1,721,021 research outputs found
Cameron, R H, 401741
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/375531Surname: CAMERON
Given Name(s) or Initials: R H
Military Service Number or Last Known Location: 401741
Missing, Wounded and Prisoner of War Enquiry Card Index Number: 57390188232
Item: [2016.0049.07839] "Cameron, R H, 401741
The rush to the poles and the role of magnetic buoyancy in the solar dynamo
No full textContext. The butterfly diagram of the solar cycle shows a poleward migration of the diffuse magnetic field resulting from the decay of trailing sunspots. It is one component of what is sometimes referred to as the ‘rush to the poles’ and is responsible for the reversal and buildup of the polar cap fields. Aims. We investigated under which conditions the rush to the poles can be reproduced in flux-transport Babcock-Leighton dynamo models. We also considered other observational consequences of the different mechanisms that reproduce the rush to the poles. Methods. We identified three main ways to achieve the rush to the poles: a flux emergence probability that decreases rapidly with latitude; a threshold for the sub-surface toroidal field strength below which the toroidal flux emerges only slowly and above which the emergence rate is high; and an emergence rate that depends on the mean magnetic field squared, mimicking magnetic buoyancy. We implemented these three mechanisms in a 2D Babcock-Leighton flux transport dynamo model that incorporates toroidal flux loss and deep downward turbulent pumping. Moreover, we directly compared the observational sunspot zone migration law with what our models predict. Results. We find that all three mechanisms lead to solar-like butterfly diagrams, but with notable differences. The shape of the butterfly diagram is very sensitive to model parameters for the threshold prescription, while most models that incorporate magnetic buoyancy converge to very similar butterfly diagrams, with butterfly wing widths of ≲ ± 30°, in very good agreement with observations. With turbulent diffusivities above 35 km 2 /s but below about 40 km 2 /s, buoyancy models are strikingly solar-like. The threshold and magnetic buoyancy prescriptions make the models non-linear and as such they can saturate the dynamo through latitudinal quenching; during this process, emergences at higher latitudes are less efficient at transporting fields across the equator and hence less efficient in reversing the polar fields – although only the magnetic buoyancy prescription can saturate the dynamo when emergence loss is turned off. The period of the models that involve buoyancy is independent of the source term amplitude, but emergence loss increases it by ≃60%. The models with the right advection amplitude and turbulent diffusivity match the observational equatorward migration law very well. Conclusions. For the rush to the poles to be visible, a mechanism suppressing (enhancing) emergences at high (low) latitudes must operate. It is not sufficient that the toroidal field be stored at low latitudes for emergences to be limited to low latitudes. Magnetic buoyancy appears to be the most promising non-linearity as models that incorporate it produce the most solar-like butterfly diagrams, with the exact width of the butterfly wings being roughly independent of model parameters. Dynamo saturation is achieved by a competition between latitudinal quenching and a quenching due to the tilt of the mean bipolar magnetic region. From these models we infer that the Sun is not in the advection-dominated regime, but nor is it in the diffusion-dominated regime. The cycle period is set through a balance between advection, diffusion, and flux emergence in accordance with the observational sunspot zone migration law. This accordance seems to imply that the toroidal field is indeed stored in the equatorial region of the lower convection zone
Surface flux transport simulations: Effect of inflows toward active regions and random velocities on the evolution of the Sun's large-scale magnetic field
No full textAims. We aim to determine the effect of converging flows on the evolution of a bipolar magnetic region (BMR), and to investigate the role of these inflows in the generation of poloidal flux. We also discuss whether the flux dispersal due to turbulent flows can be described as a diffusion process.
Methods. We developed a simple surface flux transport model based on point-like magnetic concentrations. We tracked the tilt angle, the magnetic flux and the axial dipole moment of a BMR in simulations with and without inflows and compared the results. To test the diffusion approximation, simulations of random walk dispersal of magnetic features were compared against the predictions of the diffusion treatment.
Results. We confirm the validity of the diffusion approximation to describe flux dispersal on large scales. We find that the inflows enhance flux cancellation, but at the same time affect the latitudinal separation of the polarities of the bipolar region. In most cases the latitudinal separation is limited by the inflows, resulting in a reduction of the axial dipole moment of the BMR. However, when the initial tilt angle of the BMR is small, the inflows produce an increase in latitudinal separation that leads to an increase in the axial dipole moment in spite of the enhanced flux destruction. This can give rise to a tilt of the BMR even when the BMR was originally aligned parallel to the equator
A Babcock-Leighton dynamo model of the Sun incorporating toroidal flux loss and the helioseismically-inferred meridional flow
Full text linkWe investigate whether the Babcock-Leighton flux-transport dynamo model
remains in agreement with observations if the meridional flow profile is taken
from helioseismic inversions. Additionally, we investigate the effect of the
loss of toroidal flux through the solar surface. We employ the 2D
flux-transport BL dynamo framework. We use the helioseismically-inferred
meridional flow profile, and include toroidal flux loss in a way that is
consistent with the amount of poloidal flux generated by Joy's law. Our model
does not impose a preference for emergences at low latitudes, but we do require
that the model produces such a preference. We can find solutions in general
agreement with observations, including the equatorward drift of the butterfly
wings and the cycle's 11 year period. The most important free parameters in the
model are the depth to which the radial turbulent pumping extends and the
turbulent diffusivity in the lower half of the convection zone. We find that
the pumping needs to extend to depths of about and the bulk
turbulent diffusivity needs to be around 10 km/s or less. We find that the
emergences are restricted to low latitudes without the need to impose such a
preference. The flux-transport BL model, incorporating the helioseismically
inferred meridional flow and toroidal field loss term, is compatible with the
properties of the observed butterfly diagram and with the observed toroidal
loss rate. Reasonably tight constraints are placed on the remaining free
parameters. The pumping needs to be to just below the depth corresponding to
the location where the meridional flow changes direction. Our linear model does
not however reproduce the observed "rush to the poles" of the diffuse surface
radial field resulting from the decay of sunspots -- reproducing this might
require the imposition of a preference for flux to emerge near the equator.Comment: 10 pages, 8 figures, accepted by A&
Inflows towards active regions and the modulation of the solar cycle: A parameter study
No full textAims. We aim to investigate how converging flows towards active regions affect the surface transport of magnetic flux, as well as their impact on the generation of the Sun’s poloidal field. The inflows constitute a potential non-linear mechanism for the saturation of the global dynamo and may contribute to the modulation of the solar cycle in the Babcock-Leighton framework.
Methods. We build a surface flux transport code incorporating a parametrized model of the inflows and run simulations spanning several cycles. We carry out a parameter study to assess how the strength and extension of the inflows affect the build-up of the global dipole field. We also perform simulations with different levels of activity to investigate the potential role of the inflows in the saturation of the global dynamo.
Results. We find that the interaction of neighbouring active regions can lead to the occasional formation of single-polarity magnetic flux clumps that are inconsistent with observations. We propose the darkening caused by pores in areas of high magnetic field strength as a possible mechanism preventing this flux-clumping. We find that inflows decrease the amplitude of the axial dipole moment by ~ 30%, relative to a no-inflows scenario. Stronger (weaker) inflows lead to larger (smaller) reductions of the axial dipole moment. The relative amplitude of the generated axial dipole is about 9% larger after very weak cycles than after very strong cycles. This supports the idea that the inflows are a non-linear mechanism that is capable of saturating the global dynamo and contributing to the modulation of the solar cycle within the Babcock-Leighton framework
The mean solar butterfly diagram and poloidal field generation rate at the surface of the Sun
No full textContext. The difference between individual solar cycles in the magnetic butterfly diagram can mostly be ascribed to the stochasticity of the emergence process.
Aims. We aim to obtain the expectation value of the butterfly diagram from observations of four cycles. This allows us to further determine the generation rate of the surface radial magnetic field.
Methods. We used data from Wilcox Solar Observatory to generate time-latitude diagrams of the surface radial and toroidal magnetic fields spanning cycles 21–24. We symmetrized them across the equator and cycle-averaged them. From the mean butterfly diagram and surface toroidal field, we then inferred the mean poloidal field generation rate at the surface of the Sun.
Results. The averaging procedure removes realization noise from individual cycles. The amount of emerging flux required to account for the evolution of the surface radial field is found to match that provided by the observed surface toroidal field and Joy’s law.
Conclusions. Cycle-averaging butterfly diagrams removes realization noise and artefacts due to imperfect scale separation and corresponds to an ensemble average that can be interpreted in the mean-field framework. The result can then be directly compared to αΩ-type dynamo models. The Babcock-Leighton α-effect is consistent with observations, a result that can be appreciated only if the observational data are averaged in some way
SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere
No full textIn this paper we describe the semi-spectral linear MHD (SLiM) code which we have written to follow the interaction of linear waves through an in homogeneous three-dimensional solar atmosphere. The background model allows almost arbitrary perturbations of density, temperature, sound speed as well as magnetic and velocity fields. We give details of several of the tests we have used to check the code. The code will be useful in understanding the helioseismic signatures of various solar features, including sunspots
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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