equal ratio of toroidal to poloidal magnetic fields. The relative initial speed of the flux rope with respect to the solar wind does not influence the large-scale magnetic properties up to 5 AU, although it does affect the detailed field topology.

Geometry of interplanetary magnetic clouds

Two dimensional magnetohydrodynamic simulations are presented of the distortion of a magnetic flux rope that is being accelerated through ambient solar wind plasma. The flux rope magnetic field has an axial component parallel to the solar wind field and an azimuthal component, which lies in the simulation plane. As the flux rope moves through the solar wind plasma, vortices form on its trailing edge and couple strongly to its interior. If the flux rope azimuthal field is weak, it deforms into an elongated banana-like shape after a few Alfven transit times. A strong azimuthal field component tends to inhibit this distortion. If the flux rope is taken to model a magnetic cloud, it is suggested that the shape of the cloud at 1 AU is determined by its distortion in the inner solar wind. Distortion timescales beyond 1 AU are estimated as many days. It is estimated that effective drag coefficients somewhat greater than unity are appropriate for modelling flux rope propagation.

The Kelvin-Helmholtz instability in photospheric flows - Effects of coronal heating and structure

A series of hydrodynamic numerical simulations has been used to investigate the nonlinear evolution of driven, subsonic velocity shears under a range of typical photospheric conditions. These calculations show that typical photospheric flows are susceptible to the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth times that are approximately half of a typical granule lifetime. The KHI produces vortical structures in intergranule lanes comparable to a typical fluxule radius; this is precisely the correct scale for maximum power transfer to the corona.

Magnetohydrodynamic Simulations of Alfvénic Pulse Propagation in Solar Magnetic Flux Tubes: Two-dimensional Slab Geometries

Two-dimensional magnetohydrodynamic simulations are presented of the evolution of a nonlinear Alfven wave pulse in the region between the solar photosphere and corona. A magnetic field profile that incorporates the characteristic field spreading expected in flux tubes is used. The pulse is chosen initially to have a purely Alfvenic polarization and to extend over a limited horizontal distance. It is shown that as this pulse rises in the atmosphere, it becomes wedge-shaped. The part of the pulse at the center of the flux tube reaches the transition region first, with other parts arriving at a time that is determined by the history of the Alfven speed along the path of the wave. Since field lines that spread out from the center of the flux tube spend longer in the high-density photosphere and chromosphere, and also have a smaller total field strength, waves that travel along them will take longer to reach the corona. The nonlinearity of the Alfvenic pulse drives a plasma flow both parallel to the ambient magnetic field and in a direction normal to the field, owing to transverse modulation of the Alfvenic pulse. The pulse associated with this plasma flow is also wedge-shaped, but the actual shape is different from that of the Alfvenic pulse. Since these plasma flows are compressible, they propagate at a different characteristic speed from the Alfven waves, and so can reach the transition region either before or after the Alfven pulse, the precise result depending on the plasma parameters. As the compressible pulse moves upward, a finite-sized blob of chromospheric material is injected into the corona. The relevance of this to spicules and jets is discussed.

Interacting Jets From Binary Protostars

Aims. We investigate potential models that could explain why multiple proto-stellar systems predominantly show single jets. During their formation, stars most frequently produce energetic outflows and jets. However, binary jets have only been observed in a very small number of systems. Methods. We model numerically 3D binary jets for various outflow parameters. We also model the propagation of jets from a specific source, namely L1551 IRS 5, known to have two jets, using recent observations as constraints for simulations with a new MHD code. We examine their morphology and dynamics, and produce synthetic emission maps. Results. We find that the two jets interfere up to the stage where one of them is almost destroyed or engulfed into the second one. We are able to reproduce some of the observational features of L1551 such as the bending of the secondary jet. Conclusions. While the effects of orbital motion are negligible over the jets dynamical timeline, their interaction has significant impact on their morphology. If the jets are not strictly parallel, as in most observed cases, we show that the magnetic field can help the collimation and refocusing of both of the two jets.

Extreme-Ultraviolet Transition-Region Line Emission during the Dynamic Formation of Prominence Condensations

We calculated the emission expected in EUV transition-region lines during the process of dynamic formation of prominence condensations in coronal loops, as predicted by the thermal nonequilibrium model of Antiochos et al. We selected some lines emitted by ions of carbon and oxygen because they are among the most intense and representative in the temperature range corresponding to the solar transition region. We present and discuss the principal characteristics of the line intensities and profiles synthesized from the hydrodynamic model at different times during the loop evolution. The ionization balance is computed in detail and the deviations from the ionization equilibrium caused by plasma flows and variations of temperature and density are accounted for. The atomic physics is treated using the latest atomic coefficients and the collisional-radiative theory approach. The synthesized carbon and oxygen lines exhibit a behavior significantly dependent on the variations of the plasma parameters inside the magnetic flux tube and therefore are suitable observational signatures of the processes giving rise to prominence condensations. In particular, a sizeable increase of line intensity as well as small blueshifts are expected from the loop footpoints during the first part of the evaporation phase that fills the loop with the material which subsequently condenses into the prominence. Once the condensation appears, line intensities decrease in the footpoints and simultaneously increase at the transition regions between the cool plasma of the condensation and the coronal portion of the loop. Line shifts are quite small in our symmetric model, and during most of the condensations lifetime, the nonthermal widths are relatively small. These results can be compared with detailed ultraviolet observations of filament/prominence regions obtained by recent space missions in order to test the model proposed for the formation of solar prominences.