Table Of ContentPHYSICAL PROCESSES IN SOLAR FLARES
ASTROPHYSICS AND
SPACE SCIENCE LIBRARY
A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS
OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS
PUBLISHED IN CONNECTION WITH THE JOURNAL
SPACE SCIENCE REVIEWS
Editorial Board
R. L. F. BOYD, University College, London, England
W. B. BURTON, Sterrewacht, Leiden, The Netherlands
C. DE JAGER, University of Utrecht, The Netherlands
J. KLECZEK, Czechoslovak Academy of ScienCes, Ondfejov, Czechoslovakia
Z. KOPAL, University of Manchester, England
R. LOST, Max-Planck-Institutfiir Meteorolof{ie, Hamburf{, Germany
CURRENT RESEARCH
VOLUME 172
PHYSICAL PROCESSES
IN SOLAR FLARES
by
BORIS V. SOMOV
Sternberg Astrollomicallnstitute,
Moscow State University
SPRINGER SCIENCE+BUSINESS MEDIA, B.v.
Library of Congress Cataloging-in-Publication Data
Somov, B. V. (Boris V.)
Physical processes in sclar flares I by Boris V. Somov.
~. cm.
Includes bibl iographical reFerences (p. ) and index.
ISBN 978-94-010-5056-2 ISBN 978-94-011-2396-9 (eBook)
DOI 10.1007/978-94-011-2396-9
1. Solar flares. 2. High ter;perature plasmas. 1. Title.
QB526.F6S65 1991
523.7'5--dc20 91-14565
ISBN 978-94-010-5056-2 ASTROPHYSICS AND SPACE SCIENCE LIBRARY, VOLUME 172
Printed on acid-free paper
All Rights Reserved
© 1992 Springer Science+B usiness Media Dordrecht
Originally published by Kluwer Academic Publishers in 1992
Softcover reprint of the hardcover 1s t edition 1992
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
Table of Contents
PREFACE vii
CHAPTER 1 / THE PRESENT STATE OF THE SOLAR
PROBLEM 1
1.1. Observed phenomena and the energetics of solar flares 1
1.2. Physical concepts of the nature of flares 13
1.3. Flare-induced physical processes in the solar atmosphere 35
1.4. The solar flare problem 44
CHAPTER 2 / CHROMO SPHERIC HEATING: THE
MECHANISM PRODUCING HIGH-TEMPERATURE
PLASMA IN SOLAR FLARES 49
2.1. Impulsive heating by accelerated electrons 49
2.2. Impulsive heating by intense heat flows 87
2.3. Chromo spheric heating by X-ray emission 109
CHAPTER 3 / MAGNETIC RECONNECTION AND ENERGY
RELEASE IN SOLAR FLARES 115
3.1. Reconnection as a heating mechanism for X-ray coronal
loops 115
3.2. The natural high-temperature turbulent current sheet 134
3.3. The high-temperature turbulent current sheet with a small
transverse magnetic field 156
3.4. The current sheet structure under the fast and slow
reconnection regimes 185
3.5. Stabilization oftearing instability in current sheets on the
Sun 201
3.6. The effect of the longitudinal magnetic field in the current
sheets on the Sun 206
CONCLUSION 219
REFERENCES 223
Preface
During the last three decades a new field has been developing rapidly on
the borderline between extra-atmospheric astronomy and high-temperature
plasma physics; under the name the physics of solar flares this field has in
effect acquired the status of a new branch of physics. The interest in the
investigation of solar flares has been caused by two facts. First, the solar
flare is a complex electromagnetic phenomenon of a cataclysmic nature in
the cosmic plasma with particles being accelerated to high energy, which
can be studied by a variety of methods both from the Earth and from
space. Second, flares have a strong influence on interplanetary and near
Earth space and on the Earth's atmosphere and biosphere. There are good
reasons why flares are interesting to physicists and astronomers, biologists
and physicians, specialists in energetics as well as in astronautics. The
above two facts characterize the essence of the problem of solar flares as
well as its significance for both pure and applied science.
On the way to the solution of this problem we come closer to an un
derstanding of a wide class of nonstationary phenomena in plasmas both in
space and in the nuclear fusion laboratory, and to the establishment of a sci
entifically sound and reliable method of forecasting the radiation situation
in near space. Also, the physics of solar flares now becomes an 'etalon' for
contemporary astrophysics, in particular for gamma and X-ray astronomy.
An essential step on the way to the solution of the problem is a study of
the energetics and dynamics of high-temperature plasma in the flares. It is
in the high-temperature plasma that the primary energy release takes place
in the form of fast magnetohydrodynamic plasma flows, intense heat flows,
accelerated particles and hard electromagnetic radiation. Constructing the
theory of this process is the goal of those who investigate the flares.
Achieving this goal is complicated considerably by the fact that the
information about the primary flare process (called the flare mechanism)
can be obtained only in a very indirect way - by studying various secondary
effects in the solar atmosphere caused by the flows of energy released
from the flare. These consequences of the primary energy release are what
produce the complex observational signature of a flare. The structure of this
book is derived from this fact. There are three chapters in the book, each
representing a separate stage in the solution of the problem.
VB
Preface
V111
The first chapter is a kind of bridge between contemporary observational
data and contemporary theoretical concepts. First the main observational
characteristics of flares are described. We stress the necessity of inves
tigating not only the flares themselves, but also the related nonstationary
large-scale phenomena in the solar atmosphere. Based on a generalization
of the observed facts a phenomenological systematization of energy release
and transformation is proposed. Elementary physical concepts of the flare
mechanism are then outlined - magnetic field reconnection in current sheets.
We discuss the characteristics of the current sheet as the primary source of
energy in various stages of flare development. Particular attention is paid to
the prospects for the detection of pre-flare current sheets, which is crucial
for short-time flare forecasting.
The second chapter is devoted to constructing the theory of secondary
physical processes, in particular to the study of the dynamics of high
temperature plasmas during impulsive heating of the solar atmosphere by
accelerated electrons, intense flows of heat and radiation. The main problem
here is to calculate the time-dependent physical characteristics of the flare,
such as the distribution of the electron and ion temperatures and of plasma
density and velocity, or the X-ray and UV radiation intensity. Comparing
the calculated and observed characteristics imposes restrictions on models
of the primary source of flare energy.
In the third chapter we use the results of the preceding two chapters to
choose a model and to construct a theory of non-neutral turbulent current
sheets (having a small transverse and arbitrary longitudinal magnetic field
component), which serve as the source of energy of the flares. In recent
years great progress has been achieved in this part of the physics of solar
flares. The reason is that a self-consistent theory of high-temperature tur
bulent current sheets has been formulated, which takes account of physical
processes that are essential for the field reconnection in the solar active
regions, where flares occur.
In sum, the book aims at a consistent model of physical processes in
high-temperature solar flare plasma, including the theory of the primary
release of energy in turbulent current sheets and the theory of secondary
transformations of this energy in the solar atmosphere that could account
for the observed signature of the flare. The author hopes that this model
will contribute to the solution of the global problem of solar flares in both
its theoretical and applied aspect - the forecasting of flares and their effects
in near space and on the Earth.
CHAPTER 1
The Present State of the Solar Flare Problem
1.1 Observed phenomena and the energetics of solar flares
In contrast to flares on other stars or to many analogous phenomena in the
Universe, flares on the Sun are accessible to a broad variety of observational
methods. Solar flares emit radiation which covers virtually the whole elec
tromagnetic spectrum, from kilometer radio waves to hard gamma rays, and
which can be detected by Earth-based, satellite-borne, and interplanetary
observing stations. At the same time, the particles accelerated in the flare,
the plasma ejected into the interplanetary space, and secondary ionospheric
and geomagnetic events related to the flare are detected directly. As a result,
during the last two decades solar flare research has become very comprehen
sive, in particular during the years of coordinated international programs.
An intense stream of very diverse observational data on solar flares and
secondary events allowed us to obtain new and more detailed information
about the solar flare mechanism.
This success of flare research has been achieved thanks to two char
acteristics of today's observational programs. First, the central element of
any such program - in fact its cornerstone - is a specialized complex of
scientific instruments on a satellite or an orbital station. Second, the activity
of such a complex is closely coordinated with the activities of a number of
Earth-based radio and optical observatories.
The observational and diagnostic aspects of solar flares have been de
scribed reasonably completely in many monographs and review articles (see
[335,334, 133,279, 1, 70]). The collectively written monograph [334], de
voted to the investigation of flares during the Skylab period, should be
particularly noted. Our knowledge of flares has been substantially im
proved during the international programs Solar Maximum Year [338] and
Solar Maximum Analysis [304, 157]. Many observational results will be
given below where necessary, with appropriate references to their sources.
Despite this fact and the existence of excellent monographs, it is useful to
summarize here the main observational characteristics of the flares.
1
2 Chapter 1
t
~
I-- PH '1' EP ......*. '1 HP ------...,--1
41----
Fig. 1.1.1. Typical time variation of the energy output of a flare.
1.1.1 Energy released in solar flares
Flares are the most powerful of all manifestations of solar activity. The
energy of a large flare can be as large as (1-3) x 1032 erg, which is almost
100 times more than the energy obtained from burning the known reserves
of oil and coal on the Earth. On the Sun this gigantic amount of energy is
released within several minutes to tens of minutes, which corresponds to an
average output of (1-3) x 1029 erg/so
On the basis of present observational data (see, e.g. [157, 109]) the
process of energy release in a solar flare can be conventionally divided into
four parts (Fig. 1.1.1) [69, 311]. The violent discharge of energy in the flare
is preceded by a stage of preheating (PH). The name of this phase is rather
conventional, but its existence has a crucial significance both for an early
diagnosis of flare events and for an understanding of their mechanism.
The preheating phase is followed by the phase with fast MHD plasma
flows and the local restructuring of magnetic fields. This is called the ex
plosive phase (EP). From its background we can recognize the impulsive
phase (IP) of the flare as a sequence of short powerful bursts. This phase
is usually shorter than the explosive phase; sometimes these two phases
are improperly treated as a single phase under the name of either 'explo
sive' or 'impulsive'. During each of the abovementioned bursts, which
are sometimes, also improperly (see Sec. 1.1.4), called 'elementary' [351,
352], electrons and ions are rapidly (in several seconds) accelerated to high
energies.
Then comes the hot (HP) or main phase (MP) of the flare. It is called hot
because its most prominent manifestation is the X-ray emission from high
temperature and relatively dense plasmas in the corona, and main because it
The Present State of the Solar Flare Problem 3
is apparently during this phase that the largest part of the total flare energy
is released; this energy has the fonn of an intense flow of heat. Such is the
scenario of energy release in flares.
At some moments, in particular during the impulsive phase, the rate
of energy release can be several times larger than the values given above.
However, it is easy to see that even then the energy release is of the order of
only 0.01 percent of the total output of the Sun 3.83 x 1033 erg/so Therefore
no marked increase in solar luminosity can be seen during the flare. Only
the largest flares can be seen in 'white light' , i. e. in the optical continuum,
where most solar radiation is emitted. Flares are usually observed as marked
brightness enhancements not in the photosphere (which is the source of
continuum radiation), but in the chromosphere, in chromo spheric lines, in
particular in the Hex line of hydrogen. As a consequence of this fact, for many
years the tenn 'chromospheric flare' was widely used, which, however,
does not correspond to the nature of the phenomenon, being a complex
nonstationary process encompassing a huge range of heights in the solar
atmosphere. The dimensions of 'X-ray' flares were first detennined in the
experiments [122] on satellites of the Kosmos series: the altitudes, estimated
from the limb passage of an active region, reached about 20 000 km above
the photosphere. Moreover, only a small part of the total flare energy is
released in the fonn of hydrogen Hex emission (see Table 1.1.1).
A significant part of the energy of flares is manifested in the fonn of
MHD ejections of plasma, which move with velocities sometimes exceeding
1 000 km/s in the corona and interplanetary space, as well as in the fonn
of streams of high-energy particles: electrons, protons, and heavier nuclei.
Hard electromagnetic radiation of the flare and its radio emission, different
from that of the quiet Sun, are also evidence of the existence of nonthennal
processes in the flare mechanism. However, the relative magnitude of these
processes in the flare energy budget does apparently not exceed 10 %.
1.1.2 Flare-like events in the solar atmosphere
The estimates of different fonns of energy released by the flare shown in
Table 1.1.1 represent only the characteristic values of the corresponding
quantities for the case of the largest and the smallest flares (subflares).
Despite some vagueness in its entries (they should not be treated as the
result of observation of any particular flare), Table 1.1.1 demonstrates the
rather large range of values of the quantities shown. However, let us note
that because of the observed complexity and variability of flares, their
mechanism should not be viewed as a process in which individual flares
differ from each other only in tenns of, e.g., energy output. Actually flares
