RAS "G/MIST" discussion meeting
8 December 2000

Geological Society Lecture Theatre, Burlington House, Piccadilly, London

[Report published in Astronomy & Geophysics 42, 2.22 (2001)]

A. Programme B. Report C. Abstracts

Self Organised Criticality and Turbulence in the Solar System

Organisers: Prof. S.C. Chapman (Warwick University), Dr M.P. Freeman (British Antarctic Survey) and Dr S. Oughton (University College London)


10:00 Coffee

10:30-13:00 Morning Session, Chair: S. Oughton

10:30 R Grappin and A Mangeney (DESPA, Meudon, Paris) SJ Schwartz (Queen Mary, London) WC Feldman (LANL, Los Alamos, New Mexico) Solar Wind Acceleration by Impulsive Events
10:40 Christina Pagel & André Balogh (Space and Atmospheric Physics, ICSTM, UK) Intermittency in Heliospheric Turbulence
10:50 Louise Harra, (MSSL, UK), Turbulence in solar flare plasma
11:05 Markus J. Aschwanden, (Lockheed Martin, Palo Alto, USA), Self-Organized Criticality and Solar Flares
11:20 Richard Dendy, (UKAEA Culham, UK), Sandpile paradigm and fusion plasma phenomenology
11:35 Sebastian Galtier, (Mathematics Institute, University of Warwick, U.K.) Intermittency in a 1D MHD model of solar flares
11:50 D. Hamon, H. J. Jensen, M. Nicodemi, (Dept of Mathematics, ICSTM, UK), Energy emission from the sun and dissipative SOC models
12:05 Loukas Vlahos, (Department of Physics, University of Thessaloniki, Greece) Statistical flare models
12:20 Vincenzo Carbone, (Dipartimento di Fisica, Universita' della Calabria, Italy) Waiting Time Statistics in Solar Flares
12:35 S. C. Chapman, G. Rowlands (1), N. W. Watkins(2) ((1)Univ. of Warwick, UK, (2) BAS, UK), Extreme statistics and universal fluctuations in highly correlated systems.

12:45 Discussion
13:00-14:00 Lunch and Posters

R. Bruno(1), V. Carbone(2), L.Sorriso-Valvo(2), E. Pietropaolo (3), B. Bavassano(1) (Istituto Fisica Spazio Interplanetario del CNR, Italy (2) Dipartimento di Fisica Univ. della Calabria/INFM, Italy (3) Dipartimento di Fisica, Univ. di L'Aquila, Italy) Observations on Solar Wind Intermittency: radial evolution and characterization of intermittent events
A L MacKinnon (University of Glasgow), K P Macpherson (University of Central Lancashire) , Investigating limit cycle behaviour in avalanche models with nonlocal communication
B. Hnat, S. C. Chapman (Univ. of Warwick), Complexity in sandpile models for astroplasma confinement systems
V.Antoni (1,4), V. Carbone (2,3), R. Cavazzana (1), L. Fattorini (1,4), G.Regnoli (1,4), E. Spada (1), N. Vianello(1,4),((1) Consorzio RFX, corso Stati Uniti 4, Padova, Italy (2) Dipartimento di Fisica, Universita` degli Studi della Calabria, Rende (CS), Italy (3) INFM, Unita` di Cosenza, Sez. A, Italy,(4) INFM, Unita` di Padova, Sez. A, Italy) Bursty transport in magnetically confined plasmas: avalanche-like processes or MHD turbulence?
V. Uritsky, A. Klimas, (NASA GSFC, USA), Nonlinear chaos as a source of complexity in a 1D continuum current sheet model
A. Pacini, J. -P. Raulin, P. Kaufmann, E. Correia, (Univ. Pres. Mackensie, Brasil) Statistical study of simple and impulsive solar microwave flares
P. Kaufmann, J. -P. Raulin, E. Correia, J. E. R. Costa, C. G. Gimenez de Castro, A. V. R. Silva, (Univ. Pres. Mackensie, Brasil ), H. Levato (Comp. Astronomico El Leoncito, Argentina) M. Rovira, C. Mandrini, R. Fernandez-Borda (Inst. de Astro. y Fisica del Espacio, Argentina), O. H. Bauer (MPE Garching, Germany), Fast brightenings at submillimeter waves only associated with a large solar flare
V. Krasnoselskih, O. Podladchikova (1), B. Lefebre (2), N. Vilmer (3),(1) LPCE-CNRS, France, (2) Kyuchu Univ. of Fukuoka, Japan (3) Obs. de Paris- Meudon, France Quiet sun coronal heating: sandpile reconnection model.

14:00-15:30 Afternoon Session, Chair: S. C. Chapman

14:00 Tim Horbury, (ICSTM,UK), Field aligned anisotropy in MHD turbulence
14:10 A J Klimas, V Uritsky, D Vassiliadis, and D N Baker (NASA/Goddard), USA, Multi-Scale Turbulence in a 2-Dimensional Current Sheet Model
14:25 Giuseppe Consolini, (IFSI/CNR, Italy), Self- organised criticality vz criticality in magnetotail dynamics: a critical revision
14:40 N. W. Watkins (1), M. P. Freeman (1), D. Riley(1,2), C. Rhodes (1) ,P. Wilson (1), S. C. Chapman(3), ((1) British Antarctic Survey, U.K.,(2) Cambridge University, U.K., (3) Univ. of Warwick, UK) Scaling in the magnetosphere and solar wind: where does the 'smoking gun' point?
14:55 N.B. Crosby (1 and on leave from 2) M.K. Georgoulis (3) N. Vilmer (2) (1. Mullard Space Science Laboratory-University College London, UK (2). LPSH, URA 2080 CNRS, DASOP Observatoire de Paris, Meudon (3). Section of Astrophysics, Astronomy & Mechanics, Department of Physics, University of Thessaloniki, Greece) WATCH solar X-ray bursts and the statistical flare model: waiting time distributions
15:05 Chris Brockwell, Andrew Conway (Open University, UK) Physical-Statistical Models of Solar Magnetic Field Interaction
15:15 K P Macpherson (University of Central Lancashire), A L MacKinnon (Univ Glasgow, UK) ,Remote diagnosis of conservative SOC

15:25 Discussion
15:30 Tea at Saville Row, RAS Ordinary Meeting

18:00 Drinks at Saville Row

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RAS G/MIST discussion meeting, 8 December 2000
Self Organised Criticality and Turbulence in the Solar System

Meeting Report

by S Oughton (University College London) and N W Watkins (British Antarctic Survey)

Published in Astronomy & Geophysics 42, 2.22 (2001)

An RAS G/MIST discussion meeting was held in London on December 8th at the Geological Society. The theme "Self-organised criticality and turbulence in the solar system" was chosen by the convenors---Sandra Chapman (Warwick), Mervyn Freeman (BAS) and Sean Oughton (UCL)---to reflect substantial current interest in the application of complexity ideas to astroplasmas. Many astrophysical systems are found to have processes occurring over a broad range of coupled scales and to have distribution functions (for various types of events) which are powerlaw or otherwise strongly non-Gaussian, indicating the relevance of non-equilibrium statistical mechanics. The intent of the meeting, amply borne out by the contributions and the lively debates, was to contrast relatively long established paradigms for studying turbulence with more recent ideas such as self-organised criticality (SOC).
A related aim was the elucidation of possible tests and/or diagnostics for comparing and contrasting these paradigms. Indeed, intense discussion was focused on one particular such candidate, namely the distribution of waiting times between solar flares.

The morning session was chaired by Sean Oughton (UCL). S. Schwartz (QMW) led off with a discussion of the influence on the solar wind of brief intervals of elevated coronal temperatures. Simulations indicate that the associated plasma can fill disproportionately large fractions of the heliosphere.
C. Pagel (IC) compared the quality of fit between various statistical models of intermittent turbulence and observations of magnetic fluctuations in the solar wind, concluding that the Kolmogorov p-model is a good fit and the Kraichnan model a poor one.
L. Harra (MSSL) presented the first preflare observations of non-thermal velocities. These are seen to increase and fluctuate 10 minutes before the flare proper begins and suggest that very small scale reconnection events precede and perhaps build up to the main flare event.

M. Aschwanden (Lockheed Martin) discussed current views on various solar flare distribution statistics, and showed that a simple nonlinear saturation model can produce power-law distributions.
R. Dendy (UKAEA) then introduced an artificial, rather than natural, plasma confinement system, the tokamak. He presented recent results from a 1D ``sandpile'' model showing close analogues for enhanced confinement, edge pedestals, and edge localised modes, and for the qualitative correlations between them.

Several talks on models of solar flares were then presented. S. Galtier (Warwick) employed a 1D magnetohydrodynamic (MHD) model of solar flares, with heating due to intermittent dissipation along a magnetic loop, which reproduces X-ray event statistics.
H. Jensen (IC) applied an established dissipative ``sandpile'' model, that of Olami Feder and Christensen (OFC) to the solar flare problem. He showed that the probability density of event magnitudes given by the OFC model was in good agreement with most of the flare observations, and showed that there was evidence of correlation in time between flares in the OFC model.
L. Vlahos (Thessaloniki) discussed a 3D cellular automaton (CA) model for solar flares using sub-resolution magnetic dissipation at random points inside large-scale structures to provide heating and particle acceleration.

V. Carbone (Calabria) discussed models for dissipative events in MHD turbulence and their connection with waiting-time statistics for solar flares, emphasizing that the correct nonlinear dynamics is unlikely to be obtained if the models fail to incorporate the appropriate ideal conservation laws.
Moving to the most general level of description, S. Chapman (Warwick) gave the last talk before lunch. She presented a derivation of the global probability density of events in terms of the source distribution for individual events in the system. The resulting curve was shown to be relatively insensitive to the distributions of such events, explaining its previously observed ``universality.'' However, higher-order moments should provide more sensitive discriminators.

The afternoon session was chaired by Sandra Chapman (Warwick). In the context of single-spacecraft observations of solar wind fluctuations, T. Horbury (IC) presented a promising wavelet-based method for determining the full form of the magnetic autocorrelation tensor. Knowledge of the full tensor would considerably improve understanding of the associated turbulence.
A. Klimas (NASA) then gave the first of several talks discussing whether the magnetotail can be effectively described using the formalism of SOC. He showed 1D and 2D current sheet models exhibiting both a global loading/unloading cycle and internal complex activity.

G. Consolini (IFSI/CNR) presented a critical examination of the suitability of SOC models, noting the possibility that models of quenched disorder - ``spin glasses'' - might have less restrictive conditions of application than SOC.
N. Watkins (BAS) then discussed evidence that some of the scale free properties seen in the magnetosphere might originate in the solar wind. He speculated that these might be related to the scale free properties of solar flares which were under study within several different paradigms at the meeting.

N. Crosby (MSSL) presented a numerical (SOC-based) and observational investigation of the correlation between solar flare peak intensity and the preceding waiting time between two flares, finding little support for the concept of sympathetic flaring.
C. Brockwell (Open University) presented a CA model of the photospheric magnetic field and large-scale flow which displays self-organising behaviour and produces event distributions consistent with observational ones.
A. MacKinnon (Glasgow) demonstrated a connection between sandpile models and percolation systems subject to a local conservation law (again stressing the importance of the conservation laws to the modelling process).
In such cases, useful qualitative statements may be made regarding remotely sensed avalanche processes, even when the detailed physics is uncertain, with solar flares being a prominent example.

Additional poster contributions were made by Bruno et al. (IFSI/CNR), McKinnon and Macpherson (Glasgow), Hnat and Chapman (Warwick), Uritsky and Klimas (NASA), Pacini et al. (Sao Paulo), Kaufmann et al (Sao Paulo) and Watkins et al (BAS). Sandra Chapman concluded the meeting by thanking all contributors, attendees and her co-chairs.

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