G. Y. Liang

R-matrix electron-impact excitation data for the Ne-like iso-electronic sequence

We have carried-out parallel intermediate-coupling frame transformation R-matrixcalculations for electron-impact excitation amongst the 204 close-coupling levels of the 2s x 2p y (x + y = 3), 2s 2 {3,4}l ,2 s2p{3,4}l, and 2p 2 3l configurations for all boron-like ions from C + to Kr 31+ . We have also included the configuration interaction due to the 2p 2 4l ,2 s3l3land 2p3s3l configurations. A detailed comparison has been made of the target structure and excitation data for four specific ions (viz., Ne 5+ ,A r 13+ ,F e 21+ and Kr 31+ )t hat span the sequence, so as to assess the accuracy over the entire sequence. Effective collision strengths (Ys) are presented at temperatures ranging from 2 ×10 2 (z +1) 2 Kt o 2×10 6 (z +1) 2 K( wherez is the residual charge of the ions, i.e. Z −5). Detailed comparisons for the (effective) collision strengths (Ys) Ω are made with the results of previous calculations for the four representative ions. Furthermore, we examine the iso-electronic trends of the effective collision strengths as a function of temperature.

R-matrix electron-impact excitation data for B-like Si and its application in cool stars

We present the results of an intermediate coupling frame-transformation R-matrix calculation for the electron-impact excitation of Si 9+ . The target and close-coupling expansions are both taken to be the 125 fine-structure levels (58 LS terms) belonging to the configurations 2s x 2p y (x + y = 3) and 2s α 2p β 3l (α+ β = 2, l = s, p, and d). Due to the additional resonances included in our calculation, we find significant differences at low temperatures with the widely used n = 2 → 2 excitation rates, also obtained with the R-matrix method, as well as with the n = 2 → 3 excitation rates calculated by using the distorted wave (DW) approximation. We present a list of prominent transition lines and comparisons with SERTS and Hinode/EIS EUV spectra of the solar corona, SUMER observations for the quiet sun, as well as Chandra LETG and Rocket soft X-ray spectra of the Procyon corona and solar flares, respectively. Line emissivities of some transitions are enhanced up to 40% when compared with those obtained from using the previous atomic data at the same electron density (1.6 × 10 9 cm −3 ) and temperature (1.3 × 10 6 K). The comparison with Chandra LETG observation of Procyon reveals that the 3s−2p line flux was significantly underestimated (by a factor of 4−5) in previous analyses. Some EUV and soft X-ray emission line ratios are ne-sensitive and T e-insensitive. Estimated electron densities from them shift downwards due to the new resonant-enhanced excitation data used in the present modelling.

R-matrix inner-shell electron-impact excitation of the Na-like iso-electronic sequence

We present results for the inner-shell electron-impact excitation of all Na-like ions from Mg+ to Kr25+ obtained using the intermediate-coupling frame transformation R-matrix method with both Auger and radiation damping included via the optical potential approach. For each ion, the target and close-coupling expansions are taken to be the 59 LS terms (134 levels) belonging to configurations 2s22p63l, 2s22p53s3l (l s, p, d), 2s22p53p2 and 2s22p53p3d. Radial wavefunctions were obtained using autostructure. Effective collision strengths are presented at temperatures ranging from 2 ? 102(q + 1)2 K to 2 ? 106(q + 1)2 K (where q is the residual charge of ions, i.e. Z ? 11). Comparisons for the collision strengths and effective collision strengths are made with the results of other calculations for several ions which span the sequence. The Auger and radiation damping effects along the sequence have also been explored and their importance is highlighted. We further examine iso-electronic trends of both low- and high-temperature effective collision strengths.

Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coil target

We demonstrate a novel plasma device for magnetic reconnection, driven by Gekko XII lasers irradiating a double-turn Helmholtz capacitor-coil target. Optical probing revealed an accumulated plasma plume near the magnetic reconnection outflow. The background electron density and magnetic field were measured to be approximately 1018 cm−3 and 60 T by using Nomarski interferometry and the Faraday effect, respectively. In contrast with experiments on magnetic reconnection constructed by the Biermann battery effect, which produced high beta values, our beta value was much lower than one, which greatly extends the parameter regime of laser-driven magnetic reconnection and reveals its potential in astrophysical plasma applications.

Atomic processes for astrophysical plasmas

In this review we summarize the recent calculations and improvements of atomic data that we have carried out for the analysis of astrophysical spectroscopy within the Atomic Processes for Astrophysical Plasmas (APA P) network. We briefly discuss the various methods used for the calculations, and highlight several issues that we have uncovered during such extensive work . We discuss the completeness and accuracy of the cross sections for ionic excitation by electron impact for the main isoelectronic sequences, which we have obtained with large-scale calculations. Given its astrophysical importance, we emphasize the work on iron. Some examples on the significant improvement that has been achieved over previous calculations are provided.

SOFT X-RAY EMISSION LINES OF S VII-S XIV IN PROCYON

Observational data for cool star Procyon available from the Chandra Data Public Archive are co-added and analyzed with as high a signal-to-noise ratio as possible. The soft X-ray emission lines of highly charged sulfur ions (S VII-S XV) are investigated in the 30-80 A range. A collisional-radiative model is constructed to predict line emissivities of sulfur ions using updated excitation data from the R-matrix method. Theoretical line fluxes and line intensity ratios are calculated, and theoretical spectra are constructed with a Gaussian profile with a line width of 0.06 A. By comparing predicted emission lines with observed ones, several strong emission lines are identified for the first time. Some misassignments of lines in previous works are also corrected. By comparing our results with those from the Chianti (v6) model, this work provides insight into the completeness and accuracy of the atomic data of sulfur ions in the Chianti (v6) database.

R-matrix electron-impact excitation data for astrophysically abundant sulphur ions

We present results for the electron-impact excitation of highly-charged sulphur ions (S 8+ –S 11+ ) obtained using the intermediatecoupling frame transformation R-matrix approach. A detailed comparison of the target structure has been made for the four ions to

Relativistic electrons produced by reconnecting electric fields in a laser-driven bench-top solar flare

Laboratory experiments have been carried out to model the magnetic reconnection process in a solar flare with powerful lasers. Relativistic electrons with energy up to megaelectronvolts are detected along the magnetic separatrices bounding the reconnection outflow, which exhibit a kappa-like distribution with an effective temperature of ~109 K. The acceleration of non-thermal electrons is found to be more efficient in the case with a guide magnetic field (a component of a magnetic field along the reconnection-induced electric field) than in the case without a guide field. Hardening of the spectrum at energies ≥500 keV is observed in both cases, which remarkably resembles the hardening of hard X-ray and γ-ray spectra observed in many solar flares. This supports a recent proposal that the hardening in the hard X-ray and γ-ray emissions of solar flares is due to a hardening of the source-electron spectrum. We also performed numerical simulations that help examine behaviors of electrons in the reconnection process with the electromagnetic field configurations occurring in the experiments. The trajectories of non-thermal electrons observed in the experiments were well duplicated in the simulations. Our numerical simulations generally reproduce the electron energy spectrum as well, except for the hardening of the electron spectrum. This suggests that other mechanisms such as shock or turbulence may play an important role in the production of the observed energetic electrons.

ELECTRON DENSITY DIAGNOSTIC FOR HOT PLASMAS IN THE CORONAL REGIME BY USING B-LIKE IONS

A line ratio of 3d-2p transition lines in boron-like spectra of Si X, S XII, Ar XIV, and Fe XXII has been investigated. Collisional-radiative model calculations reveal that the line ratio is sensitive to the electron density in ranges of n e = 4.0 × 107-3.0 × 1010 cm–3, 4.0 × 108-3.0 × 1011 cm–3, 3.0 × 109-4.0 × 1012 cm–3, and 2.0 × 1012-3.0 × 1015 cm–3, respectively. This complements the K-shell diagnostics of helium-like ions. By a comparison between the predicted and the measured values, effective electron densities in the electron beam ion trap (EBIT) plasmas, performed by Lepson and collaborators at the Lawrence Livermore EBIT, are estimated to be n e = 3.4+0.8 –0.6 × 1010 cm–3 and 5.6+1.0 –1.1 × 1010 cm–3 for sulfur and argon plasmas, respectively. In the case of argon, a good agreement is shown with the actual electron density derived from an N VI K-shell spectrum. We further explore the 3d-2p transition lines of Si X and S XII in the stellar coronal spectra measured with the low energy transmission grating spectrometer combined with a high resolution camera on board the Chandra X-Ray Observatory. The constrained electron densities show a good agreement with those determined from C V and O VII K-shell spectra.

R-matrix inner-shell electron-impact excitation of Fe15+ including Auger-plus-radiation damping

We present results for the inner-shell electron-impact excitation of Fe15+ using the intermediate-coupling frame transformation R-matrix approach in which Auger-plus-radiation damping has been included. The target and close-coupling expansions are both taken to be the 134 levels belonging to the configurations 2s22p63l, 2s22p53s3l, 2s22p53p2 and 2s22p53p3d. The comparison of Maxwell-averaged effective collision strengths with and without damping shows that the damping reduction is about 30-40% for many transitions at low temperatures, but up to 80% for a few transitions. As a consequence, the results of previous Dirac R-matrix calculations (Aggarwal and Keenan 2008 J. Phys. B: At. Mol. Opt. Phys. 41 015701) overestimate the effective collision strengths due to their omission of Auger-plus-radiation damping.