Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations

In this paper we discuss a dynamic flux-transport dynamo model that includes the feedback of the induced magnetic field on differential rotation and meridional flow. We consider two different approaches for the feedback: meanfield Lorentz force and quenching of transport coefficients such as turbulent viscosity and heat conductivity. We find that even strong feedback on the meridional flow does not change the character of the flux-transport dynamo significantly; however it leads to a significant reduction of differential rotation. To a large degree independent from the dynamo parameters, the saturation takes place when the toroidal field at the base of the convection zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional oscillations produced through Lorentz force feedback on differential rotation show a dominant poleward propagating branch with the correct phase relation to the magnetic cycle. We show that incorporating enhanced surface cooling of the active region belt (as proposed by Spruit) leads to an equatorward propagating branch in good agreement with observations.Comment: 15 pages, 12 figures, Accepted for publication in ApJ August 10 issue; corrected typos, corrected referenc

Comprehensive Bayesian Modeling of Tidal Circularization in Open Cluster Binaries part I: M 35, NGC 6819, NGC 188

Tidal friction has long been recognized to circularize the orbits of binary stars over time. In this study, we use the observed distribution of orbital eccentricities in populations of binary stars to probe tidal dissipation. In contrast to previous studies, we incorporate a host of physical effects often neglected in other analyses, provide a much more general description of tides, model individual systems in detail (in lieu of population statistics), and account for all observational uncertainties. The goal is to provide a reliable measurement of the properties of tidal dissipation that is fully supported by the data, properly accounts for different dissipation affecting each tidal wave on each object separately, and evolves with the internal structure of the stars. We extract high precision measurements of tidal dissipation in short period binaries of Sun-like stars in three open clusters. We find that the tidal quality factor on the main sequence falls in the range $5.7 < \log_{10}Q_\star' < 6$ for tidal periods between 3 and 7.5 days. In contrast, the observed circularization in the 150 Myr old M 35 cluster requires that pre-main sequence stars are much more dissipative: $Q_\star' < 4\times10^4$. We test for frequency dependence of the tidal dissipation, finding that for tidal periods between 3 and 7.5 days, if a dependence exists, it is sub-linear for main-sequence stars. Furthermore, by using a more complete physical model for the evolution, and by accounting for the particular properties of each system, we alleviate previously observed tensions in the circularization in the open clusters analyzed.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society 28 pages, 18 figures in main text + 7f figures in appendice

Concentration of toroidal magnetic field in the solar tachocline by eta-quenching

We show that if the turbulent magnetic diffusivity used in solar dynamos is assumed to be 'quenched' by increasing toroidal fields, much larger amplitude and more concentrated toroidal fields can be induced by differential rotation from an assumed poloidal field than if there is no quenching. This amplification and concentration mechanism is weakened and bounded by j x B feedbacks on the differential rotation. Nevertheless, it is strong enough to contribute to the creation of ~100 kG toroidal fields near the base of the convection zone, perhaps in conjunction with the 'exploding flux tube' process. Such high fields are necessary for sunspots to occur in low solar latitudes.Comment: 8 pages, 6 figures, added references, corrected typos, accepted by Ap

Transport of toroidal magnetic field by the meridional flow at the base of the solar convection zone

In this paper we discuss the transport of toroidal magnetic field by a weak meridional flow at the base of the convection zone. We utilize the differential rotation and meridional flow model developed by Rempel and incorporate feedback of a purely toroidal magnetic field in two ways: directly through the Lorentz force (magnetic tension) and indirectly through quenching of the turbulent viscosity, which affects the parametrized turbulent angular momentum transport in the model. In the case of direct Lorentz force feedback we find that a meridional flow with an amplitude of around 2 m/s can transport a magnetic field with a strength of 20 to 30 kG. Quenching of turbulent viscosity leads to deflection of the meridional flow from the magnetized region and a significant reduction of the transport velocity if the magnetic field is above equipartition strength.Comment: 8 pages, 6 figure

Measuring Tidal Dissipation in Giant Planets from Tidal Circularization

In this project, we determined the constraints on the modified tidal quality factor, $Q_{pl}'$, of gas-giant planets orbiting close to their host stars. We allowed $Q_{pl}'$ to depend on tidal frequency, accounting for the multiple tidal waves with time-dependent frequencies simultaneously present on the planet. We performed our analysis on 78 single-star and single-planet systems, with giant planets and host stars with radiative cores and convective outer shells. We extracted constraints on the frequency-dependent $Q_{pl}'$ for each system separately and combined them to find general constraints on $Q_{pl}'$ required to explain the observed eccentricity envelope while simultaneously allowing the observed eccentricities of all systems to survive to the present day. Individual systems do not place tight constraints on $Q_{pl}'$. However, since similar planets must have similar tidal dissipation, we require that a consistent, possibly frequency-dependent, model must apply. Under that assumption, we find that the value of $\log_{10}Q_{pl}'$ for HJs is $5.0\pm0.5$ for the range of tidal period from 0.8 to 7 days. We did not see any clear sign of frequency dependence of $Q_{pl}'$.Comment: Accepted for publication in MNRAS 19 pages, 11 figures, 2 table

The Origin of Solar Activity in the Tachocline

Solar active regions, produced by the emergence of tubes of strong magnetic field in the photosphere, are restricted to within 35 degrees of the solar equator. The nature of the dynamo processes that create and renew these fields, and are therefore responsible for solar magnetic phenomena, are not well understood. We analyze the magneto-rotational stability of the solar tachocline for general field geometry. This thin region of strong radial and latitudinal differential rotation, between the radiative and convective zones, is unstable at latitudes above 37 degrees, yet is stable closer to the equator. We propose that small-scale magneto-rotational turbulence prevents coherent magnetic dynamo action in the tachocline except in the vicinity of the equator, thus explaining the latitudinal restriction of active regions. Tying the magnetic dynamo to the tachocline elucidates the physical conditions and processes relevant to solar magnetism.Comment: 10 pages, 1 figure, accepted for publication in ApJ

The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone

We present three-dimensional numerical simulations of the rise and fragmentation of twisted, initially horizontal magnetic flux tubes which evolve into emerging Omega-loops. The flux tubes rise buoyantly through an adiabatically stratified plasma that represents the solar convection zone. The MHD equations are solved in the anelastic approximation, and the results are compared with studies of flux tube fragmentation in two dimensions. We find that if the initial amount of field line twist is below a critical value, the degree of fragmentation at the apex of a rising Omega-loop depends on its three-dimensional geometry: the greater the apex curvature of a given Omega-loop, the lesser the degree of fragmentation of the loop as it approaches the photosphere. Thus, the amount of initial twist necessary for the loop to retain its cohesion can be reduced substantially from the two-dimensional limit. The simulations also suggest that as a fragmented flux tube emerges through a relatively quiet portion of the solar disk, extended crescent-shaped magnetic features of opposite polarity should form and steadily recede from one another. These features eventually coalesce after the fragmented portion of the Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres

Comparing the energy spectra of ultra-high energy cosmic rays measured with EAS arrays

The energy spectra of ultra-high energy cosmic rays (CRs) measured with giant extensive air shower (EAS) arrays exhibit discrepancies between the flux intensities and/or estimated CR energies exceeding experimental errors. The well-known intensity correction factor due to the dispersion of the measured quantity in the presence of a rapidly falling energy spectrum is insufficient to explain the divergence. Another source of systematic energy determination error is proposed concerning the charged particle density measured with the surface arrays, which arises due to simplifications (namely, the superposition approximation) in nucleus-nucleus interaction description applied to the shower modeling. Making use of the essential correction factors results in congruous CR energy spectra within experimental errors. Residual differences in the energy scales of giant arrays can be attributed to the actual overall accuracy of the EAS detection technique used. CR acceleration and propagation model simulations using the dip and ankle scenarios of the transition from galactic to extragalactic CR components are in agreement with the combined energy spectrum observed with EAS arrays.Comment: Accepted for publication in Ap

Comparison of a Head Mounted Impact Measurement Device to the Hybrid III Anthropomorphic Testing Device in a Controlled Laboratory Setting

Background: Reports estimate that 1.6 to 3.8 million cases of concussion occur in sports and recreation each year in the United States. Despite continued efforts to reduce the occurrence of concussion, the rate of diagnosis continues to increase. The mechanisms of concussion are thought to involve linear and rotational head accelerations and velocities. One method of quantifying the kinematics experienced during sport participation is to place measurement devices into the athlete’s helmet or directly on the athlete’s head. Purpose: The purpose of this research to determine the accuracy of a head mounted device for measuring the head accelerations experienced by the wearer. This will be accomplished by identifying the error in Peak Linear Acceleration (PLA), Peak Rotational Acceleration (PRA) and Peak Rotational Velocity (PRV) of the device. Study Design: Laboratory study. Methods: A helmeted Hybrid III 50th percentile male headform was impacted via a pneumatic ram from the front, side, rear, front oblique and rear oblique at speeds from 1.5 to 5 m/s. The X2 Biosystems xPatch® (Seattle, WA) sensor was placed on the headform’s right side at the approximate location of the mastoid process. Measures of PLA, PRA, PRV from the xPatch ® and Hybrid III were analyzed for Root Mean Square Error (RMSE), and Absolute and Relative Error (AE, RE). Result: Seventy-six impacts were analyzed. All measures of correlation, fixed through the origin, were found to be strong: PLA R2 =0.967 p \u3c 0.01, PRA R2 =0.933 p \u3c 0.01, PRV R2 =0.999 p \u3c 0.00. PLA RMSE was 34%, RE 31.0% ± 14.0, and AE 31.1% ± 13.7. PRA RMSE was 23.4%, RE -6.7 ± 22.4 and AE 18.9% ± 13.8. PRV RMSE was 2.2%, RE 0.1 ± 2.2, and AE 1.8 ± 1.3. Conclusion: Without including corrections for effect of skin artifact, the xPatch® produces measurements highly correlated with the gold standard yet above the average error of testing devices in both PLA and PRA, but a low error in PRV. PLA measures from the xPatch® system demonstrated a high level of correlation with the PLA data from the Hybrid III mounted data collection system. Level of Evidence:

Inter-Rater Agreement and Validity of a Tackling Performance Assessment Scale in Youth American Football

Background: Long term neurologic injury and concussion have been identified as risks from participation in American football. Altering tackling form has been recommended to reduce the risk of neurologic injury caused by head accelerations when tackling. The purpose of this research is to determine the inter-rater agreement and validity of the Qualitative Youth Tackling System (QYTS), a six-item feedback scale to correct tackling form, when utilized by novice and expert raters. Hypothesis: Experienced raters will have higher levels of agreement with each other and with motion capture when compared to novice raters. Methods: Both novice and experienced raters viewed video of youth athletes (ages 9-13) tackling a dummy in a laboratory setting along. The raters identified successful performance according to a binary rating scale for each component. Analysis of both the raters\u27 agreement with each other and with an objective motion capture measure were completed. Results: Fliess\u27 Kappa measures between all raters were found to be moderate for head placement (k=.48), fair for cervical extension (k=.38), trunk inclination (k=.37), shoulder extension (k=.27) and step length (k=.29), and there was no agreement for pelvic height (k=.-16). When compared to the dichotomized validation measures of each of the five components provided by the motion capture system the average Cohen\u27s Kappa agreement was substantial for pelvic height (k=.63), fair for step length (k=.34), cervical extension (k=.40), trunk inclination (k=.35), and slight for shoulder extension (k=.16). The experienced raters out-performed the novice raters in all categories. Conclusion: The results of this study indicate that skilled raters are better able to identify the movement patterns included in the QYTS when compared to a validation measure as well have higher rates of interrater agreement than novice raters. Level of Evidence: 3