756 research outputs found
Lifetime of Surface Features and Stellar Rotation: A Wavelet Time-Frequency Approach
We explore subtle variations in disk-integrated measurements spanning
\lsim 18 years of stellar surface magnetism by using a newly developed
time-frequency gapped wavelet algorithm.
We present results based on analysis of the Mount Wilson
Ca II H and K emission fluxes in four, magnetically-active stars (HD 1835
[G2V], 82885 [G8IV-V], 149661 [K0V] and 190007 [K4V]) and sensitivity tests
using artificial data.
When the wavelet basis is appropriately modified (i.e., when the
time-frequency resolution is optimized), the results are consistent with the
existence of spatially localized and long-lived
Ca II features (assumed here as activity regions that tend to recur in
narrowly-confined latitude bands), especially in HD 1835 and 82885.
This interpretation is based on the observed persistence of relatively
localized Ca II wavelet power at a narrow range of rotational time scales,
enduring as long as \gsim 10 years.Comment: to appear in THE ASTROPHYSICAL JOURNAL LETTER
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
High School Competitive Diving Injuries: National Athletic Treatment Injury and Outcomes Network (NATION)
Purpose: Elite diving coaches and USA diving officials have become increasingly concerned about injury prevention among adolescent divers. However, little is known about such injuries. The purpose of this study was to describe the injuries among high school students who participated on high school diving teams.
Subjects: High school students who participated on the diving teams of high schools that were included in the National Athletic Treatment, Injury and Outcomes Network (NATION) for 2011–2014.
Methods: Descriptive epidemiology using injury exposure data on 56 boys\u27 Swimming and Diving teams and 55 girls\u27 Swimming and Diving teams from the National Athletic Treatment, Injury and Outcomes Network (NATION) for 2011–2014.
Results: Only 12 injuries were reported, and 8 (67%) were concussions. The incidence of concussions was the same between boys and girls.
Conclusion: Concussions are the highest reported injury among high school divers in the NATION data. Student athletes who had minor injuries may not have been evaluated by an athletic trainer. Researchers need better injury surveillance data for high school divers
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
Effects of Instructional Model on Student Attitude in an Introductory Biology Laboratory
This study assessed student attitude towards reformed laboratories featuring a factorial design of inquiry (IN) and explicit / reflective (ER) pedagogy to foster nature of science understanding. Students in thirty-one lab sections responded to pre and post semester assessments of their confidence, perception of usefulness, and effectance motivation toward the laboratories. Relative change in attitude (RCA) was not significantly different (p\u3e0.05) among the treatments or their interaction for confidence, usefulness, or effectance motivation. Student self-reports (n = 137) of factors that affected their attitude suggested that grades and TAs played a larger role in determining student attitude than the laboratory treatments. This hints at the complex interactions that impact student attitude, and which should be considered when implementing course reforms
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
Measuring Tidal Dissipation in Giant Planets from Tidal Circularization
In this project, we determined the constraints on the modified tidal quality
factor, , of gas-giant planets orbiting close to their host stars. We
allowed 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 for each
system separately and combined them to find general constraints on
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 . 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 for HJs is
for the range of tidal period from 0.8 to 7 days. We did not see any clear sign
of frequency dependence of .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
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