8 December 2000

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

*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 *

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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