Early Solar Physics

305 Pages·1970·4.681 MB·English

by  A. J. Meadows and D. ter Haar (Auth.)

EARLY SOLAR PHYSICS
BY
A. J. MEADOWS
Senior Lecturer in Astronomy
University of Leicester
1966
PERGAMON PRESS
OXFORD · LONDON · EDINBURGH · NEW YORK
TORONTO · SYDNEY · PARIS · BRAUNSCHWEIG
PERGAMON PRESS LTD.,
Headington Hill Hall, Oxford
4 & 5 Fitzroy Square, London W.l
PERGAMON PRESS (SCOTLAND) LTD.,
2 & 3 Teviot Place, Edinburgh 1
PERGAMON PRESS INC.,
Maxwell House, Fairview Park, Elmsford, New York 10523
PERGAMON OF CANADA LTD.,
207 Queen's Quay West, Toronto 1
PERGAMON PRESS (AUST.) PTY. LTD.,
19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia
PERGAMON PRESS S.A.R.L.,
24 rue des Écoles, Paris 5e
VIEWEG & SOHN GMBH,
Burgplatz 1, Braunschweig
Copyright © 1970 Pergamon Press Ltd.
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, mechanical, photocopying, recording or otherwise, without the
prior permission of Pergamon Press Ltd.
First edition 1970
Library of Congress Catalog Card No. 74-103021
Printed in Great Britain by Thomas Nelson (Printers) Ltd, London and Edinburgh
This book is sold subject to the condition
that it shall not, by way of trade, be lent,
resold, hired out, or otherwise disposed
of without the publisher's consent,
in any form of binding or cover
other than that in which
it is published.
08 006654 2 (flexicover)
08 006653 4 (hard cover)
Preface
THE growth of solar physics is a vast and intricate subject full of
twists and turns. The description of it provided in this book is no
more than a brief, selective introduction to the early period. The
emphasis here is more especially on the development of solar
spectroscopy, and on the relationships which were discovered
between the various layers of the solar atmosphere and between
the different forms of solar activity. In terms of time, the main
concentration is on the years from 1850 to 1900. This is, of course,
an arbitrary choice, but it represents, nevertheless, a period during
which knowledge of the Sun progressed from the extremely
rudimentary to the recognizably modern. The fact that a new
type of astronomy was being created received full contemporary
recognition. S. P. Langley, writing in 1888, noted that:
"Within a comparatively few years, a new branch of astronomy
has arisen. ... Its study of the Sun, beginning with its external
features (and full of novelty and interest, even as regards those),
led to the further enquiry as to what it was made of, and then to
finding the unexpected relations which it bore to the earth. . . .
This new branch of inquiry is sometimes called Celestial Physics,
sometimes Solar Physics and is sometimes . . . referred to as the
New Astronomy."
To provide some insight into the changes and developments
which a single individual might feel forced to make to his views
of the Sun during this period, the work of Sir Norman Lockyer
is described separately in rather greater detail.
This main body of the commentary is contained in Chapter II.
Chapter I is a short introduction, setting the scene for the follow-
ing period of rapid growth. Chapter III is by way of an epilogue.
If it is necessary to distinguish between early and modern solar
vii
VÜi PREFACE
physics, the division may, perhaps, be set most reasonably in
1913, when Bohr proposed the first workable theory of atomic
spectra. The third chapter selects two advances during the first
decade of the present century—one theoretical and one observa-
tional—to show how solar physics was ready to move ahead into
the modern era as soon as this vital development in physics
occurred.
I am much indebted to the following organizations for per-
mission to reprint articles in Part 2 of this book: the University
of Chicago Press for the chapter by G. E. Hale from Stellar
Evolution and for the article by W. W. Campbell in the Astro-
physicalJournal (1899); the Smithsonian Institution for the article
by G. E. Hale in the Smithsonian Report (1913); the Royal Society
for the article by J. Evershed in the Proceedings of the Royal
Society (1901).
CHAPTER I
Ideas of the Sun in the
M id-Nineteenth Century
THE initial burst of enthusiasm for solar observation, which
followed the introduction of the astronomical telescope at the
beginning of the seventeenth century, waned fairly quickly. The
discoveries made during this early period included observation
of sunspots, of faculae and of solar rotation; no further advances
of importance in solar astronomy were made subsequently until
the latter half of the eighteenth century. This lack of progress
may be attributable in part to a certain scarcity of sunspots
throughout much of this period, for it was the spots which
caused much of the early interest in the Sun, and they, too, were
involved in the resurgence of interest in the later eighteenth
century
In 1774, Alexander Wilson, a professor at Glasgow University,
published some observations which seemed to show conclusively
that sunspots were neither clouds floating above the solar surface,
nor layers of accumulated slag, nor volcanoes—all of which had
been suggested—but were, instead, depressions below the normal
level of the surface (Wilson, 1774). He obtained this result from a
study of the apparent change in appearance of a spot as it crossed
the solar disc.
"Astronomers will remember that a spot of an extraordinary
size appeared upon the sun, in the month of November 1769.. ..
on the 22nd day I had a view of the sun through an excellent
Gregorian telescope.... I then beheld the spot which at that time
was not far from the sun's western limb. . . . Next day being the
23rd, I had a curiosity to see it again.... I now found however a
3
4 EARLY SOLAR PHYSICS
remarkable change; for the umbra, which before was equally
broad all round the nucleus, appeared much contracted on that
part which lay towards the centre of the disc.... I began to suspect
that the central part, or nucleus of this spot, was beneath the
level of the sun's spherical surface; and that the shady zone or
umbra, which surrounded it, might be nothing else but the
shelving sides of the luminous matter of the sun, reaching from
his surface, in every direction, down to the nucleus; for, upon this
supposition, I perceived, that a just account could be given of the
changes, of the umbra and of the figure of the nucleus.'f
The existence of this Wilson effect was disputed by his in-
fluential French contemporary, Lalande, who preferred to believe
that spots represented mountains sticking up through the liquid
surface of the Sun. Wilson's idea was, however, taken up in
England by Sir William Herschel (1795), who developed it into
a general description of the solar constitution. He believed the
bright surface of the Sun to be the upper side of a layer of lumin-
ous clouds; the spots were regions where the clouds had been
temporarily dispersed by atmospheric currents, so that the dark,
solid body of the true Sun was exposed. In detail, his theory
became more complicated, for two cloud layers were actually
envisaged : the opening in the upper layer being larger than that
in the lower. In this way, the presence of a penumbra round the
central umbra of a spot could be explained. Herschel speculated
that, since the supposed true surface of the Sun was protected
from the heating effects of its luminous envelope by the lower-
lying clouds, therefore the solar surface could be inhabited. This
last opinion was generally disregarded, but his overall concept
of the solar constitution was widely accepted until after the
middle of the nineteenth century.
". .. according to the above theory, a dark spot in the Sun is a
place in its atmosphere which happens to be free from luminous
decompositions; and that faculae are, on the contrary, more
copious mixtures of such fluids as decompose each other. The
t Note that Wilson refers to the umbra of the spot as its nucleus, and calls
the penumbra the umbra.
IDEAS OF THE SUN IN THE MID-NINETEENTH CENTURY 5
penumbra which attends the spots, being generally depressed
more or less to about half way between the solid body of the sun
and the upper part of those regions in which luminous decomposi-
tions take place, must of course be fainter than other parts.
The sun, viewed in this light, appears to be nothing else than a
very eminent, large and lucid planet, evidently the first, or in
strictness of speaking, the only primary one of our system; all
others being truly secondary to it. Its similarity to the other
globes of the solar system with regard to its solidity, its atmosphere,
and its diversified surface; the rotation upon its axis, and the fall
of heavy bodies, leads us on to suppose that it is most probably
also inhabited like the rest of the planets, by beings whose
organs are adapted to the peculiar circumstances of that vast
globe."
Slight modifications were introduced into this scheme from
time to time. In 1851, for example, Rev. W. R. Dawes—one of the
leading British solar observers—detected small, round spots in
the umbra which were even darker than their surroundings. He
therefore assumed that the umbra of a spot did not, in fact,
represent the true solar surface. It was, instead, an even lower
cloud stratum, and only the smaller spots he had discovered pro-
vided a true glimpse of the surface (Dawes, 1852).
The apparent similarity between the solar and terrestrial
atmospheres suggested a comparison between solar and terrestrial
meteorology. Sir John Herschel (1847), for example, thought that
solar rotation would produce a higher temperature at the equator
of the Sun than at the poles. He was thus led to suppose that
atmospheric zonation occurred on the Sun, as on the Earth.
Sunspots were then comparable with terrestrial cyclones: they
represented regions where the cloudy strata of the Sun were
penetrated by downward motions of the solar atmosphere. (This
contrasted with the opposing view at the time that the clearings
were due to an upward motion of the atmosphere as a result of
volcanic explosions on the solar surface.) The idea that sunspots
could be identified with some kind of vortex motion continued to
be more or less influential throughout the next hundred years.
6 EARLY SOLAR PHYSICS
The picture of the Sun as a relatively cool, solid body surrounded
by layers of luminous clouds gradually disappeared as the full
implications of new work, particularly in the field of spectroscopy,
were understood. Nevertheless, it lingered on for a surprisingly
long time: Sir John Herschel's standard textbook—Outlines of
Astronomy—still reproduced Sir William Herschel's scheme at
the end of the 1860's.
It was apparent from the earliest solar observations that the
number of sunspots visible varied with time, but the first detailed,
long-term survey of these variations was not undertaken until
well into the nineteenth century. In the early 1840's, S. H. Schwabe,
an amateur astronomer of Dessau, announced the results from
seventeen years observations of sunspot numbers (the paper is
reprinted in Part 2). He showed that the number of spots visible
on the Sun's surface, instead of varying randomly as had usually
been supposed, rose and fell with a period of about ten years.
His initial announcement in the Astronomische Nachrichten
aroused only slight interest, but Humboldt was struck by his
results and published them in the volume of his book Kosmos
which appeared in 1851. This work was very widely read, so that
the concept of a sunspot cycle quickly became general currency.
At about the same time, Lamont (1852) at the University of
Munich published an analysis of observations of terrestrial
magnetism made in Germany over the previous 15 years. He
pointed out that the amplitude of the daily variation was itself
variable, with a period of 10£ years. He did not, however, relate
this discovery to Schwabe's sunspot cycle (indeed, he later opposed
the belief that any such connection existed). Very shortly after-
wards, Sir Edward Sabine (1852) began an examination of
magnetic observations from Canada. He concentrated on the
occurrence of magnetic storms, and found that they varied in
frequency and amplitude with a periodicity of about ten years.
Unlike Lamont, he recognized the relationship between his
results and those of Schwabe for sunspots, and pointed out
further that the magnetic fluctuations he had been examining
were greatest when sunspot numbers were at a maximum.
IDEAS OF THE SUN IN THE MID-NINETEENTH CENTURY 7
. . new and important features have presented themselves in
the comparison of the frequency and amount of the disturbances
in different years, apparently indicating the existence of a periodical
variation, which, either from a causal connection (meaning thereby
there being possibly joint effects of a common cause), or by a
singular coincidence, corresponds precisely both in period and
epoch, with the variation in the frequency and magnitude of the
solar spots, recently announced by M. Schwabe as the result of
his systematic and long-continued observations."
Sabine's identification of the connection between the sunspot
and the magnetic periods was published in mid-1852. Within the
next few months the same identification was also made on the
Continent by Wolf (1852) and, independently, by Gautier (1852)
These publications mark the beginning of research into solar-
terrestrial relations.
As we have remarked, spots were not the only solar features
noted by early telescopic observers. Faculae—small bright
patches, best seen near the Sun's limb—were recognized at about
the same time. Similarly, it was soon observed that the Sun's
surface was mottled in appearance rather than uniform. By the
middle of the nineteenth century the darkening of the solar disc
towards the limb was also well known, although still slightly
controversial—Arago, for example, believed that genuine limb-
darkening only existed to a very limited extent. This question
was to some extent settled when the first good daguerreotype of
the Sun was obtained by Fizeau and Foucault in 1845 (at Arago's
request). This clearly showed limb-darkening, as well as the
umbrae and penumbrae of some spots present (see the reproduc-
tion in G. de Vaucouleurs (1961), Plate 1).
A few measurements of the overall brightness of the Sun had
been made during the eighteenth century (e.g. by Bouguer in 1725
and by Wollaston in 1799). The method used was very elementary
—a visual comparison of a candle flame with the Sun in the
meridian—and the results were correspondingly inaccurate. There
was also considerable doubt as to the magnitude of many of the
corrections (such as that for the atmospheric absorption of
8 EARLY SOLAR PHYSICS
sunlight) which had to be made. In 1844, Foucault and Fizeau tried
to obtain a more refined estimate of the solar brightness by com-
paring daguerreotype images of the Sun and incandescent lime-
light. Their results, however, proved to be no more reliable than
the earlier visual estimates, which were therefore still being quoted
at the mid-century.
The first quantitative estimate of the heat emitted by the Sun
was made by Newton (who also reasoned that the Sun must be
red-hot throughout). The first significant measurements, however,
were not made until well into the nineteenth century. Then in
1837-8 Sir John Herschel (1847), during his stay at the Cape of
Good Hope, carried out some observations with an actinometer.
(This consisted essentially of a bowl of water which was exposed
to the Sun for a short period of time, and the resulting rise in
temperature noted.) About the same time, C. S. M. Pouillet in
France began a series of measurements with a pyrheliometer
(based on essentially the same principle as the actinometer,
though considerably different in appearance). Their results for
what was named the solar constant agreed quite well, but were, in
fact, only slightly over half the value accepted later in the century.
(For a description and comparison of Herschel's and Pouillet's
work, see: Young, 1896, chapter 9.) Shortly afterwards, J. D.
Forbes (1842) obtained a much higher value, but it was considered
at the time to be less reliable. As in the measurements of solar
brightness, one of the main experimental difficulties lay in the
allowance for atmospheric absorption.
An important reason for determining the solar constant was
that it could, theoretically, be used to derive a surface temperature
for the Sun. The major difficulty, which plagued scientists
throughout the nineteenth century, concerned the exact relation-
ship between the temperature of a surface and the amount of
radiation it emitted. Until the early part of the nineteenth century,
Newton's law of cooling had been universally accepted, i.e. the
radiation emitted was supposed to be directly proportional to
the temperature. Then, however, the French scientists, Dulong
and Petit (1817), carried out a series of experiments which showed