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 propor
tional 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.
In this review we discuss some observational aspects and theoretical models of astrophysical collisionless shocks in partly ionized plasma with the presence of non-thermal components. A specific feature of fast strong collisionless shocks is their ab
ility to accelerate energetic particles that can modify the shock upstream flow and form the shock precursors. We discuss the effects of energetic particle acceleration and associated magnetic field amplification and decay in the extended shock precursors on the line and continuum multi-wavelength emission spectra of the shocks. Both Balmer-type and radiative astrophysical shocks are discussed in connection to supernova remnants interacting with partially neutral clouds. Quantitative models described in the review predict a number of observable line-like emission features that can be used to reveal the physical state of the matter in the shock precursors and the character of nonthermal processes in the shocks. Implications of recent progress of gamma-ray observations of supernova remnants in molecular clouds are highlighted.
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-l
imb 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.
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.
We provide a guideline to interpret the UVCS emission lines (in particular O VI and Si XII) during shock wave propagation in the outer solar corona. We use a numerical MHD model performing a set of simulations of shock waves generated in the corona a
nd from the result we compute the plasma emission for the O VI and Si XII including the effects of NEI. We analyze the radiative and spectral properties of our model with the support of a detailed radiation model including Doppler dimming and an analytical model for shocks, and, finally, we synthesize the expected O VI 1032A line profile. We explain several spectral features of the observations like the absence of discontinuities in the O VI emission during the shock passage, the brightening of Si XII emission and the width of the lines. We use our model also to give very simple and general predictions for the strength of the line wings due to the ions shock heating and on the line shape for Limb CMEs or Halo CMEs. The emission coming from post-shock region in the solar corona roughly agrees with the emission from a simple planar and adiabatic shock, but the effect of thermal conduction and the magnetic field may be important depending on the event parameters. Doppler dimming significantly influences the O VI emission while Si XII line brightens mainly because of the shock compression. Significant shock heating is responsible for the wide and faint component of the O VI line usually observed which may be taken as a shock signature in the solar corona.
