Table Of ContentLecture Notes in Physics
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Editors
Arnold .O Benz
Institut ffir Astronomie, ETH-Zentrum
CH-8o92 Zfirich, Switzerland
Albrecht Krfiger
Astrophysikalisches Institut
D-14482 Potsdam, Germany
Frontispiece:
Dynamic evolution of a current sheet following the localized occurrence of anomalous
resistivity (here applied at x = ,31 ,72 53,Y = o, and o < t < 2 in units of the current sheet
half width and the Alfv6n time, respectively). Current density maxima (bright) are
localized at the X-points formed by the magnetic field (curves). Induced tearing leads to
the formation of three current filaments by t = lo. Subsequently, complete coalescence
occurs, connected with the induction of an electric field. Both processes are supposed to
lead to rapid energy release in the corona (courtesy .B Kliem).
ISBN 3-540-59109-5 Springer-Verlag Berlin Heidelberg New York
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© Springer-Verlag Berlin Heidelberg 5991
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Typesetting: Camera-ready by the authors
SPIN: 10481062 55/3142-54321o - Printed on acid-free paper
Field Lines ond Current Dens;ty ot t=10
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0 t0 20 30 40 50 60
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FTetd Lines and Current Dens;ty ot t=55
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0.5
-> 0.0
-0.5
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0 10 20 30 40 50 60
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FieId Lines ond Current Density ot t=105
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Preface
In the last ten years it has become increasingly clear that more dynamical phe-
nomena occur in the solar corona than just flares. Not only are there different
kinds of flares, as classified for example in hard X-rays, but magnetic energy
is released in several different forms that are not necessarily related to some
brightening in Ha, the traditional definition of flares. Radio emissions, such as
S-component and noise storms, transient brightenings of active regions in radio
and X-rays, X-ray bright points, filament eruptions, coronal mass ejections, and
others have been recognized as non-flare releases of magnetic energy stored in
the corona. New instruments, too many to be listed, with higher sensitivity, bet-
ter resolution or longer observing time have been put into operation during the
present solar cycle. They have recorded not only smaller and smaller flares, but
also a wealth of information on various kinds of coronal dynamics. The coronae
of more rapidly rotating stars exhibit even more activity and seem' to be in a
state of very frequent or continuous flaring.
The Board of CESRA, the Community (formerly named "Commission") of
European Solar Radio Astronomers, decided to organize a workshop with a uni-
fying scope on magnetic energy releases. Not excluding that coronae may also
be fed by other forms of energy, focusing on magnetic energy limits the range
of topics and observations to phenomena of possibly similar physics. These ex-
citing, but poorly understood, events comprise instabilities of electric currents,
acceleration of nonthermal particles, shock waves, and heating processes.
The generation and maintenance of hot coronae around central gravitating
astrophysical bodies, the processes of their heating and activity are intimately
related to the question of energy release and also pose a burning problem of
contemporary cosmic physics. One of the key questions is expected to be in the
understanding of the role which magnetic fields are playing in this respect.
The experimental base to explore coronal magnetic energy releases includes
the methods of radio astronomy and space research. Thus the combined dis-
cussion of ground-based and space-borne projects, complementing each other,
appears useful. For this reason special attention was paid at the workshop to
the presentation of actual satellite experiments, in particular the SOHO project
and the CORONAS-I mission. Other sessions were devoted to the time, space,
and energy scaling of flares, radio observations, theory, time-series analyses and
IIV
statistics, coronal and interplanetary shock waves, new radio instrumentation,
software, and data bases. The Scientific Organizing Committee consisted of A.O.
Senz (Ziirich, chair), B. Fleck (Noordwijk), A. Kriiger (Potsdam), A. Magun
(Bern), M. Pick (Meudon), G. Trottet (Meudon), and P. Zlobec (Trieste).
This volume contains invited reviews and invited contributions of the work-
shop. There were 82 participants from 41 European countries, and also from the
USA, China, and Japan. The program comprised 78 contributions, 59 of which
orally presented, among them were 41 invited reviews and 01 specific invited
contributions, most of them contained in this issue. (The other contributed pa-
pers and posters have been recommended to be published in a volume of "Solar
Physics" .)
CESRA, which took the initiative for the present workshop, was founded in
the beginnings of the seventies and one of its major activities is the stimula-
tion of scientific meetings devoted to actual topics of solar radio physics and
neighbouring disciplines. The present meeting was organized by the Astrophys-
ical Institute Potsdam and the University of Potsdam under the auspices of the
European Physical Society. We are grateful that the workshop could be spon-
sored by the "Deutsche Forschungsgemeinschaft", the Commission of European
Communities, the International Science Foundation, the Astrophysical Institute
Potsdam, and the University of Potsdam. We also thank the members of the
Local Organizing Committee (H. AuraB, J. Hildebrandt, B. Kliem, J. Kurths,
G. Mann) and many assistants (G. Haase, L. Kurth, D. Lehmann, A. Marks, and
A. Trettin) for their untiring help in making the workshop a success. Moreover
the editors of this volume are indebted to a number of referees for evaluating
the contributions. Also the assistance of Ljudmila Kurth in editing the volume
is gratefully acknowledged.
Ziirich and Potsdam, December 1994 Arnold O. Benz
Albrecht Kriiger
IIIV
Contents
1 Solar Flares and the Corona
Flares and Coronal Heating in the Sun and Stars
A.O. Benz
Imaging, Stereoscopy, and Tomography of the Solar Corona
in Soft X-Rays and Radio
M.J. Aschwanden 31
Initial Results from the Nobeyama Radioheliograph
S. Enome 35
Long-Duration Non-Thermal Energy Release in Flares
and Outside Flares
K.-L. Klein 55
Characteristics of Two Simple Microwave Bursts
M.R. Kundu, S.M. White, N. Nitta, K. Shibasaki, S. Enome 75
Retrieving Information from Digital Solar Radio Spectrograms
A. Csillaghy 83
2 Flare Theory and Statistics
Coupled Magnetohydrodynamic and Kinetic Development
of Current Sheets in the Solar Corona
B. Kliem 93
Acceleration and Radiation from a Complex Active Region
L. Vlahos 115
Flares in Accretion Disks
J. Kuijpers 135
Non-Linear Data Analysis and Statistical Techniques
in Solar Radio Astronomy
J. Kurths, .U Schwarz, A. Witt 159
3 Coronal Ejections and Shocks
Interplanetary Scintillation Imaging of Disturbances
in the Solar Wind
A. Hewish, G. Woan 173
XI
Theory and Observations of Coronal Shock Waves
G. Mann 381
Numerical Simulations of Shock Electron Acceleration
in Solar Physics
B. Lembege 102
Surprises in the Radio Signatures of CMEs
N. Gopalswamy, M.R. Kundu 223
4 Space Projects (SOHO): Goals, Instruments and Collaboration
The SOHO Mission
B. Fleck 233
SUMER Solar Ultraviolet Measurements of Emitted Radiation
K. Wilhelm - 245
EIT: The Extreme Ultraviolet Imaging Telescope. Synoptic
Observations of Small and Large-Scale Coronal Structures
F. Clette, J.-P. Delaboudini~re, K.P. Dere, P. Cugnon
and the EIT Science Team 152
The Ultraviolet Coronagraph Spectrometer
G. Nod, J.L. Kohl, M.C.E. Huber, E. Antonucci, S. Fineschi,
L.D. Gardner, G. Naletto, P. Nicolosi, J.C. Raymond, M. Romoli,
D. Spadaro, L. Strachan, G. Tondello, A. nav Ballegooijen 162
The Charge, Element, and Isotope Analysis System CELIAS
on SOHO
D. Hovestadt, P. Bochsler, .t1 ,tdlacvnJirG F. Gliem,
M. Hilchenbach, F.M. Ipavich, D.L. Judge, W.L Axford, H. ~regislaB
A. B~rgi, M. Coplan, A.B. Galvin, J. Geiss, G. Gloeckler, K.C. Hsieh,
R. Kallenbach, B. Klecker, M.A. Lee, S. Livi, G.G. Managadze,
E. Marsch, E. Mfbius, M. Neugebauer, K.-U. Reiche, M. Scholer,
M.L Verigin, D. Wilken, P. Wurz 172
Participants
Flares and Coronal Heating in the Sun and
Stars
Arnold O. Benz
Institute of Astronomy, ETH-Zentrum, Ctt-8092 Zfirich, Switzerland
Abstract. Many forms of energy input into coronae have been proposed as
the dominant heating mechanism. Here I review topical aspects of impulsive re-
leases of magnetic energy. Several solar phenomena from bright points to coronal
mass ejections are attributed to free magnetic energy apparently available in the
corona. The possibility that magnetic energy release is the dominant energy in-
put into the corona is discussed for the Sun with special emphasis on small radio
events, with negative results. The evidence is better, however, for active stars
where a correlation between thermal radiation and gyrosynchrotron emission by
energetic electrons has been found recently. It suggests that a flare-like release
of magnetic energy is the dominant coronal heating process of active, rapidly
rotating stars. However, the required cadence of flares has not (yet) been found.
The link between stellar coronal heating and magnetic energy release is not clear
as long as the various flare-like phenomena in the solar corona are not better
understood.
1 What Heats the Corona?
At about the same time -- in the middle of this century m when astrophysicists
started to understand the nuclear beating process in the interior of the Sun, W.
Grotrian and B. Edl~n suggested that the corona has a temperature of some
million degrees. It raises a new, major problem. What heats the solar corona?
Recent reviews are contained in a book edited by Ulmschneider et .M (1991). For
an overview (but not a review), the many ideas may be grouped into six classes:
.1 MHD waves driven by the convection below the photosphere.
2. Body waves in dense flux tubes or surface waves in boundary layers of the
inhomogeneous corona.
3. Gradual dissipation of currents in magnetic flux emerged into the corona
from below.
4. Impulsive release of magnetic energy in flares, microflares, or flare-like events.
5. Bulk motion (spicules etc.) in the transition layer and into the lower corona.
