38 research outputs found
Charge exchange and ionisation in N, N, C - H() collisions studied systematically by theoretical approaches
The introduction of gases like nitrogen or neon for cooling the edge region
of magnetically confined fusion plasmas has triggered a renewed interest in
state selective cross sections necessary for plasma diagnostics by means of
charge exchange recombination spectroscopy. To improve the quality of
spectroscopic data analysis, charge exchange and ionisation cross sections for
N + H() have been calculated using two different theoretical
approaches, namely the atomic-orbital close-coupling method and the classical
trajectory Monte Carlo method. Total and state resolved charge exchange cross
sections are analysed in detail.
In the second part, we compare two collision systems involving equally
charged ions, C and N on atomic hydrogen. The analysis of the
data lead to the conclusion that deviations between these two impurity ions are
practically negligible. This finding is very helpful when calculating cross
sections for collision systems with heavier not completely stripped impurity
ions.Comment: 21 pages, 10 figures, 6 data table
Non-LTE calculations for neutral Na in late-type stars using improved atomic data
Neutral sodium is a minority species in the atmospheres of late-type stars,
and line formation in local thermodynamic equilibrium (LTE) is often a poor
assumption, in particular for strong lines. We present an extensive grid of
non-LTE calculations for several NaI lines in cool stellar atmospheres,
including metal-rich and metal-poor dwarfs and giants. For the first time, we
constructed a Na model atom that incorporates accurate quantum mechanical
calculations for collisional excitation and ionisation by electrons as well as
collisional excitation and charge exchange reactions with neutral hydrogen.
Similar to LiI, the new rates for hydrogen impact excitation do not affect the
statistical equilibrium calculations, while charge exchange reactions have a
small but non-negligible influence. The presented LTE and non-LTE
curves-of-growth can be interpolated to obtain non-LTE abundances and abundance
corrections for arbitrary stellar parameter combinations and line strengths.
The typical corrections for weak lines are -0.1...-0.2dex, whereas saturated
lines may overestimate the abundance in LTE by more than 0.5dex. The non-LTE Na
abundances appear very robust with respect to uncertainties in the input
collisional data.Comment: 9 pages, 8 figures, accepted for publication in A&
Cluster observations of ULF waves with pulsating electron beams above the high latitude dusk-side auroral region
We report observations by the four Cluster satellites of particle acceleration associated with ULF (Alfven) waves at an altitude of 6R(E) above the dusk-side auroral region. All satellites observed upward accelerated ions and upgoing electron beams, which coincided with the upward field-aligned current around the plasmasheet boundary region. Here we study in detail one region of Alfvenic ULF waves observed together with upward electron beams, both having a quasi-periodicity of about 2 minutes. The ULF waves have a downward Poynting flux. Comparing data from different spacecraft, the observed electron beams are likely caused by the ULF waves in localized (0.5degrees latitude extension) flux tubes in the plasmasheet boundary region. The high-energy keV plasmasheet dispersive ion signatures showed similar periodicity, which suggests that the generation region of the ULF Alfven waves is near the magnetospheric flank, and in turn induce time-varying particle energization