Arctic Communications. Proceedings of the Eighth Meeting of the AGARD Ionospheric Research Committee, Athens, Greece, July 1963

281 Pages·1964·11.63 MB·English

by  B. Landmark (Eds.)

ARCTIC
COMMUNICATIONS
Proceedings of the Eighth Meeting of the
AGARD Ionospheric Research Committee
Athens, Greece, July 1963
Edited by
B. LANDMARK
Published for and on behalf of
ADVISORY GROUP FOR
AERONAUTICAL RESEARCH AND DEVELOPMENT
NORTH ATLANTIC TREATY ORGANIZATION
by
PERGAMON PRESS
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1964
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1964
ADVISORY GROUP FOR
AERONAUTICAL RESEARCH AND DEVELOPMENT
NORTH ATLANTIC TREATY ORGANIZATION
Library of Congress Catalogue Card Number 64-17190
Set in Monotype Baskerville 10 on 11 pt.
and printed in Great Britain by
Blackie and Son Ltd., Bishopbriggs, Glasgow
WELCOME ADDRESS
AND
ACTIVITIES OF THE IONOSPHERIC
INSTITUTE OF THE NATIONAL
OBSERVATORY OF ATHENS
PROF. DR. MICHAEL ANASTASSIADES
National Delegate of Greece to AGARD
Director of the Ionospheric Institute
THE scientific world of Greece and especially the Members of the Upper
Atmosphere and Ionospheric Institute of the National Observatory of
Athens wish you welcome to Greece.
This is the second time the IRC meets in Athens. The first meeting, in
July 1960, was a most pleasant and stimulating event for us here. I hope
that this second gathering will make you realize how much your presence
was then of help.
Since your first visit, in 1960, the Ionospheric Institute has become an
efficient Research center, as well as a center of International cooperation.
The panoramic recorder of 30 kw, which you will have the opportunity to
see this evening, during your visit to the Scaramanga station, has been
installed partly thanks to economic and scientific support offered by our
dear colleague Professor Dr. K. Rawer.
The receiver for oblique incidence studies between Breisach and Athens
was established some time later and the logarithmic Ionospheric recorder
of 30 kw for the study of oblique incidence path Boston-Breisach-Athens
was put into operation two years ago.
These achievements allowed the Ionospheric Institute of Athens to
make a contribution to the study of the Upper atmosphere in middle
latitudes. I hope that you all receive our monthly Bulletin containing our
data.
Actually, the existence of these two recorders at Scaramanga has per­
mitted us to proceed further. Thus, with the help of NATO's Scientific
Committee, we have been able to organize an important network of five
special receivers giving normal ionograms of the Scaramanga recorder.
These are set up at a distance of 500 km around Athens, and will
permit us to make a more extended study of sporadic-^ over Greece.
The described research program becomes even more important by the
installation of an Observatory for the study of nightglow on the island of
Poros, a few kilometers southwest of Athens. The undertaking is made in
cooperation with our eminent President Professor Vassy and our dear colleague
Mrs. Vassy. The results that we have already obtained, prove that we
can look forward to clarifying the correlation between sporadic-E and F
spread with nightglow.
vii
PROFESSOR DR. MICHAEL ANASTASSIADES
As our Ionospheric Institute has the rare privilege to observe the Sun
320 days per year and to obtain excellent optical images due to our clear
sky, we have extended our work towards specializing in the solar Radio-
Astronomy as well. You will have the opportunity to visit our establishments
for optical observations of the Sun, situated on the Observatory hill,
opposite to the Acropolis.
You will also have a chance to pay a pleasant as well as useful visit to
our establishments on Pentelli hill. There, you will be able to see our
riometers for measurements of ionospheric absorption by the cosmic noise
method. These riometers have been obtained thanks to the precious aid of
the Office of Aerospace Research of USAF.
The continuous operation since 1959 of riometers working simultaneously
at five different frequencies, has enabled us to develop the method of
multifrequency riometer technique. I will have the pleasure to present to
you some results of this technique during our morning session. A solar
noise device on 3000 Mc/s added to our equipment by a research grant of
the NATO Science Committee will allow us to make an advanced study
of solar disturbances correlated with variations of the Earth magnetic
field. An improved magnetometer was put into operation only a few
months ago.
On the same hill you will also visit our establishment for the study of
upper atmosphere phenomena by means of artificial satellites. As you
probably know the Ionospheric Institute of Athens is a member of a group
of eight European and American Laboratories that have undertaken such
a research program under the sponsorship of NATO.
I will not mention the activities which you will not have the opportunity
to see during your short stay in Athens. Most of them are functioning far
from the city. However, I should mention our network of three stations
for the study of radioactive fallout that has been undertaken in collabora­
tion with the Laboratoire de Physique de l'Atmosphere in Paris, and our
extensive network for radiometeorological studies performed in collaboration
with GNET. Finally I should add the valuable collaboration of our eminent
Program Director Dr. B. Landmark and the contribution of the Norwegian
Defense Research Establishment under the direction of Dr. Lied, for the
installation on Crete island of a cross-modulation and backscatter device.
It is due to the Ionospheric family of AGARD, to Dr. Philip Newman,
who during his presidentship stimulated an earnest desire in all of us, to
the eminent President Professor Vassy, who is undoubtedly the Father of
the ionospheric movement in the Mediterranean area, to Dr. Diemeinger
who has proved an excellent President of the Avionic Panel, to Colonel
Lairmore and Colonel Dukes former and present Executives of our Com­
mittee, that Athens has become in so short a time an ionospheric center
able to contribute to the general movement of upper atmosphere research.
Gentlemen, in the name of my Government, on behalf of the National
Observatory of Athens and my collaborators, I wish you welcome to the city
of Athens, and I hope that you will enjoy a profitable stay and we shall
do our best to make it as pleasant as possible for you.
vin
INTRODUCTION
E. VASSY
President du Comite de Richerches Ionospheriques de l'AGARD
EN ouvrant la Huitieme Reunion du Comite de Recherches Ionospheriques
de l'AGARD, je manquerais a mon devoir si je n'evoquais la memoire du
Pt von KARMAN disparu recemment. G'est une grande lumiere qui s'est
eteinte et qui sans eclipses a brille d'un vif eclat jusqu'a la fin.
Vous me permettrez d'evoquer quelques souvenirs recents, temoins de
sa claire intelligence, dominant, avec une etonnante lucidite et une largeur
de vue peu commune, des problemes souvent ecartes de sa specialite.
II y a cinq ans, les resultats de la recherche spatiale se faisaient de plus
en plus nombreux, etaient dissemines dans trois Unions Scientifiques et
a la Federation Astronautique Internationale. II fallait mettre de Pordre
dans les resultats, donner aux specialistes engages dans ces recherches des
moyens d'organiser leur travail. Charge par le Gonseil International des
Unions Scientifigues de participer a une reunion de quatre personnes avec
mission de presenter un rapport sur la question, c'est aupres de lui que je
trouvais une aide a la fois efficace et bienveillante, une vision claire des
besoins des specialistes, un echange de vues vraiment constructif. Et
c'est a la suite du rapport de Moscou que fut le COSPAR (Committee for
Space Research) organisme debordant de vitalite.
L'annee derniere, la celebration du X e anniversaire de l'AGARD fut
un pretexte a un travail d'inventaire et de prevision qu'il repartit admirable-
ment entre les divers specialistes. Les consignes precises et detaillees qu'il
nous donna, l'orientation qu'il imprima aux debats en assurerent le plein
succes.
Enfiri plus recemment, en mars dernier, lors de la presentation au budget
de l'AGARD de credits destines a alimenter les travaux de recherches
effectues en commun par des scientifigues de divers pays de l'OTAN, il
s'engagea fermement dans cette voie, appuya le projet malgre les textes
de reference. Avec sa longue experience d'homme de science, il avait
tout de suite vu l'interet de ces recherches et pese de tout son poids dans
la discussion.
Personnalite attachante par beaucoup de points particuliers, je regrette
beaucoup l'homme, mais notre Comite a perdu en lui le guide le plus sur.
Je voudrais ensuite rendre hommage a l'oeuvre de mon predecesseur, le
Dr. Newman. Sous des dehors simples, avec beaucoup de discretion et
avec une bonhommie souriante, il a conduit le destinee du Comite d'une
main ferme. II a su choisir les sujets d'etudes et de travail de telle maniere
que nos discussions s'accompagnent d'un progres. Aujourd'hui, la pro­
duction scientifique est telle que chacun, par une publication, apporte
sa pierre a l'edifice, mais la depose en vrac sur le chantier. II faut ensuite
ix
PROFESSEUR DR. E. VASSY
mettre de I'ordre, assembler suivant un plan logique et c'est alors qu'apparait
l'edifice. Le Dr. Newman a ete un excellent architecte. II a eu de plus le
merite d'exercer ses fonctions pendant quatre ans et d'assurer deux presi-
dences successives. De cela, j'en suis le responsable, car lorsque vou3
m'avez propose la succession ici meme a Athenes en 1960, j'ai demande a
n'etre d'abord qu'un vice-president, de fagon a apprendre peu a peu le
metier, a avoir le temps de m'initier aux affaires de notre Comite. Pendant
cette vice-presidence, le Dr. Newman a toujours ete pour moi le guide le
plus sur et le plus amical tout en me fournissant des occasions pour le
remplacer. C'est en mon nom et en celui du Gomite tout entier que je
lui adresse des vifs remerciements pour l'oeuvre qu'il a accomplie et pour
la maniere avec laquelle il l'a accomplie.
Le sujet de notre Reunion est " Gommunications radio a l'interieur et a
travers les regions arctiques". Pourquoi ce choix? G'est ici meme a Athenes
qu'en est I'origine. En effet, il y a trois ans, en traitant la question generale
de I'absorption, nous fumes quelque peu surpris de voir la place que tenait
I'absorption aux latitudes elevees et la question des "blacks-out" polaires.
D'autre part avec le souci de satisfaire aux terms de reference de notre
Gomite, j'avais dit au cours des remarques finales "puisque nous avons la
chance et l'honneur d'avoir parmi nous un representant due SHAPE et
qui est une sommite dans le domaine qui nous preoccupe (il s'agissait de M.
Pierre David), il serait bon qu'il nous disait sur quels points son administra­
tion porte plus particulierement de l'interet de facon a ce que nous puissions
orienter nos efforts". C'est ainsi que cette question avait ete avancee,
avec d'autres d'ailleurs. Aussi a-t-elle pu etre reprise a Munich et etre
mise au programme. Ce faisant, nous pensons faire oeuvre utile plutot qu'en
traitant de questions de science pure faire double emploi avec les nombreuses
Societes Associations et Unions Internationales.
Le Dr. Landmark, par la position geographique de son laboratoire, son
importante contribution personnelle dans cette matiere et sa haute compe­
tence etait tout designe pour etre responsable du programme. Nous le
remercions d'avoir bien voulu l'accepter.
Enfin, nous sommes tres heureux d'avoir recu l'invitation des autorites
helleniques. Le Comite de Recherches Ionospheriques a deja l'experience
de la legendaire hospitalite de ce pays. Mon cher Anastassiades, en ce qui
me concerne personnellement, vous avez parle tout a l'heure comme votre
ancetre Homere, vous avez enjolive la verite; en realite j'ai ete un simple
catalyseur. Soyez notre interprete aupres de votre administration pour la
remercier de nous avoir invites.
Beaucoup de travail nous attend; je declare ouverte la Huitieme Reunion
du Comite de Recherches Ionospheriques.
x
CHAPTER 1
SOME EXPERIMENTAL STUDIES OF THE
ARCTIC D-REGION
O. HOLT and B. LANDMARK
Norwegian Defence Research Establishment, Kjeller, Norway
ABSTRACT
High latitude absorption phenomena may be divided in three types, (a) Auroral absorption,
(b) Polar cap absorption and (c) Sudden commencement absorption. Experimental studies of these
phenomena are reviewed, including their morphology, duration, structure and relation to other
geophysical phenomena, as well as direct measurements of D-region electron densities during the
absorption periods.
1. INTRODUCTION
THE absorption of radio waves reflected from or penetrating through the
ionosphere, takes place mainly in the D-region—the altitude interval from
40 to 90 km. Normally, the electron density in this region is low, but
there are periods of increased electron density, when ordinary radio com­
munications are difficult, or impossible, to maintain. Particularly, such
periods are frequent at high latitudes, where energetic, charged particles,
probably of solar origin, are guided to D-region levels along the geomagnetic
field lines, and ionize the atmospheric constituents there.
Apart from its obvious practical aspects, the study of such events presents
a vast amount of interesting, physical problems, most of which are yet far
from solved. Though closest to the earth, the D-region has perhaps, until
recently, been the part of the ionosphere of which one has had the poorest
knowledge. This is connected with the methods of observation. Ordinary
ionosonde records give very little quantitative information on the D-region.
The A 1 absorption method gives only the integrated absorption, and since
it is difficult to separate deviative and non-deviative absorption, the absorp­
tion vs. frequency measurements did not make it possible to determine
JV (A)-profiles with any reliability. The use of v.l.f.-observations required
a full wave solution, which is extremely complicated, and was practically
impossible before the advent of large computing machines.
These difficulties have been overcome by refined echo methods, such as
ionospheric cross modulation and partial reflection observations, and by
the use of rockets.
In the statistical studies of the absorption periods, the installation of
riometers at a large number of sites, represents a great improvement. By a
method developed by Little, Lerfald and Parthasarethy 1 multifrequency
riometer observations may also be used to determine electron densities
during disturbed periods. In this paper some results obtained during
3
O. HOLT AND B. LANDMARK
different types of high latitude Z)-region disturbances will be summarized.
The paper is mainly based on observations made at the NDRE, but also
some other pertinent data are included. Before continuing, we will state
the characteristics of the different types of absorption.
(a) Auroral absorption (AA). This type of absorption is closely correlated
to visual aurora and geomagnetic disturbances. The ionization is mainly
due to electrons in the energy range 10-100 keV, incident on the atmos­
phere. The origin of these energetic electrons is not clear. The frequency
of occurrence has a maximum in the zone of maximum auroral activity, or
slightly south of this.
(b) Polar cap absorption (PCA). This absorption takes place simulta­
neously over the polar cap, i.e. at geomagnetic latitudes higher than about
60°. It is caused by protons of energies up to 100 MeV, almost certainly
of solar origin. The occurrence is related to solar disturbances. The
absorption may last for days, and is connected with the magnetic activity
during the period.
(c) Sudden commencement absorption (SCA). At the time of sudden com­
mencement geomagnetic storms, the absorption often increases rather
abruptly, and returns to its normal value in less than one hour. This
seems to be caused by a short burst of energetic electrons bombarding the
atmosphere. The events are limited in latitude to a region round the zone
of maximum auroral activity, but occur simultaneously over wide ranges
in longitude. In the following these three types of absorption will be treated
separately.
2. AURORAL ABSORPTION
In this section we shall deal with observations of different aspects of auroral
absorption. First we will discuss the temporal and spatial occurrence
patterns, as revealed by the riometer records from a north-south chain
of riometers through the auroral zone. The duration and structure
of the disturbances are treated in the next subsection. The correlation
between absorption observed on riometers and the occurrence of visual
aurora is discussed. Finally, results from electron density measurements
during auroral absorption are presented.
2.1. Large Scale Morphology. In the period from October 1958 to June
1959 five riometers were set up in a north-south chain from Longyearbyen
(74-1° geomagnetic latitude) to Kjeller (59-8° geomagnetic latitude).
The diurnal variation of occurrence of absorption at each of the five
stations, and the variation with geomagnetic latitude, were evaluated and
published by Holt, Landmark and Lied. 2 In a similar study from a
Canadian chain of riometers, Hartz 3 plotted the occurrence frequency in
polar coordinates of geomagnetic latitude and geomagnetic time. The
resulting pattern is very similar to that of a theoretical model developed
by Axford and Hines4 to explain several geophysical phenomena. This
model is based on a pattern fixed in geomagnetic time, and Hartz there­
fore evaluated the Norwegian data in the same way. Again a similar
pattern was found and it seems therefore justified to discuss some
aspects of the theory of Axford and Hines from observations made at one
longitude only.
4
ARCTIC D-REGION
Hartz3 used the occurrence frequency of absorption exceeding 0-5 dB.
It is interesting to see whether the pattern is changed when this lower
limit is increased. The observations were therefore redrawn in the way
introduced by Hartz, for absorption exceeding 0-5 dB and 1 dB and 2 dB.
The results are shown in Fig. 2.1 a, b and c. There is no obvious differ­
ence between the pattern in the three cases. The characteristic features
Fig. 2.1 a. Percentage of total time with absorption exceeding 0-5 dB at different
geomagnetic time and latitude.
are the maximum of occurrence in the morning hours at a geomag­
netic latitude of about 67°, and the minimum before midnight at all
latitudes except the highest. Further there seems to be a tendency for
the absorption occurrence to extend southwards during the hours after
geomagnetic midday.
To detect possible seasonal effects, the data from the summer, equinox
and winter months were separated. The results are shown in Fig. 2.2 a,
b and c. The auroral zone morning maximum is characteristic of all seasons.
Also a minimum occurs some hours before geomagnetic midnight, earlier
at the lower latitudes than in the conventional zone of maximum auroral
5
O. HOLT AND B. LANDMARK
activity. At the highest latitudes of observation, this minimum is not
present. The time difference of the minimum from lower to higher latitudes
had the effect that the variation of absorption occurrence frequency with
geomagnetic latitude, in these hours show a double maximum. The
broadening towards lower latitudes in the early afternoon is apparent in the
equinox and winter months. In the winter there is, in fact, a secondary
maximum at the rather low geomagnetic latitude of 60° during these
hours. In the summer, no southward extension is observed, but the maxi­
mum occurrence is shifted to a lower latitude (63°) at this time of the day.
The morning maximum also seems to be shifted southwards in the summer,
Fig. 2.1 b. Percentage of total time with absorption exceeding 1 dB at different
geomagnetic time and latitude.
a little less than 2°. In the winter a maximum is seen at 01 00 geomagnetic
time in the zone of maximum auroral activity, with a secondary maximum
at approximately 74° geomagnetic latitude. This feature is absent in the
equinox months, but is seen again in the summer, from 1 to 2 hr before
midnight. The two maxima are then shifted southwards, to 64° and 71°
geomagnetic latitude, respectively.
6