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SEIKEN SYMPOSIUM
Helicity Thinkshop 3 Free Conference is closed
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helicity thinkshop 3
Affiliation University of Tokyo
Tokyo (Japan)
19.11.2017 - 24.11.2017

Organizing institutions

Institute of Industrial Science (IIS), University of Tokyo

Institute for Space-Earth Environmental Research (ISEE), Nagoya University

National Astronomical Observatory of Japan (NAOJ)

The Kajima Foundation

Main category Natural Sciences (Astrophysics and Astrononmy)
Alternative category Natural Sciences (Physics)
Conference/Workshop objectives

Scope:
Helicities (kinetic, magnetic, current, cross, etc.), as well as energies, are fundamental quantities of hydrodynamics (HD) and magnetohydrodynamics (MHD). In the presence of certain combination of anisotropies (caused by, e.g., rotation, gravity, magnetic fields, etc.), mirror and reflectional symmetries in HD and MHD turbulence are broken. Symmetry-breaking in turbulence is measured by pseudoscalars such as helicities, which represent the topological properties of turbulence. In helical turbulence, mean-field structures (global vorticity, mean magnetic field, etc.) can be generated through a dynamo action by turbulent motions. Therefore, the dynamic and magnetic activities of the Sun and stars are intimately related to turbulent helicities. These relationships have been extensively investigated, both theoretically and observationally.
In the past, two Helicity Thinkshops, mainly on solar physics, were held in 2009 and 2013 at the National Astronomical Observatory of China (NAOC) at Beijing, China (Chair: Hongqi Zhang). They originated from a Chapman Conference on Magnetic Helicity in Space and Laboratory Plasmas held at Boulder, USA, in 1998 (Chair: Alexei Pevtsov). This time we organize a Helicity Thinkshop in Tokyo, Japan.

Aims and Topics:
The aims of this workshop (Helicity Thinkshop 3) are

• to share frontier knowledge on the topic of helicity stemming from observational investigations in astrophysics and geophysics, and from numerical simulations and experiments in fluids and plasmas;
• to promote closer collaboration between different research fields involved in helicity studies (e.g., solar/stellar/geo, theory/modeling/experimental/observations);
• to construct models of phenomena potentially influenced by helicity whose underlying physical mechanisms are not entirely understood.

Topics to be discussed include

• insights on and estimates of helicity in the Sun and solar wind, helical structures on Earth and other astrophysical bodies, as well as in the fluid and plasma experiments;
• role of helicities in solar and stellar flares and in coronal mass ejections with an emphasis on space weather phenomena and their coupling with the Earth environment;
• role of helicities in dynamo theories and numerical modelling;
• sources of helicities in astro/geophysical context;
• future directions in helicity studies.

In order to enhance interdisciplinary communication, all speakers in Helicity Thinkshop 3 are expected to present their talks in plain, generic physics language.

Contact:

• Nobumitsu Yokoi: nobyokoi (at) iis.u-tokyo.ac.jp
• Kirill Kuzanyan: kuzanyan (at) gmail.com
Local organizing committee

Nobumitsu Yokoi (Chair, University of Tokyo), Shoji Koyama (University of Tokyo), Takashi Sakurai (National Astronomical Observatory of Japan), Yoichiro Hanaoka (National Astronomical Observatory of Japan), Masaoki Hagino (National Astronomical Observatory of Japan), Shin Toriumi (National Astronomical Observatory of Japan)

Scientific organizing committee (SOC)

Axel Brandenburg (Sweden/USA), Manolis Georgoulis (Greece), Kirill Kuzanyan (Co-Chair, Russia), Raffaele Marino (France), Alexei Pevtsov (USA/Finland), Takashi Sakurai (Japan), Dmitry Sokoloff (Russia), Nobumitsu Yokoi (Chair, Japan), Hongqi Zhang (China)

Sessions
• Sun., 19 Nov.: Registration and reception at IIS
• Mon., 20 Nov.: Talks and discussion at IIS
• Tue., 21 Nov.: Talks and discussion at IIS
• Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park
• Thu., 23 Nov.: Talks and discussion at IIS
• Fri., 24 Nov.: Visit to Solar Observatory and free discussion at NAOJ
Programme
Invited speakers
Important dates
• 18 May 2017: Start of on-line registration
• 23 October 2017: Deadline of abstract submission
• 31 October 2017: Program announcement
• 19 November 2017: Registration and reception
• 20-24 November 2017: Scientific talks and discussions
Registration and payment information

Registration fee is 10,000 yen (approx. 80 Euro or 90 USD) excluding the conference dinner (approx. 5,000 yen).

Registration fee should be paid in cash by Japanese yen on-site at the conference reception.

Request for an invitation letter, as well as other requests, if any, should be noted in the "Information" field of the Registration form.

Conference venue

Institute of Industrial Science, Univ. of Tokyo: Komaba, Tokyo

National Astronomical Observatory of Japan: Mitaka, Tokyo

Hotel information

University accommodations:

Some block of the following university accommodations are reserved for the workshop participants. Those who wish to stay there should name the accommodation and stay period (check-in and -out dates) in the "Information" field of the Registration form. Because of the limitation of the number of rooms, the accommodations and rooms are allocated on a first-come, first-served basis. The allocation will be individually announced later.

1. Komaba Faculty House: Located at the Komaba Main Campus of University of Tokyo, close to the Komaba-Todai-mae station of the Keio Inokashira Line, 7-minute walk from the conference venue
2. Sanjo Conference Hall Tatsuokamon Annex: Located at the Hongo Campus of University of Tokyo, close to Hongo-sanchome subway station in the Central Tokyo
3. Sanjo Conference Hall Main: Located at the Hongo Campus of University of Tokyo, close to the Hongo-sanchome subway station in the Central Tokyo
4. Mukougaoka Faculty House: Located at the Yayoi Campus of University of Tokyo, close to the Todai-mae and Nezu subway station in the Central Tokyo

Other hotels:

Hotels in Shibuya and Shinjuku areas are closer to IIS, but the access to IIS from hotels in the downtown Tokyo is in general easy with public transportation.

Travel information

Institute of Industrial Science (IIS), University of Tokyo

National Astronomical Observatory of Japan (NAOJ)

Financial support:

We have some limited funds for the travel support. In particular, for students, early career scientists and scientists from less developed countries. Those who wish to get partial financial support should tell us their support request in the "Information" field of the Registration form.

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Session: Mon., 20 Nov.: Talks and discussion at IIS

1849 views
22.11.2017
Co-author:
G. Hornig, M.H. Page
Abstract:
What if there were a way to identify **where** the magnetic helicity is concentrated within a three- dimensional magnetic field? At first sight this question appears meaningless, since magnetic helicity is an integral over the whole volume of the magnetic field. But, in fact, it is possible to decompose this total helicity as an integral over individual "field line helicities" for each magnetic field line in the domain. All of these are ideal-invariant, topological quantities, and they allow us to quantify in a meaningful way how magnetic helicity is distributed within the domain. In this talk, I will show how this idea can be practically applied to typical extrapolations of the Sun's coronal magnetic field that are used in solar physics.

Session: Tue., 21 Nov.: Talks and discussion at IIS

687 views
22.11.2017
Co-author:
H. Hotta, T. Yokoyama
Abstract:
High-resolution solar convection simulations have been suffering a problem called ‘convective conundrum’; they tend to fail in reproducing the observed solar-like differential rotation owing to an over-estimation of convective velocities and thus the Rossby number (e.g., Hanasoge et al. 2012, Gastine et al. 2013). One possible culprit for this problem is the prevailing small-scale magnetism generated by small-scale dynamo that cannot be fully resolved by current numerical simulations (Hotta et al. 2015, 2016). Several recent works, in fact, have demonstrated that the convective velocities can be suppressed if the essentially-magnetized solar convection operates in an effectively high-Prandtl number regime (O'Mara et al. 2016, Bekki et al. 2017). However, no rotational effects were considered in these previous studies. Here, we report the results of compressible convection simulations using a local f-plane box model with different Prandtl numbers and discuss how the deep convective amplitudes and stratification are influenced by rotation. The properties of the turbulent Reynolds stress and the resulting large-scale mean flows are also investigated and will be compared with the results of full-spherical convection simulations.

Session: Tue., 21 Nov.: Talks and discussion at IIS

1057 views
23.11.2017
Co-author:
Abstract:
Helicities as well as the turbulent energies are key players of the dynamo process. From viewpoint of turbulent transport, helicities mainly suppress the effective transports. In the presence of inhomogeneous large-scale flow, the turbulent cross helicity (cross-correlation between the velocity and magnetic-field fluctuations) enters the turbulent electromotive force and its suppression/generation effect tends to be balanced with the turbulent magnetic diffusivity. At the same time, the turbulent cross helicity coupled with the mean magnetic strain affects the momentum transport. In addition to these effects of helicities, in strong compressible magnetohydrodynamic (MHD) turbulence, the density variance contributes to the turbulent electromotive force as the coupling coefficient of the obliqueness of the mean magnetic field to the density gradient. This means that the density variance as well as the helicities alters the turbulent transport. This density-variance effect is expected to enhance the intensity of turbulence across the slow MHD shock, which may contribute to the realization of a localized fast magnetic reconnection.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

650 views
23.11.2017
Co-author:
G. J. D. Petrie, & N. K.:Singh
Abstract:
The solar magnetic helicity has opposite signs not only in the two hemispheres, but also at large and small length scales. The latter can be detected by computing magnetic helicity spectra, but this must be done separately in each hemisphere. Here we utilize a two-scale method from mean-field dynamo theory that allows us to compute magnetic helicity spectra as a function of two different wavenumbers: one corresponding to rapidly varying scale and one corresponding to a slowly varying one. We generalize this method to spherical harmonics and compute in that way global magnetic helicity spectra for that part of the field that shows a global dipolar symmetry. We present results from simple one-dimensional model calculations, three-dimensional dynamo simulations, and the two-dimensional magnetic field from synaptic vector magnetograms.

Session: Thu., 23 Nov.: Talks and discussion at IIS

551 views
23.11.2017
Co-author:
Abstract:
Electromotive force plays a central role in turbulent dynamo mechanisms and carries important information on the nature of the helical turbulent fields. An analysis method is proposed for the electromotive force to evaluate the transport coefficients for the alpha effect and the turbulent diffusivity directly from the measurements. The method is applied to a magnetic cloud event observed by the Helios-2 spacecraft in the inner heliosphere. The electromotive force is enhanced together with the magnetic cloud event by one order of magnitude or two, suggesting that the magnetic field may be amplified in the heliosphere for a short time. Relation between the helicity quantities and the electromotive force is discussed on the basis of the observation.

Session: Thu., 23 Nov.: Talks and discussion at IIS

531 views
23.11.2017
Co-author:
Abstract:
The coronal heating problem is also tied to the change of magnetic helicity, because the dissipation of magnetic energy is a key energy source in the corona. MHD models suggest that the photospheric field-line braiding mechanism (Parker, 1972) is the driver of the coronal heating. As a consequence, the helicity change rate should be a marker of the actual dissipation. Though, such a process may only operate, if plasma beta is sufficiently small in order to build up magnetic stress within a magnetic flux bundle. On the other hand, a different kind of heating is expected, when coronal current sheets arise between large-scale magnetic field topologies that interact, which we call coronal tectonics. The latter may also operate if plasma beta is large. From simulation data, we test plasma beta in the solar atmosphere and relate helicity changes with coronal thermal energy input in order to identify the heating mechanisms. We find that the heating along hot coronal EUV-bright loops indeed correlates with local magnetic helicity changes. Ultimately, the solar atmosphere releases helicity on large spatial scales into the solar wind that is later observed, e.g., by the Helios-2 spacecraft at 0.4 AU. We fit a model of helical magnetic field and vortical plasma flows to an observed magnetic transient event.

Session: Tue., 21 Nov.: Talks and discussion at IIS

556 views
23.11.2017
Co-author:
Abstract:
The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. We simulate slowly to rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives large-scale magnetic field. We apply the test-field method to characterise the dynamo mechanisms acting in this simulations by determining turbulent transport coefficients of the electromotive force. We find that the alphaeffect has a complex nature and does not follow the profile expected from kinetic helicity. However, the alpha effects in these simulations show strong rotational dependency resulting in highly anisotropic tensors and vanish alpha_zz components for rapid rotation. Furthermore, I will present the determination of magnetic helicity fluxes across the equator and through the surface, which are important quantities for the alleviation of catastrophically alpha quenching. Unlike in simulation of forced turbulence of Warnecke et al. (2011), the helicity fluxes across the equator are found to be much weaker in convection simulations. Moreover, I discuss the the relevants of magnetic and current helicity production in the dynamo region for the coronal heating process as well as to understand the activity-rotation-relation of main-sequence stars.

Session: Mon., 20 Nov.: Talks and discussion at IIS

553 views
23.11.2017
Co-author:
Gunnar Hornig, University of Dundee
Abstract:
I will consider some new developments in absolute measures of helicity (as opposed to measures relative to a vacuum field). These measures are defined for arbitrary foliations of space by simply connected surfaces (e.g. start with a set of nested spheres with spherical coordinates, then deform the spheres). They are based on generalizations of Poloidal-Toroidal decompositions. One application lies in measuring the helicity contained within the Northern hemisphere interior of the sun (or Southern hemisphere).

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

582 views
23.11.2017
Co-author:
Abstract:
The 1980s and 1990s saw seminal works on the role of magnetic helicity in the magnetized solar atmosphere. Thanks to helicity, our understanding of the quiescent and eruptive solar magnetism has progressed substantially since then. From the plausible necessity of coronal mass ejections (CMEs) as sinks of excess solar magnetic helicity in the heliosphere to the solar cycle-independent hemispheric helicity preference, likely imposed by the steady solar differential rotation, to the actual estimates of active-region, quiet Sun, and CME helicities, we have come to place magnetic helicity on equal footing with the electric-current-induced (i.e., non-potential) magnetic energy that fundamentally fuels solar instabilities and eruptions. In spite of this tremendous progress, however, there is still is a lot to learn: first, we need to optimize the way magnetic helicity is practically calculated in local and global solar scales. Then, we need to determine the interplay between different helicity terms that seem to hold distinct aspects of the physics of the system. Furthermore, the role of spectral helicity characteristics, as well as the competition between opposite senses of helicity in a single magnetic structure and its role to eruptions, need to be further clarified. We attempt a resume of what we know, what we are hinted about, and what we should aim to achieve in hopes of spurring a discussion between involved researchers that could further advance the state of the art in the field. This account will be attempted in a plain, physical language to hopefully enable cross-fertilization between different physical domains where helicity is deemed to play a role.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

634 views
23.11.2017
Co-author:
Abstract:
By using the photospheric vector magnetogram and subsurface vector velocity field, we studied the systematic behavior of current helicity and relevant quantities of subsurface flow. Mainly it contains three aspects: 1) The spatial distribution and temporal evolution of current helicity. 2) The connection between the current helicity and subsurface kinetic helicity. 3) The spatial distribution ant temporal evolution of subsurface vorticity and divergence.

Session: Thu., 23 Nov.: Talks and discussion at IIS

462 views
24.11.2017
Co-author:
Abstract:
Solar flares and coronal mass ejections (CMEs) are the catastrophic releasing of magnetic energy and helicity. It is known that strong eruptions take place in complex active regions (ARs). We surveyed all ARs that produced >M5.0-class events for 6 years from May 2010 to April 2016. Morphologically, these ARs can be classified into four categories, namely, (1) Spot-Spot, a complex AR with an extended magnetic neutral line, (2) Spot-Satellite, in which a newly-emerging field appears next to the pre-existing sunspot, (3) Quadrupole, where two emerging fields collide against each other, and (4) Inter-AR, the flares occurring between two separated ARs. We found, for example, that the flare durations are longer for the Spot-Spot events than the Spot-Satellite ones. We then reproduced these four AR groups by conducting a series of 3D MHD flux emergence simulations and found that the sheared magnetic structures in these ARs are created through the stretching and advection of horizontal magnetic fields due to relative spot motions. As ARs develop, free magnetic energy becomes stored in the higher atmosphere, which could be eventually released through flare eruptions.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

477 views
26.11.2017
Co-author:
Tomoya Iju, Muhamad Johan, Satoshi Inoue, Naoyuki Ishiguro, Yuki Asahi, Yuta Mizuno, KD Leka, and Sung-Hong Park
Abstract:
Solar eruptions manifesting as solar flares and coronal mass ejections (CMEs) are the explosive liberations of magnetic energy contained in the solar corona. However, the onset mechanism of solar eruptions is not yet clearly explained, although many different models have been proposed so far. In particular, what triggers solar eruptions is an important question for improving the predictability of solar eruptions. Here, we propose a new scheme of prediction of solar eruptions based on the critical condition of eruptive instability in the solar corona, which was recently proposed by Ishiguro and Kusano (2017). Their analysis indicated that a new parameter kappa, which is given by the magnetic twist, provides a critical condition to destabilize the sheared magnetic field. We build a new predictive scheme of solar eruptions using the analysis of magnetic twist and demonstrate how the structure of magnetic twist in solar active regions correlates with the activities of solar eruptions based on the space-born data of several active regions.

Session: Thu., 23 Nov.: Talks and discussion at IIS

547 views
27.11.2017
Co-author:
KD Leka, Kanya Kusano
Abstract:
We investigated statistical characteristics of magnetic helicity injected per unit time through the active region photosphere using vector magnetic field data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The photospheric helicity injection rate of a given active region was calculated from the formula of the gauge-invariant relative helicity flux derived by Berger and Field (1984), applying the optical flow technique Differential Affine Velocity Estimator for Vector Magnetogram (DAVE4VM) to co-registered pairs of vector magnetograms sampled 12 minutes apart. We determined helicity injection rates for a total of 30,628 SHARP vector magnetogram pairs sampling 866 unique active regions from 2012 September to 2016 December. Using this large data set, we studied long-term, large-scale characteristics of helicity injection rates in the active region photosphere. We will present how helicity injection rates are distributed with respect to Heliographic latitude, Carrington longitude, solar cycle epoch and Hale magnetic class. It will be also discussed how helicity injection rate is related to other magnetic parameters and flare productivity of active regions.

Session: Tue., 21 Nov.: Talks and discussion at IIS

538 views
27.11.2017
Co-author:
Nobumitsu YOKOI, Fujihiro HAMBA
Abstract:
Recent numerical simulation revealed that large-scale flow is generated in a rotating system with inhomogeneous helicity. However its mechanism was not examined in terms of the Reynolds stress transport. In this study, the large-eddy simulation (LES) of the rotating inhomogeneous turbulence with/without helicity injection is performed. As a result, it is shown that the mean flow is only generated when both the system rotation and helicity injection exist. In the Reynolds stress transport equation, the pressure diffusion term has predominant contribution in the way that it supports the generated mean flow. The close relationship between the pressure diffusion term and inertial wave is discussed.

Session: Thu., 23 Nov.: Talks and discussion at IIS

532 views
27.11.2017
Co-author:
Takashi Sakurai
Abstract:
The fine structure in solar filaments has been known to show the chiral nature depending on the hemisphere, and it is presumed to be a manifestation of the helicity in the magnetic field generated by the solar dynamo. To investigate the chirality of the filament magnetic field, we carried out a statistical study of the magnetic field orientation in solar filaments based on our daily full-Sun, full-Stokes spectropolarimetric observations with the He I 10830 line. The analysis of more than 400 filaments revealed that the average direction of the magnetic field in filaments generally deviates from their axis by 10-30 degrees, and the direction of the deviation strongly depends on the hemisphere where the filaments appear. This hemispheric pattern is consistent with the well-known chirality pattern of the fine structure seen in filaments, and for some of the filaments we can confirm that the magnetic field direction is parallel to their fine structure.

Session: Mon., 20 Nov.: Talks and discussion at IIS

619 views
30.11.2017
Co-author:
Abstract:
*Quadratic Helicity in MHD* We discuss the definitions and properties of the helicity $\chi$ and of the quadratic helicities \chi^{[2]} and \chi^{(2)} in MHD. A formula for the practical computation of the quadratic helicity density $\chi^{(2)}$ is presented. We also explain why the examples of geodesic flows by P. Dehornoy determine magnetic fields in a 3D domain $\Omega$ of minimal magnetic energy with a constant helicity density and minimal quadratic helicity: $\chi^{(2)} = \chi^2/Vol(\Omega)$. This is a joint result with Simon Candelaresi and Alexandr Smirnov.

Session: Mon., 20 Nov.: Talks and discussion at IIS

553 views
04.12.2017
Co-author:
Abstract:
In our conventional understanding, large-scale magnetic fields are thought to originate from an inverse cascade in the presence of magnetic helicity, differential rotation or a magneto-rotational instability. However, as recent simulations have given strong indications that an inverse cascade (transfer) may occur even in the absence of magnetic helicity, the physical origin of this inverse cascade is still not fully understood. We here present two simulations of freely decaying helical and non-helical magnetohydrodynamic (MHD) turbulence. We verified the inverse transfer of helical and non-helical magnetic fields in both cases, but we found the underlying physical principles to be fundamentally different. In the former case, the helical magnetic component leads to an inverse cascade of magnetic energy. We derived a semi-analytic formula for the evolution of large-scale magnetic field using α coefficient and compared it with the simulation data. But in the latter case, the α effect, including other conventional dynamo theories, is not suitable to describe the inverse transfer of non-helical magnetic energy. To obtain a better understanding of the physics at work here, we introduced a `field structure model' based on the magnetic induction equation in the presence of inhomogeneities. This model illustrates how the curl of the electromotive force leads to the build up of a large-scale magnetic field without the requirement of magnetic helicity. And we applied a quasi-normal approximation to the inverse transfer of magnetic energy.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

469 views
06.12.2017
Co-author:
Abstract:
In this talk, we would like to analyze the configuration and evolution of magnetic fields and the corresponding relationship with the magnetic non-potentiality and helicity of active regions by means of observations of solar vector magnetograms. We present the magnetic helicity distribution and the change with the solar cycles based on the statistical analysis of the magnetic field in solar active regions. We also discuss some questions on the solar vector magnetic field and helicity from the solar observations.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

504 views
06.12.2017
Co-author:
Abstract:
In this talk, we would like to analyze the configuration and evolution of magnetic fields and the corresponding relationship with the magnetic non-potentiality and helicity of active regions by means of observations of solar vector magnetograms. We present the magnetic helicity distribution and the change with the solar cycles based on the statistical analysis of the magnetic field in solar active regions. We also discuss some questions on the solar vector magnetic field and helicity from the solar observations.

Session: Mon., 20 Nov.: Talks and discussion at IIS

756 views
08.12.2017
Co-author:
Mitchell Berger
Abstract:
It is known that the poloidal field is at its maximum during solar minima, and that its behaviour during this time acts as a strong predictor of the strength of the following solar cycle. This relationship relies on the action of differential rotation (the Omega effect) on the poloidal field, which generates the toroidal flux observed in sunspots and active regions. We measure the helicity flux into both the northern and southern hemispheres using a model that takes account of the Omega effect, which we apply to data sets covering a total of sixty years. We find that the helicity flux offers a strong prediction of solar activity up to 5 years in advance of the next solar cycle.

Session: Tue., 21 Nov.: Talks and discussion at IIS

600 views
09.12.2017
Co-author:
N. YOKOI
Abstract:
We study effects of the cross-helicity in the full-sphere large-scale mean-field dynamo models of the $\mathrm{0.3M_{\odot}}$ star rotating with the period of 10 days. In exploring several dynamo scenarios which are stemming from the cross-helicity generation effect, we found that the cross-helicity provide the natural generation mechanisms for the large-scale scale axisymmetric and non-axisymmetric magnetic field. Therefore the rotating stars with convective envelope can produce the large-scale magnetic field generated solely due to the turbulent cross-helicity effect (we call it $\gamma^{2}$-dynamo). Using mean-field models we compare properties of the large-scale magnetic field organization that stem from dynamo mechanisms based on the kinetic (associated with the $\alpha^{2}$ dynamos) and cross-helicity. For the fully convective stars both generation mechanisms can maintain a large-scale dynamos even for the solid body rotation law inside the star. The non-axisymmetric magnetic configurations become preferable when the cross-helicity and the $\alpha$-effect operate independently of each other. This corresponds to situations of the purely $\gamma^{2}$ or $\alpha^{2}$ dynamos. Combination of these scenarios, i.e., the $\gamma^{2}\alpha^{2}$ dynamo can generate preferably axisymmetric, dipole-like magnetic field of strength several kG. Thus we found a new dynamo scenario which is able to generate the axisymmetric magnetic field even in the case of the solid body rotation of the star. We discuss the possible applications of our findings to stellar observations.

Session: Wed., 22 Nov.: Talks and discussion at IIS, conference dinner at Ueno Park

439 views
11.12.2017
Co-author:
Abstract:
In Masada & Sano (2016), we reported the successful simulation of spontaneous formation of surface magnetic structures from a large-scale dynamo in strongly-stratified plane-layer convection. The large-scale dynamo observed in our model had physical properties similar to those in earlier weakly- stratified convective dynamo simulations (Kapyla et al. 2013; Masada & Sano 2014a,b), suggesting that the α^2-type mechanism is responsible for it. In this talk, we will present our recent results on the Rossby number (Ro) dependence of the large-scale dynamo in the strongly-stratified convection. From our study, we found that the critical Rossby number that separates the success and failure of the dynamo is in the range 0.02 < Ro < 0.04, which is compatible with the recent global convective dynamo simulation of solar-like stars by Kapyla et al. (2013). We also find that the Ro-dependence of the large-scale dynamo can be reproduced by the mean-field α^2-type dynamo model constructed with using turbulent velocity profiles extracted from the DNS results. We finally discuss the reason why the model with the higher Rossby number fails to sustain the dynamo with focusing on the profiles of the turbulent transport coefficients.

Session: Mon., 20 Nov.: Talks and discussion at IIS

636 views
17.12.2017
Co-author:
David Pontin, Gunnar Hornig, Christopher Berg-Smiet, Dirk Bouwmeester
Abstract:
Magnetic helicity is a conserved quantity under an ideal evolution. Here we present methods for simulating such topology conserving systems. We make use of Lagrangian grids and mimetic differential operators. It is shown that the magnetic field topology is exactly conserved. This method is then used to study equilibria of configurations like the Hopf fibration.

Session: Tue., 21 Nov.: Talks and discussion at IIS

1143 views
10.01.2018
Co-author:
Abstract:
Collision-less large-Reynolds-number astrophysical plasmas are prone to turbulence. In this context, it is necessary to consider the impact of turbulence during believed magnetic reconnection events in solar and stellar flares or in planetary magnetospheres. Magnetic reconnection is a multi-scale process and turbulence can be the key to bridge the gap between the magnetic energy release at large scales due to magnetic diffusion at small scales through the Richardson's picture of direct and inverse energy cascade. Moreover, diffusion of magnetic field by turbulence at small scales might lead to fast reconnection. Such an interaction between turbulence and fast magnetic reconnection in weakly dissipative plasmas is considered through the plasmoid instability. The turbulent transport coefficients are characterize by a turbulent mean-field model and are identified as a turbulent diffusion, crosshelicity and a residual helicity. These turbulent coefficients are found to lead to fast reconnection for a single 'X'-point current sheet as well as in the case of multiple 'X'-points, as present in plasmoid unstable current sheets. For the plasmoid instability, the turbulent coefficients are also found to be responsible for fast reconnection. In addition, the dynamics between diffusion and sustainment of magnetic field, related to the turbulent diffusion and residual helicity effects, are shown to be important for fast magnetic reconnection in presence of strong guide-magnetic field perpendicular to the reconnection plane. For residual helicity intensities stronger than turbulent diffusive ones, a time delay in reaching fast reconnection regime is observed. Finally, this turbulence dynamics obtained during the fast magnetic reconnection phase of the plasmoid instability is used to relate energetic electrons often found near astrophysical current sheets and magnetic reconnection. We obtain that fast energetic particles are accelerated by turbulence during fast reconnection processes if residual helicity intensities are weaker than the strength of the diffusion of magnetic field by turbulence.

Session: Tue., 21 Nov.: Talks and discussion at IIS

538 views