TALKS
Solar
Wind Acceleration by Impulsive Events.
R
Grappin (1), A Mangeney (1), SJ Schwartz (2) and WC Feldman (3)
(1)
DESPA, Meudon, Paris
(2)
Queen Mary, London
(3)
LANL, Los Alamos, New Mexico
e-mail:S.J.Schwartz@qmw.ac.uk
We
have studied the influence of brief intervals of elevated coronal temperatures
on the solar wind. The results of even the simplest fluid modelling reveal
that such events have a disproportionally large impact on the entire solar
wind, such that the solar wind may be filled completely by plasma originating
from them. We discuss the importance of these ideas in understanding both
the gross solar wind features (e.g., flow speeds) and also microphysical
aspects (temperature anisotropies, minor species temperatures, and particle
beams).
Christina
Pagel and André Balogh
Space
and Atmospheric Physics, The Blackett Laboratory, Imperial College, London
SW7 2BU
e-mail:c.pagel@ic.ac.uk
The
solar wind is a highly turbulent and intermittent medium at frequencies
between 10-3 and 10-1 Hz. Various models have been
put forward in an attempt to understand this process better, and tested
against solar wind data.We have
used the Ulysses fast latitude scan data to perform a wide-ranging comparison
of three intermittency models - the well-known P model, in both its Kolmogorov
and Kraichnan form, the lognormal cascade model and a model adapted from
atmospheric physics, the G infinity model.They
were tested using fits to graphs of the structure function exponents g(q),
comparison with a non-linear measure of the deviation of g(q) from the
non-intermittent straight line, and using extended self similarity techniques,
over a large range of heliolatitudes.Tests
of all three models indicated a high level of intermittency in the fast
solar wind, and showed a varied structure in the slow wind, with regions
of practically no intermittency next to regions of high intermittency,
implying that the slow wind has no uniform origin. The lognormal and Kolmogorov
P Model performed the best over all the tests, indicating that inhomogeneous
energy transfer in the cascade is a good description.The
Kraichan model performed relatively poorly, and the overall results show
that Kraichnan model of turbulence is not well-supported. The G infinity
model fitted the results surprisingly well and showed that there may very
well be important universal geometrical aspects of intermittency over many
physical systems.
Louise
Harra
Mullard
Space Science Laboratory, UCL
e-mail:lkh@mssl.ucl.ac.uk
Excess
soft X-ray line broadening has been observed for many years during solar
flares. Its interpretation has been debated to be either (a) the energy
release site (b) response to energy deposition (c) response to energy release.
Evidence has been found for both response to energy release in the form
of evaporating, turbulent plasma, and also for turbulence at the loop top
which may produce acceleration. Recently evidence has been found to show
that pre-flare turbulence exists which increases slowly over the period
of 10 minutes before the flare proper occurs. This will be discussed in
terms of small scale reconnection events through emerging flux which build
up to induce the flare trigger.
Markus
J. Aschwanden
Lockheed-Martin
ATC, Solar & Astrophysics Lab., Palo Alto, CA 94304, USA
e-mail:aschwanden@lmsal.com
Solar
Flares exhibit power-laws up to eight orders of magnitude in energy, if
one includes the tiniest nanoflares detected now in extreme-ultraviolet
with the TRACE telescope. This is probably the largest dynamic range of
power-law behaviour observed in astrophysics to-date. Despite of this overwhelmingly
clean manifestation of self-organized criticality, the relation of the
underlying physics of magnetic energy release processes in the solar corona
to the principle of self-organized criticality is subject of intense debates.
We show that (1) a power-law distribution is a universal property of every
nonlinear process with saturation limits set by random parameters, and
that (2) the power-law slope can be used as diagnostic of coherent versus
incoherent processes. We review obervations of frequency distributions
of solar phenomena and how they were related to various models of self-organized
criticality.
Sandpile Paradigm and
Fusion Plasma Phenomenology.
R. Dendy, UKAEA Culham,
U.K.
e-mail:richard.dendy@ukaea. org.uk
No abstract
S.
Galtier
Mathematics
Institute, University of Warwick, U.K.
e-mail:galtier@maths.warwick.ac.uk It
is becoming increasingly clear that solar flares and the heating of the
solar corona are related to magnetic activity on the Sun, an activity illustrated
for instance in the movies of the dynamical corona made from observations
with the YOHKOH spacecraft. As the sensitivity of instrumentation increases,
a large spectrum of flares is recorded, in intensity, in duration and in
numbers. Such histograms are known to obey power laws, the origin of which
is not clear. Substantial progress was made in the wake of the work of
Lu and Hamilton [1] (see eg Georgoulis and Vlahos [2]); they view solar
flares as stemming from a purely statistical approach using sand-pile models
of Self-Organised Critical (SOC) phenomena. In this context I will present
a simplified one-dimensional MHD model of solar flares [3-4] in which the
coronal heating is due to an intermittent dissipation along a magnetic
loop. The model is able to reproduce the statistics observed in X-rays.
The differences between MHD turbulence and SOC will be discussed. [1]
Lu and Hamilton, ApJ, 380, pL89 (1991). [2]
Georgoulis and Vlahos, A&A, 336, p721 (1998). [3]
Galtier and Pouquet, Solar Phys., 179, p141 (1998). [4]
Galtier, ApJ, 521, p483 (1999). D.
Hamon, H. J. Jensen, and M. Nicodemi Department
of Mathematics, ICSTM, U.K. e-mail: No
abstract Loukas
Vlahos Department
of Physics, University of Thessaloniki, 54006 Thessaloniki, Greece. http://www.astro.auth.gr/~vla hos It
is common belief that many still unsolved problems in solar and stellar
physics (coronal heating, plasma flows and dynamo, flares) are due to MHD
turbulence driven magnetic dissipation. Sub-photospheric convection motions
generate and move the magnetic fields, forming the active region complexes.
The turbulence driven active region reaches a Self Organized Critical (SOC)
state which we have simulated with a 3-D deterministic CA. We propose a
new flare model called the statistical flare were sub-resolution magnetic
dissipation heats and accelerates particles in random points inside large
structures with still undefined filling factors. We have constructed recently
a new setup were CA models can become compatible with MHD equations and
yield also secondary variables such as currents and local E-fields. Heat
conduction and particle diffusion along magnetic field lines spread the
energy in large structures in a fraction of a second. Vincenzo
Carbone Dipartimento
di Fisica, Universita' della Calabria, 87036 Rende (CS), Italy e-mail:carbone@fis.unical.it Solar
flare activity is investigated by looking at the statistics of the waiting
times between hard X-ray bursts. The results of the SOC-paradigm for solar
flares are then compared with models describing dissipative events in MHD
turbulence. Extreme Statistics and Universal
Fluctuations in Highly Correlated Systems.
S.
C. Chapman, G. Rowlands (1), and N. W. Watkins(2) (1)
Physics Department, University of Warwick, Coventry, CV4 7AL, U. K. (2)
British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET, U. K. e-mail:Sandra.chapman@astro.warwick.ac.uk Complex
systems can often be characterized by long range correlations, a lack of
scale, and in particular, by the exponents, of the power law probability
distributions (PDF) of patches of activity in the system. Examples of these
are energy dissipated in avalanches in sandpiles, in turbulent fluids and
magnetization in a ferromagnet close to the critical point. Determining
the exponents is nontrivial, requiring in- situ measurements over many
decades. Here we consider what may be a more readily accessible measure,
a global average quantity such as the total energy dissipation or magnetization.
The PDF of this global measure is non Gaussian and here we derive its functional
form in terms of the PDF of the individual energy dissipation events in
the system. We find a single parameter distinguishes this global PDF and
obtain its relationship to the source PDF. We derive the moments of the
global PDF in terms of this parameter; these then provide a powerful tool
to probe the degree of complexity of the system. Tim
Horbury Imperial
College, London, U. K. e-mail:t.horbury@ic.ac.uk The
presence of a background magnetic field in a turbulent plasma produces
an anisotropy in the fluctuations, in contrast to the globally isotropic
nature of neutral fluid turbulence: this anisotropy is a key property of
turbulence in plasmas. The field-aligned rotational symmetry of the turbulence
restricts the form of the two-point correlation tensor, making it easier
to determine the structure of this physically important parameter. Spacecraft
observations of MHD turbulence in the solar wind allow field-aligned anisotropy
to be studied, but it is difficult to measure 3D structure with single
spacecraft data. Previous work by others has shown that there is considerable
power in wavevectors that are not parallel to the background field. We
present a new, wavelet based analysis method using Ulysses polar magnetic
field data, which is particularly well suited to this work. Extending earlier
work, we present early results of this analysis including measurements
of the diagonal elements of the correlation tensor and its scale dependence.
The method offers the prospect of determining for the first time the full
form of the correlation tensor: we discuss the consequences of such measurements. A
J Klimas, V Uritsky, D Vassiliadis, and D N Baker NASA
Goddard Space Flight Center, Greenbelt, Maryland, USA e-mail:alex@bokeh.gsfc.nasa.gov Chang
[Phys. Plasmas, 1999 and references therein] has suggested that the magnetospheric
plasma sheet may be in self-organized criticality (SOC), or in a state
that is closely related to SOC.New
observational evidence continues to support this suggestion [Lui et al.,
GRL, 2000]. Early cellular automaton studies [e.g., see Chapman et al.,
GRL, 1998] have confirmed the compatibility of a global loading-unloading
cycle with internal complex activity in the magnetosphere. Focusing on
the plasma sheet, Klimas et al. [JGR, 2000] have studied the behavior of
a 1-dimensional current sheet model containing a magnetic field reversal.
They have shown that the current sheet model also exhibits a loading-unloading
cycle and Uritsky et al. [in preparation] have shown that when executing
the loading-unloading cycle the model dynamics is in a near-SOC domain.
An extension of this current sheet model will be introduced. The behavior
of a 2-dimensional MHD current sheet, coupled to an idealized current-driven
instability that produces localized anomalous resistivity when and where
the current density exceeds a critical value, will be demonstrated and
discussed. The model is driven by convection of plasma and magnetic flux
into a field reversal region and it unloads through the ejection of plasma
and flux though an open boundary following field line reconnection in the
reversal. The dynamics of this process will be discussed within the framework
of the SOC paradigm. Self-Organised
Criticality vz Criticality in Magnetotail Dynamics: A Critical Revision. Giuseppe
Consolini IFSI/CNR,
Italy e-mail:giuseppe@sunago2.ifsi.rm.cnr.it No
abstract Scaling
in the magnetosphere and solar wind:Where
does the “smoking gun” point? N
W Watkins (1), M P Freeman (1), D J Riley (1,2), C S Rhodes (1,2), P S
Wilson (1), and S C Chapman (3) (1)
British Antarctic Survey (NERC), High Cross, Madingley Road, Cambridge,
CB3 0ET, UK (2)
University of Cambridge, Cambridge, UK (3)
University of Warwick, Coventry, CV4 7AL, UK e-mail: nww@bas.ac.uk The
last 3 years have seen aresurgence
of interest in the proposal by Chang (1992) that the Earth's magnetosphere
may exhibit forced or self-organized criticality (F/SOC). In particular
the observation of temporal scaling in the AE index by Consolini (1997)
coupled with the demonstration by Chapman et al. (1998) that substorms
might correspond to systemwide outputs of an otherwise scale-free system
helped motivate sandpile-like MHD-derived models such as those developed
by Klimas and co-workers. More recently Lui et al. (2000) have found evidence
of spatial scaling in auroral dissipation measured by ultraviolet images.
We examine whether the power law distribution of burst lifetimes in the
AE index is evidence that the magnetosphere is a Self-Organized Critical
(SOC) system. To do this we compare the burst lifetime distributions of
the AU and |AL| indices with those of the vBs and epsilon solar wind input
functions. We show that both the vBs and epsilon burst lifetime distributions
are of power law form with an exponential cut-off, consistent with the
solar wind being an SOC or turbulent system. Furthermore, the power law
of the epsilon burst lifetime distribution is not significantly different
to that of the AU and |AL| indices, indicating that this scale-free property
of the AE indices could arise from the solar wind input and may not be
an intrinsic property of the magnetospheric system. We discuss the implications
of this result. [Chapman
and Watkins, Space. Sci. Rev., in press, 2000 (astr-ph/0007086); Freeman
et al., Geophys Res Lett, 27, 1087, 2000; Freeman
et al., Phys Rev E, December, 2000 (@astro-ph/0003114); Watkins
et al., J. Atmos. Sol.-Terr. Phys., in press, 2000 (astr-ph/0002359)] WATCH Solar X-ray Bursts and the
Statistical Flare Model: Waiting Time Distributions.
N.B.
Crosby (1 and on leave from 2), M.K.Georgoulis
(3) and N. Vilmer (2) (1)
Mullard Space Science Laboratory, University College London, UK (2)
DASOP Observatoire de Paris, France (3)
Department of Physics, University of Thessaloniki, Greece e-mail:norma_crosby@hotmail.com The
current debate concerning waiting time distributions between solar flares
will be given. In the WATCH solar flare database no correlation was found
between the elapsed time interval between successive flares arising from
the same active region and the peak intensity of the flare. This result
will be discussed with respect to what has been found in other observations.
Furthermore our simulations based on a self-organised critical (SOC) cellular
automaton statistical flare model, which includes a highly variable non-linear
driver, reproduces this result. This will be compared to other results
based on similar types of SOC models. The implication of the various results
(observations and simulations) in regard to the above will be discussed:
is there a correlation and/or is there not a correlation between solar
flares? Chris
Brockwell and Andrew Conway e-mail:c.p.brockwell@open.ac.uk Theoretical understanding
of solar activity such as reconnection, dynamo and flux tube theory are
now well developed and have been used to explain phenomena recorded in
a range of satellite and ground-based data.Much
of this modelling deals directly with local (both spatial and temporal)
physical information such as velocities, densities, temperatures and magnetic
fields. On a larger scale, statistical data on frequencies and sizes of
solar flares, CMEs and other events are also available. Modelling of this
data has been less common, and has not been concerned with local physical
quantities. We believe that
whilst physical modelling is mostly (but not exclusively) appropriate for
explaining local details, a statistical approach is needed to assemble
the larger picture. To this end we use a one-dimensional cellular automaton
model to formalize the photospheric magnetic fields and bulk flows.The
local rules governing interaction between cells are guided by considerations
of physical realism.The overall
behaviour, which displays self-organising behaviour, is purely a consequence
of collective effects.The model
obeys simple one-dimensional fluid laws and represents magnetic reconnection
in a parameterised way. Two notable results are the formation of large
unipolar regions of magnetic fields, and events that have a power law distribution
across 30 orders of magnitude with a power law index consistent with that
deduced from observations. Remote Diagnosis of Conservative
SOC.
K
P Macpherson (1) and A L MacKinnon (2) (1)
University of Central Lancashire (2)
University of Glasgow e-mail:a.mackinnon@educ.gla.ac.uk Percolation
systems provide an understandable mathematical structure which can represent
avalanche type processes. Remotely sensed, under conditions and for reasons
that we describe, they may display much of the same phenomenology as the
sandpile models usually representative of SOC (power-law event size distributions).
We demonstrate that sandpile models may
be recast in percolation terms, with a significant difference which arises
ultimately from the mediating role of an underlying, local conservation
law. While event size distributions do not distinguish these possibilities,
statistics describing event temporal behaviour do (with some caveats about
radiation processes). The data in the solar flare case favour an avalanche
process in which a conservation law plays such a mediating role. Thus useful
qualitative statements may be made regarding remotely sensed avalanche
processes, even if the detailed physics is still uncertain. POSTERS Bursty
Transport in Magnetically Confined Plasmas: Avalanche-like Processes or
MHD Turbulence? 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 No
abstract Observations
on Solar Wind Intermittency: Radial Evolution and Characterization of Intermittent
Events. R.
Bruno(1), V. Carbone(2), L.Sorriso-Valvo(2), E. Pietropaolo (3), and B.
Bavassano(1) (1)
Istituto Fisica Spazio Interplanetario del CNR, 00133 Roma, Italy (2)
Dipartimento di Fisica Universit\`a della Calabria/INFM, 87036 Rende (Cs),
Italy (3)
Dipartimento di Fisica, Universit\`a di L'Aquila, 67100 L'Aquila, Italy e-mail:roberto.bruno@sunwks.ifsi.rm.cnr.it Interplanetary
solar wind velocity and magnetic field observations have been analyzed
within the inner heliosphere in order to highlight their intermittent behavior.
The radial evolution of intermittency, the different intermittent character
of magnetic and velocity fluctuations, differences between fast and slow
wind and, finally, the characterization of intermittent events are the
main focus of this work. N.
W. Watkins and M. P. Freeman British
Antarctic Survey, Cambridge, U.K. e-mail:
nww@bas.ac.uk No
abstract Complexity in Sandpile
Models for Astroplasma Confinement Systems. B.
Hnat and S. C. Chapman University
of Warwick, U.K. e-mail:Sandra.chapman@astro.warwick.ac.uk) No
abstract Fast
brightenings at submillimeter waves only associated with a large solar
flare P.
Kaufmann, J.-P. Raulin, E. Correia, J.E.R. Costa, C.G. Gimenez de Castro,
A.V.R. Silva (all at 1), H. Levato (2), M. Rovira, C. Mandrini, R. Fernandez-Borda
(all at 3), and O.H. Bauer (4) (1)
CRAAM/CRAAE, Universidade Presbiteriana Mackenzie, Sao Paulo, Brasil. (2)
Complejo Astronomico El Leoncito, San Juan, Argentina. (3)
Instituto de Astronomia y Fisica del Espacio, Buenos Aires, Argentina. (4)
Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany. We
present high time resolution observations of AR 8910 obtained simultaneously
at 212 GHz and 405 GHz during a large H-alpha flare, which produced a soft
X-ray X 1.1 class event. Data were obtained with the new solar submillimeter
telescope (SST) recently installed at El Leoncito observatory, to explore
this poorly known part of the solar emission spectrum.A
small slow submm enhancement ( < 100 sfu) was associated to bulk emissions
at X-rays, H-alpha and microwaves. The event exhibited numerous submm-w
100-300 millisecond duration spikes, the larger ones with fluxes of the
order of 50 and 180 sfu (+/- 20%) at 212 GHz and 405 GHz respectively.
A dramatic increase in the rate of submm spikes incidence sets in as a
new large loop system appears in AR 8910 and X-ray emission increases nearly
one hour before the large flare. A maximum in the brightenings incidence
rate (~ 30 per minute) correlates with the large flare light curves at
X-rays and H-alpha. We suggest that the spikes observed only at submillimeter
waves are produced by optically thick emissions of > 10 Mev electrons in
compact synchrotron sources which may be representative of discrete primary
energetic releases in flares. Quiet sun coronal heating: sandpile
reconnection model.
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 No
abstract Investigating
the Limit Cycle Behaviour in Avalanche Models With Nonlocal Communication. A
L MacKinnon (1) and K P Macpherson (2) (1)
University of Glasgow (2)
University of Central Lancashire e-mail:k.macpherson@uclan.ac.uk Starting
from a nonlinear cellular automaton model used to model the occurrence
of solar flares, we recall the physical motivation for modifying such models
to include non-local communication between sites. An initial study showed
that the introduction of too many remote sites may destroy the self-organised
critical (SOC) state of the CA model. Indeed as the degree of nonlocal
inter-connectedness is increased, a transition from SOC behaviour to limit
cycle behaviour is observed. Using 3-D numerical simulations, we investigate
further here the circumstances governing this transitional behaviour. We
comment on parallels between this class of model and other popular avalanche
models for complex dynamical systems. A.
Pacini, J.P. Raulin, P. Kaufmann, E. Correia CRAAM/CRAAEUniversidade
Presbiteriana Mackenzie, Sao Paulo, Brasil We
have analyzed 1248 simple solar flares at 17 GHz detected by the Nobeyama
radioheliograph (NRH) with 1 second time resolution. Only simple impulsive
events have been chosen during the period 1991 and 1997.We
present statistical distributions of the impulsive phase flare durations,
T, (elapsed time between start and peak), the peak fluxes F and the time-integrated
fluxes (E, fluence) over T. For each distribution of the parameters T,
F and E, a clear maximum is obtained.These
maxima are real, and well above any instrumental cutoff which would be
given by the temporal resolution and the sensitivity of the data sample
used. Moreover, the peak in the distribution of F values, is in agreement
with similar past studies done at 22 GHz. The presence of real maxima in
the distributions of T, F and E suggests, contrary to what has been deduced
from similar X-ray studies, the existence of a minimum value for each of
these parameters. The possibility of having lower limit values for T, F
and E has already been proposed in the "quasi-quantized" hypothesis of
the energy released during solar flares. Non-Local
Self Organised Critcality: A Scalar Model with Random Neighbours Graeme
A Stewart Department
of Physics and Astronomy, University of Glasgow, G12 8QQ email: graeme@astro.gla.ac.uk There
has been some interest since the introduction of SOC models in non-local
communication between sites during avalanche events. Such models may provide
a sounder basis for the analysis of systems such as solar flares, where
it is known that accelerated particles may communicate with far faster
than the Alfven time, triggering "subevents" at different spatial locations.
To study this idea we introduce a finite state scalar field model with
random neighbour interactions during avalanche events. We show that neglecting
all spatial information greatly simplifies the problem, both analytically
and numerically. Numerical simulations show that the system exhibits power
law scaling in avalanche size and duration, and that system tends towards
a uniform distribution of cells in each possible state. We analyse the
statistics of avalanches which are far smaller than the system size and
show that avalanche statistics are controlled by a Galton-Watson process. Nonlinear
Chaos as a Source of Complexity in a 1-D Continuum Current Sheet Model. V.
Uritsky and A. Klimas NASA
Goddard Space Flight Center, Greenbelt, Maryland, USA e-mail:vadim@roselott.gsfc.nasa.gov We
analyze the dynamics of 1-D continuum current sheet model due to A.Klimas at
al (JGR, 2000). The turbulent activity of the model shows a hierarchy of
scale-invariant turbulent effects consistent with the self-organized criticality
(SOC) mechanism. On the global scale, the model displays critical scaling
of active site density and zero-field susceptibility. On the intermediate
scale, it demonstrates a multiscale pattern of spatiotemporal fluctuations
characterized by power-law distribution functions. In the present study,
we show numerically that the multiscale model behavior is strongly affected
by nonlinear chaotic dynamics due to local interactions of neighboring
current sheet elements. Depending on the model parameters, these dynamics
can either randomize or regularize the conditions for the propagation of
instability waves associated with SOC avalanches.Based
on the results obtained, we suggest that the nonlinear chaos on the microscopic
level of activity can play a crucial role in the formation of SOC regime
in continuum systems without external noise sources.
Energy emission from the sun and dissipative SOC models.
Statistical flare models.
Waiting Time Statistics in Solar Flares.
Field-Aligned Anisotropy in Magnetohydrodynamic Turbulence.
Multi-Scale Turbulence
in a 2-Dimensional Current Sheet Model
Physical-Statistical
Models of Solar Magnetic Field Interaction
Testing
for Criticality in the Coupled Geospace System
Statistical study of
simple and impulsive solar microwave flares
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