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Register a new userModeling magnetohydrodynamics and non equilibrium SoHO/UVCS line emission of CME shocks
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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 and 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.
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Outflows are a pervasive feature of mechanical feedback from super star clusters (SSC) in starburst galaxies, playing a fundamental role in galaxy evolution. Observations are now starting to confirm that outflows can undergo catastrophic cooling, suppressing adiabatic superwinds. Here we present a suite of one-dimensional, hydrodynamic simulations that study the ionization structure of these outflows and the resulting line emission generated by the cooling gas. We use the non-equilibrium atomic chemistry package within MAIHEM, our modified version of FLASH, which evolves the ionization state of the gas and computes the total cooling rate on an ion-by-ion basis. We find that catastrophically cooling models produce strong nebular line emission compared to adiabatic outflows. We also show that such models exhibit non-equilibrium conditions, thereby generating more highly ionized states than equivalent equilibrium models. When including photoionization from the parent SSC, catastrophically cooling models show strong C IV {lambda}1549 and O VI {lambda}1037 emission. For density bounded photoionization, He II {lambda}1640, {lambda}4686, C III] {lambda}1908, Si IV {lambda}1206, and Si III {lambda}1400 are also strongly enhanced. These lines are seen in extreme starbursts where catastrophic cooling is likely to occur, suggesting that they may serve as diagnostics of such conditions. The higher ionization generated by these flows may help to explain line emission that cannot be attributed to SSC photoionization alone.
Ultraviolet spectra of the extended solar corona have been routinely obtained by SoHO/UVCS since 1996. Sudden variations of spectral parameters are mainly due to the detection of Coronal Mass Ejections (CMEs) crossing the instrumental slit. We present a catalog of CME ultraviolet spectra based upon a systematic search of events in the LASCO CME catalog, and we discuss their statistical properties. Our catalog includes 1059 events through the end of 2005, covering nearly a full solar cycle. It is online available at the URL http://solarweb.oato.inaf.it/UVCS_CME and embedded in the online LASCO CME catalog (http://cdaw.gsfc.nasa.gov/CME_list). The emission lines observed provide diagnostics of CME plasma parameters, such as the light-of-sight velocity, density and temperature and allow to link the CME onset data to the extended corona white-light images. The catalog indicates whether there are clear signatures of features such as shock waves, current sheets, O VI flares, helical motions and which part of the CME structures (front, cavity or prominence material) are detected. The most common detected structure is the cool prominence material (in about 70% of the events). For each event, the catalog also contains movie, images, plots and information relevant to address detailed scientific investigations. The number of events detected in UV is about 1/10 of the LASCO CMEs, and about 1/4 of the halo events. We find that UVCS tends to detect faster, more massive and energetic CME than LASCO and for about 40% of the events events it has been possible to determine the plasma light-of-sight velocity.
We report on the properties of halo coronal mass ejections (HCMEs) in solar cycles 23 and 24. We compare the HCMEs properties between the corresponding phases (rise, maximum, and declining) in cycles 23 and 24 in addition to comparing those between the whole cycles. Despite the significant decline in the sunspot number (SSN) in cycle 24, which dropped by 46% with respect to cycle 23, the abundance of HCMEs is similar in the two cycles. The HCME rate per SSN is 44% higher in cycle 24. In the maximum phase, cycle-24 rate normalized to SSN increased by 127% while the SSN dropped by 43%. The source longitudes of cycle-24 HCMEs are more uniformly distributed than those in cycle 23. We found that the average sky-plane speed in cycle 23 is ~16% higher than that in cycle 24. The size distributions of the associated flares between the two cycles and the corresponding phases are similar. The average speed at a central meridian distance (CMD) = 600 for cycle 23 is ~28% higher than that of cycle 24. We discuss the unusual bump in HCME activity in the declining phase of cycle 23 as due to exceptional active regions that produced many CMEs during October 2003 to October 2005. The differing HCME properties in the two cycles can be attributed to the anomalous expansion of cycle-24 CMEs. Considering the HCMEs in the rise, maximum and declining phases, we find that the maximum phase shows the highest contrast between the two cycles.
The radiation magnetohydrodynamics (RMHD) system couples the ideal magnetohydrodynamics equations with a gray radiation transfer equation. The main challenge is that the radiation travels at the speed of light while the magnetohydrodynamics changes with the time scale of the fluid. The time scales of these two processes can vary dramatically. In order to use mesh sizes and time steps that are independent of the speed of light, asymptotic preserving (AP) schemes in both space and time are desired. In this paper, we develop an AP scheme in both space and time for the RMHD system. Two different scalings are considered. One results in an equilibrium diffusion limit system, while the other results in a non-equilibrium system. The main idea is to decompose the radiative intensity into three parts, each part is treated differently with suitable combinations of explicit and implicit discretizations guaranteeing the favorable stability conditionand computational efficiency. The performance of the AP method is presented, for both optically thin and thick regions, as well as for the radiative shock problem.
The Interface Region Imaging Spectrograph (IRIS) routinely observes the Si IV resonance lines. When analyzing observations of these lines it has typically been assumed they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron beam driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres we solved the radiation transfer equations to obtain the non-LTE, non-equilibrium populations, line profiles, and opacities for a model Silicon atom, including charge exchange processes. This was achieved using the `minority species version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si IV at cooler temperatures than those predicted by ionisation equilibrium. All simulations with an injected energy flux $F>5times10^{10}$ erg cm$^{-2}$ s$^{-1}$ resulted in optical depth effects on the Si IV emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to $Fapprox5times10^{9}$ erg cm$^{-2}$ s$^{-1}$) also resulted in Si IV emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of $5times10^{-6}$ g cm$^{-2}$ over the temperature range $40$ to $100$ kK, and the Si IV formation temperatures were between $30$ and $60$ kK. We urge caution when analyzing Si IV flare observations, or when computing synthetic emission without performing a full radiation transfer calculation.
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