New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of high-resolution X-ray spectra in which hydrogen- and helium-like ions are usually strong features. We present results from a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic data in the companion Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains the requisite atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. We compare the APEC results to other plasma codes for hydrogen- and helium-like diagnostics and test the sensitivity of our results to the number of levels included in the models. We find that dielectronic recombination with hydrogen-like ions into high (n = 6-10) principal quantum numbers affects some helium-like line ratios from low-lying (n = 2) transitions.

https://iopscience.iop.org/article/10.1086/322992/pdf

Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions

Many classes of transient solar phenomena, such as flares, flare sprays, and eruptive prominences, cause major disruptions in the magnetic geometry of the overlying corona. Typically, the results from Skylab indicate that pre-existing closed magnetic loops in the corona are torn open by the force of the disruption. We examine here some of the theoretical consequences to be expected during the extended relaxation phase which must follow such events. This phase is characterized by a gradual reconnection of the outward-distended field lines. In particular, the enhanced coronal expansion which occurs on open field lines just before they reconnect appears adequate to supply the large downward mass fluxes observed in Ha loop prominence systems that form during the post-transient relaxation. In addition, this enhanced flow may produce nonrecurrent high speed streams in the solar wind after such events. Calculations of the relaxation phase for representative field geometries and the resulting flow configurations are described.

Magnetic reconnection in the corona and the loop prominence phenomenon

A B ST R A C T Radiativecooling iscentralto a widerangeofastrophysicalproblem s.Despiteits im portance,cooling ratesaregenerally com puted using very restrictiveassum ptions, such as collisionalionization equilibrium and solar relative abundances.W e sim ul- taneously relax both assum ptions and investigate the eects ofphoto-ionization of heavy elem entsby them eta-galacticUV/X-ray background and ofvariationsin rela- tiveabundanceson thecoolingratesofopticallythin gasin ionization equilibrium .W e �nd thatphoto-ionization by them eta-galacticbackground radiation reducesthenet cooling ratesby up to an orderofm agnitudeforgasdensitiesand tem peraturestyp- icalofthe shock-heated intergalactic m edium and proto-galaxies(10 4 K< T < 10 6 K, �=hi < 100).In addition,photo-ionization changesthe relative contributionsofdif- ferentelem entstothecoolingrates.W econcludethatphoto-ionization by theionizing background and heavy elem entsboth need to be taken into accountin orderforthe cooling ratesto be correctto orderofm agnitude.M oreover,ifthe ratesneed to be known to better than a factor ofa few,then departures ofthe relative abundances from solar need to be taken into account.W e propose a m ethod to com pute cool- ing rateson an elem ent-by-elem entbasisby interpolating pre-com puted tables that take photo-ionization into account.W e provide such tables for a popular m odelof the evolving UV/X-ray background radiation,com puted using the photo-ionization package cloudy. K ey w ords: atom icprocesses| plasm as| coolingows| galaxies:form ation | intergalacticm edium

The effect of photoionization on the cooling rates of enriched, astrophysical plasmas

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections We also discuss flare soft X-ray spectroscopy and the energetics of the process The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations

An Observational Overview of Solar Flares

Steady-state radiative cooling rates for low-density, high-temperature plasmas

Ionization equilibrium and radiative cooling of high temperature low density plasma, noting cosmic gas cooling curve of line emission from oxygen ion transitions

Ionization equilibrium and radiative cooling of a low-density plasma.

Ratio of Line Intensities in Helium-Like Ions as a Density Indicator.

Solar coronal X ray spectrum calculation of high- temperature low-density plasma, considering line emission from electron collisional excitation and radiation

Radiation from a high-temperature, low-density plasma - The X-ray spectrum of the solar corona

Heating of solar active regions by magnetic energy dissipation: the steady- state case

We report on a Chandra observation of the Crab Nebula that gives the first clear view of the faint boundary of the Crab's X-ray-emitting pulsar wind nebula. There is structure in all directions. Fingers, loops, bays, and the south pulsar jet all indicate that either filamentary material or the magnetic field is controlling the relativistic electrons. In general, spectra soften as distance from the pulsar increases but do not change rapidly along linear features. This is particularly true for the pulsar jet. The termination of the jet is abrupt; the east side is close to an [O III] optical filament, which may be blocking propagation on this side. We argue that linear features have ordered magnetic fields and that the structure is determined by the synchrotron lifetime of particles diffusing perpendicular and parallel to the magnetic field. We find no significant evidence for thermal X-rays inside the filamentary envelope.

https://iopscience.iop.org/article/10.1086/508532/pdf

Wallace H. Tucker

Papers

Ionization equilibrium and radiative cooling of a low-density plasma.

Ratio of Line Intensities in Helium-Like Ions as a Density Indicator.

Radiation from a high-temperature, low-density plasma - The X-ray spectrum of the solar corona

Heating of solar active regions by magnetic energy dissipation: the steady- state case

Faint X-Ray Structure in the Crab Pulsar Wind Nebula