Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere

The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona. Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) Non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Halpha, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Halpha also at low temperature.Comment: 10 pages, 4 figure

Radial Velocity Jitter in Stars from the California and Carnegie Planet Search at Keck Observatory

I present an empirical model for predicting a star's radial velocity jitter from its B-V color, activity level, and absolute magnitude. This model is based on observations of 450 well- observed stars from Keck Observatory for the California and Carnegie Planet Search Program. The model includes noise from both astrophysical sources and systematic errors, and describes jitter as generally increasing with a star's activity and height above the main sequence.Comment: 16 pages, 7 figures, PASP in pres

Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars

The objective of this paper is to investigate whether a convective dynamo can account quantitatively for the observed lower limit of X-ray surface flux in solar-type main sequence stars. Our approach is to use 3D numerical simulations of a turbulent dynamo driven by convection to characterize the dynamic behavior, magnetic field strengths, and filling factors in a non-rotating stratified medium, and to predict these magnetic properties at the surface of cool stars. We use simple applications of stellar structure theory for the convective envelopes of main-sequence stars to scale our simulations to the outer layers of stars in the F0--M0 spectral range, which allows us to estimate the unsigned magnetic flux on the surface of non-rotating reference stars. With these estimates we use the recent results of \citet{Pevtsov03} to predict the level of X-ray emission from such a turbulent dynamo, and find that our results compare well with observed lower limits of surface X-ray flux. If we scale our predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with the observed lower limit of chromospheric emission in K dwarfs. This suggests that dynamo action from a convecting, non-rotating plasma is a viable alternative to acoustic heating models as an explanation for the basal emission level seen in chromospheric, transition region, and coronal diagnostics from late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure

H-alpha features with hot onsets. II. A contrail fibril

The solar chromosphere observed in H-alpha consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from multiple telescopes to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to high temperature as in the spicule-II phenomenon. After this passage a dark H-Halpha fibril appeared as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset and subsequent visibilities in various diagnostics and conclude that such H-alpha fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic happenings before. They do not connect across internetwork cells but represent launch tracks of heating events and chart magnetic field during launch, not at present.Comment: Accepted for Astronomy & Astrophysic

Sit down at the ball game: how trade barriers make the world less food secure

This paper analyses the impacts of trade policy responses to rising world food prices by carrying out a series of stylised experiments in the wheat market using a world trade model, GTAP. The sequence of events that is modelled comprises a negative wheat supply shock and subsequent implementation of an export tax by a major net exporter and a reduction in import tariffs by a small importer. The effects of trade policy responses are contrasted with those of full liberalisation of the wheat market. At the core are the (opposite) effects on producers and consumers, as well as the terms-of-trade and trade tax revenue effects. Food security is shown to depend crucially on changes in prices but also in incomes that are associated with changes in factor returns. The results reveal that major net exporters are generally better off when implementing export taxes for food security purposes. Large exporting countries export price instability causing world food prices to rise further. Net importing countries lose out and have limited leeway to reduce tariffs or subsidise imports. Liberalising wheat trade mitigates rising prices and contributes to food security, but to the detriment of production in Africa and Asia, making them more dependent on and vulnerable to changes in the world market. Concerted action at the WTO forum is required, notably clarifying and sharpening the rules regarding export measures.food security; world food crisis; international grain trade; trade measures; trade liberalisation; CGE modelling

The Quiet-Sun Photosphere and Chromosphere

The overall structure and the fine structure of the solar photosphere outside active regions are largely understood, except possibly important roles of a turbulent near-surface dynamo at its bottom, internal gravity waves at its top, and small-scale vorticity. Classical 1D static radiation-escape modelling has been replaced by 3D time-dependent MHD simulations that come closer to reality. The solar chromosphere, in contrast, remains ill-understood although its pivotal role in coronal mass and energy loading makes it a principal research area. Its fine structure defines its overall structure, so that hard-to-observe and hard-to-model small-scale dynamical processes are the key to understanding. However, both chromospheric observation and chromospheric simulation presently mature towards the required sophistication. The open-field features seem of greater interest than the easier-to-see closed-field features.Comment: Accepted for special issue "Astrophysical Processes on the Sun" of Phil. Trans. Royal Soc. A, ed. C. Parnell. Note: clicking on the year in a citation opens the corresponding ADS abstract page in the browse

Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs

We have measured the profiles of the Ca II H and K chromospheric emission lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar masses) using multiple high resolution spectra obtained during six years with the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM, equivalent widths, and line luminosities of Ca II H and K increase by a factor of 3 with increasing stellar mass over this small range of stellar masses. We fit the H and K lines with a double Gaussian model to represent both the chromospheric emission and the non-LTE central absorption. Most of the sample stars display a central absorption that is typically redshifted by ~0.1 km/s relative to the emission, but the nature of this velocity gradient remains unknown. The FWHM of the H and K lines increase with stellar luminosity, reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent widths and FWHM exhibit modest temporal variability in individual stars. At a given value of M_v, stars exhibit a spread in both the equivalent width and FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters including rotation rate, age, and possibly metallicity, and to the spread in stellar mass at a given M_v. The K line is consistently wider than the H line, as expected, and its central absorption is more redshifted, indicating that the H and K lines form at slightly different heights in the chromosphere where the velocities are slightly different. The equivalent width of H-alpha correlates with Ca II H and K only for stars having Ca II equivalent widths above ~2 angstroms, suggesting the existence of a magnetic threshold above which the lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for publication in PAS