Showing papers by "Henk C. Spruit published in 2006"
TL;DR: Detailed analysis of small output variations in the Sun's total energy output has greatly advanced understanding, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century.
Abstract: Variations in the Sun's total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun's output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present.
326 citations
TL;DR: In this paper, the role of kink instability in magnetically driven jets was explored through numerical one-dimensional steady relativistic MHD calculations, and the instability was shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets.
Abstract: The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances >1000 rg because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ∼10 16 cm where the deceleration of the ejecta due to interaction with the external medium is expected. The energy released by the instability can power the compact “blazar zone” emission and the prompt emission of GRB outflows with high radiative efficiencies.
198 citations
TL;DR: In this article, the authors interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above.
Abstract: We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund & Stein (2005, in preparation). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines.
183 citations
TL;DR: In this paper, the photosphere of an ultrarelativistic flow with internal dissipation of energy was explored by calculating the spectra for a large range of the characteristics of the flow, and an accurate fitting formula was given that provided the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of flow.
Abstract: We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E*f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.
165 citations
TL;DR: Braithwaite et al. as discussed by the authors used numerical MHD to look at the stability of a possible poloidal field in neutron stars and follow its unstable evolution, which leads to the complete decay of the field.
Abstract: We use numerical MHD to look at the stability of a possible poloidal field in neutron stars (Flowers & Ruderman 1977, ApJ, 215, 302), and follow its unstable evolution, which leads to the complete decay of the field. We then model a neutron star after the formation of a solid crust of high conductivity. As the initial magnetic field we use the stable "twisted torus" field which was the result of our earlier work (Braithwaite & Nordlund 2006, A&A, 450, 1077), since this field is likely to exist in the interior of the star at the time of crust formation. We follow the evolution of the field under the influence of diffusion, and find that large stresses build up in the crust, which will presumably lead to cracking. We put this forward as a model for outbursts in soft gamma repeaters.
142 citations
TL;DR: In this article, numerical 2D magnetostatic models for sunspot penumbrae consisting of radially aligned field-free gaps in a potential magnetic field, as proposed by Spruit & Scharmer (2006, AA), were presented.
Abstract: We present numerical 2D magnetostatic models for sunspot penumbrae consisting of radially aligned field-free gaps in a potential magnetic field, as proposed by Spruit & Scharmer (2006, AA the gaps should thus appear as noticeably elevated features. This structure explains the large variations in field strength in the inner penumbra inferred from magnetograms and two-component inversions, and the varying appearance of the inner penumbra with viewing angle. In the outer penumbra, on the other hand, the gaps are flat-topped with a horizontal magnetic field above the middle of the gap. The magnetic field has large inclination variations horizontally, but only small fluctuations in field strength, in agreement with observations.
112 citations
TL;DR: In this paper, the role of kink instability in magnetically driven jets was explored through numerical one-dimensional steady relativistic MHD calculations, and the instability was shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets.
Abstract: The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances larger than ~1000 r_g because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~10^{16} cm where the deceleration of the ejecta due to its interaction with the external medium is expected. The energy released by the instability can power the compact ``blazar zone'' emission and the prompt emission of GRB outflows with high radiative efficiencies.
10 citations