Paleointensity of the geomagnetic field in the Cretaceous (from Cretaceous rocks of Mongolia)

A representative collection of Cretaceous rocks of Mongolia is used for the study of the magnetic properties of the rocks and for determination of the paleodirections and paleointensities H _anc of the geomagnetic field. The characteristic NRM component in the samples is recognized in the temperature interval from 200 to 620–660°C. The values of H _anc are determined by the Thellier-Coe method with observance of all present-day requirements regarding the reliability of such kind of results. Comparison of data in the literature on paleointensity in the Cretaceous superchron and in the Miocene supports the hypothesis of the inverse correlation between the average intensity of the paleofield and the frequency of geomagnetic reversals. The increase in the average intensities is accompanied by an appreciable increase in the variance of the virtual dipole moment (VDM). We suggest that the visible increase in the average VDM value in the superchron is due to the greater variability of VDM in this period compared to the Miocene.     

Determination of the paleointensity in the early proterozoic from granitoids of the Shumikhinskii complex of the Siberian craton

The extreme scarcity of data on the behavior of the paleointensity H _an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H _an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H _an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 10²² A m². Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 10²² A m², which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.     

Secular geomagnetic variations and volcanic pulses in the Permian-Triassic traps of the Norilsk and Maimecha-Kotui provinces

Detailed paleomagnetic studies have shown that the effusive Permian-Triassic traps in the Kotui River valley were formed as the result of volcanic activity, which occurred in the form of volcanic pulses and individual eruptions with net duration of at most 7000–8000 years, excluding the periods of volcanic quiescence. According to the analysis of the paleomagnetic data earlier obtained by Heunemann and his coauthors [2004b] on the Abagalakh and Listvyanka sections in the Norilsk region, those geological units were formed during 25 volcanic pulses and separate eruptions, which all lasted up to 8000 years altogether, whereas the total time of formation (including the periods of volcanic quiescence) exceeded 10000–100000 years for the Norilsk section and was probably a bit shorter for the Kotui section. Comparison of the positions of virtual geomagnetic poles calculated for the Norilsk and the Kotui sections provides no grounds to suggest that these sections were formed at different geological times. The scatter in the positions of the virtual geomagnetic poles (VGP) for the directional groups and individual directions (58 altogether) jointly for the two sections (more than 160 lava flows) indicates that the secular geomagnetic variations at the Permian-Triassic boundary had similar amplitudes to those that occurred in the past 5 Ma.     

Geomagnetic field paleointensity in the cretaceous from Upper Cretaceous rocks of Georgia

A representative collection of Upper Cretaceous rocks of Georgia (530 samples from 24 sites) is used for the study of magnetic properties of the rocks and the determination of the paleodirection and paleointensity (H _an) of the geomagnetic field. Titanomagnetites with Curie points of 200–350°C are shown to be carriers of natural remanent magnetization (NRM) preserving primary paleomagnetic information during heatings to 300–350°C. The characteristic NRM component of the samples is identified in the interval 120–350°C. The Thellier and Thellier-Coe methods are used for the determination of H _an meeting modern requirements on the reliability of such results. New paleointensity determinations are obtained and virtual dipole magnetic moment (VDM) values are calculated for four sites whose stratigraphic age is the Upper Cretaceous (Cenomanian-Campanian). It is shown that, in the interval 99.6–70.6 Ma, the VDM value was two or more times smaller than the present value, which agrees with the majority of H _an data available for this time period. According to our results, the H _an value did not change at the boundary of the Cretaceous normal superchron.     

Paleointensity and paleodirection of the geomagnetic field in the middle Miocene: Evidence from late cenozoic volcanites of primorye

We present the results of analyzing a representative collection of the middle Miocene 12.4–10.0 Ma basalts that compose the volcanic cover of the Shufan and Sovgavan plateaus, namely the Nikolo-L’vovsk (NL) and Sovetskaya Gavan (SG) volcanic fields. Preliminary data are obtained about the relicts of some volcanic edifices within the West and East Sikhote-Alin volcanic belts, namely the Shishlovskii, Malyshevo, and Truzhenik objects. It is established that the volcanic rocks from these localities are characterized by similar petrologic and magnetic properties. Thermal cleaning of the samples is carried out, and the coordinates of the paleomagnetic pole are determined as Λ = 190.2°E, Φ = 71.3°N for basalts of the Nokolo-L’vovsk area and Λ = 180.4°E, Φ = 71.9°N for rocks from the Sovgavan locality. These values are consistent with the data for coeval volcanics from other regions of Eurasia. Reliable determinations of the paleointensity H _pal for a representative collection of samples were obtained using the Thellier method. The corresponding values of the virtual dipole moment (VDM) are almost half its present-day value. The analysis of the Miocene VDM values available from the world database revealed a low average field 5.06 × 10²² Am² characterized by high variance σ = 2.13 × 10²² Am² at that time. The similarity of VDM values for the Miocene characterized by frequent inversions and for the Cretaceous Superchron supports the hypothesis of the lack of a correlation between the VDM values and the frequency of geomagnetic inversions.     

Paleointensity in the Quaternary West Eifel volcanic field, Germany: preliminary results

Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.     

On the intensity of the geomagnetic field in the geological past

A detailed analysis of the data on the intensity of the geomagnetic dipole and frequency of its reversals presented in the world’s paleointensity databases provided the arguments in favor of the hypothesis of the negative correlation between the average virtual dipole moment (VDM) and the frequency of the reversals on the interval from 5 Ma to 100 Ma ago. However, the statistical confidence level of this hypothesis is only 60–70%, which is far below 95%, the standard required confidence level of a hypothesis to be considered statistically reliable. At a high level of confidence (above 99%), the presence of a positive correlation between the mean value and variance of VDM for a number of intervals of stable polarity in the Cenozoic and Mesozoic is confirmed. This finding means that the distribution of VDM on these time intervals is certainly non-Gaussian and is rather described by the gamma- or lognormal law. At the same time, in contrast to the earlier intervals, the histogram of VDM for the Brunhes epoch is closer to the normal distribution. Compared our conclusions with the published results on the numerical modeling of the geodynamo, we found that they are consistent in terms of a probable negative correlation between the average VDM and reversal frequency, as well as the lack of correlation between the average VDM and the length of the interval of stable polarity.     

On the variation in the geomagnetic dipole over the geological history of the Earth

The global database on the paleointensity, containing determinations of the virtual dipole moment (VDM) for a stable (normal) regime of the geomagnetic field in a time interval of up to 3.5 Ga, is supplemented by new VDM determinations and analyzed. The field generation process started no later than 3–3.5 Ga (earlier data are absent) at the stage of the Earth’s core formation. Since that time, the dipole value has differed from its present value by no more than an order of magnitude, and the deviations that have already been detected tend toward smaller values. The distribution of VDM values in the time interval 0–400 Ma is bimodal, which apparently reflects the presence of two different generation levels of the geomagnetic field distinguished by a relatively large value (close to the present field value) and a relatively small value (approximately half as large as the present value). The total duration of decreased VDM values appreciably exceeds that of increased VDM values (179.1 and 28.6 Myr, respectively). On the whole, data on the paleointensity do not contradict the hypothesis about the dipole nature of the field over the last 400 Myr; however, the number of determinations at high paleolatitudes is too small to draw decisive conclusions on the validity (or invalidity) of the dipole field approximation based solely on paleointensity data.     

The effect of solar activity on the secular variation of the geomagnetic field in Romania

The 11-year solar cycle effect in the geomagnetic components H and Z is made clear for Surlari Observatory and 19 repeat stations for the interval 1952–1974. The correlation with Wolf number and its time derivative is discussed in terms of the effects of the external and induced current systems.The $\text{H}\text{?}$ data available for solar cycle 20 (1964–1976) were processed to give the geographical distribution of the secular variation impulse for epoch 1969.5 in Romania. It is suggested that this distribution might reflect the deep internal structure of the area considered.A qualitative correlation is noted between long-period solar activity and variation of the horizontal component of the geomagnetic field at some repeat stations.     

Paleomagnetic studies and estimation of geomagnetic paleointensity at the early/middle Riphean boundary in rocks of the Salmi Formation (North Ladoga area)

The territory of Karelia (Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval (1650—1350 Ma). As regards the paleointensity H _an, the huge interval 1–2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1–1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15° S and 40° S. The inferred value of H _an is close to the lower boundary of the interval (1.36–11.56) × 10²² A m², encompassing previously published intensity values of the paleofield 1–1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003].     

Magnetostratigraphic timescale of the Phanerozoic and its description using a cumulative distribution function

A method is proposed for quantified structuring of a magnetochronological scale of the Phanerozoic, i.e., the construction of a magnetostratigraphic timescale on the basis of a cumulative function of geomagnetic field asymmetry with regard for the polarity sign. Analysis of the cumulative curve reveals basic characteristic patterns of the field evolution in the Phanerozoic: the reversed polarity being predominant in this epoch, three megachrons of variable polarity are identified against this background: Paleozoic R13 (468-315 Ma), Mesozoic N6 (258-123 Ma), and Cenozoic R10 (83-0 Ma). The megachrons are subdivided into hyper-and superchrons and are separated by single polarity hyperchrons. Most important are changes in the general trend of the polarity bias in the Middle Triassic and at the Paleogene/Cretaceous boundary. Data of fractal and wavelet analyses suggest the presence of two regimes of geomagnetic field generation: chaotically distributed frequent reversals (geodynamo) and a stable polarity.     

Parameters of the near-earth interplanetary medium under quiet and disturbed geomagnetic conditions

The distributions of the parameters of the solar wind, IMF, and physical fields (E _y component of the SW electric field, compression field DCF) and the rms errors (σ) of measurements, depending on the daily characteristic of geomagnetic disturbance (Cp), are considered. The scatter of parameters in the interplanetary medium (IM) is actually considerable even during a long interval of geomagnetic quiet. It has been indicated that an unambiguous correspondence between the IM parameters and the characteristic of geomagnetic activity on the Earth is absent, and we have only tendencies toward an increase (decrease) in the parameter of the near-Earth medium (physical quantity) with increasing geomagnetic activity. These tendencies are transformed into linear relationships only after the three-fold averaging of values (hourly, daily, annual), which corresponds to numerous equations of relation between IM parameters and different geomagnetic indices, obtained by many researchers based on statistical analyses.     

The geomagnetic field at the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries and lower mantle plumes

The data on the amplitude of variations in the direction and paleointensity of the geomagnetic field and the frequency of reversals throughout the last 50 Myr near the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries, characterized by peaks of magmatic activity of Siberian and Deccan traps, and data on the amplitude of variations in the geomagnetic field direction relative to contemporary world magnetic anomalies are generalized. The boundaries of geological eras are not fixed in recorded paleointensity, polarity, reversal frequency, and variations in the geomagnetic field direction. Against the background of the “normal” field, nearly the same tendency of an increase in the amplitude of field direction variations is observed toward epicenters of contemporary lower mantle plumes; Greenland, Deccan, and Siberian superplumes; and world magnetic anomalies. This suggests a common origin of lower mantle plumes of various formation times, world magnetic anomalies, and the rise in the amplitude of geomagnetic field variations; i.e., all these phenomena are due to a local excitation in the upper part of the liquid core. Large plumes arise in intervals of the most significant changes in the paleointensity (drops or rises), while no correlation exists between the plume generation and the reversal frequency: times of plume formation correlate with the very diverse patterns of the frequency of reversals, from their total absence to maximum frequencies, implying that world magnetic anomalies, variations in the magnetic field direction and paleointensity, and plumes, on the one hand, and field reversals, on the other, have different sources. The time interval between magmatic activity of a plume at the Earth’s surface and its origination at the core-mantle boundary (the time of the plume rise toward the surface) amounts to 20–50 Myr in all cases considered. Different rise times are apparently associated with different paths of the plume rise, “delays” in the plume upward movement, and so on. The spread in “delay” times of each plume can be attributed to uncertainties in age determinations of paleomagnetic study objects and/or the natural remanent magnetization, but it is more probable that this is a result of the formation of a series of plumes (superplumes) in approximately the same region at the core-mantle boundary in the aforementioned time interval. Such an interpretation is supported by the existence of compact clusters of higher field direction amplitudes between 300 and 200 Ma that are possible regions of formation of world magnetic anomalies and plumes.     

Geomagnetically induced currents in power lines according to data on geomagnetic variations

The time derivative (d H/dt) of the geomagnetic field horizontal component (H) for seven intervals of high geomagnetic activity in 2003–2005 has been calculated, based on the data of Alma-Ata, Novosibirsk, and Irkutsk observatories, in order to estimate the probability of appearance of geomagnetically induced current (GIC), the value of which is linearly dependent on d H/dt, in power lines on the territory of Kazakhstan. The distributions of the H and d H/dt directions have been constructed; in this case it was most interesting that these distributions were narrow and extended along the magnetic meridian for Alma-Ata and were wider angular for Novosibirsk and Irkutsk. It has been indicated that large H values, determining significant GIC values, took place at a sudden commencement of strong storms, which had a character of a pulsed disturbance of the geomagnetic field, and during large-amplitude geomagnetic field pulsations. The duration of the periods of large |d H/dt| values exceeding the threshold (30 nT/min), when GIC could cause unwanted consequences in power circuits, has been determined.     

Brunhes-Matuyama polarity transition

A paleomagnetic record of the geomagnetic field during its change of polarity from the reversed Matuyama epoch to the normal Brunhes epoch has been obtained from sediments of ancient Lake Tecopa in southeastern California. The polarity switch occurs in siltstone of uniform composition, and anhysteretic magnetization experiments indicate that the magnetic mineralogy does not change markedly across the transition. Within the transition interval, intensity of the magnetization drops to a minimum of 10% of the intensity after the transition. The interval of low field intensity preceded and lasted longer than the interval during which the field direction reversed, the latter being shorter than the interval of low intensity by a factor of at least 2.5. The VGP's make a smooth transit from reversed to normal polarity, with the path lying in the sector of longitude between 30°E and 60°W. Pole paths for the Brunhes-Matuyama transition recorded in California and Japan are completely different, indicating that the dipole field decayed. The transition field appears to be nondipolar, and there is no evidence for an equatorial component. Since there is little dispersion of the VGP's about a great circle path, it is possible that large-scale drift of the nondipole field ceased during this polarity transition.     

Paleomagnetic constraints on Miocene rotation in the central Japan Arc

We present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning‐induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual‐polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α₉₅ = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A₉₅ = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.     

The geomagnetic field at the Proterozoic-Paleozoic boundary and lower-mantle plumes

Data on the amplitude of variations in the direction of the geomagnetic field and the frequency of reversals in the Vendian-Cambrian are presented. It has been established from these data that (a) distributions of variations in the direction of the geomagnetic field S _p are bimodal (modes 9° and 11°); (b) the maximum of the average amplitude S _p takes place by 5–10 Myr later than the Vendian-Cambrian boundary; (c) S _p tends to increase as plume epicenters are approached; and (d) the plume formation is more often confined to intervals with different frequencies of geomagnetic reversals than to the interval of a stable state of the geomagnetic field without reversals (Vendian hyperchron). The listed features of the geomagnetic field behavior are repeated near all boundaries of geological eras of the Phanerozoic.     

Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

Dependence of time derivative of horizontal geomagnetic field on sunspot number and aa index

This work investigated an interrelationship between the monthly means of time derivatives of horizontal geomagnetic field, dH/dt, sunspot number, R _z , and aa index for the period of substorms (from ?90 to ?1800 nT) during the years 1990–2009. A total of 232 substorms were identified during the period of study. The time derivative of horizontal geomagnetic field, dH/dt, used as a proxy for geomagnetically induced current (GIC) exhibited high positive correlation with sunspot number (0.86) and aa index (0.8998). The obtained geomagnetic activity is in 92.665% explicable by the combined effect of sunspot number and aa index. The distribution of substorms as a function of years gives a strong support for the existence of geomagnetic activity increases, which implies that as the sunspot number increases the base level of geomagnetic activity increases too.     

Solar flare effect on the geomagnetic field and ionosphere

This paper studies the ionospheric and geomagnetic response to an X6.2 solar flare recorded at 14:30 UT on December 13, 2001, in quiet geomagnetic conditions which allow the variations in the geomagnetic field and ionosphere measurements to be easily related to the solar flare radiation.By using measurements from the global positioning system (GPS) and geomagnetic observatories, the temporal evolution of ionospheric total electron content variation, vTEC_V, and geomagnetic field variations, δB, as well as their rates of variation, were obtained around the subsolar point at different solar zenith angles. The enhancement of both parameters was recorded one to three minutes later than the Geostationary Operational Environmental Satellite (GOES) programme recording; such delay tends to depend on the latitude, longitude, and solar zenith angle of the observatory's observations.The vTEC_V is related to the local time and the δB to the intensity and position of the ionospheric currents.The vTEC_V′s maximum value is always recorded later than the maximum values reached by δB and the X-ray intensity. The maximum δB is larger in the local morning than in the afternoon.The rates of vTEC_V and δB have two maximum values at the same time as the maximum values recorded by Hα (for each ribbon).This work shows the quantitative and qualitative relations between a solar flare and the ionospheric and geomagnetic variations that it produces.