Spectropolarimetric multi line analysis of stellar magnetic fields

In this paper we study the feasibility of inferring the magnetic field from polarized multi-line spectra using two methods: The pseudo line approach and The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines of a stellar spectrum contribute to obtain a polarization signature. The use of multiple lines dramatically increases the signal to noise ratio of these polarizations signatures. Using one technique, the pseudo-line approach, we construct the pseudo-line as the mean profile of all the individual lines. The other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006) for the detection of polarized signals, combines Principle Components Analysis (PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a main advantage: the polarized signature is extracted using cross correlations between the stellar spectra nd functions containing the polarization properties of each line. These functions are the principal components of a database of synthetic spectra. The synthesis of the spectra of the database are obtained using the radiative transfer equations in LTE. The profiles built with the PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in the study of stellar and solar magnetic fields. The information of the physical parameters that governs the line formation is contained in the final polarized profiles. In particular, using inversion codes, we have shown that the magnetic field vector can be properly inferred with both approaches despite the magnetic field regime.Comment: Accepted for publication in Astronomy and Astrophysic

The coronal structure of AB Dor determined from contemporaneous Doppler imaging and X-ray spectroscopy

We obtain contemporaneous observations of the surface and corona of AB Dor using ground-based circularly polarised spectra from the Anglo-Australian Telescope and X-ray data from the Chandra satellite. The ground-based data are used to construct surface magnetic field maps, which are extrapolated to produce detailed models of the quiescent corona. The X-ray data serve as a new test for the validity of these coronal models. The high coronal density and complex multi-polar magnetic field indicate a compact X-ray corona, which is concentrated close to the surface, with a height, H~0.3-0.4R*. There is also significant correlation between the surface and coronal active region locations. At this epoch AB Dor appears to possess one very large active longitude region; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. Finally, the level of rotational modulation and shape of the X-ray lightcurve depend on the distribution of magnetic field in the obscured hemisphere. The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray lightcurve and spectra require the magnetic field in the obscured hemisphere to be of the same polarity as that in the observed hemisphere. The Sun shows different behaviour, with the leading polarity reversed in the opposite hemisphere. The X-ray observations provide a unique constraint on the magnetic structure in the obscured hemisphere.Comment: 17 pages, 14 figures, accepted by MNRAS. This version has cropped figures. For a preprint with the original figures please go to http://star-www.st-and.ac.uk/~gajh/papers0

Mechanisms of internal Atlantic multidecadal variability in HadGEM3-GC3.1 at two different resolutions

This study broadly characterises and compares the key processes governing internal AMV in two resolutions of HadGEM3-GC3.1: N216ORCA025, corresponding to ~60km in the atmosphere and 0.25 degrees in the ocean, and N96ORCA1 (~135km / 1 degree). Both models simulate AMV with a timescale of 60-80 years, which is related to low frequency ocean and atmosphere circulation changes. In both models, ocean heat transport convergence dominates polar and subpolar AMV, whereas surface heat fluxes associated with cloud changes drive subtropical AMV. However, details of the ocean circulation changes differ between the models. In N216 subpolar subsurface density anomalies propagate into the subtropics along the western boundary, consistent with the more coherent circulation changes and widespread development of SST anomalies. In contrast, N96 subsurface density anomalies persist in the subpolar latitudes for longer, so circulation anomalies and the development of SST anomalies are more centred there. The drivers of subsurface density anomalies also differ between models. In N216, the NAO is the dominant driver, while upper-ocean salinity-controlled density anomalies that originate from the Arctic appear to be the dominant driver in N96. These results further highlight that internal AMV mechanisms are model dependent and motivate further work to better understand and constrain the difference

The influence of anthropogenic aerosol on multi-decadal variations of historical global climate

Analysis of single forcing runs from CMIP5 (the fifth Coupled Model Intercomparison Project) simulations shows that the mid-twentieth century temperature hiatus, and the coincident decrease in precipitation, is likely to have been influenced strongly by anthropogenic aerosol forcing. Models that include a representation of the indirect effect of aerosol better reproduce inter-decadal variability in historical global-mean near-surface temperatures, particularly the cooling in the 1950s and 1960s, compared to models with representation of the aerosol direct effect only. Models with the indirect effect also show a more pronounced decrease in precipitation during this period, which is in better agreement with observations, and greater inter-decadal variability in the inter-hemispheric temperature difference. This study demonstrates the importance of representing aerosols, and their indirect effects, in general circulation models, and suggests that inter-model diversity in aerosol burden and representation of aerosol–cloud interaction can produce substantial variation in simulations of climate variability on multi decadal timescales

The first magnetic maps of a pre-main sequence binary star system - HD 155555

We present the first maps of the surface magnetic fields of a pre-main sequence binary system. Spectropolarimetric observations of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) system were obtained at the Anglo-Australian Telescope in 2004 and 2007. Both datasets are analysed using a new binary Zeeman Doppler imaging (ZDI) code. This allows us to simultaneously model the contribution of each component to the observed circularly polarised spectra. Stellar brightness maps are also produced for HD 155555 and compared to previous Doppler images. Our radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes. We find rings of azimuthal field on both stars, most of which are found to be non-axisymmetric with the stellar rotational axis. We also examine the field strength and the relative fraction of magnetic energy stored in the radial and azimuthal field components at both epochs. A marked weakening of the field strength of the secondary star is observed between the 2004 and 2007 epochs. This is accompanied by an apparent shift in the location of magnetic energy from the azimuthal to radial field. We suggest that this could be indicative of a magnetic activity cycle. We use the radial magnetic maps to extrapolate the coronal field (by assuming a potential field) for each star individually - at present ignoring any possible interaction. The secondary star is found to exhibit an extreme tilt (~75 deg) of its large scale magnetic field to that of its rotation axis for both epochs. The field complexity that is apparent in the surface maps persists out to a significant fraction of the binary separation. Any interaction between the fields of the two stars is therefore likely to be complex also. Modelling this would require a full binary field extrapolation.Comment: 17 pages, 12 figures, accepted for publication in MNRA

Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies

Spectroscopic and spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007, 2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. Brightness and surface magnetic field topologies were reconstructed for the star using the technique of least-squares deconvolution to increase the signal-to-noise of the data. The reconstructed brightness maps show that HD 141943 had a weak polar spot and a significant amount of low latitude features, with little change in the latitude distribution of the spots over the 4 years of observations. The surface magnetic field was reconstructed at three of the epochs from a high order (l <= 30) spherical harmonic expansion of the spectropolarimetric observations. The reconstructed magnetic topologies show that in 2007 and 2010 the surface magnetic field was reasonably balanced between poloidal and toroidal components. However we find tentative evidence of a change in the poloidal/toroidal ratio in 2009 with the poloidal component becoming more dominant. At all epochs the radial magnetic field is predominantly non-axisymmetric while the azimuthal field is predominantly axisymmetric with a ring of positive azimuthal field around the pole similar to that seen on other active stars.Comment: 18 pages, 17 figures, accepted by MNRA

Predictability of the quasi-biennial oscillation and its northern winter teleconnection on seasonal to decadal timescales

Journal ArticlePublished version used with permision of the publisher.The predictability of the quasi-biennial oscillation (QBO) is examined in initialized climate forecasts extending out to lead times of years. We use initialized retrospective predictions made with coupled ocean-atmosphere climate models that have an internally generated QBO. We demonstrate predictability of the QBO extending more than 3 years into the future, well beyond timescales normally associated with internal atmospheric processes. Correlation scores with observational analyses exceed 0.7 at a lead time of 12 months. We also examine the variation of predictability with season and QBO phase and find that skill is lowest in winter. An assessment of perfect predictability suggests that higher skill may be achievable through improved initialization and climate modeling of the QBO, although this may depend on the realism of gravity wave source parameterizations in the models. Finally, we show that skilful prediction of the QBO itself does not guarantee predictability of the extratropical winter teleconnection that is important for surface winter climate prediction. Key Points The QBO is skilfully predicted in seasonal-decadal forecast systems Further improvements in predictions of the QBO are possible The QBO winter surface teleconnection is reproduced with mixed succes

Persistence in diving American mink

Background American mink forage on land and in water, with aquatic prey often constituting a large proportion of their diet. Their long, thin body shape and relatively poor insulation make them vulnerable to heat loss, particularly in water, yet some individuals dive over 100 times a day. At the level of individual dives, previous research found no difference in dive depth or duration, or the total number of dives per day between seasons, but mink did appear to make more dives per active hour in winter than in summer. There was also no difference in the depth or duration of individual dives between the sexes, but there was some evidence that females made more dives per day than males. However, because individual mink dives tend to be extremely short in duration, persistence (quantified as the number of consecutive dives performed) may be a more appropriate metric with which to compare diving behaviour under different scenarios. Results Mink performed up to 28 consecutive dives, and dived continually for up to 36 min. Periods of more loosely aggregated diving (termed ‘aquatic activity sessions’) comprised up to 80 dives, carried out over up to 162.8 min. Contrary to our predictions, persistence was inversely proportional to body weight, with small animals more persistent than large ones, and (for females, but not for males) increased with decreasing temperature. For both sexes, persistence was greater during the day than during the night. Conclusions The observed body weight effect may point to inter-sexual niche partitioning, since in mink the smallest animals are females and the largest are males. The results may equally point to individual specialism’s, since persistence was also highly variable among individuals. Given the energetic costs involved, the extreme persistence of some animals observed in winter suggests that the costs of occasional prolonged activity in cold water are outweighed by the energetic gains. Analysing dive persistence can provide information on an animal’s physical capabilities for performing multiple dives and may reveal how such behaviour is affected by different conditions. Further development of monitoring and biologging methodology to allow quantification of hunting success, and thus the rewards obtained under alternative scenarios, would be insightful