Table Of ContentSponsors
THE COMMITTEE ON SPACE RESEARCH (COSPAR)
THE INTERNATIONAL ASTRONOMICAL UNION (IAU)
and
THE POLISH ACADEMY OF SCIENCES*
Program Committee
D.E. Page (ESA/JPL) (Chairman)
S. Grzedzielski, Poland (Executive Chairman)
W.I. Axford (F.R.G.)
V.B. Baranov (U.S.S.R.)
J.L. Bertaux (France)
S. Bowyer (U.S.A.)
H.J. Fahr (F.R.G.)
L.A. Fisk (U.S.A.)
P.C. Frisch (U.S.A.)
K.I. Ciringauz (U.S.S.R.)
A. Hewish (U.K.)
J. Kota (Hungary)
R.R. Meier (U.S.A.)
M. Nicolet (Belgium)
A. Nishida (Japan)
E.J. Smith (U.S.A.)
E.C. Stone (U.S.A.)
K.-P. Wenzel (ESA/ESTEC)
Local Organizing Committee
S. Grzedzielski*
R. Ratkiewicz*
D. Rucinski*
J. Ziemkiewicz*
Editorial Assistance
A. Czechowski*
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KEY TO PHOTOGRAPH OF PARTICIPANTS
1. V.B. Baranov; 2. K. Scherer; 3. W.I. Axford; 4. D.E. Page; 5. S. Grzedzielski; 6. S.A. Grib; 7. M.S. Ruderman;
8. J. Ziemkiewicz; 9. M. Verigin; 10. G. Cini Castagnoli; 11. H.J. Fahr; 12. E.C. Roelof; 13. R.B. McKibben; 14. K. Sauer;
15. W.S. Kurth; 16. H. Lehmann; 17. S.V. Chalov; 18. Badruddin; 19. A. Czechowski; 20. K.-P. Wenzel; 21. M. Serio;
22. Β. Popielawska; 23. Κ. Baumgärtel; 24. Ν. Kömle; 25. J.F. McKenzie; 26. R.A. Burger; 27. E. Möbius; 28. E.J. Smith;
29. LS. Veselovsky; 30. F. Mariani; 31. O.V. Likin; 32. A.G Krainiuk; 33. A. Somogyi; 34. GN. Zastenker; 35. A.J. Lazarus;
36. M. Gruntman; 37. P. Kiraly; 38. Z. Kobylinski; 39. H. Pérez-de-Tejada; 40. M. Ostrowski; 41. M. Banaszkiewicz;
42. J. Juchniewicz; 43. W.-H. Ip; 44. A. Barnes; 45. M.S. Potgieter; 46. P. Dyal; 47. Y.C. Whang; 48. M. Lee; 49. J.R. Jokipii;
50. D.L. Judge; 51. M.V. Alaniya; 52. —; 53. J. Kota; 54. M. Bzowski; 55. S. Bowyer; 56. M.K. Bird.
PHYSICS OF THE OUTER
HELIOSPHERE
Proceedings of the 1st COSPAR Colloquium
held in Warsaw, Poland, 19-22 September
1989
Edited by
S. GRZEDZIELSKI
Space Research Centre, Polish Academy of Sciences,
Ordona 21, 01-237 Warsaw, Poland
and
D.E. PAGE
European Space Agency and Jet Propulsion Laboratory,
4800 Oak Grove Drive, MS 169/506, Pasadena,
CA 94035, U.S.A.
Published for
THE COMMITTEE ON SPACE RESEARCH
by
PERGAMON PRESS
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253 College Street, Toronto, Ontario, Canada M5T1R5
Copyright © 1990 COSPAR
All Rights Reserved. No part of this publication may be reproduced,
stored in a retrieval system or transmitted in any form or by any means:
electronic, electrostatic, magnetic tape, mechanical, photocopying,
recording or otherwise, without permission in writing from the
copyright holders.
First edition 1990
Library of Congress Cataloging in Publication Data
COSPAR Colloquium (1st: 1989: Warsaw, Poland)
Physics of the outer heliosphere: proceedings of the 1st COSPAR
Colloquium held in Warsaw, Poland, 19-22 September, 1989/edited
by S. Grzedzielski and D.E. Page.
Sponsors, the Committee on Space Research (COSPAR), The
International Astronomical Union (IAU), and the Polish Academy
of Sciences.
1. Heliosphere—Congresses 2. Astrophysics—Congresses.
3. Solar Wind—Congresses. 4. Interstellar matter—
Congresses. 5. Galactic cosmic rays—Congresses.
I. Grzedzielski, S. II. Page, D. Edgar. III. COSPAR.
IV. International Astronomical Union. V. Polska Akademia
Nauk. VI. Title.
QC881.2.H43C67 1989 523.7—dc20 90-7920
ISBN 0-08-040780-3
In order to make this volume available as economically and as rapidly
as possible the author's typescript has been reproduced in its original
form. This method unfortunately has its typographical limitations but
it is hoped that they in no way distract the reader.
Printed in Great Britain by BPCC Wheatons Ltd, Exeter
THE INTERNATIONAL HELIOSPHERIC
STUDY AND THE FIRST COSPAR
COLLOQUIUM — PREFACE
The International Heliospheric Study (IHS) was approved by COSPAR as long ago as 1982. At the time it
was expected that the study would centre on the ESA/NASA Ulysses spacecraft but unfortunately the start
of that mission has been delayed until October 1990. It was decided that the study should in any case begin
in 1986 in order to exploit the existence of the Pioneer 10 and 11 and the Voyager 1 and 2 spacecraft in the
outer heliosphere, and results from other spacecraft will of course be included as they become available.
The scientific topics of greatest interest to the IHS are the three dimensional structure of the solar wind and
the interplanetary magnetic field carried by it, the propagation of energetic solar particles throughout the
heliosphere and the modulation of galactic cosmic rays by the heliosphere. The Pioneer and Voyager
spacecraft at great distances from the Sun present the possibility of detecting a boundary between our solar
system and the local interstellar medium.
The IHS task group set up by COSPAR aims to strengthen links between those scientists interested in the
heliosphere so that the best possible use can be made of the missions , facilities and data that become
available. The intention is not to create any sort of bureaucratic coordinating machinery but only to
facilitate the flow of information. The members of the Task Group are W. I. Axford (Germany), R. V.
Bhonsle (India), Κ. I. Gringauz (U.S.S.R.), S. Grzedzielski (Poland), M. Oda (Japan), D. E. Page
(ESA/JPL) as Chairman, E. J. Smith (U.S.A.), E. C. Stone (U.S.A.) and K. P. Wenzel (ESA).
This task group was given the honour of organising the very first COSPAR Colloquium and called this
"Physics of the Outer Heliosphere". They were happy to accept the invitation of Professor S. Grzedzielski,
Director of the Space Research Centre, Polish Academy of Sciences, to hold the meeting in Warsaw and
would like to thank the local organising committee (S. Grzedzielski, R. Ratkiewicz, D. Rucinski and J.
Ziemkiewicz) for an excellent job carried out in less than ideal circumstances. The Colloquium programme
committee was S. Grzedzielski (Poland, Executive Chairman), W. I. Axford, (F.R.G), V. A. Baranov
(U.S.S.R), J. L. Bertaux (France), R. V. Bhonsle (India), S. Bowyer (U.S.A.), H. J. Fahr (F.R.G.), L. A. Fisk
(U.S.A.), P. C. Frisch (U.S.A.), Κ. I. Gringauz (U.S.S.R.), A. Hewish (U.K.), J. Kota (Hungary), R. R.
Meier (U.S.A.), M. Nicolet (Belgium), A. Nishida (Japan), D. E. Page (ESA, Chairman), E. J. Smith
(U.S.A.), E. C. Stone (U.S.A.), K. P. Wenzel (Netherlands).
Thanks are due to those referees who helped collect and edit the papers arriving from the five Colloquium
sessions. Particular mention should be made of Andrzej Czechowski (Space Research Centre, Warsaw)
who assisted in a major way in bringing together the material for the Colloquium Proceedings which are to
be published by Pergamon Press in September 1990.
A highlight of the Colloquium was the report on the encounter of the Voyager 2 spacecraft with Neptune
which had occurred just a few weeks earlier. We are grateful to N.F. Ness who stepped in at short notice
when it was found that the Voyager project scientist was unable to be present. The report is not available
for the Colloquium Proceedings.
D. Edgar Page
Chairman COSPAR IHS Task Group
1
PRESIDENTIAL ADDRESS
This is the first volume in the series "COSPAR Colloquia" which, it is anticipated, will become a permanent
and vigorous component of COSPAR's programme of publications on all aspects of scientific space research.
It should be a useful adjunct to "Advances in Space Research" which records the proceedings of the biennial
COSPAR Assemblies. The Colloquia are intended to be specialized meetings to be held as a need is perceived
and generally in countries where an Assembly may not be expected to take place during the following few
years. It is hoped that on the one hand, the influence of COSPAR, in a positive sense, can thereby be more
widely felt than it might be otherwise, and, on the other hand, that communities with specialized interests
can also be better recognized and served in COSPAR.
The first Colloquium held in Poland has been a success in every respect. The topic chosen was a timely one,
in view of the continuation of the Voyager and Pioneer missions beyond their last planetary encounters and
eventually perhaps into the undisturbed interstellar medium, and also because of the planned launch of the
Ulysses mission in 1990. The topic has also long been of special interest to space scientists in Poland who
have made many important contributions to its study, some of which are described in this volume.
This Colloquium will serve as a model for all subsequent Colloquia both in terms of the quality of the sci-
entific presentations and the efficient organization. I wish to record, on behalf of COSPAR, our gratitude to
Professor Grzedzielski and his colleagues and also to Dr. Page, representing the International Heliospheric
Study, for the great efforts they have made in ensuring its success and in providing COSPAR with an excellent
start in a new venture.
W.I. Axford
President of COSPAR
3
INTRODUCTORY LECTURE —
THE HELIOSPHERE
W.I. Axford
Max-Planck-Institut für Aeronomie, D-3411 Katlenburg-Lindau, F.R.G.
ABSTRACT
A brief review is given of the historical ideas dealing with the development of our present concept of the
heliosphere, together with a summary of the present status, particularly with regard to the distance to the
solar wind shock termination.
INTRODUCTION
The "heliosphere" is defined to-day as the region of space around the Sun where the solar wind plasma and
magnetic field have a controlling effect on the state and behaviour of the medium. The heliosphere must
therefore contain a shock which marks the termination of the solar wind as a supersonic flow and there should
be an outer boundary, termed the "heliopause", separating plasma of interstellar and solar origins. In fact
the definition may be over simple in view of the probable importance of reconnection between the solar and
interstellar magnetic fields and the transparency of the boundary to neutral particles coming from either side.
Nevertheless the definition is easily understood and should be retained as long as it remains useful. The likely
configuration of the heliosphere is sketched in Figure 1. By analogy with magnetospheric terminology the
region between the termination shock and the heliopause is termed the "heliosheath".
This is not intended as an exhaustive review of the literature of heliospheric physics but rather as an intro-
duction to some of the more interesting early history of the subject, which is now being gradually forgotten,
together with a brief update on some recent developments which seem to be of particular significance. The
reader is referred to the remaining papers in this volume for more detailed accounts and also to the reviews
by Hölzer, Suess, Baranov /!/ and, from a more historical perspective, Simpson /2/.
Outer boundary of heliosphere = "heliopause"
Region of subsonic flow = "heliosheath"
Fig. 1 The configuration of the heliosphere shown schematically.
7
8 W.I. Axford
THE FIRST PHASE OF STUDIES OF THE HELIOSPHERE
The first suggestion that the heliosphere (then not defined as such) might have important observable effects
was made by Alfvén /3/ who, following Teller, proposed that cosmic rays have a local (solar) origin and are
accelerated somehow by the irregular solar "corpuscular streams" believed to cause geomagnetic storms. A
first version of the required acceleration mechanism /4/ involved radial diffusion in the solar dipole magnetic
field in a manner similar to that believed to occur in a magnetosphere; this would of course lead to the
particles having their highest energies close to the Sun and therefore a negative radial gradient which is not
observed. A second version involved pitch angle scattering and "magnetic pumping" and was successful in
producing a power law spectrum.
In fact, it is obvious from a closer consideration of the nature of cosmic rays that they cannot be of local origin,
although there are components of the energetic particle population observed near the Earth that have been
accelerated in the solar system (e.g. solar flare particles, the "anomalous" component, particles accelerated
by interplanetary shocks and in planetary magnetospheres). Alfvén however does not give up so easily as is
evident from an amusing exchange with Ginzburg /5/:
Ginzburg: "The arguments against a local/solar origin of cosmic rays are numerous I have worked in
this field for some years and I can say in the course of time the argument for a galactic origin slowly improves.
So I hope during my lifetime I shall see the full victory of this theory."
Alfvén: "I hope you will live very long!"
The first serious discussion of the nciture of the heliosphere as such was made by Davis /6,7/ whose ideas
are effectively summarized in Figure 2. Davis pointed out that the solar corpuscular radiation described by
Biermann /8/ and now termed the solar wind, would produce a cavity in the galactic magnetic field permitting
energetic solar flare particles to reach the Earth. (A magnetic field of galactic origin between the Earth and
the Sun would shield the former from solar particles having energies less than 45 GeV if the field strength
were 1 nT.) In order to estimate the size of the field-free cavity Davis simply equated the dynamic pressure
of the solar wind at R au to the interstellar magnetic pressure:
'•lie = Β]
and took B = 1 nT and V = 1000 km/sec. The values of n , the electron density at 1 aw, derived by Biermann
g e
on the basis of an incorrect idea concerning the interaction between solar wind and cometary plasma (viz.
n = 103 - 10scm"3) gave uncomfortably large values for R but Davis nevertheless took R — 200 au as a
e
working value. This later proved acceptable on considerations of the scattering of sunlight by interplanetary
electrons by Kiepenheuer /9/ and of the amplitude of geomagnetic storm sudden commencements (i.e. n =
e
1-10 cm -3). Even so Davis was fairly cautious in his second paper /7/ and suggested that the boundary
might be beyond the orbit of Jupiter or even of Uranus. Our present estimates, which involve modifications
of (1), suggest that R is of the order of 100 au with an uncertainty of perhaps 30% and that this refers only
to the minimum distance to the solar wind termination shock rather than to the heliopause.
Davis was most interested in the effects of the solar wind cavity on galactic cosmic rays. He suggested that
cosmic rays with energies less than about 10 12eV could be trapped in the cavity for long periods and ac-
cordingly that changes in the size of the cavity could produce the observed solar cycle modulation effect and
that the electron component would be removed by its Compton scattering of sunlight /10/. An attempt to
explain the time variation of the cosmic ray spectrum on this basis was made by Beiser /ll/; such a theory
implies that the radial gradient of the cosmic ray intensity is zero, which we now know is not the case. In fact,
it is clear that the interplanetary (solar) magnetic field plays an important role in the modulation process
/12/ as well as in the guiding of flare particles following their release from the solar corona; Davis had some
conceptual difficulties in dealing with this field as can be seen from Figure 2.
The next step in the development of our understanding of the nature of the heliosphere was made by Meyer,
Parker and Simpson /13/, who constructed a composite model to account for the initial anisotropy and the
temporal behaviour of solar flare particle events. This involved a field-free region extending beyond the orbit
of the Earth allowing direct propagation from the Sun to the Earth, a region of disordered fields through which
the particles are lost diffusively and, on the outside, the galactic field. From the decay phase of the events
it could be estimated that the outer boundary of the disordered region, where the particles were assumed to
escape freely, should be at about 5 au.
Following his account of the true nature of the solar wind, Parker /14/ was able to modify this model in a
significant way by including a smooth spiral interplanetary magnetic field in the inner region, thus providing
a simple explanation of the east-west asymmetry in the behaviour of flare particle events. He also suggested
that the disordered field could be considered as being the consequence of hydromagnetic (firehose, mirror,
The Heliosphere 9
cyclotron) instabilities which might naturally arise in a collisionless solar wind having a spiral magnetic field.
In fact, with the introduction of the concepts of coronal transport /15/ and anisotropic diffusion in the largely
ordered interplanetary magnetic field /16/ the need for the main feature of the Meyer et al. model is obviated
and indeed, subsequent observations have shown that there is no isotropically diffusing region beyond the
orbit of the earth, at least out to 40 au.
(a) (b) (c)
Fig. 2 Various versions of the heliosphere according to Davis /6,7/. It is argued that the region between
the Earth and the Sun is free of magnetic field, although a general weak solar field could be accommodated
as shown in (b) and (c).
With the development of the concept of the solar wind as a supersonic flow resulting from the continual
heating of the solar corona, it was natural for a quite different approach to be made to the problem of cosmic
ray modulation /17/. It became possible to account for the solar cycle and irregular short term variations
(Forbush decreases) on the basis of diffusion in the expanding solar wind with occasional modifications induced
by expanding shock associated with flares, for example /12,17,18/, and the diurnal variations on the basis
of anisotropic diffusion in this spiral interplanetary magnetic field pattern which must corotate with the Sun
/19/.
It is quite obvious to those with a background in gas dynamics that the solar wind, which was shown by Parker
to have a certain analogy to flow in a Laval nozzle, must terminate in a shock front at a point where its dynamic
pressure has dropped to that of the external medium /20/. This condition is, with minor modifications /21/
that given by equation (1), which should not therefore be regarded as giving the distance to the heliopause as
tacitly assumed by Davis. With more up-to-date values for the various parameters involved it is difficult to
place the termination of the supersonic solar wind closer than about 100 au if B = 0.3 nT /22/. Nonetheless
g
arguments have been advanced for putting it much closer: for example, Brandt and Michie /23/, using an
incorrect interpretation of the nature of comet tails argued that the supersonic wind would change to a solar
breeze at about 2 au in rough agreement with the inner boundary of the Meyer et al. diffusing region, while
Lanzerotti and Schulz /24/, Jovian radio emissions, argued that 5 au might be appropriate. There has never
been any basis for assuming that the pressure in the interstellar medium could be high enough to produce such
small values of R (equivalent to B = 300 nT and 50 nT, respectively) and we know now from observations
g
from the Pioneer and Voyager spacecraft that R exceeds 40 au.
If the Sun had always been at rest with respect to the interstellar medium, the position of the termination
shock would always have been stationary but the heliopause would have grown in radius as t~ 1^. At the
present time the radius of the spherical region occupied by solar plasma would be a few parsecs if there had
been no great variations in the solar wind in the past. If all stars were equally privileged it would be expected
that the whole of interstellar space would be filled by hot bubbles of stellar wind plasma with little room
left for the rest of the interstellar medium. In fact this result does not depend on the stars being stationary
since the plasma tails of moving stars would occupy roughly the same volume. Cooling as a consequence
of interaction with neutral component of the interstellar medium changes this as far as our heliosphere is
concerned, although in the case of more powerful stellar winds the hot plasma may be much more resistant
and therefore make an important contribution to the very hot (10 6K) component of the interstellar medium.
With the assumption that the region of supersonic flow in the heliosphere is small, Parker /17/ has examined
the possible shape of the heliopause in the case where the Sun moves subsonically with respect to the exter-
nal medium and the contribution of the magnetic field to the pressure of this medium is small (Figure 3a).
In the case of supersonic motion one would expect a similar shape but there would be a bow shock in the
interstellar medium upstream. The presence of a weak magnetic field would lead to the tail of the heliosphere
