Near-surface stellar magneto-convection: simulations for the Sun and a metal-poor solar analog

We present 2D local box simulations of near-surface radiative magneto-convection with prescribed magnetic flux, carried out with the MHD version of the CO5BOLD code for the Sun and a solar-like star with a metal-poor chemical composition (metal abundances reduced by a factor 100, [M/H]=-2). The resulting magneto-hydrodynamical models can be used to study the influence of the metallicity on the properties of magnetized stellar atmospheres. A preliminary analysis indicates that the horizontal magnetic field component tends to be significantly stronger in the optically thin layers of metal-poor stellar atmospheres.Comment: Proc. IAU Symposium 259, Cosmic Magnetic Fields: from Planets, to Stars and Galaxies, K.G. Strassmeier, A.G. Kosovichev and J.E. Beckman, eds. (2009) p.23

Exchange rate predictability and dynamic Bayesian learning

This paper considers how an investor in foreign exchange markets might exploit predictive information in macroeconomic fundamentals by allowing for switching between multivariate time series regression models. These models are chosen to reflect a wide array of established empirical and theoretical stylized facts. In an application involving monthly exchange rates for seven countries, we find that an investor using our methods to dynamically allocate assets achieves significant gains relative to benchmark strategies. In particular, we find strong evidence for fast model switching, with most of the time only a small set of macroeconomic fundamentals being relevant for forecasting

The H.E.S.S. multi-messenger program

Based on fundamental particle physics processes like the production and subsequent decay of pions in interactions of high-energy particles, close connections exist between the acceleration sites of high-energy cosmic rays and the emission of high-energy gamma rays and high-energy neutrinos. In most cases these connections provide both spatial and temporal correlations of the different emitted particles. The combination of the complementary information provided by these messengers allows to lift ambiguities in the interpretation of the data and enables novel and highly sensitive analyses. In this contribution the H.E.S.S. multi-messenger program is introduced and described. The current core of this newly installed program is the combination of high-energy neutrinos and high-energy gamma rays. The search for gamma-ray emission following gravitational wave triggers is also discussed. Furthermore, the existing program for following triggers in the electromagnetic regime was extended by the search for gamma-ray emission from Fast Radio Bursts (FRBs). An overview over current and planned analyses is given and recent results are presented.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherland

Precision spectroscopy of the 3s-3p fine structure doublet in Mg+

We apply a recently demonstrated method for precision spectroscopy on strong transitions in trapped ions to measure both fine structure components of the 3s-3p transition in 24-Mg+ and 26-Mg+. We deduce absolute frequency reference data for transition frequencies, isotope shifts and fine structure splittings that are in particular useful for comparison with quasar absorption spectra, which test possible space-time variations of the fine structure constant. The measurement accuracy improves previous literature values, when existing, by more than two orders of magnitude

Grand minima and maxima of solar activity: New observational constraints

Using a reconstruction of sunspot numbers stretching over multiple millennia, we analyze the statistics of the occurrence of grand minima and maxima and set new observational constraints on long-term solar and stellar dynamo models. We present an updated reconstruction of sunspot number over multiple millennia, from $^{14}$C data by means of a physics-based model, using an updated model of the evolution of the solar open magnetic flux. A list of grand minima and maxima of solar activity is presented for the Holocene (since 9500 BC) and the statistics of both the length of individual events as well as the waiting time between them are analyzed. The occurrence of grand minima/maxima is driven not by long-term cyclic variability, but by a stochastic/chaotic process. The waiting time distribution of the occurrence of grand minima/maxima deviates from an exponential distribution, implying that these events tend to cluster together with long event-free periods between the clusters. Two different types of grand minima are observed: short (30--90 years) minima of Maunder type and long ($>$110 years) minima of Sp\"orer type, implying that a deterministic behaviour of the dynamo during a grand minimum defines its length. The duration of grand maxima follows an exponential distribution, suggesting that the duration of a grand maximum is determined by a random process. These results set new observational constraints upon the long-term behaviour of the solar dynamo.Comment: 10 Figure

CRISP Spectropolarimetric Imaging of Penumbral Fine Structure

We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16 arcsec. Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows 0.3 arcsec wide spines, separated by 1.6 arcsec (1200 km) and associated with 30 deg inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be be identified with individual flux tubes. A structure with nearly 10 deg more vertical and weaker magnetic field is seen midways between two spines. This structure is co-spatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below.Comment: Accepted for publication in ApJL 17 Oct 2008. One Figure adde

Efficiency of choice set generation methods for bicycle routes

The current study analyses the efficiency of choice set generation methods for bicycle routes and proposes the extension of cost functions to bicycle-oriented factors not limited to distance and time. Three choice set generation methods for route choice were examined in their ability to generate relevant and heterogeneous routes: doubly stochastic generation function, breadth first search on link elimination, and branch & bound algorithm. Efficiency of the methods was evaluated for a high-resolution network by comparing the performances with four multiattribute cost functions accounting for scenic routes, dedicated cycle lanes, and road type. Data consisted of 778 bicycle trips traced by GPS and carried out by 139 persons living in the Greater Copenhagen Area, in Denmark. Results suggest that both the breadth first search on link elimination and the doubly stochastic generation function generated realistic routes, while the former outperformed in computation cost and the latter produced more heterogeneous routes

Morphology and evolution of umbral dots and their substructures

Substructures - dark lanes and tails - of umbral dots (UDs) were predicted by numerical simulations of magnetoconvection. We analyse a 6 h 23 min time series of broadband images of a large umbra in the active region NOAA 10634, acquired with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A 43 min part of this series was reconstructed with the MFBD method, reaching a spatial resolution of 0.14". We measure brightness, size, lifetime, and horizontal velocities of various umbral structures. Most (90 %) of UDs and bright point-like features in faint LBs split and merge, and their median lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is considered as the end of their life. Both UDs and features in faint LBs that do not split or merge are clearly smaller (0.15") than the average size (0.17") of all features. Horizontal motions of umbral bright small-scale features are directed either into the umbra or along faint LBs with mean horizontal velocities of 0.34 km/s. Features faster than 0.4 km/s appear mostly at the periphery of the umbra. The intensity of dark lanes, measured in four bright central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs. The width of dark lanes is probably less than the resolution limit 0.14". The characteristic time of substructure changes of UDs is ~4 min. We observe narrow (0.14") bright and dark filaments connected with PUDs. Usually one dark and two bright filaments form a 0.4" wide tail attached to one PUD, resembling a short dark-cored penumbral filament. Our results indicate the similarity between PUDs and PGs located at the tips of bright penumbral filaments. The features seen in numerical MHD simulations are consistent with our observations of dark lanes in CUDs and tails attached to PUDs.Comment: 8 pages, 8 figures, Astronomy & Astrophysics, in pres