Geomagnetism. Annals of The International Geophysical Year, Vol. 4 PDF
Preview Geomagnetism. Annals of The International Geophysical Year, Vol. 4
PART 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.
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