Table Of ContentPART IV
GEOMAGNETISM PART I
GENERAL REMARKS ON GEOMAGNETIC OBSERVATORIES
J. Bartels
TECHNIQUE OF SCALING INDICES Κ AND Q OF GEOMAGNETIC ACTIVITY
/. Bartels
THE GEOMAGNETIC MEASURES FOR THE TIME-VARIATIONS OF SOLAR
CORPUSCULAR RADIATION, DESCRIBED FOR USE IN CORRELATION
STUDIES IN OTHER GEOPHYSICAL FIELDS
/. Bartels
INSTRUMENTAL EQUIPMENT FOR THE RECORDING OF SPACE GRADIENTS
OF THE MAGNETIC ELEMENTS
S. Chapman and J. H. Nelson
PERGAMON PRESS
London New York Paris
ANNALS OF THE
INTERNATIONAL GEOPHYSICAL YEAR
VOLUME IV
IGY INSTRUCTION MANUALS
NUCLEAR RADIATION
AURORA AND AIRGLOW
LONGITUDES AND LATITUDES
GEOMAGNETISM—Part I
GEOMAGNETISM—Part II
SEISMOLOGY
COSMIC RADIATION
International Council of Scientific Unions
Comitι Special de VAnnee Gιophysique Internationale
(CSAGI)
Published by
PERGAMON PRESS
LONDON · NEW YORK · PARIS
PUBLISHED BY
PERGAMON PRESS
4 ώ 5 Fitzroy Square, London W.l
122 E. 55th St., New York 22, N.Y.
24 Rue des Ε coles, Paris Ve
Printed in Great Britain by J. W. Arrowsmith Ltd., Bristol
VOLUME IV
ANNALS OF THE
INTERNATIONAL GEOPHYSICAL YEAR
Instruction Manuals
PART I. NUCLEAR RADIATION TECHNIQUES FOR RADIOACTIVITY
MEASUREMENTS
PART II. AURORA AND AIRGLOW
PART III. LONGITUDES AND LATITUDES
PART IV. GEOMAGNETISM PART I
PART V. GEOMAGNETISM PART II
PART VI. SEISMOLOGY
PART VII. COSMIC RADIATION
I—INTRODUCTION
GENERAL REMARKS ON GEOMAGNETIC OBSERVATORIES
by
J. BARTELS
Göttingen
(Revised text of an introductory talk delivered at the opening meeting
of the Arctic Conference, Stockholm, May 1956.)
THE purpose of a geomagnetic observatory is to provide complete records of the
variations of the magnetic field. It is customary to restrict the records to the slower
variations and to leave those electromagnetic phenomena with periods shorter than,
say, 0 1 sec, to the attention of others working, for instance, on lightning discharges
and atmospherics.
Details of the instruments needed and their operation are given in text-books.
The magnetographs should be sheltered in houses protected against the daily vari
ations of air temperature, and/or provided with temperature-compensated vario
meters.
An ideal observatory should have at least four complete sets of variometers, each
set for the three field components X (geographic north), Y (east), and Ζ (vertical down
ward) . These geographically oriented components will be preferred in polar regions
(H less than 10,000y), while other stations may record horizontal intensity Η and
declination D instead of X and Y. The sets are:
Standard variograph
Normal sensitivity, scale-values 2 to 5 y/mm, paper speed 15 to 20 mm/hour,
base-lines controlled by absolute observations.
Storm variograph
Low sensitivity, scale values 30 to 50 y/mm, able to record fully any variations
up to 2000y or more either side of the normal, paper speed 15 or 20 mm/hour. The
storm variograph shall provide complete records in the times of greatest disturbance,
when the standard magnetograph fails, either because the variations are too rapid
or confused or because the deviations from normal are too big. It pays to operate a
storm magnetograph even if it is really needed on very few occasions during the Geo
physical Year, because those occasions will be the most interesting in the history of
geomagnetism. This holds for equatorial stations too.
The observatory Potsdam-Niemegk has three sets, instead of the two sets men
tioned with scale values 2, 8, and 32 y/mm: standard, intermediate, and storm. This
is of advantage in case of failure of one set.
Quick-run recorders of the La Cour type: high sensitivity, with scale values of the
order 1 y/mm, paper speed 6 mm/min or faster, exact time marks every minute.
209
210 GEOMAGNETISM—PART I
For polar observatories a second quick-run recorder with lower sensitivity (.10 y/mm)
will be of value to register the rare giant pulsations.
Induction variometers to record the most rapid variations.
The observatory needs, in addition, absolute instruments for the control of the
base-lines.
The time-scales mentioned (paper speeds) are those for photographic recording
which is still the best way. Although photographic recording was introduced as long
ago as 1846 at Greenwich, it is no secret that many records of inferior quality are
still being made. The best optical conditions should be aimed at.
Before going on an expedition, it is of advantage to produce at home, by means
of large Helmholtz coils, the magnetic field to be expected, and to set up and adjust
in that field the set of variometers, in order to prepare for the conditions to be en
countered at the station.
The advantage of photography is, of course, that the blackening of the trace is
an indication of the rapidity of the light spot; sudden commencements can, for
instance, easily be detected even if the rate of change of the component is already high
before the onset. Magnetograms otherwise produced, with even traces (for instance,
pen and ink), need therefore a wider time scale.
A perfect observatory will also look after the correct orientation of the variometer
magnets, so that the variometer really records the variations of that force com
ponent for which it is intended. The instruments will also be installed so that there
is no interaction of the variometer magnets. The induction variometers with perm
alloy cores, for instance, must be kept several metres away from the other vario
meters, because of their large magnetic moments. Possible parallaxes between the
various recording light-spots and the time-marks should be accurately measured.
Electronic variometers of the airborne magnetometer type are now being operated
at some stations. It will be of considerable interest to compare their records with
standard variographs in order to judge the stability (base-lines, scale-values) and
reliability of their performance.
Another type of variograph with direct visible recording has been developed for
ionospheric observatories in order to indicate instantaneously the degree of magnetic
disturbance: the light-spot from a horizontal-intensity variometer is thrown on a
divided photo-cell and the difference in current recorded by a line-writer. The vario
graph can be run at different sensitivities and is provided with an alarm functioning
in case of big disturbances. Such instruments are sufficient for the purpose of current
comparisons with ionospheric observations, although the traces are not quite accurate
enough for the purposes of a magnetic observatory, so that the visible trace vario
graph must be regarded as a welcome supplement, but not as a substitute for ordin
ary variographs.
An ideal magnetic station requires much space and attention. At any time,
accurate to about one second, it will furnish the absolute values of the three field
components to the nearest y, and variations to 0*1 y. It will publish complete lists
of hourly means as well as reproductions of the magnetograms, like the observatories
of the U.S. Coast and Geodetic Survey, and it will participate in the evaluations of
its records with respect to magnetic activity (three-hour range indices Κ and, for
polar stations in latitudes higher than 58°, quarter-hourly indices Q) and the various
GENERAL REMARKS ON GEOMAGNETIC OBSERVATIONS 211
effects (storm sudden commencements, bays, pulsations, sudden impulses, solar-flare
effects) as suggested by the respective Committees of the International Association
of Geomagnetism and Aeronomy.
The exacting requirements of such an absolute observatory tend to deter would-be
operators from planning such a costly station. So the question arises: Is it possible
to gain a reasonably high percentage of the ideal information with less effort? The
answer to that question involves quite a responsibility, because we cannot claim to
know all the future problems for which material might be looked for. (Of course, it
is clear that for all stations a series of absolute values should be measured to obtain
an annual mean for the determination of secular variation, with an error comparable
with that of ordinary survey field stations, say, 10 y.)
With our present knowledge, one may venture the answer that—although we
should have as many ideal or absolute stations as possible—it would likewise be
desirable to supplement the network of observatories by variation stations which
may dispense with correct absolute values. This holds especially for polar regions
where the immediate effects of solar corpuscular radiation dominate the magneto-
grams. Here the slow changes of the base-line values need be known only within,
say, 10 or 20 y.
The requirements for such variometer stations are that they should provide full
records of the variations. While the base-lines may not be so accurate, the scale-
values must be correct, the alignment of the variometer magnets must be checked
with the greatest care, and provision should be taken that no records are lost even
during the most intense disturbance. A moderate diurnal change of temperature
in the station and imperfections in the temperature compensation of the variometers
might also be tolerated, provided that the light-points are not allowed to drift too
far towards the edge of the paper. If only one set of variometers can be made avail
able, the question is whether it should be of standard sensitivity, so that one records
the smaller variations but loses the big ones occasionally—or insensitive, a storm
magnetograph. My own bias would be in favour of a storm variograph because it
provides information about the most interesting times, namely, those of great dis
turbance. The variation station will then help to fill in the gaps in the maps showing
the magnetic field variations during storms and bays.
In order to realize why a few polar stations might restrict themselves to recording
the variations only, we should mention the two reasons why we could not dispense
with accurate absolute values at equatorial stations. The accurate level of horizontal
and vertical intensity in the tropics is important in the study of post-perturbation,
the effect of the hypothetical equatorial ring-current with its relations to cosmic-ray
intensity. The second reason is that the amplitudes of the solar and lunar daily
variations at tropical stations, which can be derived only from absolute hourly
means, give information on the fluctuations of the solar wave-radiation. Neither
of these two phenomena are of the same importance at polar stations. A hint
to equatorial stations with limited means: They may dispense with the storm
variometers for the two components D and Z, but they should have one insensi
tive H.
How dense must the net of stations be? In polar regions, in the auroral zone, the
time-variations change much from place to place, indicating that the ionospheric
212 GEOMAGNETISM—PART I
currents themselves have a complicated geographical distribution. This alone justi
fies a dense network.
But an additional justification has been realized in recent years: there are regions
where the induced part of the magnetic field changes rapidly from place to place.
RIKITAKE has given an example for Japan, and we have found another remarkable
case in north-west Germany during magnetic bays. It appears as if there are local
areas of increased electrical conductivity underground, in Germany a strip at a
depth estimated as at least 30 km below the Mohórovicic-layer (of 8 km/sec velocity
of compressional seismic waves). The induced part of the magnetic variations in a
bay is especially clear in the variations of the vertical component which it can change
even in sign! Such effects will occur also elsewhere, although it may be presumed that
regions exist where the underground does not contain such structures but is flatly
stratified, for instance around the magnetic observatory of Tucson (Arizona), where
the Z-variations are extremely phlegmatic.
The effects of these inhomogeneities in the deeper crust have two aspects:
(1) they make it more difficult to deduce the ionospheric currents from the
geomagnetic time-variations, and
(2) they allow information on the deeper crust to be derived, from a dense net
work of semi-permanent variation stations, geomagnetic depth sounding as a
natural extension of Cagniard's magneto-telluric prospecting method.
Pulsations are more or less regular variations, of periods between, say, 1 and 100
seconds which will receive much attention in the IGY. The standard equipment
often shows them clearly enough but allows only statistical data about frequency of
their appearance, their prevailing periods, amplitudes, and daily variations to be de
duced. Valuable as this statistical material is, it should be the aim of the IGY to
provide more exact data on the actual field variations, say, second by second, so
that the exact and complete geographical distribution of the vectorial field varia
tions during selected typical pulsations will be known.
For instance, the so-called bp—the pulsations with which many (if not all)
bays start—seem to be world-wide in nature; although they are much more intense
at certain places, they may perhaps be distinguished in sensitive quick-run records
all over the world. The exact phase-relations of the individual quasi sine-waves have
never been determined, on a world-wide scale, with the accuracy needed for the
localization of the electric currents causing them—which presumably will, in certain
regions, contain local effects due to enhanced induction, even more so than in the
case of bays mentioned above. Work with sensitive quick-run recorders, in the polar
regions too, will be highly desirable to clear up these old problems.
A serious problem concerning the evaluation of the quick-run records, especially
of the induction or Grenet types, arises from the fact that the natural periods
of pulsations are in the neighbourhood of the free periods of magnet systems
and of the galvanometers. This introduces shifts in phase and changes of scale values
with period, which make it difficult to infer from the records the exact time-vari
ations of the magnetic-field components. This problem is the same as in the deduc
tion of the "real movement" 5 of the soil from records of, say, a Galitzin seismometer.
The safest ways to deal with such questions seem to be adequate calibrations for
GENERAL REMARKS ON GEOMAGNETIC OBSERVATIONS 213
various periods, and the experimental study of the response of the recorder to arti
ficial imitations of the natural field variations:
Let F(t) be the natural geomagnetic variation (unknown), r(t) the recorded trace
of the variometer, and f(t) the geomagnetic variation inferred from r(t). Then the
conclusion that f(t) equals F(t) is, within the limitations set by the instrument, most
definitely proven if the experimental application of an artificial magnetic variation
f(t) to the variometer produces exactly the record r(t).
Earth-currents
In the discussions of the Working Group on Geomagnetism at the Arctic Con
ference, Stockholm, May 1956, the excellent earth-current records at U.S.S.R.
stations, as demonstrated by Mrs. V. TROITSKAYA (Geophysical Institute, Academy
of Sciences, Moscow) aroused much interest in the question whether earth-currents
should be added to the regular programme of geomagnetic observatories. The
simplicity of the instrumental arrangements and the very expressive records ob
tained—especially with regard to small short-period pulsations which appear simul
taneously at stations thousands of km apart—support such a request.
It was generally agreed that an ideal station should have, in addition to quick-run
geomagnetic recorders and induction variometers, earth-current recorders as well;
since earth-currents are much more sensitive to industrial disturbances, it may not
be possible to record them successfully in populated regions. But even where earth-
current recording is possible it should not be a substitute for the sensitive quick-run,
although earth-current recorders are more easily operated. The reason is that earth-
currents give two components only, while what are primarily needed are the exact
phase-relations of the pulsations in the three geomagnetic-field components which
the quick-run furnishes, at least for the many pulsations with not too small ampli
tudes and with periods of a few seconds or longer. The arrangement for recording
earth-currents is, so-to-say, a natural induction variometer, whose detailed and
possibly complicated circuit is not known. Therefore earth-currents express local
peculiarities of the underground structure more distinctly than world-wide pheno
mena.
Utilization of the magnetograms
Copies of the magnetograms will be made available, through duplication
arrangements to be organized by the CSAGI, to all investigators. All stations will be
asked to evaluate their records with respect to special effects and the absolute
observatories, especially those in non-polar regions, will be asked to scale and publish
hourly means of three components. But even during the IGY, certain abstracts will
be asked from all those stations which will be able to furnish them. Thus, equatorial
stations will be asked to supply current hourly means of horizontal intensity to
some central agency which will combine them into a measure for the intensity of the
ring-current (ERC) according to a project which is now being started.
But the most important contribution of geomagnetic observatories, especially
those in polar regions, to the joint effort in the IGY will be based on the fact that
they are able to record most directly the effects of incoming solar corpuscular radiation
of the type producing "geomagnetic activity''. Auroral observations do that too;
214 GEOMAGNETISM—PART I
magnetic and auroral observations supplement each other. The aurora indicates in
greater detail the distribution over the sky, both in the horizontal and in the vert
ical directions; geomagnetic variations are an integral effect of all electric currents
(which may not vary proportionally to the auroral light intensity). The horizontal
magnetic components emphasize the currents directly overhead, while the vertical
magnetic component records currents nearer to the horizon, possibly of long line-
currents far away. The great advantage of the geomagnetic records is, of course, that
they can be obtained independently of daylight or clouds.
A special article will be devoted to the scaling of indices expressing geomagnetic
activity: the three-hour-range index, K, and a new quarter-hourly index, Q, to be
introduced for polar observatories in latitudes higher than 58°. It may be added
that equatorial observatories, unless they already have a long series of if-indices,
may be excused from taking up if-scaling, if they devote the time saved to the
derivation of reliable hourly means.
Magnetic recording is basically simple, but it needs care to be brought to perfec
tion. May all magnetic stations devote their best efforts to the success of their
magnetic task and not neglect them in favour of studies involving more electronics.
The International Association of Geomagnetism and Aeronomy has a Committee
on Observatories (Chairman: Prof. E. LAHAYE, 3 Avenue Circulaire, Uccle, Belgium)
and a Committee on Magnetic Instruments (Chairman: Mr. J. H. NELSON, Division
of Geophysics, U.S. Coast and Geodetic Survey, Washington 25, D.C., U.S.A.).
These Committees will be glad to help if they are asked. General questions may be
directed to the Secretary of the Association, Dr. V. LAURSEN, Meteorologisk Institut,
Charlottenlund, Denmark.
