Geomagnetic Intensity Variations Research Articles

Periodicities of palaeomagnetic intensity and palaeoclimatic variations: a Walsh spectral approach

The Walsh power spectral technique has been applied to the non-sinusoidal palaeoclimatic (the Globorotalia menardii climatic curve) and palaeomagnetic intensity variation records of the last 1.2 Myr. The analysis reveals, in the case of palaeomagnetic variations, statistically significant peaks (at 90% confidence level) centered around 150, 60 and 24 kyr. The palaeoclimatic spectrum, however, indicates only two significant peaks of 60 and 24 kyr. The identical periodicities in both the phenomena may suggest a causal link between the geomagnetic intensity variations and climatic changes.

Thermoluminescent dating of Lake Mungo geomagnetic polarity excursion

ARCHAEOMAGNETIC studies made by Barbetti and McElhinny1,2 on prehistoric aboriginal fireplaces at Lake Mungo, Australia, indicate the occurrence of a polarity excursion about 30,000 years ago during which the geomagnetic field intensity increased to three times its presentday value; this main excursion was followed by a second one during which the intensity decreased to about one third of the present day value. This note reports thermoluminescent (TL) dating measurements that were carried out on fireplace material at the request of Dr M. F. Barbetti. Comparison of the TL dating with the radiocarbon ages3,4 is of particular interest because geomagnetic intensity variations may be the dominant cause of long term distortion of the radiocarbon time scale5–7. The geomagnetic field gives the Earth partial shielding from cosmic rays, particularly from the lower energy, component that reaches only the upper atmosphere and is responsible for most of the radiocarbon production. If the geomagnetic intensity was lower than usual, then the production rate would have been higher and this would cause relevant radiocarbon dates to be too recent. In this comparison of TL and radiocarbon ages we have not, however, found evidence for a disturbance of the radiocarbon time scale caused by geomagnetic polarity excursions.

古地磁気層序，地磁気と気候変化に関する二・三の問題点

(1) Magnetization acquired after deposition is called “post-depositional detrital remanent magnetization” (PD-DRM). Natural remanent magnetization (NRM) of very fine-grained deposits is regarded as a kind of PD-DRM, since the loose unconsolidated upper part of fine-grained sediments-say several tens of centimeters-does not have the stable NRM, the direction of which usually does not change with depth. The thickness of this upper part generally coincides with the depth at which magnetic particles are fixed by friction due to compaction, and it should be noted, especially in case of stratigraphic correlation, that there is a time lag in the magnetic record in a sediment. This time lag can be obtained dividing thickness by depositional rate. Since the depth at which magnetic particles are fixed is a function of the internal friction due to compaction as well as the applied magnetic field, it is possible to use this depth as a convenient parameter to indicate the degree of compaction.(2) There are two contrary speculations concerning the correlation between climatic changes and geomagnetic intensity-variations. One maintains that an increase of the magneic moment corresponds to the surface coldness of the Earth. The other view insists that a decrease of the geomagnetic field is a cause of an ice-age. Both vieupointo are derived from paleomagnetic studies of fine-grained sediments; the former from investigations of deep-sea cores, and the latter mainly from study of a 200m-long core taken from Lake Biwa.Since the rate of sedimentation is usually a few hundred to thousands of times greater in lakes than in the ocean, there must be some difference in time scale in these presumptive arguments. Moreover, there is another important difference in the nature of their arguments. The former concept regards the intensity of magnetization of sediments as a linear function of that of the past geomagnetic field, but the latter does not. To normalized the intensity variation of magnetization due to the difference in the quantity of the magnetic substances in the sediments, the latter concept requires as an indicator of the intensity of the past geomagnetic field, that the value be obtained through dividing the intensity of magnetization by that of saturation remanent magnetization.It is pointed out that these differences in the two viewpoints lead to contrary conclusions.

A new longitude effect in the geomagnetic solar daily variation

The daily variation of geomagnetic vertical intensity (Z) at Koror, Palau Islands, slightly south of the dip equator, is characterized by a positive salient before 1100 hours and a negative one after 1500 hours. This feature, which persists throughout the year, appears to reflect a component of the Z curve that simulates the time derivative of the horizontal-intensity (H) curve. A similar though less pronounced effect may be discerned at Huancayo and at Trivandrum; at Addis Ababa its sign is reversed. This component probably makes inapplicable to Koror (and possibly to some other localities) the suggestion that the day-to-day fluctuations in the ratio of Z and H ranges in western Africa may reflect shifts in the latitude of the electrojet. To account for the observed Z effect it is suggested that there is superimposed on the conventional S/sub q/ pattern of ionospheric currents a north-south flow of current confined to the daylight side of the earth, that this current reverses its sign in a systematic relation with the earth's rotation, and that it may tend to channelize along the paths established by the S/sub q/ patterns, so as to become manifest in the daily- variation curves at stations in highermore » latitudes as well as along the dip equator. On the basis of the relative amplitudes of the eastward and westward salients of magnetic declination curves, the suggestion is strongly supported as regards Europe and the western hemisphere; but equivocal results are obtained for Australasia and the western Pacific, notwithstanding that Koror in this region exhibits the Z effect more strongly than any other equatorial station. Neither the source of the postulated current nor the path by which its circuit is completed is clear. It is conjectured that a persistent leakage of charged particles trapped in the magnetosphere, falling from their mirror points into the upper air, may give rise to vacillating potential differences between the northern and southern auroral zones, and that the phenomena here reported represent the ebb and flow of ionospheric current required to equalize these potential differences.« less

Lunar daily variation of geomagnetic horizontal intensity at Alibag

Following the mathematical development of Chapman and Miller, Tschu has described a practical method of determining the lunar daily variation of geophysical elements. By application of the Chapman-Miller method as described by Tschu, the lunar semidiurnal variation of geomagnetic horizontal intensity at Alibag is worked out for the equinoctial season by making use of the hourly values of the element for the period 1940 to 1944. The expression L(H)=1.2 sin(2τ+62°)±0.3 where L(H) is the amplitude of the lunar variation in the horizontal intensity H and τ is the lunar time reckoned from lower transit of mean moon at Alibag is obtained for lunar semidiurnal variation. L(H) is a maximum when the lunar time is 2 lunar hours past midnight or noon. The solar diurnal variation is also determined up to 4 harmonics. A comparison of these results with similar results for Kodaikanal suggests that solar and lunar variations are independent of each other.

Variation of Geomagnetic Intensity with Depth

THE formula giving the variation of the earth‘s horizontal field with depth quoted in the letter of Hales and Gough involves an approximation concerning densities which is only applicable to spheres the density distribution of which is only slightly different from a uniform one, such as is roughly true for the earth. Thus it is not to be expected that the formula should give the dipole variation law when ρ 1 = 0, but I am grateful to Prof. Chapman for pointing out a more general formula. It should be noted, however, that in the present state of the theory of Blackett‘s formula, , the treatment of densities must remain ambiguous, various assumptions leading to slightly different values of the predicted variation with depth. This point will be dealt with in detail elsewhere.

Variation of Geomagnetic Intensity with Depth

IN a recent letter1, A. L. Hales and D. I. Gough compared their measurements of Hd, the horizontal magnetic intensity (H) at depth d, made in the Witwatersrand at d = 4,800 ft., with the formula Hd/H0 = 1 - 2 (5Ï1/Ïp + 1) d/a (1) obtained by S. K. Runcorn from Blackett‘s theory2. In (1), Ho denotes the value of H at the overlying point on the surface (d = 0), a denotes the earth‘s radius, Ï1the mean density of the rocks down to depth d, and Ï the overall mean density of the earth. The observations were compared also with the formula Hd/H0 = a3/(a-d)3 = 1 + 3d/a (approximately), (2) corresponding to any âcoreâ theory which places the source of the field wholly below the depth d.