Nicholas A. Murphy

Prevalence of small-scale jets from the networks of the solar transition region and chromosphere

As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.

NUMERICAL EXPERIMENTS ON FINE STRUCTURE WITHIN RECONNECTING CURRENT SHEETS IN SOLAR FLARES

We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibrium that separates two regions of the magnetic field with opposite polarity which are line-tied at the lower boundary representing the photosphere. Reconnection commences gradually due to an initially imposed perturbation, but becomes faster when plasmoids form and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. Among these X-points, a principal one exists at which the reconnection rate reaches maximum. A fluid stagnation point (S-point) in the sheet appeared where the reconnection outflow bifurcates. The S-point and the principal X-point (PX-point) are not co-located in space though they are very close to one another. Their relative positions alternate as reconnection progresses and determine the direction of motion of individual magnetic islands. Newly formed islands move upward if the S-point is located above the PX-point, and downward if the S-point is below the PX-point. Merging of magnetic islands was observed occasionally between islands moving in the same direction. Reconnected plasma flow was observed to move faster than blobs nearby.

Asymmetric Magnetic Reconnection in Solar Flare and Coronal Mass Ejection Current Sheets

We present two-dimensional resistive magnetohydrodynamic simulations of line-tied asymmetric magnetic reconnection in the context of solar flare and coronal mass ejection current sheets. The reconnection process is made asymmetric along the inflow direction by allowing the initial upstream magnetic field strengths and densities to differ, and along the outflow direction by placing the initial perturbation near a conducting wall boundary that represents the photosphere. When the upstream magnetic fields are asymmetric, the post-flare loop structure is distorted into a characteristic skewed candle flame shape. The simulations can thus be used to provide constraints on the reconnection asymmetry in post-flare loops. More hard X-ray emission is expected to occur at the footpoint on the weak magnetic field side because energetic particles are more likely to escape the magnetic mirror there than at the strong magnetic field footpoint. The footpoint on the weak magnetic field side is predicted to move more quickly because of the requirement in two dimensions that equal amounts of flux must be reconnected from each upstream region. The X-line drifts away from the conducting wall in all simulations with asymmetric outflow and into the strong magnetic field region during most of the simulations with asymmetric inflow. There is net plasma flow across the X-line for both the inflow and outflow directions. The reconnection exhaust directed away from the obstructing wall is significantly faster than the exhaust directed toward it. The asymmetric inflow condition allows net vorticity in the rising outflow plasmoid which would appear as rolling motions about the flux rope axis.

Plasma Heating During a Coronal Mass Ejection Observed By the Solar and Heliospheric Observatory

We perform a time-dependent ionization analysis to constrain plasma heating requirements during a fast partial halo coronal mass ejection (CME) observed on 2000 June 28 by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO). We use two methods to derive densities from the UVCS measurements, including a density sensitive O V line ratio at 1213.85 and 1218.35 A, and radiative pumping of the O VI λλ1032, 1038 doublet by chromospheric emission lines. The most strongly constrained feature shows cumulative plasma heating comparable to or greater than the kinetic energy, while features observed earlier during the event show plasma heating of order or less than the kinetic energy. SOHO Michelson Doppler Imager observations are used to estimate the active region magnetic energy. We consider candidate plasma heating mechanisms and provide constraints when possible. Because this CME was associated with a relatively weak flare, the contribution from flare energy (e.g., through thermal conduction or energetic particles) is probably small; however, the flare may have been partially behind the limb. Wave heating by photospheric motions requires heating rates to be significantly larger than those previously inferred for coronal holes, but the eruption itself could drive waves that heat the plasma. Heating by small-scale reconnection in the flux rope or by the CME current sheet is not significantly constrained. UVCS line widths suggest that turbulence must be replenished continually and dissipated on timescales shorter than the propagation time in order to be an intermediate step in CME heating.We perform a time-dependent ionization analysis to constrain plasma heating requirements during a fast partial halo coronal mass ejection (CME) observed on 2000 June 28 by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO). We use two methods to derive densities from the UVCS measurements, including a density sensitive O V line ratio at 1213.85 and 1218.35 Angstroms, and radiative pumping of the O VI 1032,1038 doublet by chromospheric emission lines. The most strongly constrained feature shows cumulative plasma heating comparable to or greater than the kinetic energy, while features observed earlier during the event show cumulative plasma heating of order or less than the kinetic energy. SOHO Michelson Doppler Imager (MDI) observations are used to estimate the active region magnetic energy. We consider candidate plasma heating mechanisms and provide constraints when possible. Because this CME was associated with a relatively weak flare, the contribution by flare energy (e.g., through thermal conduction or energetic particles) is probably small; however, the flare may have been partially behind the limb. Wave heating by photospheric motions requires heating rates significantly larger than those previously inferred for coronal holes, but the eruption itself could drive waves which heat the plasma. Heating by small-scale reconnection in the flux rope or by the CME current sheet is not significantly constrained. UVCS line widths suggest that turbulence must be replenished continually and dissipated on time scales shorter than the propagation time in order to be an intermediate step in CME heating.

THE KECK APERTURE-MASKING EXPERIMENT: NEAR-INFRARED SIZES OF DUSTY WOLF-RAYET STARS

We report the results of a high angular resolution near-infrared survey of dusty Wolf-Rayet stars using the Keck I Telescope, including new multiwavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112. Angular sizes were measured for an additional eight dusty Wolf-Rayet stars using aperture-masking interferometry, allowing us to probe characteristic sizes down to � 20 mas (� 40 AU for typical sources). With angular sizes and specific fluxes, we can directly measure the wavelength-dependent surface brightness and size relations for our sample. We discovered tight correlations of these properties within our sample that could not be explained by simple spherically symmetric dust shells or even the more realistic ‘‘pinwheel nebula’’ (three-dimensional) radiative transfer model, when using Zubko’s optical constants. While the tightly correlated surface brightness relations we uncovered offer compelling indirect evidence of a shared and distinctive dust shell geometry among our sample, long-baseline interferometers should target the marginally resolved objects in our sample in order to conclusively establish the presence or absence of the putative underlying colliding-wind binaries thought to produce the dust shells around WC WolfRayet stars. Subject headingg binaries: general — circumstellar matter — instrumentation: interferometers —

On the design of a new CPU architecture for pedagogical purposes

Ant-32 is a new processor architecture designed specifically to address the pedagogical needs of teaching many subjects, including assembly language programming, machine architecture, compilers, operating systems, and VLSI design. This paper discusses our motivation for creating Ant-32 and the philosophy we used to guide our design decisions and gives a high-level description of the resulting design.

Global axisymmetric simulations of two-fluid reconnection in an experimentally relevant geometry

To address the interplay between local and global effects in magnetic reconnection, axisymmetric numerical simulations for the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] are performed using the NIMROD code [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)]. The “pull” and “push” modes of the device are simulated both with and without two-fluid effects in the generalized Ohm’s law. As in experiment, the pull reconnection rate is slowed due to the presence of downstream pressure associated with the outflow. Effects induced by toroidicity include a radially inward drift of the current sheet during pull reconnection and a radially outward displacement of the X-point during push reconnection. These effects result from the inboard side of the current sheet having less volume than the outboard side, facilitating the formation of large scale pressure gradients since the inboard side is more susceptible to a buildup or depletion of density. Toroidicity also leads to asy...

Resistive magnetohydrodynamic simulations of X-line retreat during magnetic reconnection

To investigate the impact of current sheet motion on the reconnection process, we perform resistive magnetohydrodynamic simulations of two closely located reconnection sites that move apart from each other as reconnection develops. This simulation develops less quickly than an otherwise equivalent single perturbation simulation but eventually exhibits a higher reconnection rate. The unobstructed outflow jets are faster and longer than the outflow jets directed toward the magnetic island that forms between the two current sheets. The X-line and flow stagnation point are located near the trailing end of each current sheet very close to the obstructed exit. The speed of X-line retreat ranges from ∼0.02–0.06, while the speed of stagnation point retreat ranges from ∼0.03–0.07 in units of the initial upstream Alfven velocity. Early in time, the flow stagnation point is located closer to the center of the current sheet than the X-line, but later on the relative positions of these two points switch. Consequently,...

NON-EQUILIBRIUM IONIZATION MODELING OF THE CURRENT SHEET IN A SIMULATED SOLAR ERUPTION

The current sheet that extends from the top of flare loops and connects to an associated flux rope is a common structure in models of coronal mass ejections (CMEs). To understand the observational properties of CME current sheets, we generated predictions from a flare/CME model to be compared with observations. We use a simulation of a large-scale CME current sheet previously reported by Reeves et al. This simulation includes ohmic and coronal heating, thermal conduction, and radiative cooling in the energy equation. Using the results of this simulation, we perform time-dependent ionization calculations of the flow in a CME current sheet and construct two-dimensional spatial distributions of ionic charge states for multiple chemical elements. We use the filter responses from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory and the predicted intensities of emission lines to compute the count rates for each of the AIA bands. The results show differences in the emission line intensities between equilibrium and non-equilibrium ionization. The current sheet plasma is underionized at low heights and overionized at large heights. At low heights in the current sheet, the intensities of the AIA 94 angstrom and 131 angstrom channels are lower for non-equilibrium ionization than for equilibrium ionization. At large heights, these intensities are higher for non-equilibrium ionization than for equilibrium ionization inside the current sheet. The assumption of ionization equilibrium would lead to a significant underestimate of the temperature low in the current sheet and overestimate at larger heights. We also calculate the intensities of ultraviolet lines and predict emission features to be compared with events from the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory, including a low-intensity region around the current sheet corresponding to this model.

The Effects of Clumps in Explaining X-Ray Emission Lines from Hot Stars

It is now well established that stellar winds of hot stars are fragmentary and that the X-ray emission from stellar winds has a strong contribution from shocks in winds. Chandra high spectral resolution observations of line profiles of O and B stars have shown numerous properties that had not been expected. Here we suggest explanations by considering the X-rays as arising from bow shocks that occur where the stellar wind impacts on spherical clumps in the winds. We use an accurate and stable numerical hydrodynamic code to obtain steady state physical conditions for the temperature and density structure in a bow shock. We use these solutions plus analytic approximations to interpret some major X-ray features: the simple power-law distribution of the observed emission measure derived from many hot star X-ray spectra and the wide range of ionization stages that appear to be present in X-ray sources throughout the winds. Also associated with the adiabatic cooling of the gas around a clump is a significant transverse velocity for the hot plasma flow around the clumps, and this can help to understand anomalies associated with observed line widths, and the differences in widths seen in stars with high and low mass-loss rates. The differences between bow shocks and the planar shocks that are often used for hot stars are discussed. We introduce an on the shock approximation that is useful for interpreting the X-rays and the consequences of clumps in hot star winds and elsewhere in astronomy.