3
DAMTP Astro Seminars
http://sms.cam.ac.uk/collection/3309632
Recordings of the DAMTP Astro Seminar Series, held live on Mondays @ 2 pm via Zoom. Organised by the Astrophysical Fluid Dynamics, Nonlinear Dynamics and Atomic Astrophysics Groups based at DAMTP (Department of Applied Mathematics and Theoretical Physics).
1440
2024
Mon, 10 Jun 2024 15:20:12 +0100
Fri, 16 Oct 2020 14:21:36 +0100
en
smssupport@uis.cam.ac.uk
DAMTP Astro Seminars
http://sms.cam.ac.uk/collection/3309632
http://rss.sms.cam.ac.uk/itunesimage/3341340.jpg
http://video.search.yahoo.com/mrss
DAMTP Astro Seminars
Recordings of the DAMTP Astro Seminar Series, held live on Mondays @ 2 pm via Zoom. Organised by the Astrophysical Fluid Dynamics, Nonlinear Dynamics and Atomic Astrophysics Groups based at DAMTP (Department of Applied Mathematics and Theoretical Physics).
DAMTP Astro Seminars
Recordings of the DAMTP Astro Seminar Series, held live on Mondays @ 2 pm via Zoom. Organised by the Astrophysical Fluid Dynamics, Nonlinear Dynamics and Atomic Astrophysics Groups based at DAMTP (Department of Applied Mathematics and Theoretical Physics).
Cambridge University
Dr Cleo Loi
http://sms.cam.ac.uk/collection/3309632
DAMTP Astro Seminars
20201016T14:21:36+01:00
DAMTP
002295
no

20240304 Martin Laming  The First Ionization Potential Effect in Solar/Stellar Coronae and Winds
ucs_sms_3309632_4557053
http://sms.cam.ac.uk/media/4557053
20240304 Martin Laming  The First Ionization Potential Effect in Solar/Stellar Coronae and Winds
Seminar given by Martin Laming (Naval Research Laboratory) to the DAMTP Astrophysics group on Mon 4th March 2024
Mon, 04 Mar 2024 17:48:37 +0000
University of Cambridge
Martin Laming
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Seminar given by Martin Laming (Naval Research Laboratory) to the DAMTP...
Seminar given by Martin Laming (Naval Research Laboratory) to the DAMTP Astrophysics group on Mon 4th March 2024
Cambridge University
3465
http://sms.cam.ac.uk/media/4557053
20240304 Martin Laming  The First Ionization Potential Effect in Solar/Stellar Coronae and Winds
Seminar given by Martin Laming (Naval Research Laboratory) to the DAMTP Astrophysics group on Mon 4th March 2024
Since 1963, we have known, or at least suspected, that element abundances in the solar corona and wind are different to those in the photosphere. Elements that are predominantly ionized in the photosphere and chromosphere, e.g. Fe, Si, Mg, are observed to be enhanced in abundance once transported to the corona and wind by a factor of typically 34, while elements that are mainly neutral are relatively unaffected. Due to the dependence on the ionization potential, this phenomenon has been dubbed the First Ionization Potential (FIP) Effect. A model capturing the FIP and effect and the various modifications to it seen in different coronal regions and the solar wind invokes ionneutral separation by the ponderomotive force due to Alfven and magnetosonic waves propagating through the chromosphere. This acts on ions, but not neutrals, and depends on the interaction of the waves with the magnetic geometry of the solar atmosphere.
The launch of the Extreme Ultraviolet Explorer (EUVE) in 1992 afforded us the first observations of abundances in stellar coronae. This, and succeeding missions like Chandra, XMM Newton and Suzaku, have shown that solarlike stars show a similar FIP effect, which transitions to Inverse FIP (IFIP), i.e. a coronal depletion of Fe, Si, Mg, etc., as the activity level increases.
In this presentation, I will review the theoretical basis of FIP and IFIP fractionation by the ponderomotive force. Fractionation by such means is new to solar physics and astrophysics, but is not new to science. Manipulation of atoms, molecules, biological samples, etc. by the forces due to refraction of photons from lasers, known as “optical tweezers”, has a rather long history in optical sciences, and won Nobel Prizes for Steven Chu (in 1997) and Arthur Ashkin (in 2018). Our model is a precise analog of this work, but with magnetohydrodynamic waves instead of optical photons. This connection leads to a better physical understanding of the mechanisms at work in FIP fractionated plasma.
Work supported by NASA Heliophysics Supporting Research Program (80HQTR20T0076), and by Basic Research Funds of the Office of Naval Research.
20240314T13:10:41+00:00
3465
4557053
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Dr. Adrian Barker (University of Leeds)
ucs_sms_3309632_3653063
http://sms.cam.ac.uk/media/3653063
Dr. Adrian Barker (University of Leeds)
Tidal dissipation in stars, with new results on the interaction between fast tides and convection
Thu, 23 Sep 2021 03:10:32 +0100
University of Cambridge
Zhao Guo
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Tidal dissipation in stars, with new results on the interaction between fast...
Tidal dissipation in stars, with new results on the interaction between fast tides and convection
Cambridge University
3600
http://sms.cam.ac.uk/media/3653063
Dr. Adrian Barker (University of Leeds)
Tidal dissipation in stars, with new results on the interaction between fast tides and convection
I will present new results on tidal dissipation in stars and planets, mainly focussing on tides in convection zones. The interaction between tidal flows and turbulent convection is often considered important for tidal evolution of binary stars and planetary systems but its efficiency for fast tides (when the tidal frequency exceeds the convective turnover frequency) has long been controversial. I will first describe a new analysis of the energy transfer terms between tidal flows and convection that is verified by idealised Boussinesq and anelastic simulations. One result is that Reynolds stresses involving tidal flow components are unlikely to contribute to dissipation of equilibrium tides. I will then review the results of direct numerical simulations in both local and global models (that study Reynolds stresses involving correlations between convective flow components, which likely dominate the interaction), which demonstrate that convection can act like an effective viscosity which falls off quadratically with tidal frequency for fast tides. I will then present calculations studying tidal dissipation in stellar models with masses in the range 0.11.6 M_\odot throughout their evolution. These models incorporate turbulent viscosity acting on equilibrium tides and inertial waves in convection zones, and internal gravity waves in radiation zones. I will use these results to compute tidal quality factors following stellar evolution, and tidal evolutionary timescales, for the orbital decay of hot Jupiters, and the spin synchronization and circularization of binary stars.
20210923T03:10:32+01:00
3600
3653063
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Dr. George Mamatsashvili, April 26 2021
ucs_sms_3309632_3503403
http://sms.cam.ac.uk/media/3503403
Dr. George Mamatsashvili, April 26 2021
Zero net flux MRIturbulence in disks – specific anisotropy of nonlinear processes, sustenance and dependence on magnetic Prandtl number
Mon, 26 Apr 2021 17:30:02 +0100
disk,turbulence,magnetic
University of Cambridge
Zhao Guo
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Zero net flux MRIturbulence in disks – specific anisotropy of nonlinear...
Zero net flux MRIturbulence in disks – specific anisotropy of nonlinear processes, sustenance and dependence on magnetic Prandtl number
Cambridge University
3538
disk,turbulence,magnetic
http://sms.cam.ac.uk/media/3503403
Dr. George Mamatsashvili, April 26 2021
Zero net flux MRIturbulence in disks – specific anisotropy of nonlinear processes, sustenance and dependence on magnetic Prandtl number
We study the sustenance and effect of magnetic Prandtl (Pm) number for MRI turbulence in accretion disks with a zero net magnetic flux in the shearingbox. The zero flux case is unique, as there is no characteristic lengthscale for MRI that would grow purely exponentially and hence the instability is instead of a subcritical type, being energetically powered by linear nonmodal/transient mechanism of perturbation growth. This transient growth of MRI is “imperfect” in the sense that it is not able to ensure longterm sustenance of the turbulence. This should be compensated by a necessary positive nonlinear feedback, regenerating nonmodally growing MRI modes. To examine the existence of such a feedback and ultimately understand the selfsustenance process, we first perform numerical simulations with SNOOPY code and then do a detailed analysis of the turbulence dynamics in Fourier space. Our main results can be summarized as:
1. It was shown that the flow shear leads to anisotropy of nonlinear processes in Fourier space. A key factor for the sustenance is the existence and dominance of a topologically new type of a nonlinear process – an angular (i.e., over wavevector orientations) redistribution of modes in Fourier space, which we call the nonlinear transverse cascade in contrast to the classical direct/inverse cascade in classical (Kolmogorov, IroshnikovKraichnan) theories of isotropic turbulence without mean shear flow. The transverse cascade that we revealed is the generic nonlinear process of decisive importance in different kinds of shear flows.
2. The sustenance of the MRI turbulence is achieved by the interplay of the linear nonmodal growth of MRI and the nonlinear transverse cascade. These two processes mainly operate at length scales comparable to the box size (disk scale height) which we refer to as the vital area of the turbulence in Fourier space. The usual direct cascade merely transfers the energy of these modes from the vital area to large dissipative wavenumbers (small scales).
3. At large Pm, the transverse cascade prevails over the direct one, keeping most of the mode energy contained in small wavenumber (large scales) modes. With decreasing Pm, the action of the nonlinear transverse cascade weakens, so that it can no longer oppose the action of the direct cascade, which transfers energy of small wavenumber modes to higher wavenumber ones, where it is efficiently dissipated, leading to the decay of the turbulence. Thus, decreasing of Pm results in the topological rearrangement of the nonlinear processes when the action of the direct cascade begins to dominate over the action of the key agent for the turbulence sustenance – the nonlinear transverse cascade.
20210426T17:30:02+01:00
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Dr. Sacha Brun CEA ParisSaclay  seminar (Mon 7th June 2021)
ucs_sms_3309632_3548808
http://sms.cam.ac.uk/media/3548808
Dr. Sacha Brun CEA ParisSaclay  seminar (Mon 7th June 2021)
Powering Stellar Magnetism: Energy Transfers in Convective Dynamo
of Sunlike Stars
Mon, 07 Jun 2021 20:23:48 +0100
University of Cambridge
Zhao Guo
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Powering Stellar Magnetism: Energy Transfers in Convective Dynamo
of Sunlike...
Powering Stellar Magnetism: Energy Transfers in Convective Dynamo
of Sunlike Stars
Cambridge University
3960
http://sms.cam.ac.uk/media/3548808
Dr. Sacha Brun CEA ParisSaclay  seminar (Mon 7th June 2021)
Powering Stellar Magnetism: Energy Transfers in Convective Dynamo
of Sunlike Stars
We present recent advances made to understand the origin of
stellar magnetism thanks to an extensive 3D MHD study of convective
dynamo of solartype stars. We aim at characterising the detailed energy
transfers occurring in these model stars and the type of dynamos
realised (statistically steady, cyclic on either long or short time
scales), depending on the stellar global parameters considered. We
confirm that the Rossby number is a good proxy to identify key
transitions for the different dynamical behaviours found in our study.
We also propose various scaling laws for the large scale field, magnetic
flux and surface differential rotation that are qualitatively in good
agreement with observations of stellar magnetism. We also quantify the
amount of stellar energy converted into magnetic energy finding that
there is plenty of energy (up to 3% of the stellar luminosity) made
available to power surface eruptive events. We conclude by discussing
the socalled spotdynamo paradox, e.g.are starspots playing or not a
key role in the operation of stellar cyclic dynamos?
20210607T20:23:48+01:00
3960
3548808
true
4x3
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Modelling and Interpreting NonLTE Emission from Isolated Solar Structures
ucs_sms_3309632_4651398
http://sms.cam.ac.uk/media/4651398
Modelling and Interpreting NonLTE Emission from Isolated Solar Structures
Seminar given by Chris Osborne from Glasgow University to the DAMTP astrophysics group on 10th June 2024
Mon, 10 Jun 2024 15:20:11 +0100
University of Cambridge
Chris Osborne
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Seminar given by Chris Osborne from Glasgow University to the DAMTP...
Seminar given by Chris Osborne from Glasgow University to the DAMTP astrophysics group on 10th June 2024
Cambridge University
3311
http://sms.cam.ac.uk/media/4651398
Modelling and Interpreting NonLTE Emission from Isolated Solar Structures
Seminar given by Chris Osborne from Glasgow University to the DAMTP astrophysics group on 10th June 2024
Modern simulations of isolated structures such as solar prominences and filaments demonstrate increasingly complex and realistic behaviour down to fine spatial scales. However, the lack of selfconsistent optically thick radiation treatment limits their comparison to observations. This is primarily due to the difficulty converting magnetohydrodynamic (MHD) simulation cubes into emergent spectra for spectral lines that form outside of local thermodynamic equilibrium (LTE). Whilst complex, these spectral lines are the primary vector by which these structures can be interpreted and are key to exploiting cutting edge solar observatories.
In this talk I will discuss the formation of spectral lines outside of LTE (nonLTE), and the application of these techniques to models of isolated solar structures. I will also touch on potential pitfalls of a columnbycolumn approach to synthesis and discuss a new approach for scaling to threedimensional integrated radiative MHD simulations for these structures.
20240610T15:20:12+01:00
3311
4651398
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4x3
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3309632