International Quiet Days Research Articles

The Daily Variation of Irregular Disturbances of the Earths's Magnetic Field at Bombay

Besides the quiet‐day solar diurnal variation and the variations associated with magnetic storms, the intensity of the Earth's magnetic field at any place is subject to many irregular, short‐period fluctuations. The paper contains an analysis of the diurnal variation of these irregular disturbances during the eleven‐year period 1923–33. As a measure of the disturbance, the departures of the hourly ranges of the horizontal forces from the mean hourly range on the five international quiet days in that particular month were used. In calculating the disturbance, due regard was paid to the sign of the change of the element during the hour. The diurnal variations of the irregular disturbances depend on the season of the year and on the sunspot‐activity. These variations are analyzed and compared with the results of disturbance diurnal variation in higher latitudes. The main conclusions that emerge from the analysis are:(i) The hour of minimum disturbance occurs early in the morning usually between 04h to 05h or 05h to 06h local time and that of the maximum near about noon.(ii) There are two secondary maxima, one at 16h to 18h and another at about 22h to 23h local time.(iii) Analyzing the results according to month, the day maximum is most pronounced in the months April to August. In the winter months, there is a tendency for the variation to approach the European type (as shown by the analysis of the data of Eskdalemuir (magnetic latitude Φ=59°.5) and of Wilhelmshaven (Φ = 54°.5) in which there is one minimum in the morning at about 09h local time and a maximum in the evening at about 22 h.(iv) If days of magnetic character 2 are excluded, the diurnal variations are similar, but the late evening maximum is suppressed, leaving the noon maximum more pronounced.It is generally believed that “in the latitudes between the two auroral zones, that is, up to at least 65° magnetic latitude, Di has a simple diurnal variation with its maximum in the evening; as the auroral zone is approached the hour of maximum gets later, from about 21h at 55° to midnight at 70°. Up to this latitude the form of the daily variation of Di does not vary much either with season or with the general intensity of magnetic disturbance” [Chapman]. The present analysis of the Bombay data shows that in low magnetic latitudes, the main variation is a maximum near noon with a minimum a little before sunrise. The evening maximum of temperate latitudes is superposed on this simple variation.It is suggested that the maxima of disturbance‐variation at Bombay at about noon and in the afternoon are associated with the maxima of ion‐density in the E‐ and F1‐layers and in the F2‐layer, respectively. The late evening maximum is presumably due to fluctuations in F2‐layer caused by electrified particles from the Sun concentrating on the night side of the Earth on account of the deflecting action of its magnetic field.

Concerning the magnetic numerical character‐numbers

In a communication presented to the Assembly of the Association of Terrestrial Magnetism and Electricity held at Lisbon September 19331 the author gave the results of a comparison of numerical character‐numbers with international character‐numbers. It was shown that the numerical character‐number corresponding to the international character‐number 0.2 for the magnetic observatory at Rude Skov was about 120 in the winter (December) and about 240 in the summer (June).In the present note the magnitude of the numerical character‐numbers for magnetically quiet days for observatories at different latitudes is examined. The numerical character‐numbers for the five international quiet days of June and of December for the years 1932 and 1933 are considered.

Notes

In a communication presented to the Assembly of the Association of Terrestrial Magnetism and Electricity held at Lisbon September 19331 the author gave the results of a comparison of numerical character‐numbers with international character‐numbers. It was shown that the numerical character‐number corresponding to the international character‐number 0.2 for the magnetic observatory at Rude Skov was about 120 in the winter (December) and about 240 in the summer (June).In the present note the magnitude of the numerical character‐numbers for magnetically quiet days for observatories at different latitudes is examined. The numerical character‐numbers for the five international quiet days of June and of December for the years 1932 and 1933 are considered.

Note on the Mean Monthly Values and the Annual Inequalities of the Magnetic Elements

The ultimate cause of the present note is a recent paper by Dr. la Cour, Director of the Danish Meteorological Service, which dealt with mean monthly values of the magnetic elements from 1911 to 1920 at Rude Skov (near Copenhagen) and ten other European observatories, including Greenwich, Kew, and Eskdalemuir. Dr. la Cour applied corrections for secular change assumed to proceed at a uniform rate throughout the year, and discussed the residuals left in the changes of the magnetic elements from month to month. The present paper is devoted mainly to the phenomena exhibited at Kew and Eskdalemuir. At these and other stations in North-Western Europe secular change of declination underwent a large acceleration during the period considered. The consequences of this acceleration and of a variety of other special features, especially the use of the international quiet days at Kew, are discussed.

A comparison of magnetic declination changes at British Observatories

In spite of the very varied use of the compass needle for purposes of navigation and surveying, there is little really known about the closeness in the parallelism of daily magnetic changes in different parts of the United Kingdom. The mean annual values from magnetic observatories show that, since 1910, secular change of declination has been at least approximately the same throughout England, the south of Scotland, and the south-west of Ireland. Thus the conditions have been favourable for an enquiry into the parallelism of other changes. Diurnal inequalities from five selected quiet days a month were published for Falmouth as well as Kew up to 1912. A comparison of results from a number of years combined had shown little difference between the diurnal inequalities at the two stations as regards the range. Difference in local time produced a visible effect, but it was small. In more recent years, corresponding diurnal inequalities for Eskdalemuir and Kew from the five international quiet days of each month had shown a close similarity.

Concerning procedure at magnetic observatories

If I rightly follow Mr. Hazard (pages 145–148), the diurnal inequalities published of late years at the observatories of the United States Coast and Geodetic Survey for the five quiet days a month are derived from 24‐hour periods which commence at 0h G.M.T. of the international quiet days; also an approximate correction has been applied for non‐cyclic change. This is so far satisfactory. But I venture to express a hope that an exact, not an approximate, non‐cyclic correction may be applied in future, and that it will be extended to all diurnal inequalities.For those who have no fondness for mental arithmetic, I would recommend the use of a table of non‐cyclic corrections. Using a table, it is just as easy to apply the exact correction at each hour to the nearest 0.01 or 0.1, as the case may be, as it is to apply the approximate correction described by Mr. Hazard. The use of zonal time instead of L.M.T. is obviously advantageous for international term days. It has always been the practice for such purposes to use G.M.T. at all the British magnetic observatories. It undoubtedly facilitates the intercomparison of curves. When diurnal inequalities are analyzed in Fourier series, there are advantages in referring the phase angles to local time. But the difference between the phase angles calculated for Local and Greenwich time is constant, being determined by the difference of longitude. Thus the phase angles referred to local time can be derived at once from those calculated using Greenwich or Zonal time.

The Diurnal Variation of Terrestrial Magnetism

PUBLICATION No. 102 of the Royal Meteorological Institute; of the Netherlands consists of a doctor's dissertation in Dutch by Miss Annie van Vleuten “On the Diurnal Variation of Terrestrial Magnetism” and two short papers in English from vol. xxvi. (1917). of the Proceedings of the Science Section of Kon. Ak. v. Wet. of Amsterdam. The dissertation which extends to 106 pages, contains numerous tables of diurnal variation data for the magnetic elements, and the corresponding Fourier coefficients for a number of stations, more especially for Pavlovsk, Sitka, Irkutsk, De Bilt, Cheltenham, U.S., Zi-ka-wei, Honolulu, Bombay, Buitenzorg, and Samoa, and for the group of years 1906–8. The Fourier coefficients, based on the data from these ten stations from the international quiet days, five a month, are used to furnish answers to the questions advanced in the two short papers in English: (1) Does the internal magnetic field to which the diurnal variation is partly ascribed depend on induced electric currents? (2) Do the forces causing the diurnal variation possess a potential? These are problems chiefly associated in England with the name of Prof. Schuster, to whose work there are many references, while abroad they have occupied, amongst others, Profs. Fritsche and Steinef. Schuster and Fritsche, using totally different observational data, separated the forces causing the diurnal variation into one set having a source external to the earth, and a second set having an. internal source. Schuster suggested that the second set arise from currents induced in the earth by the former set. Steiner, employing Fritsche's results, decided against Schuster's hypothesis. Miss van Vleuten's material,}s at once more homdgeneotis than Fritsche's, and more representative than Schuster's. She concludes that. while the terms of higher order accord pretty fairly on the whole with Schuster's hypothesis, this is not true of the principal terms of lower order. The natural inference is that the hypothesis is, at best, not a complete explanation of the phenomena. To the second question the answer obtained is that the forces causing the diurnal variation do not possess a potential; part, but only part, of the diurnal variation may be derived from a potential. Besides the main data mentioned above, data from a number of other stations are utilised, and there is, besides, a good deal of mathematical theory. While the publication makes most direct appeal to theorists, it contains much valuable information as to facts not otherwise readily accessible.