Surface flux transport modeling for solar cycles 15--21: effects of cycle-dependent tilt angles of sunspot groups

We model the surface magnetic field and open flux of the Sun from 1913 to 1986 using a surface flux transport model, which includes the observed cycle-to-cycle variation of sunspot group tilts. The model reproduces the empirically derived time evolution of the solar open magnetic flux, and the reversal times of the polar fields. We find that both the polar field and the axial dipole moment resulting from this model around cycle minimum correlate with the strength of the following cycle.Comment: Accepted for publication by Ap

The Role of Subsurface Flows in Solar Surface Convection: Modeling the Spectrum of Supergranular and Larger Scale Flows

We model the solar horizontal velocity power spectrum at scales larger than granulation using a two-component approximation to the mass continuity equation. The model takes four times the density scale height as the integral (driving) scale of the vertical motions at each depth. Scales larger than this decay with height from the deeper layers. Those smaller are assumed to follow a Kolomogorov turbulent cascade, with the total power in the vertical convective motions matching that required to transport the solar luminosity in a mixing length formulation. These model components are validated using large scale radiative hydrodynamic simulations. We reach two primary conclusions: 1. The model predicts significantly more power at low wavenumbers than is observed in the solar photospheric horizontal velocity spectrum. 2. Ionization plays a minor role in shaping the observed solar velocity spectrum by reducing convective amplitudes in the regions of partial helium ionization. The excess low wavenumber power is also seen in the fully nonlinear three-dimensional radiative hydrodynamic simulations employing a realistic equation of state. This adds to other recent evidence suggesting that the amplitudes of large scale convective motions in the Sun are significantly lower than expected. Employing the same feature tracking algorithm used with observational data on the simulation output, we show that the observed low wavenumber power can be reproduced in hydrodynamic models if the amplitudes of large scale modes in the deep layers are artificially reduced. Since the large scale modes have reduced amplitudes, modes on the scale of supergranulation and smaller remain important to convective heat flux even in the deep layers, suggesting that small scale convective correlations are maintained through the bulk of the solar convection zone.Comment: 36 pages, 6 figure

Limits to solar cycle predictability: Cross-equatorial flux plumes

Within the Babcock-Leighton framework for the solar dynamo, the strength of a cycle is expected to depend on the strength of the dipole moment or net hemispheric flux during the preceding minimum, which depends on how much flux was present in each hemisphere at the start of the previous cycle and how much net magnetic flux was transported across the equator during the cycle. Some of this transport is associated with the random walk of magnetic flux tubes subject to granular and supergranular buffeting, some of it is due to the advection caused by systematic cross-equatorial flows such as those associated with the inflows into active regions, and some crosses the equator during the emergence process. We aim to determine how much of the cross-equatorial transport is due to small-scale disorganized motions (treated as diffusion) compared with other processes such as emergence flux across the equator. We measure the cross-equatorial flux transport using Kitt Peak synoptic magnetograms, estimating both the total and diffusive fluxes. Occasionally a large sunspot group, with a large tilt angle emerges crossing the equator, with flux from the two polarities in opposite hemispheres. The largest of these events carry a substantial amount of flux across the equator (compared to the magnetic flux near the poles). We call such events cross-equatorial flux plumes. There are very few such large events during a cycle, which introduces an uncertainty into the determination of the amount of magnetic flux transported across the equator in any particular cycle. As the amount of flux which crosses the equator determines the amount of net flux in each hemisphere, it follows that the cross-equatorial plumes introduce an uncertainty in the prediction of the net flux in each hemisphere. This leads to an uncertainty in predictions of the strength of the following cycle.Comment: A&A, accepte

Waves as the source of apparent twisting motions in sunspot penumbrae

The motion of dark striations across bright filaments in a sunspot penumbra has become an important new diagnostic of convective gas flows in penumbral filaments. The nature of these striations has, however, remained unclear. Here we present an analysis of small scale motions in penumbral filaments in both simulations and observations. The simulations, when viewed from above, show fine structure with dark lanes running outwards from the dark core of the penumbral filaments. The dark lanes either occur preferentially on one side or alternate between both sides of the filament. We identify this fine structure with transverse (kink) oscillations of the filament, corresponding to a sideways swaying of the filament. These oscillations have periods in the range of 5-7 min and propagate outward and downward along the filament. Similar features are found in observed G-band intensity time series of penumbral filaments in a sunspot located near disk center obtained by the Broadband Filter Imager (BFI) on board {\it Hinode}. We also find that some filaments show dark striations moving to both sides of the filaments. Based on the agreement between simulations and observations we conclude that the motions of these striations are caused by transverse oscillations of the underlying bright filaments.Comment: Accepted for publication in Astrophysical Journal on 8th April 201

No evidence for intense, cold accretion onto YSOs from measurements of Li in T-Tauri stars

We have used medium resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion Nebula Cluster for strong lithium depletion through analysis of veiling corrected 6708 angstrom lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M_dot > 5 x 10-4 M_sol yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M < 1.9 M_sol stars. We also find that the dispersion in the strength of the 6708 angstrom lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour magnitude diagrams. Evidence for weak lithium depletion (< 10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 x 10-4 M_sol yr-1 are responsible.Comment: 13 pages, 10 figures; Accepted for publication in Monthly Notices of the Royal Astronomical Society, 2013 June 0

Stegosaurus chirality

We explain that Stegosaurus exhibited exterior chirality and observe that the largest plate in particular of USNM 4394, USNM 4714, DMNS 2818 and NHMUK R36730 appears to have tilted to the right rather than to the left in each case. Several instances in which Stegosaurus specimens have been confused with their distinct, hypothetical mirror-image forms are highlighted. We believe our findings to be consistent with the hypothesis that Stegosaurus's plates acted primarily as display structures. A collection of more than one Stegosaurus might be referred to henceforth as a 'handful' of Stegosaurus

The Waldmeier Effect in Sunspot Cycles

We discuss two aspects of the Waldmeier Effect, namely (1) the rise times of sunspot cycles are anti-correlated to their strengths (WE1) and (2) the rates of rise of the cycles are correlated to their strengths (WE2). From analysis of four different data sets we conclude that both WE1 and WE2 exist in all the data sets. We study these effects theoretically by introducing suitable stochastic fluctuations in our regular solar dynamo model.Comment: Magnetic Coupling between the Interior and Atmosphere of the Sun; Astrophysics and Space Science Proceeding

Surface differential rotation and prominences of the Lupus post T Tauri star RX J1508.6-4423

We present in this paper a spectroscopic monitoring of the Lupus post T Tauri star RX J1508.6-4423 carried out at two closely separated epochs (1998 May 06 and 10) with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Applying least-squares convolution and maximum entropy image reconstruction techniques to our sets of spectra, we demonstrate that this star features on its surface a large cool polar cap with several appendages extending to lower latitudes, as well as one spot close to the equator. The images reconstructed at both epochs are in good overall agreement, except for a photospheric shear that we interpret in terms of latitudinal differential rotation. Given the spot distribution at the epoch of our observations, differential rotation could only be investigated between latitudes 15° and 60°. We find in particular that the observed differential rotation is compatible with a solar-like law (i.e., with rotation rate decreasing towards high latitudes proportionally to sin 2l, where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we find that the equator of RX J1508.6-4423 does one more rotational cycle than the pole every 50 ±10 d, implying a photospheric shear 2 to 3 times stronger than that of the Sun. We also discover that the Hα emission profile of RX J1508.6-4423 is most of the time double-peaked and strongly modulated with the rotation period of the star. We interpret this rotationally modulated emission as being caused by a dense and complex prominence system, the circumstellar distribution of which is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, precisely at the corotation radius. We use the total Hα and Hβ emission flux to estimate that the mass of the whole prominence system is about 10 20g. From our observation that the whole cloud system surrounding the star is regenerated in less than 4 d, we conclude that the braking time-scale of RX J1508.6-4423 is shorter than 1 Gyr, and that prominence expulsion is thus likely to contribute significantly to the rotational spindown of young low-mass stars

A 10-micron Search for Inner-Truncated Disks Among Pre-Main-Sequence Stars With Photometric Rotation Periods

We use mid-IR (primarily 10 $\mu$m) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre-main sequence stars in Orion and Taurus-Auriga for which rotation periods are known and which do not show evidence for inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence for excess mid-IR emission. While these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk-locking, and there exist very young stars already rotating near breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr, which may allow the models to explain the rotational evolution of low-mass pre-main sequence stars in a way that does not depend upon braking by disks.Comment: 20 pages, 4 figure