Nordita Program on Turbulence and Dynamos (17 March 2008 to 11 April 2008)

Final Report

Overall structure of the program

The overall duration was 4 weeks. It started with a 4 day conference on the same topic. The actual program ran during the following 3 weeks. The total number of participants was 52, and 8 of them were women. Of the 52 participants, 12 came from the Nordic countries, 24 from Europe, 7 from America, 3 from India, 3 from Russia, 2 from Israel, and one from Australia. During the long-term program, we had 21 people, of whom 14 stayed for more than about 3 weeks. Additional funding for the conference was attracted from the Swedish Science Foundation (VR) and the European Science Foundation (ESF) through the AstroSim program.

The conference programme had 36 talks of 30 min duration plus the weekly joint AlbaNova and Nordita Colloquium that was given by Bryan Gaensler on the Square Kilometer Array. In addition, there were 4 scheduled discussion sessions that where hosted by one or two senior participants. The conference received positive feedback throughout. Particularly worthwhile was the time set aside for discussions. This provided an excellent opportunity to focus attention on the big outstanding issues that we all hope will be addressed in the next few months and years. Many participants will meet again at the upcoming program at the KITP on Dynamo Theory.

During the long-term program, we had 5 regular seminar talks, of which two were presented by people in Stockholm area (Erik Lindborg from KTH and Andris Vaivads from the IRF in Uppsala), who were not participants of the program, and one was a talk of general interest for everybody at Nordita and the Physics Department. This talk also served as a summary talk of the program. The talk was announced like all other Nordita talks, but with an explicit comment that it is of general interest to all Nordita staff.

We also had 4 open discussion sessions that were, like the other talks, publicized on the web site. These discussion sessions had a duration of 2 hours each and where triggered by provocative statements by 2 or 3 participants for each session. The topics covered include the question of dynamically important large-scale magnetic fields in galaxies, incoherent alpha-shear dynamos, convective dynamos, and turbulent transport.

Each day we had group coffees at 10-10:30 and 16-16:30. These events provided a convenient opportunity for everybody to get out of their offices and to discuss the latest results. Some people came with their laptops and showed their latest simulation, and others discussed around the white board. The question of catastrophic alpha quenching was a recurrent topic during several coffee gatherings. The informal coffee sessions were also the time when new ideas where born.

Scientific impact so far

One of the big scientific questions in dynamo theory that was still quite open by the end of the conference concerned the possibility of magnetic Reynolds number-dependent quenching of the turbulent diffusivity. Several aspects to this question were known, but nothing conclusive yet. A breakthrough came in the third week when several participants came to the conclusion that the test field method does give valid results also in the nonlinear regime. This and other such highlights are, of course, signs of success of the program.

  • Theoretical work involved different effects in mean-field dynamo, e.g., shear-current effect, kinetic helicity fluctuations with a zero mean, and vorticity dynamo using renormalization approach. In particular: joint work on formation of meso-granular structures in turbulent convection in numerical simulations, laboratory experiments and theory, helical dynamos in turbulent convection in numerical simulations and theory, as well as the vorticity dynamo in turbulent convection in numerical simulations and theory (Kapyla, Kleeorin, Rogachevskii et al.); work on the formation of large-scale inhomogeneous structures in solar convective zones (Brandenburg, Kleeorin, Rogachevskii), work on the origin of magnetic fields in Ap-stars (Blackman, Kleeorin, Rogachevskii), and discussion on applications of second-order correlation approximation to sheared turbulence and magnetic helicity effects in mean-field dynamos (Kleeorin, Rogachevskii, with Raedler and Rheinhardt) and the effects of shear on dynamos and turbulence (with Eun-jin Kim), as well as discussions on non-helical mean-field dynamos and effects of kinetic helicity fluctuations with a zero mean (Kleeorin, Rogachevskii, with Blackman, Brandenburg, Hughes, Kapyla, Korpi, Nazarenko, Proctor, Sur, Subramanian, Tavakol, Tobias, Vishniac, Yousef), on the small-scale dynamo (Kleeorin, Rogachevskii, with Brandenburg, Pouquet, Shukurov), and on formation of inhomogeneous magnetic fields in galaxies (Kleeorin, Rogachevskii, with Beck and Shukurov).

  • Numerical work on dynamo action in the interstellar medium, in particular work on dynamically dominant large-scale magnetic fields in diffuse interstellar gas (Brandenburg, Korpi, Shukurov), dynamo action in the multi-phase interstellar medium (Brandenburg, Korpi, Shukurov), observational estimates of the current helicity in the Milky Way (Baggaley, Brandenburg, Shukurov), new approaches to the flux rope dynamo (Baggaley, Subramanian, Shukurov), density wave theory and galactic dynamos (Sridhar, Subramanian, Shukurov), telegraph equation in dynamo theory (Subramanian, Sur, Shukurov), and the global structure of magnetic and velocity fields in M51 (Beck, Fletcher, Shukurov). Shukurov and Subramanian also worked on their joint book on astrophysical magnetic fields to be published by Cambridge Univ. Press.

  • Small-scale turbulence (Blackman, Brandenburg, Kim, Pouquet, Sridhar, Subramanian),

  • Numerical determination of turbulent transport coefficients in the presence of shear (Brandenburg, Kim, Mitra, Tavakol, Kapyla) and in particular in the nonlinear regime (Brandenburg, Raedler, Rheinhardt, Subramanian).

  • Suppression of the magneto-rotational instability due to supernova explosions in galactic disks (manuscript in preparation); this project greatly benefited from the discussions with Shukurov, Brandenburg and Blackman.

  • Turbulent stresses from the magnetorotational instability in disk systems with varying shear rate (manuscript Liljestrom, Korpi, Kapyla, Brandenburg in preparation); discussions with Lyra and Blackman are acknowledged.

  • Dynamically dominant large-scale magnetic fields in diffuse interstellar gas (manuscript Shukurov, Fletcher and Korpi in preparation); one of the discussion sessions of the program was dedicated to the topic, and very useful comments were obtained.

  • Dynamo action in the multi-phase ISM (Shukurov, Brandenburg and Korpi); planned to continue the SN-forced turbulence calculations with the Pencil Code with the aim to explore magnetic fields in various phases of the ISM.

  • Large-scale dynamo action in shearing convection (Kapyla, Brandenburg, Korpi): during the program conference Hughes and Tobias presented results of Boussinesq convection with rotation and shear, where they found only small-scale dynamo action to occur even though the flow exhibits net helicity. Simultaneously, we have found large-scale dynamo action in stratified shearing convection, the most important difference being the lower magnetic Reynolds number of our calculations. Calculations with higher Rm have now been initiated to resolve this issue. A lot of important feedback on this project was obtained during the discussion sessions from various long-term participants.

  • On the test field method in the non-linear regime (Subramanian, with Brandenburg, Raedler and Rheinhardt). It is remarkable that the same method which works for kinematic regime has a chance of working for a non-linear regime.

  • Density wave/dynamo wave interaction (Subramanian, with Shukurov and Sridhar). Sridhar has some notes on this, which we will have to persue further.

  • The effect of strong shear in the "vorticity" dynamo, and eventually in non-helical dynamos (Subramanian with Sridhar). Sridhar has some notes setting up the same equations which I had derived last-year; he may progress better in different way of solution by going to shearing co-ordinates.

  • Trying to understand further when flutuating alpha-effect can work in galactic context; seem to need very strong shear, but seem to circumvent Kleeorin-Rogachevskii arguments, although it is not quite clear exactly why so (Subramanian with Sur). Draft will soon be written summarizing findings.

  • Further discussions on the flux rope dynamos (Subramanian, with Shukurov and Baggaley). Baggaley already had a draft.

  • The causes of fluctuations in alpha/eta_t in kinematic regime, even in isotropic helical turbulence (Subramanian with Brandenburg). More ideas emerged based on Green's function formulation for both homogneous/particular solution of the "b" equation.

    Feedback from the participants

    The program was excellent and beneficial, providing ample opportunities for exchanging ideas and active discussions with other participants and also for exploring the possibility for collaborations. In particular, discussion sessions on various aspects of turbulence and dynamos were very useful and brain-storming while morning and afternoon coffee/tea times and lunch/dinner enabled us to be engaged in more informal and casual discussions. I myself have presented a talk in the conference and also contributed to the discussion session.

    The program also provided excellent working environment in terms of office space, computing facilities, etc. In particular, I was able to catch up with what was happening at my home institute through the laptop/Internet connection. The housing was superb with a spacious bedroom, modern facilities, etc. It could have been even better with a phone!

    I benefited from the discussions organized during the program - the topics were carefully chosen, and the participants were providing deep, first-hand insights. However, the frequency of discussions and seminars might be too high (especially when seminars organized in the University and Nordita independently of the program are also included), with at least one event (and often, more) each day. Even though all the discussions and seminars were very interesting and useful, there was not much time left for joint work with the participants. However, I heard from several program participants that they were especially happy about the number and intensity of the discussions -- my opinion may be just subjective.

    Nordita's facilities and accommodation were excellent. I especially appreciated that good coffee was available at all times! The apartments are in perfect state, and they are conveniently located with easy metro link to Nordita. It is very important that all the apartments are in the same building, so that the participants can interact easily as much as they wish. It would be useful to provide telephone connection between different the apartments. This was discussed between the participants, and one of the ideas that emerged was to offer the participants SIM cards with Stockholm phone numbers for their mobile phones for the duration of the program.

    The discussions organized during the program were very useful, and in my opinion there could have been more of them (perhaps twice a week) throughout the program. The amount of seminars, on the other hand, was maybe too high, especially towards the end of the program.

    Even though I did not myself stay at the apartments, I heard a lot of positive comments about them. Actually, I think it would have been more useful for myself to stay there too, as many people continued discussing after dinner at the apartments as they were all close to each other. The only negative feedback I received was due to the reimbursement, either because the emailed instructions of how and what to fill in were slightly confusing, and for the quite long delays of receiving them. All in all the Nordita facilities were excellent.

    The discussions during the program were very good, although I did feel that one could have had bit more time for work. With morning and evening coffee break, long discussion session, and occasional seminar, the day sometimes ended without being able to concretely calculate. Of course, we were all party to the overall structure. Also it may be better to err on the side of having more discussion than less. So I don't have suggestion for better framework.

    The practical facilities at the Nordita were excellent. The accommodation was superbly convenient, very comfortable, near the metro, shops, pizza place etc! I agree a public phone would be a good idea. Laila was particularly very approachable and helpful. It is really a boon for program organizers that Nordita has these 20 odd flats. The fact that most people stayed at one place was also very conducive to further interactions and discussions.

    Nordic impact:

    Large proportions of Nordic participants came from Finland and Sweden. This reflects the existing distribution of Nordic expertise in the field of magnetohydrodynamics. Except for an unexpected absence of participants from Denmark, the current numbers are satisfactory and consistent with our prior expectations. The number of Nordic participants that stayed for the full program was 6, which corresponds to about 1/3 of the long term participants.

    Technical problems during the conference:

    Some technical problems were anticipated, but our expectations have been vastly exceeded.

    1. The wireless network was flaky on Tuesday and Wednesday and was fixed during Wednesday lunch by a technician from IT&Media.

    2. The projector in Oskar Klein did not work on Wednesday and Thursday, so we used the spare one during those days. Later on this turned out to be an even bigger problem and an entirely new projector is now being ordered.

    3. The sound faded away for brief spells of 2-5 seconds. This problem was fixed during the third day by replacing microphone-1 by microphone-2. Earlier suggestions to replace the batteries turned out to be of no use.

    4. The screen jammed on Thursday, and Goesta was not available to help. This may have been an error in using the system, but it was not clear at the time.

    5. We ran out of bottled water since Tuesday. Three bottles were delivered on the first day, but that was all. A daily inspection of the auditorium would have been appreciated.

    6. One laser pointer was lost by Wednesday morning and was replaced by the Nordita laser pointer. That one too was almost lost at one point, but it was recovered later.

    Suggestions for future programs

    Many people complained about the absence of any phones in the apartments. This meant that people who where picked up by taxi in the early morning could not be notified by the taxi driver. Also, of course, the apartment was the only place where the participants could have carried out extended telephone conversations that would not have disturbed the other roommates. There was internet available, so they could have used skype, but only a small number of people was familiar with skype-out and skype-in and had no headsets. Skype also does not work on the 112 number. A pay phone somewhere in the building would have been great. Prepaid SIM cards would be another possibility.

    There were apparently no instructions in many apartments. Some participants panicked when they found out that the door key jammed. Later this turned out to be a known problem, but not to the participants. This, and any general information regarding the apartments should also be made available on the internet on the Nordita wiki pages. This way the information would always be up-to-date (at least for people using the internet version) and could be inspected by program organizers who don't live in those apartments.

    All three programs so far had identified one talk that is of general interest to all Nordita staff and others in the physics department. It is good if this tradition could be kept up, but an attempt should be made to increase the attendance.