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  • 1
    Electronic Resource
    Electronic Resource
    The role of vorticity in 3D magnetic field annihilation (1994)
    Springer
    Space science reviews 68 (1994), S. 115-116 
    Details
    ISSN: 1572-9672
    Keywords: Magnetohydrodynamics ; Reconnection ; Magnetic Annihilation ; Vorticity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We show that magnetic field annihilation depends strongly on the behaviour of the vorticity and is quite different in 2D and 3D. In 3D the vorticity can be increased locally by the stretching of vortex lines (an effect that is absent in 2D). This leads to the onset ofcellular flow at quite low vorticities and the fragmentation of the simple current sheet.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00749125
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  • 2
    Electronic Resource
    Electronic Resource
    Magnetostatic equilibria for coronal loops on rotating stars (1991)
    Springer
    Solar physics 131 (1991), S. 269-289 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have investigated magnetostatic equilibria for coronal loops embedded in a potential magnetic field on a rotating star. We find that for any given star, there is a maximum value of the plasma pressure inside a single loop, above which no equilibrium exists. This maximum internal pressure depends on the ratio of the temperatures inside and outside the loop, and on the ratio of the plasma pressure to the magnetic pressure at the base of the external field. Thus, any loop of a large-scale field which is heated or cooled to a different temperature from its immediate surroundings, or which experiences a change in its internal pressure may eventually lose equilbrium. For some values of the base pressure and temperature ratio the relation between summit height and footpoint separation is double-valued. As the summit height of a loop is increased, its footpoint separation increases to a critical value, then decreases to zero at the maximum possible summit height. At the critical footpoint separation the slope of the loop height-footpoint separation relation becomes infinite, and no equilibrium solution exists for greater footpoint separations. We find also that the strength and scale of the field external to the flux tube is the most important factor in determining its maximum height. The effects of varying the stellar rotation rate - and, hence, the variation in pressure with height - are comparatively unimportant, even for very high rotation rates at which the point of balance between gravitational and centrifugal forces lies close to the stellar surface. In this case it is possible to find equilibrium loop solutions whose summits lie outside the centrifugal balance point. We have also investigated the effects of varying the stellar surface gravity. For stellar of fixed mass and rotation rate, the loop dimensions scale approximately linearly with the stellar radius.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151638
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  • 3
    Electronic Resource
    Electronic Resource
    Removing the Pressure Limitations on Flux Pile-Up Reconnection (1998)
    Springer
    Solar physics 177 (1998), S. 411-414 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In a recent Letter, Litvinenko, Forbes, and Priest (1996) claimed that the rate of the very fast flux-pile up reconnection is severely limited by the effects of plasma pressure. They considered however only two-dimensional, zero-vorticity flows. Here we show that this limitation is a feature of these restrictive assumptions and can be removed by relaxing either of them.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004944420328
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  • 4
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    A small survey of the magnetic fields of planet-host stars (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-26
    Description: Using spectropolarimetry, we investigate the large-scale magnetic topologies of stars hosting close-in exoplanets. A small survey of 10 stars has been done with the twin instruments Télescope Bernard Lyot /NARVAL and Canada–France–Hawaii Telescope/ESPaDOnS between 2006 and 2011. Each target consists of circular polarization observations covering 7–22 d. For each of the seven targets in which a magnetic field was detected, we reconstructed the magnetic field topology using Zeeman–Doppler imaging. Otherwise, a detection limit has been estimated. Three new epochs of observations of Boo are presented, which confirm magnetic polarity reversal. We estimate that the cycle period is 2 yr, but recall that a shorter period of 240 d cannot still be ruled out. The result of our survey is compared to the global picture of stellar magnetic field properties in the mass–rotation diagram. The comparison shows that these giant planet-host stars tend to have similar magnetic field topologies to stars without detected hot Jupiters. This needs to be confirmed with a larger sample of stars.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Influence of surface stressing on stellar coronae and winds (2013)
    Oxford University Press
    Details
    Publication Date: 2013-04-03
    Description: The large-scale field of the Sun is well represented by its lowest energy (or potential) state. Recent observations, by comparison, reveal that many solar-type stars show large-scale surface magnetic fields that are highly non-potential – that is, they have been stressed above their lowest energy state. This non-potential component of the surface field is neglected by current stellar wind models. The aim of this paper is to determine its effect on the coronal structure and wind. We use Zeeman–Doppler surface magnetograms of two stars – one with an almost potential, one with a non-potential surface field – to extrapolate a static model of the coronal structure for each star. We find that the stresses are carried almost exclusively in a band of unidirectional azimuthal field that is confined to mid-latitudes. Using this static solution as an initial state for a magnetohydrodynamic (MHD) wind model, we then find that the final state is determined primarily by the potential component of the surface magnetic field. The band of azimuthal field must be confined close to the stellar surface, as it is not compatible with a steady-state wind. By artificially increasing the stellar rotation rate, we demonstrate that the observed azimuthal fields cannot be produced by the action of the wind but must be due to processes at or below the stellar surface. We conclude that the background winds of solar-like stars are largely unaffected by these highly stressed surface fields. Nonetheless, the increased flare activity and associated coronal mass ejections that may be expected to accompany such highly stressed fields may have a significant impact on any surrounding planets.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    On the environment surrounding close-in exoplanets (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-12
    Description: Exoplanets in extremely close-in orbits are immersed in a local interplanetary medium (i.e. the stellar wind) much denser than the local conditions encountered around the Solar system planets. The environment surrounding these exoplanets also differs in terms of dynamics (slower stellar winds, but higher Keplerian velocities) and ambient magnetic fields (likely higher for host stars more active than the Sun). Here, we quantitatively investigate the nature of the interplanetary media surrounding the hot Jupiters HD 46375b, HD 73256b, HD 102195b, HD 130322b and HD 179949b. We simulate the three-dimensional winds of their host stars, in which we directly incorporate their observed surface magnetic fields. With that, we derive mass-loss rates (1.9–8.0 x 10 –13 M yr –1 ) and the wind properties at the position of the hot Jupiters’ orbits (temperature, velocity, magnetic field intensity and pressure). We show that these exoplanets’ orbits are supermagnetosonic, indicating that bow shocks are formed surrounding these planets. Assuming planetary magnetic fields similar to Jupiter's, we estimate planetary magnetospheric sizes of 4.1–5.6 planetary radii. We also derive the exoplanetary radio emission released in the dissipation of the stellar wind energy. We find radio fluxes ranging from 0.02 to 0.13 mJy, which are challenging to be observed with present-day technology, but could be detectable with future higher sensitivity arrays (e.g. Square Kilometre Array). Radio emission from systems having closer hot Jupiters, such as from  Boo b or HD 189733b, or from nearby planetary systems orbiting young stars, are likely to have higher radio fluxes, presenting better prospects for detecting exoplanetary radio emission.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Time-scales of close-in exoplanet radio emission variability (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-23
    Description: We investigate the variability of exoplanetary radio emission using stellar magnetic maps and 3D field extrapolation techniques. We use a sample of hot Jupiter hosting stars, focusing on the HD 179949, HD 189733 and Boo systems. Our results indicate two time-scales over which radio emission variability may occur at magnetized hot Jupiters. The first is the synodic period of the star–planet system. The origin of variability on this time-scale is the relative motion between the planet and the interplanetary plasma that is corotating with the host star. The second time-scale is the length of the magnetic cycle. Variability on this time-scale is caused by evolution of the stellar field. At these systems, the magnitude of planetary radio emission is anticorrelated with the angular separation between the subplanetary point and the nearest magnetic pole. For the special case of Boo b, whose orbital period is tidally locked to the rotation period of its host star, variability only occurs on the time-scale of the magnetic cycle. The lack of radio variability on the synodic period at Boo b is not predicted by previous radio emission models, which do not account for the co-rotation of the interplanetary plasma at small distances from the star.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The energy budget of stellar magnetic fields (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-19
    Description: Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5 M . We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5 M having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Magnetic activity and hot Jupiters of young Suns: the weak-line T Tauri stars V819 Tau and V830 Tau (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri stars (wTTSs) V819 Tau and V830 Tau within the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets) programme, involving the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope. At ~=3 Myr, both stars dissipated their discs recently and are interesting objects for probing star and planet formation. Profile distortions and Zeeman signatures are detected in the unpolarized and circularly polarized lines, whose rotational modulation we modelled using tomographic imaging, yielding brightness and magnetic maps for both stars. We find that the large-scale magnetic fields of V819 Tau and V830 Tau are mostly poloidal and can be approximated at large radii by 350–400 G dipoles tilted at ~=30° to the rotation axis. They are significantly weaker than the field of GQ Lup, an accreting classical T Tauri star (cTTS) with similar mass and age which can be used to compare the magnetic properties of wTTSs and cTTSs. The reconstructed brightness maps of both stars include cool spots and warm plages. Surface differential rotation is small, typically ~=4.4 times smaller than on the Sun, in agreement with previous results on wTTSs. Using our Doppler images to model the activity jitter and filter it out from the radial velocity (RV) curves, we obtain RV residuals with dispersions of 0.033 and 0.104 km s –1 for V819 Tau and V830 Tau, respectively. RV residuals suggest that a hot Jupiter may be orbiting V830 Tau, though additional data are needed to confirm this preliminary result. We find no evidence for close-in giant planet around V819 Tau.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    The energy budget of stellar magnetic fields (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1–1.5 M . We find that the energy contained in toroidal fields has a power-law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5 M having power indices of 0.72 ± 0.08 and 1.25 ± 0.06, respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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