Ultraviolet Coronagraph Spectroscopy: A Key Capability for Understanding the Physics of Solar Wind Acceleration
Understanding the physical processes responsible for acce lerating the solar wind requires detailed measurements of the collisionless plasma in the extended solar corona. Some key clues about these processes have come from instruments that combine the power of an ultraviolet (UV) spectrometer with an occulted telescope. This combination enables measurements of ion emission lines far from the bright solar disk, where most of the solar wind acceleration occurs. Although the UVCS instrument on SOHO made several key discoveries, many questions remain unanswered because its capabilities were limited. This white paper summarizes these past achievements and also describes what can be accomplished with next-generation instrumentation of this kind. 1.  The hot, ionized outer atmosphere of the Sun is a unique testbed for the study of magnetohydrodynamics (MHD) and plasma physics, with ranges of parameters that are inaccessible on Earth. Although considerable progress has been made during the last few decades, we still do not know the basic physical processes responsible for heating the million-degree solar corona an d accelerating the solar wind. Identifying these processes is important not only for understanding the origi ns and impacts of space weather (e.g., Schwenn 2006; Eastwood 2008), but also for establishing a baseline of knowledge about a well-resolved star that is directly relevant to other astrophysical systems. In order to construct and test theoretical models, a wide ran ge of measurements of relevant plasma parameters must be available. In the low-density, open-field r egions that reach into interplanetary space, more parameters need to be measured (in comparison to collision-dominated regions) because the plasma becomes collisionless. In other words, individual particle species—e.g., protons , electrons, helium, and minor ions— can exhibit different properties from one another. Such dif ferences in particle velocity distributions are valuable probes of “microscopic” processes of heating and acceleration. In interplanetary space, such kinetic properties have been measured for decades by in situ particle instruments (e.g., Marsch 1999, 2006; Kasper et al. 2008). However, measurements in the near-Sun regions that are being actively heated and accelerated have been more limited in scope. Remote-sensing measurements of plasma properties in the so-called “extended solar corona” (above about 1.5 R⊙ measured from Sun-center) are difficult to make because the p hoton emission at large heights is fainter by many orders of magnitude than the emission from the solar disk. Standard telescopes, that do not explicitly block out the solar disk, typically contai n enough scattered light from the disk to totally mask the faint off-limb emission. Because the corona is highly ionized, the dominant spectral features are at wavelengths accessible only from space. For these reasons, a series of Ultraviolet Coronagraph Spectrometer (UVCS) instruments have been built and flown on rockets, on a S huttle-deployed Spartan payload, and as an instrument on the Solar and Heliospheric Observatory(SOHO) spacecraft; see, e.g., Kohl et al. (1978,

title: Deep Chandra Observations of the Crab-like Pulsar Wind Nebula G54.1+0.3 and Spitzer Spectroscopy of the Associated Infrared Shell
abstract: G54.1+0.3 is a young pulsar wind nebula (PWN), closely resembling the Crab, for which no thermal shell emission has been detected in X-rays. Recent Spitzer observations revealed an infrared (IR) shell containing a dozen point sources arranged in a ring-like structure, previously proposed to be young stellar objects. An extended knot of emission located in the NW part of the shell appears to be aligned with the pulsar's X-ray jet, suggesting a possible interaction with the shell material. Surprisingly, the IR spectrum of the knot resembles the spectrum of freshly formed dust in Cas A, and is dominated by an unidentified dust emission feature at 21 μm. The spectra of the shell also contain various emission lines and show that some are significantly broadened, suggesting that they originate in rapidly expanding supernova (SN) ejecta. We present the first evidence that the PWN is driving shocks into expanding SN ejecta and we propose an alternative explanation for the origin of the IR emission in which the shell is composed entirely of SN ejecta. In this scenario, the freshly formed SN dust is being heated by early-type stars belonging to a cluster in which the SN exploded. Simple dust models show that this interpretation can give rise to the observed shell emission and the IR point sources.

title: Molecular Cloud Formation behind Shock Waves
abstract: Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the moleculargas from the destructive effects of UVradiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, AV k0:7. The local ratio of atomic to molecular gas (4:1), coupled with short estimates for the lifetimes of molecular clouds (3‐5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12‐20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must benvk20 cm � 3 km s � 1 . In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures(gasplusmagneticpluscosmicrays).CoupledwiththerapidformationofCOonceshieldingissufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H2 formation that may be of use in numerical hydrodynamic calculations. Subject headingg ISM: clouds — ISM: evolution — ISM: kinematics and dynamics — ISM: molecules — shock waves — stars: formation

title: A DETAILED ANALYSIS OF A CYGNUS LOOP SHOCK-CLOUD INTERACTION
abstract: The XA region of the Cygnus Loop is a complex zone of radiative and nonradiatve shocks interacting with interstellar clouds. We combine five far ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra, and a high-resolution long-slit Hα spectrum to study the spatial emission line variations across the region. These spectral data are placed in context using ground-based optical emission-line images of the region and a far-UV image obtained by the Ultraviolet Imaging Telescope (UIT). The presence of high-ionization species (O VI, N V, and C IV) indicates a shock velocity near 170 km s-1 while other diagnostics indicate vsh ≈ 140 km s-1. It is likely that a large range of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing C IV λ1550, C III λ977, and C III] λ1909, we explore resonance scattering across the region. We find that a significant column depth is present at all positions, including those not near bright optical or UV filaments. Analysis of the O VI doublet ratio suggests an average optical depth of about unity in that ion, while flux measurements of [Si VIII] λ1443 suggest a hot component in the region at just below 106 K. Given the brightness of the O VI emission and the age of the interaction, we rule out the mixing-layer interpretation of the UV emission. Furthermore, we formulate a picture of the XA region as that of an encounter of the blast wave with a finger of dense gas protruding inward from the pre-supernova cavity.

title: OBSERVATIONS AND INTERPRETATION OF A LOW CORONAL SHOCK WAVE OBSERVED IN THE EUV BY THE SDO/AIA
abstract: Taking advantage of both the high temporal and spatial resolutions of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we studied a limb coronal shock wave and its associated extreme ultraviolet (EUV) wave that occurred on 2010 June 13. Our main findings are: (1) the shock wave appeared clearly only in the channels centered at 193 angstrom and 211 angstrom as a dome-like enhancement propagating ahead of its associated semi-spherical coronal mass ejection (CME) bubble; (2) the density compression of the shock is 1.56 according to radio data and the temperature of the shock is around 2.8 MK; (3) the shock wave first appeared at 05: 38 UT, 2 minutes after the associated flare has started and 1 minute after its associated CME bubble appeared; (4) the top of the dome-like shock wave set out from about 1.23 R-circle dot and the thickness of the shocked layer is similar to 2 x 10(4) km; (5) the speed of the shock wave is consistent with a slight decrease from about 600 km s(-1) to 550 km s(-1); and (6) the lateral expansion of the shock wave suggests a constant speed around 400 km s(-1), which varies at different heights and directions. Our findings support the view that the coronal shock wave is driven by the CME bubble, and the on-limb EUV wave is consistent with a fast wave or at least includes the fast wave component.

title: THE SOLAR WIND CHARGE-TRANSFER X-RAY EMISSION IN THE 1/4 keV ENERGY RANGE: INFERENCES ON LOCAL BUBBLE HOT GAS AT LOW Z
abstract: We present calculations of the heliospheric solar wind charge-exchange (SWCX) emission spectra and the resulting contributions of this diffuse background in the ROSAT 1/4 keV bands. We compare our results with the soft X-ray background (SXRB) emission detected in front of 378 identified shadowing regions during the ROSAT All-Sky Survey. This foreground component is principally attributed to the hot gas of the so-called Local Bubble (LB), an irregularly shaped cavity of ~50-150 pc around the Sun, which is supposed to contain ~106 K plasma. Our results suggest that the SWCX emission from the heliosphere is bright enough to account for most of the foreground emission toward the majority of low galactic latitude directions, where the LB is the least extended. On the other hand, in a large part of directions with galactic latitude above 30°, the heliospheric SWCX intensity is significantly smaller than the measured one. However, the SWCX R2/R1 band ratio differs slightly from the data in the galactic center direction, and more significantly in the galactic anticentre direction where the observed ratio is the smallest. Assuming that both SWCX and hot gas emission are present and their relative contributions vary with direction, we tested a series of thermal plasma spectra for temperatures ranging from 10 5 to 10 6.5 K and searched for a combination of SWCX spectra and thermal emission matching the observed intensities and band ratios, while simultaneously being compatible with O VI emission measurements. In the frame of collisional equilibrium models and for solar abundances, the range we derive for hot gas temperature and emission measure cannot reproduce the Wisconsin C/B band ratio. This implies that accounting for SWCX contamination does not remove these known disagreements between data and classical hot gas models. We emphasize the need for additional atomic data, describing consistently EUV and X-ray photon spectra of the charge-exchange emission of heavier solar wind ions.

title: Radiation effects in detector electronics
abstract: Radiation effects produced in electronics for detectors operated at intense high-energy particle accelerators are reviewed. Both displacement damage in semiconductor material and long-lived ionization effects produced by charge trapping in dielectrics near interfaces are potentially serious. Hardening and test methods must recognize the particular circuit's functional requirements. The dominant defects and appropriate hardening method depend on the important parameter (e.g., offset, leakage, noise) as well as the operating frequency.