Solar System Radio Astronomy: Lectures presented at the NATO Advanced Study Institute of the National Observatory of Athens: Cape Sounion August 2–15, 1964

415 Pages·1995·10.97 MB·English

by  G. Righini (auth.), Jules Aarons (eds.)

SOLAR SYSTEM
RADIO ASTRONOMY
SOLAR SYSTEM
RADIO ASTRONOMY
Lectures presented at
the ]jATO Advanced Study Institute
of the National Observatory of Athens:
Cape Sounion August 2-15, 1964
Edited by
Jules Aarons
Air Force Cambridge Research Laboratories
Bedford, Massachusetts
Distributed by
PLENUM PRESS
NEW YORK
1965
ISBN-13: 978-1-4615-8605-0 e-ISBN-13: 978-1-4615-8603-6
DOl: 10.1007/978-1-4615-8603-6
Library of Congress Catalog Card Number 65-14086
©1965 Ionospheric Institute of the National Observatory
of Athens, Greece
Softcover reprint of the hardcover 1st edition 1965
AU rights reserved
No part of this publication may be reproduced in any
form without written permission from the publisher
Contents
Preface: M. ANASTASSIADES •.•...•..•..••.•..•.••...•..•...••• vii
Introduction: J. AARONS . . . . . . . . • • • • . . . . • • • . • . • . . . • . . . . . •• ix
Opening Address: R. SCHRADER. . . . . • . • . . • . • • . • . . . . • • . • . . xi
SOLAR CHARACTERISTICS
Chapter 1: Solar Characteristics from Optical Observations, G. RIGHINI 1
Chapter 2: Introduction to the Study of Solar Radio
Emission, J. F. DENISSE .•.•.••..•..•........•••• 39
THE QUIET SUN
Chapter 3: A Survey of Radio Observations of Solar Eclipses,
J. CASTELLI AND J. AARONS.. . . • •• • . . • . . • .. . .•• . .• 49
Chapter 4: The Slowly Varying Component of Solar Radiation, M. PICK 81
Chapter 5: Radio Frequency Emissions of the Sun in the Centimeter
Wavelength Range: The Slowly Varying Sunspot Component,
O. HACHENBERG ..••.••.•..•.•...•....•..... 95
Chapter 6: Study of the Slowly Varying Component at Three Centimeters
as a Function of Solar Activity, C. CAROUBALOS. • . . • . . •. 109
THE DISTURBED SUN
Chapter 7: The Solar Flare Phenomenon as Seen at Radio
Frequencies, D. J. McLEAN ., • . • . • . . . . . . . . . • . . . .. 117
Chapter 8: Sweep Frequency Measurements of Solar Bursts,
J. W. WARWICK. . . . . . . . • . . . . . . . . • . • . . . . • . . . 131
Chapter 9: Noise Storms, A. D. FOKKER. . . . . . . . . • . . . . . . . . . . .. 171
Chapter 10: Narrow-Band Studies of Solar Bursts, 6. ELGAROY 201
Chapter 11: Solar Noise Measurements by the Riometer
Technique, M. ANASTASSIADES •. . • . • • • • . . • • . • • • • •. 225
Chapter 12: Radio Frequency Emission of the Sun in the Centimeter
Wavelength Range: Microwave Bursts, O. HACHENBERG ..• 241
vi CONTENTS
THE INTERPLANET ARY MEDIUM
Chapter 13: Radio Investigation of the Solar Corona and
the Interplanetary Medium, A. HEWISH . . . . . . . . . . . • . . •. 255
Chapter 14: Radio Astronomy of Solar System Plasmas, V. R. ESHLEMAN. 267
THE MOON
Chapter 15: The Interpretation of Thermal Emission from
the Moon, H. WEAVER .•...•............•...•... 295
Chapter 16: Lunar Radar Reflections, G. PETTENGILL 355
THE PLANETS
Chapter 17: Some Problems of Planetary Radio Astronomy, H. WEAVER .. 371
Chapter 18: Planetary Radar Astronomy, G. PETTENGILL ..•.•.•.... 401
Abstract: Distribution of the Ionizing Radiation on the Solar Disk
During the Solar Eclipse of Feb. 15, 1961, D. ILIAS ..••...• 413
Index . ........•....•..•....•.•......•..•..•.•••..•. 415
Preface
The Ionospheric Institute of the National Observatory of Athens has had two
interests in recent years: the study of the ionosphere and the study of the sun.
In our previous Advanced Study Institutes in 1960, 1961, and 1962, we have em
phasized the ionosphere. For the Advanced Study Institute of 1964, however, we
invited Dr. Jules Aarons of the Air Force Cambridge Research Laboratories to
collaborate in preparing and directing a program of studies of the sun, the moon,
the planets, and the interplanetary medium.
The lectures of this Advanced Study Institute form essentially an advanced
course in radio astronomy. Without being a textbook on the matter, we feel that
the present book can be considered as an excellent reference for those students
starting their research work in the field of solar system radio astronomy. All
lecturers tried to present their subjects in a simple form based upon their exten
sive personal experience, but without emphasizing their personal research. We
must recognize that it was an excellent achievement for them to keep their text
exactly at the level indicated by the Program Director, and outlined by the gen
eral program of Advanced Study Institutes of NATO. We are deeply grateful to
all the invited scientists for their outstanding contributions in lecturing on their
subjects in a clear and authoritative manner.
The Scientific Affairs Division of NATO, in its aid to basic research, spon
sors various programs. Among them is the Program of Advanced Study Institutes.
As a Research Director in a country undergoing scientific growth, I can state
that this Summer School is one of the most successful programs of aid. It pro
vides a new form of meeting, different from the well-known conferences, sym
posia, or colloquia. Scientists from developed countries stimulate and aid re
search undertaken by scientists in countries under development. Living together
in one pleasant place during the period of the Summer Schools provides the Insti
tute members with many opportunities for worthwhile discussions not only in the
conference rooms, but during leisure time as well. I feel that this informal and
amicable exchange of ideas on scientific matters is of great value and most fruit
ful. Each individual is given the greatest possible opportunity for an exchange
of creative ideas through the organization of Advanced Study Institutes.
In behalf of the participants, the lecturers, and especially the young team
of radio astronomers of the National Observatory of Athens, I express my grati
tude to NATO's Scientific Affairs Division for the foresight shown in sponsoring
the Cape Sounion Advanced Study Institute on Solar System Radio Astronomy.
Michael Anastassiades
Professor, University of Athens
Director of the Advanced Study Institute of NATO
on Solar System Radio Astronomy
vii
Introduction
In the latter part of the summer of 1964, the National Observatory of Athens
directed and hosted an Advance Study Institute on Solar System Radio Astronomy.
The aim was to satisfy the dual need of supplying a written technical review of
radio and radar astronomy as it related to the physics of the solar system and of
conducting a school plus a conference. Invitations were extended to astronomers
who had contributed to the field; they were asked to deliver lectures on the sub
jects proposed to them as well as write papers for a comprehensive volume on
Solar System Radio Astronomy. The result is this volume.
In planning the publication, stress was placed on areas which had not been
adequately covered by the plethora of review articles which have appeared re
cently. If reviews had appeared in the literature, a quite different approach to
the subject was sought. Two examples among many are Weaver's review of pas
sive lunar observations and Righini's advanced introduction to physical charac
teristics of the sun as obtained by optical techniques.
At the end of the Institute, both the lecturers and the participants are satu
rated with knowledge and discussion. One has the feeling that a great body of
knowledge has been gathered by the tools of radio and radar astronomy, but it
also was apparent that there are nagging and recurring questions that must be
answered.
The final meeting of the Institute, a round table discussion, brought to light
the ideas of the lecturers as to what might constitute future studies in solar sys
tem radio and radar astronomy.
The observations of the sun are varied both in type and in number, but there
is open season on ideas as to the means by which flares generate radio frequency
emission. Even within this volume, there are several theories as to the genera
tion and radiation mechanisms of various burst types. Some observations of the
sun have as yet inadequate materiel. The studies of magnetic field changes near
sunspots during times before and after flare emission have limited data; new
equipment in solar observatories is being set up to make these measurements.
New observations of solar X-rays have just become available; these must be
added to the background of observations that provide the foundation for theoreti
cal studies. Millimeter observations of both the solar slowly varying component
and solar bursts will become important in the future.
Absolute flux measurements are now the subjects of many discussions. The
use of radio measurements from the solar centers of activity has increased with
the correlation of solar flux values with atmospheric density, E -layer critical
frequency, and other atmospheric parameters. The need is growing for both know
ing the absolute value of the flux from the sun at a particular wavelength and
ix
x INTRODUCTION
comparing these measurements at various phases of the sunspot cycle. Both
these solar data and others must be integrated from a worldwide point of view;
observatories with radio telescopes on the same frequency report greatly varying
values for a particular day. The integration of data on a worldwide basis is nec
essary in order to understand the evolution of a series of flares from a particular
sunspot region.
The addition of polarization observations, absolute flux measurements, and
spectral index recordings (of bursts) will aid in the construction of models of
solar flare emission. Solar physicists were in agreement as to the need for both
theoretical studies in magnetohydrodynamics, laboratory plasma experiments as
well as additional measurements to understand the mechanism of the solar flare
and its emissions.
The Cavendish laboratories, using their large interferometer, have recently
observed scintillation of radio stars which" they attribute to the motion and struc
ture of the interplanetary medium. However, their derived velocities are some
what low for the solar wind. Possibilities exist using this technique to recon
struct three-dimensional spatial variations and, of course, to study the outflow
of large solar features and the solar corpuscular wind.
The participants agreed that the field of planetary studies was just begin
ning, both from the viewpoint of ground observations as well as deep space ve
hicles. Weaver has stressed the importance of good infrared studies of both the
moon and the planets. The possibility of microwave spectroscopy yielding re
sults was discussed; molecular spectra may be obtainable from ground measure
ments. Radar astronomy, of course, has been making great strides in determining
the astronomical unit and the rotational period of the planets as well as the sur
face reflectivity.
Lunar studies, in spite of the possibility of an astronaut scooping up the
surface material, will increase in the future from the earth. Infrared observations,
polarization measurements (both by passive radio measurements and active radar),
and long wavelength radar reflectivity studies (7 -10 m) as well as the resolution
of small regions, were areas where the participants felt research could and would
be extended.
One comes away from a meeting of this type with two apparently diametrical
ly opposed points of view; on one hand, a vast amount of observation and theo
retical material on solar, lunar, and planetary processes has been gathered, but
opposing this a vast amount needs to be done to answer the simplest of questions
about solar flares and general solar activity, lunar surfaces, and planetary sur
faces and atmospheres.
It is hoped that this book will serve as an introduction to solar system radio
astronomy, just as the work done to date on solar physics will merely be an in
troduction to future understanding of solar processes. I would like to thank John
Castelli for his assistance and advice in the preparation of this volume.
Jules Aarons
Editor
Air Force Cambridge Research Laboratories
U.S.A.
Opening Address
It is a great privilege and pleasure for me to attend, as a representative of the
NATO Science Committee, the opening session of the Advanced Study Institute
on Solar System Radio Astronomy. This institute is one of forty-three schools
sponsored this year by the Science Committee within the framework of its Ad
vanced Study Institute Program.
It is worth remembering that the Advanced Study Institute Program was
started with five schools in 1959, and has been gradually expanded to the present
level. Accordingly, the budget allocated by NATO for this purpose has been in
creased from 100,000 to 650,000 dollars over the same period.
The Advanced Study Institute Program is not intended for the support of regu
lar scientific meetings such as conferences, symposia, or colloquia, but for
courses or seminars at an advanced level, which last from two weeks to two
months. There is no special form of organization, but it is expected that the
schools provide for concentrated studies in a restricted scientific field.
Study institutes are held at any time of the year, although most of them take
advantage of holiday periods during the summer months, so as to be able to bring
together faculty staff and students from various nations. The schools are geo
graphically situated in a country which is a member of the Atlantic Alliance; how
ever, participation is not limited to citizens of NATO nations.
The subject of a summer school is chosen from any of the natural sciences
and technology, including mathematics. No special form of application for a NATO
grant is required if a scientist wishes to organize an institute. Eligible applica
tions are considered by the staff of the Division for Scientific Affairs, and a small
panel of three members of the Science Committee makes the selection of recipients
and determines the amount of money. Decisions are taken on the basis of the an
ticipated quality of the study, the proposed degree of international partiCipation
concerning both faculty and students, and the desire to achieve a reasonable dis
tribution of grants among different sciences and different countries.
The NATO grants are made completely unrestricted so as not to influence the
management of the school. As a matter of policy, the director of the institute
bears full responsibility and enjoys the liberty to establish the program and select
teaching staff and students. Incidentally, many schools Simultaneously receive
money from other sources.
Since the introduction of the Advanced Study Institute Program in 1959 until
today, NATO has provided financial assistance to about a hundred and fifty sum
mer schools. Approximately fifty of these have published their courses in the
form of brochures and books which are available in the literature. The Program
is now well established and well known in the Western scientific community and
will continue in the years to come.
Rudol f Schrader
Deputy Assistant Secretary General for Scientific Affairs
North Atlantic Treaty Organization
Paris, France
xi
Chapter 1
Solar Characteristics
from Optical Observations
G. Righini
Osservatorio Astrofisico di Arcetri
Florence, Italy
INTRODUCTION
The sun is a dwarf star, type G 2 in the Harvard spectral classification, whose
absolute visual magnitude is Mv = + 4. 79. It is therefore a typical main sequence
star in the H -R diagram; in addition, its place in the Galaxy is rather peripheral
since its distance from the center is about 27,000 light years, i.e., two-thirds of
the radius of the disk. The sun revolves around the galactic center with a velocity
of approximately 200 km/sec; the period of this motion is of the order of 108 years,
i.e., one order of magnitude shorter than the admitted life of our solar system.
Since the average distance from the earth is 150 x 106 km, the sun is the only
star which shows a visible radius; this, at a mean distance subtends an angle of
960" = 0.005 rad. Since the radius Ro is 7 x 105 km, it follows that 1" = 725 km.
Other physical parameters of the sun - the mass M = 2 X 1033 g, the mean den
p
sity = 1.41 g/ cm 3, the surface temperature T, and the luminosity L - will be
discussed in the next section.
It is customary to distinguish three zones in the whole solar body: the photo
sphere, the chromosphere, and the corona. The prominences are features which
have their "roots" in the chromosphere but they develop in the low corona.
THE PHOTOSPHERE
Solar Constant, Luminosity, and Temperature
The solar constant S is the flux received by a l_cm2 surface placed outside
our atmosphere. Its value at the mean sun - earth distance is
S = 1.39 X 106 erglsec-cm2
The luminosity L is the total flux emitted by the sun, which is readily obtained
by multiplication, with the surface of the sphere having a radius equal to the sun
earth distance. Its value is
L 3.90 x 10 33 erg/sec