Collisionless Shocks in the Heliosphere: Reviews of Current Research

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GeophysicalM onograph3 5
Collisionless Shocks
in the Heliosphere:
Reviews of Current Research
Bruce T. Tsurutani and Robert G. Stone
Editors
American Geophysical Union
Washington,D .C.
Published under the aegis of the AGU Geophysical
Monograph Board: Donald Eckhardt, Chairman;
Elaine Oran, James Papike, John Schaake, and
Sean Solomon, members.
Collisionless Shocks in the Hellosphere:
Reviews of Current Research
Library of Congress Cataloging in Publication Data
Main entry under title:
Collisionless shocks in the heliosphere(cid:127)
(Geophysical monograph, ISSN 0065-8448; 35)
Includes bibliographies.
1. Heliospherc Addresses, essays, lectures.
2. Shock waves--Addresses, essays, lectures.
I. Tsurutani, Bruce T. II. Stone, Robert G. (Robert
Gilbert), 1928- . III. Series.
QC881.2.H43C65 1985 551.5'11 85-13508
ISBN 0-87590-061-5
ISSN: 0065-8448
Copyright 1985 by the American Geophysical Union, 2000 Florida Avenue,
NW, Washington, DC 20009
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Printed in the United States of
CONTENTS
Introduction vii
MACROSTRUCTURE
Theories of Shock Formation in the Solar Atmosphere R.S. Steinolfson 1
Observations of Shock Formation and Evolution in the Solar Atmosphere Jean-Louis Bougeret 13
Review of Interplanetary Shock Phenomena Near and Within 1 AU A.K. Richter, K. C. Hsieh, A. H.
Luttrell, E. Marsch, and R. Schwenn 33
Interplanetary Shocks on the Large Scale: A Retrospective on the Last Decade's Theoretical
Efforts V. J. Pizzo 51
Interplanetary Shock Phenomena Beyond 1 AU Edward J. Smith 69
Magnetohydrodynamic and Gasdynamic Theories for Planetary Bow Waves John R. Spreiter and
Stephen S. Stahara 85
Planetary Bow Shocks C.T. Russell 109
MICROSTRUCTURE
Subcritical Collisionless Shock Waves M.M. Mellott 131
Ion Reflection, Gyration, and Dissipation at Supercritical Shocks J.T. Gosling and A. E.
Robson 141
Numerical Simulations of Quasi-Perpendicular Collisionless Shocks C.C. Goodrich 153
Oblique, Parallel, and Quasi-Parallel Morphology of Collisionless Shocks Eugene W. Greenstadt 169
Simulation of Quasi-Parallel Collisionless Shocks Kevin B. Quest 185
Electron Velocity Distributions Near Collisionless Shocks William C. Feldman 195
Plasma Waves and Instabilities Donald A. Gurnett 207
Microtheory of Collisionless Shock Current Layers D. Winske 225
The Electron Foreshock Alexander J. Klimas 237
Upstream Suprathermal Ions M.F. Thomsen 253
PARTICLE ACCELERATION
Shock Drift Acceleration Thomas P. Armstrong, Mark E. Pesses,a nd Robert B. Decker 271
Diffusive Acceleration Manfred Scholer 287
Notation
Introduction
Violent expansions of the solar corona cause transient shock waves which propagate outward from
the sun at hundreds to thousands of kilometers per second; simple solar wind velocity gradients at the
surface of the sun lead to high-speed streams overtaking slower streams, forming corotating shocks;
and steady state supermagnetosonic solar wind flow past objects such as the planets lead to standing
bow shocks. However, the solar wind plasma is so hot and tenuous that charged particle Coulomb
collisions produce negligible thermalization or dissipation on scale sizes less than 0.1 AU. The irrevers-
ible plasma heating by these shocks is accomplished by wave-particle interactions driven by plasma
instabilities. Hence these shocks are described as (cid:127)collisionless."
Collisionless shocks are interesting and important for numerous reasons. Collisionless shocks are the
simplest configuration in which a macroscopic flow is regulated by microscopic dissipation, a problem
common to many different plasma processes.C ollisionless shocks are therefore of basic plasma physical
interest. There are also many important ways in which shocks affect the near-earth environment.
Coronal shocks are believed to be responsible for the acceleration of solar flare energetic particles,
which then propagate outward to fill the heliosphere. Shock propagation into the outer heliosphere
may be a principal cause of the solar cycle dependent cosmic ray modulation. Interplanetary shock
interactions with the earth's magnetosphere cause magnetic storms, intense low-latitude aurorae, and
radio blackouts. Recent observations of fields and particles near interplanetary shocks and upstream of
the earth's bow shock allow us to study particle acceleration processesi n situ, giving us first-hand
knowledge of processesw hich are occurring not only in our heliosphere but which may give us
important insights into plasma processesw hich are occurring near distant interstellar shocks, pro-
cessesw hich are believed to create cosmic rays.
These two volumes update our current knowledge of collisionless shocks in the heliosphere, an area
in which recent major advances have been provided by space probes from NASA, the European Space
Agency, and the Federal Republic of Germany. Individual papers in these volumes will take you from a
detailed look at processeso ccurring at and near the shock itself (plasma instabilities responsible for
dissipation of the streaming energy, and heating and acceleration of the plasma) to the characteristics
of the upstream and downstream particles and their role in the shock structure. They discussh ow these
processes change as the strength of the shock and the properties of the upstream plasma and the
orientation of the magnetic field are varied. The past development and present state of gasdynamic,
MHD, multifluid, and hybrid numerical simulation codes are summarized, and numerous applications
to collisionless shocks are discussed.S everal papers deal with the development of shocks near the sun
and the earth and their outward propagation and eventual interactions with slower solar wind and/or
other shocks in the outer heliosphere. Particle acceleration mechanisms for parallel and perpendicular
shocks are described. The relationship between theory and observations forms the central dialogue of
these volumes.
The monographs are intended to serve as summaries of the present status of our knowledge of
collisionless shocks in the heliosphere. Volume 34, composedo f four tutorials, serves as a general text
for entering graduate students and, for a scientist from a related field, provides the background needed
to understand fully the topical reviews of volume 35. The 19 papers of volume 35 focus on specific topics
of collisionless shocks and are intended to be reviews (and a source of references) of the present status
of research in this field.
Acknowledgments. We wish to thank the referees, listed below, for the countless hours they devoted
to critical review of the papers appearing in the two volumes: K. A. Anderson, T. P. Armstrong, A.
Barnes, L. F. Burlaga, D.C. Ellison, D. H. Fairfield, M. A. Forman, D. W. Forslund, R. W. Fredricks, B.
E. Goldstein, M. L. Goldstein, J. T. Gosling, E. W. Greenstadt, C. A. Gurgiolo, T. Hada, J. D. Huba, M.
K. Hudson, J. R. Kan, P. J. Kellogg, A. J. Lazarus, M. A. Lee, R. P. Lepping, R. P. Lin, W. H.
Matthaeus, K. Papadopoulos, G. K. Parks, G. Paschmann, K. B. Quest, P. Rodriguez, C. T. Russell, F. L.
Scarf, J. D.-Scudder, D. D. Sentman, N. R. Sheeley, Jr., G. L. Siscoe, E. J. Smith, S.S. Stahara, S. T.
Suess, J. A. Van Allen, K. P. Wenzel, R. T. Woo, and C. S. Wu. The Chapman conference, held in Napa,
California, February 20-24, 1984, and these books owe a major portion of their successt o the tireless
efforts of Christina Brokl of the Jet Propulsion Laboratory and Barbara Holland of the Goddard Space
Flight Center. These books could not have been possible without the support of the National Science
Foundation and the National Aeronautics and Space Administration and, in particular, the moral and
financia! support of the International-Sun-Earth-Explorer Project. We also wish to express our special
appreciation to the American Geophysica! Union's publication staff, who provided exceptiona! manage-
ment support throughout this endeavor.
BRUCE T. TSURUTANI
Jet Propulsion Laboratory
California Institute of Technology
Pasadena
ROBERT G. STONE
NASA Goddard Space Flight Center
Greenbelt, Maryland
Geophysical Monograph Series Collisionless Shocks in the Heliosphere: Reviews of Current Research Vol. 35
Theories of Shock Formation in the Solar Atmosphere
R. S. STEINOLFSON 1
High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado 80307
Models for the generation of coronal shocks by large-scale, disruptive solar events
are reviewed. The theoretical requirements for shock formation in the corona are
discussed.V arious approaches used to model the shock-initiating events are analyzed
with distinctions drawn between the driving mechanisms in the theories. The impor-
tance of the pre-event coronal magnetic field configuration and magnitude in influ-
encing the subsequent coronal response and shock structure is also discussed. Partic-
ular emphasis is placed on the information furnished by the theories which can be
compared with observations and, similarly, the observational results which provide
critical tests for the theory. Such comparisons will be shown to suggest a revised
representation of the shock-transient association. Recent models of the observed spa-
tial relationship of type IIs and transients are examined.
1. Introduction The ultimate test for any theory of coronal shock for-
Large-scale disruptions of the solar corona have been mation is a direct comparison with available observa-
studied extensively since the start of data acquisition tions. Correlative studies, demonstrating that interplan-
from orbiting white-light coronagraphs. These disrup- etary shocks can often be associated with coronal mass
tions are generally referred to as coronal transients (the ejections [Gosling et al., 1975; Sheeley et al., 1983] and
term used herein) or, for the subclass which involves ex- solar activity [Hundhausen, 1972; Gosling, 1975], indi-
pulsion of coronal material, coronal mass ejections. Com- cate that at least some white-light observations may in-
pressive waves form as a natural consequence of such clude shocks. Data from three orbiting coronagraphs
energetic transient phenomena and, under suitable con- (Skylab [MacQueen et al., 1974], Solwind [Sheeley et al.,
ditions, steepen into shocks in either the corona or inter- 1980], and SMM [House et al., 1981]) and the ground-
planetary space. The term corona will be used here to based K-coronameter [Fisher and Poland, 1981] are cur-
refer to that portion of the solar atmosphere within ap- rently available. These instruments record electron scat-
proximately 5 solar radii of the surface. tered light and thereby provide information on the spa-
Potential sources of transients include flares, eruptive tial and temporal evolution of the line-of-sight coronal
prominences, and unstable or rapidly evolving coronal mass. A density increase due to a sufficiently strong
magnetic field configurations. Transitory coronal shock would be observed by coronagraphs, although the
changes also occur on a much slower time scale than in shock usually would not appear as a sharp discontinuity
the above impulsive events, although this review will be in the integrated measurement. Weak shocks and shocks
limited, for the most part, to the more energetic, rapid with a very thin compressed region could be lost in in-
events that are likely to result in shocks. We will not strument noise. White-light images, then, furnish infor-
consider other potential sources of coronal shocks, such mation on the transient producing the shock and, possi-
as standing shocks that occur in some solar wind solu- bly, on the shock wave itself.
tions [Hasan and Venkatakrishnan, 1982; Whang, 1982; Another source of observations is metric type II radio
Habbal and Tsinganos1,9 83]a nd shocksp roduced bursts (such as those recorded by the Clark Lake Radio
subphotospheric oscillations [Hollweg et al., 1982]. Observatory and the Nancay and Culgoora radiohelio-
graphs), which are generally interpreted as being associ-
'Permanent affiliation: Department of Physics,U niversity of ated with shocks. Satellite observations of kilometer
California, Irvine, California 92717. wavelength solar radio bursts also provide information
Copyright American Geophysical Union
Geophysical Monograph Series Collisionless Shocks in the Heliosphere: Reviews of Current Research Vol. 35
2 SHOCK FORMATION IN THE SOLAR ATMOSPHERE
i i i 1 tions 4.2 and 4.3. The final section contains a brief sum-
t = :30 rntn 45 min
1.0 - (a) ---• . • mary.
2. Shock Formation in the Corona
0.5 Ai- =r 15• min •, The essential requirement for finite-amplitude distur-
, \ bances to steepen into shocks (at least in a one-
dimensional model) is that the local characteristic (fast
0
mode) speed in the disturbance exceed the characteristic
0.5 - (b) speed in the medium through which it is traveling.
Consequently, compressive MHD waves, formed during
•,T i = 15m ln '•.• eruptive solar events, should steepen as they propagate
i away from their point of origin. Whether or not the
2
waves evolve into fully developed shocks in the corona,
SOLARR ADIUS(R /R s)
or even in the interplanetary medium, is another matter
since the steepening process requires a certain time
Fig. 1. Shock formation in the lower corona for (a) rela-
tively large-amplitude and (b) small-amplituded isturbances.D is- period. We are neglecting here the possibility of the de-
turbances were initiated at t- 0 at the solar surface, and the velopment of solitons, in which dissipative processesb al-
curves represent the steepening profiles at two successive times ance the nonlinear steepening.
for several cases. For instance, in Figure lb the wave does not From a theoretical standpoint, a shock forms when
steepen into a shock for a 5 min rise time A•i, while a shock does characteristics of the same family cross so the solution at
form when A•i is reducedt o 1 min. The initial atmospherew as the point of intersection is no longer unique [Zel'dovich
assumed to be in hydrostatic equilibrium with an embedded
and Razier, 1966]. This principle has been used to derive
magnetic field parallel to the surface. Average values of sound
analytic expressions for shock formation time for MHD
speeda nd beta are 150 km s-' and 0.04, and 7 = 1.05.
waves by Montgomery [1959] and Cohen and Kulsrud
[1974]. Numerical simulations by Steinolfson [1981] pre-
on electron acceleration by coronal shocks [Cane et al., dict that the steepening process may occur somewhat
1981]. Recent efforts to analyze simultaneousr adio emis- faster than these theories estimate.
sion and white-light data for the same transient [e.g., The time of shock formation in the above analyses de-
Stewart et al., 1982; Gergely et al., 1984; Gary et al., 1984] pendso n the ambientm ediuma nd the spatial gradienti n
are extremely useful in establishing the shock-transient the initial disturbance. A complication occurs in the
relationship. solar atmosphere because the energy source producing
Some additional data sources( such as X ray and EUV), the propagating disturbance may increase in amplitude
while not furnishing any information on the shock or with time. If the increase is too slow, the disturbance
transient directly, are, nonetheless,v aluable in the study may propagate out of the corona before a shock forms. A
of shock formation. They place constraints on the simple estimate for how rapid the energy increase must
amount and time-dependenceo f the energy released in be, in order that a shock form before the disturbance has
the solar event and the magnetic field before and follow- traveled a specified distance, can be derived using the
shock intersection criteria for fast mode MHD character-
ing the eruption. Ha observationsm ay be useful for un-
derstanding transients in those casesw here the material istics. If a solar event increases the velocity in the re-
emitting in white light can be identified as originating in sulting disturbance by an amount Av above the ambient
a prominence. velocity v, then a shock forms within l providing the time
The steepening of compressivem agnetohydrodynamic required to reach Av is less than Azi given by
(MHD) waves into shocksd ependso n the magnitudeo f lAv
the wave disturbance, gradients in the wave, and the am- A•i = (1)
(C• + v)(C• + v + Av)
bient medium. This general problem is addressedi n the
following section,w ith specifica pplication to shock for- whereC s = Cs(1+ 2/7•)' /2. The quantitiesC sa nd fl are
mation in the solar atmosphere.S ome of the models that the ambient sound speed and plasma beta (ratio of ther-
have been suggesteda s representative of coronal tran- mal to magnetic pressures),r espectively.
sients are discussed in section 3. Most of these models do The above expression was evaluated in the corona
not include the transient-atmosphere interaction (and, using a one-dimensionalM HD code, which solves the
therefore, shockst hat may form in that interaction) as an complete nonlinear, time-dependent equations. The re-
integral part of the analysis.T hus, in relating theory and sults, in a static atmosphere,f or two values of Av, repre-
observations in section 4, the quantitative comparisons sentative of the energy input in the solar event, are
are limited to models which include shock formation. shown in Figure 1. The evolving velocity profiles near
These data and a critical evaluation of the theory sug- the leading edge of the disturbances are shown at two
gest revised pictures of the shock-transienta nd the different times following their initiation at the solar sur-
shock-transient-typeI I associationsa s discussedi n sec- face. For the lower energy event in Figure lb, the profile
Copyright American Geophysical Union
Geophysical Monograph Series Collisionless Shocks in the Heliosphere: Reviews of Current Research Vol. 35
STEINOLFSON 3
shows no indication of steepening for the slowest rise It must be kept in mind that the analysis of this section
time (Azi - 15 min). The profile is clearly steepening for is based on a one-dimensional model in which the ambi-
the 5-min rise time, although it has not yet shocked. Re- ent characteristic speeds are relatively well-behaved.
ducing A•i to I min causes the shock to be fully formed, That is, they do not undergo large variations on small
as indicated by the steepness of the profile and by its (solar radius) spatial scales. Multi-dimensional effects
faster propagation speed. These results agree with (1), would tend to retard shock formation, and significant
which predicts that the shock should form before reach- changes in the ambient atmosphere would certainly alter
ing 5 solar radii if A• _<2 min for this case. The larger the results given here.
amplitude disturbance in Figure la shocks for a 5-min
3. Models of Coronal Transients
rise time, which also agrees with the (1) estimate of A• i _<
6 min. There are two main areas where the model predictions
Having established that (1) may provide a reasonable and the observations overlap. First, the mass excess com-
approximation of theoretical shock formation times, it is puted in a particular model can be converted to bright-
of interest to determine if the specified relationships in ness and compared with the brightness of a transient
the equation coincide with observations. First of all, observed by a white-light coronagraph. Thus, the simu-
since A• is an upper limit, one would expect that a very lated and observed transient shapes and their motion are
rapid energy deposition, which occurs on, for instance, directly comparable. The actual brightness levels provide
the time scale of the impulsive phase of the flare, would a quantitative comparison of somewhat less value due to
produce shocks in the corona. This is consistent with assumptions of transient line-of-sight depth and back-
correlative studies which show that type II bursts (in- ground density. Second, if shocks are predicted in the
dicative of coronal shocks) tend to be associated with the model, they can be compared with radio observations.
faster ( > 400 km s-1) coronal mass ejections [Sheeley et A common denominator in all models is the magnetic
al., 1984], which, in turn, are preferentially associated field. The mechanism driving the transient generally in-
with flares [Gosling et al., 1976]. On the other hand, a volves this quantity, either directly or indirectly. The
slow energy input, representative of that which may atmospheric response is also substantially influenced by
occur during an eruptive prominence, may not produce a the ambient magnetic field magnitude and configuration.
shock in the corona, although it may eventually steepen The shock formation process is generally not included
in interplanetary space. Once again, this is in agreement as a constituent part of the transient formation and prop-
with the above correlations. agation in the models discussed here. Two exceptions are
Another significant quantity affecting coronal shock the stationary-driver models and the self-similar solu-
formation is the value of fi in the ambient atmosphere. tions of Low [1982]. This serious shortcoming must be
Smaller values of fi, larger magnetic fields, require a addressed at some point in the development of each indi-
more impulsive event, in order to obtain the necessary vidual model before it can be used in quantitative com-
steepening. Although the sound speed does not vary ap- parison with observations. In the self-similar solutions,
preciably in the lower corona, (1) predicts that shocks which cannot model the early stages of transient devel-
form later in hotter atmospheres. opment, the problem of shock formation is avoided by
The above formulation applies to solar events in which assuming that the shock is sufficiently ahead of the tran-
the major driving mechanism remains essentially station- sient.
ary near the solar surface. Some models for coronal tran- All models contain enough free parameters that they
sients, as discussed in the following section, require that can reasonably reproduce at least one observed
the driver be an integral part of the transient. The driver quantity--the leading-edge velocity along a single radial
motion must be accounted for, in this case, in deriving a line. This necessary requirement, however, is virtually
relation similar to that in (1). useless in determining whether a model can satisfactorily
There are a couple of points worth emphasizing with describe the physics of the driver-transient-shock interac-
respect to Figure 1. Until the shock forms, the leading tion throughout the entire disturbance.
edge of the transient always travels at the local fastest A major simplifying assumption in the models, at least
characteristic speed, independent of the magnitude of the in their mathematical description, is that a complete
velocity increase in the disturbance (providing the dis- three-dimensional treatment is not necessary. There is, of
turbed velocity is less than the fastest characteristic course, no observational evidence to support a two-
speed). Note that it is not necessary for the fluid velocity dimensional representation, although a strictly one-
in the disturbance to exceed any characteristic speed in dimensional treatment is clearly inappropriate. One ad-
the ambient medium in order that shocks form. For exam- ditional mutual simplification is that dissipative effects
ple, the velocity increase in the case in Figure lb is less can be neglected, except at shocks.
than half the ambient sound speed. Finally, for a rapidly Some of the models are now described with a critical
increasing driving energy so large that the disturbed ve- examination of their advantages and disadvantages in
locity exceeds the ambient fast speed, shocks form almost their current state of development. Recent reviews that
immediately. discuss theories for transient formation have been given
Copyright American Geophysical Union