Carbon, Helium and Proton Kinetic Temperatures in a Cygnus Loop Shock Wave

Observations of SN1006 have shown that ions and electrons in the plasma behind fast supernova remnant shock waves are far from equilibrium, with the electron temperature much lower than the proton temperature and ion temperatures approximately proportional to ion mass. In the ~360 km/s shock waves of the Cygnus Loop, on the other hand, electron and ion temperatures are roughly equal, and there is evidence that the oxygen kinetic temperature is not far from the proton temperature. In this paper we report observations of the He II lambda 1640 line and the C IV lambda 1550 doublet in a 360 km/s shock in the Cygnus Loop. While the best fit kinetic temperatures are somewhat higher than the proton temperature, the temperatures of He and C are consistent with the proton temperature and the upper limits are 0.5 and 0.3 times the mass-proportional temperatures, implying efficient thermal equilibration in this collisionless shock. The equilibration of helium and hydrogen affects the conversion between proton temperatures determined from H alpha line profiles and shock speeds, and that the efficient equilibration found here reduces the shock speed estimates and the distance estimate to the Cygnus Loop of Medina et al. (2014) to about 800 pc.

Grain Destruction in a Supernova Remnant Shock Wave

Dust grains are sputtered away in the hot gas behind shock fronts in supernova remnants, gradually enriching the gas phase with refractory elements. We have measured emission in C IV $lambda$1550 from C atoms sputtered from dust in the gas behind a non-radiative shock wave in the northern Cygnus Loop. Overall, the intensity observed behind the shock agrees approximately with predictions from model calculations that match the Spitzer 24 micron and the X-ray intensity profiles. Thus these observations confirm the overall picture of dust destruction in SNR shocks and the sputtering rates used in models. However, there is a discrepancy in that the CIV intensity 10 behind the shock is too high compared to the intensities at the shock and 25 behind it. Variations in the density, hydrogen neutral fraction and the dust properties over parsec scales in the pre-shock medium limit our ability to test dust destruction models in detail.

Transition Region Emission from Solar Flares during the Impulsive Phase

There are relatively few observations of UV emission during the impulsive phases of solar flares, so the nature of that emission is poorly known. Photons produced by solar flares can resonantly scatter off atoms and ions in the corona. Based on off-limb measurements by SOHO/UVCS, we derive the O VI $lambda$1032 luminosities for 29 flares during the impulsive phase and the Ly$alpha$ luminosities of 5 flares, and we compare them with X-ray luminosities from GOES measurements. The upper transition region and lower transition region luminosities of the events observed are comparable. They are also comparable to the luminosity of the X-ray emitting gas at the beginning of the flare, but after 10-15 minutes the X-ray luminosity usually dominates. In some cases we can use Doppler dimming to estimate flow speeds of the O VI emitting gas, and 5 events show speeds in the 40 to 80 $rm km s^{-1}$ range. The O VI emission could originate in gas evaporating to fill the X-ray flare loops, in heated chromospheric gas at the footpoints, or in heated prominence material in the coronal mass ejection. All three sources may contribute in different events or even in a single event, and the relative timing of UV and X-ray brightness peaks, the flow speeds, and the total O VI luminosity favor each source in one or more events.

Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves

Many fast supernova remnant shocks show spectra dominated by Balmer lines. The H$alpha$ profiles have a narrow component explained by direct excitations and a thermally Doppler broadened component due to atoms that undergo charge exchange in the post-shock region. However, the standard model does not take into account the cosmic-ray shock precursor, which compresses and accelerates plasma ahead of the shock. In strong precursors with sufficiently high densities, the processes of charge exchange, excitation and ionization will affect the widths of both narrow and broad line components. Moreover, the difference in velocity between the neutrals and the precursor plasma gives rise to frictional heating due to charge exchange and ionization in the precursor. In extreme cases, all neutrals can be ionized by the precursor. In this paper we compute the ion and electron heating for a wide range of shock parameters, along with the velocity distribution of the neutrals that reach the shock. Our calculations predict very large narrow component widths for some shocks with efficient acceleration, along with changes in the broad- to-narrow intensity ratio used as a diagnostic for the electron-ion temperature ratio. Balmer lines may therefore provide a unique diagnostic of precursor properties. We show that heating by neutrals in the precursor can account for the observed H$alpha$ narrow component widths, and that the acceleration efficiency is modest in most Balmer line shocks observed thus far.

Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks

The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the post-shock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular, neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the H alpha line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from H alpha profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tychos supernova remnant.