Geomagnetic Signals Research Articles

Natural magnetic disturbance fields, not precursors, preceding the Loma Prieta earthquake

Available records of the magnetic indices Dst and ap together with standard observatory recordings of 1‐min field levels were examined for the period preceding the earthquake of October 1989, centered near Loma Prieta, California. The magnetic records showed that the Fraser‐Smith et al. (1990) report claiming the existence of a 100‐s (ultralow frequency) geomagnetic field precursor signal at Corralitos, California, foretelling a nearby earthquake is not valid. My study shows that the Stanford ULF signal was not local but rather widespread throughout the western United States and, therefore, expected to be due to a coincidental geomagnetic solar‐terrestrial disturbance field.

UNDERSTANDING THE GEOMAGNETIC PRECURSOR OF THE SOLAR CYCLE

Geomagnetic activity late in the sunspot cycle has been used successfully to forecast the amplitude of the following cycle. This success is somewhat surprising, however, because the recurrent high-speed wind streams that trigger the activity are not proxies of the Sun's polar fields, whose strength is a critical factor in many solar dynamo models. Instead, recurrent geomagnetic activity signals increases in the Sun's equatorial dipole moment, which decays on the ~1-2 yr timescale of the surface meridional flow and does not survive into the next cycle. In accordance with the original empirical method of Ohl, we therefore argue that solar cycle predictions should be based on the minimum level of geomagnetic activity, which is determined by the Sun's axial dipole strength, not on the peak activity during the declining phase of the cycle. On physical grounds, we suggest that an even better indicator would be the total open flux (or strength of the radial interplanetary field component) at sunspot minimum, which in turn can be derived from the historical aa index by removing the contribution of the solar wind speed. This predictor yields a peak yearly sunspot number R max = 97 ± 25 for solar cycle 24.

Denoising gravity and geomagnetic signals from Etna volcano (Italy) using multivariate methods

Abstract. Multivariate methods were applied to denoise the gravity and geomagnetic signals continuously recorded by the permanent monitoring networks on the Etna volcano. Gravity and geomagnetic signals observed in volcanic areas are severely influenced by meteorological variables (i.e. pressure, temperature and humidity), whose disturbances can make the detection of volcanic source effects more difficult. For volcano monitoring it is necessary, therefore, to reduce the effects of these perturbations. To date filtering noise is a very complex problem since the spectrum of each noise component has wide intervals of superposition and, some times, traditional filtering techniques provide unsatisfactory results. We propose the application of two different approaches, the adaptive neuro-fuzzy inference system (ANFIS) and the Independent Component Analysis (ICA) to remove noise effects from gravity and geomagnetic time series. Results suggest a good efficiency of the two proposed approaches since they are capable of finding and effectively representing the underlying factors or sources, and allow local features of the signal to be detected.

Performance of PAC1-R Heterozygous Mice in Memory Tasks-II

An involvement of pituitary adenylate cyclase activating polypeptide-specific receptor type 1 (PAC1-R) in behavioral performance of retrieval of memory upon the stimulation of perception of sensory modality was investigated in mice. Eighteen heterozygous (-/+) transgenic mutant PAC1-R-deficient mice ((-/+) mice) and 18 wild-type (+/+) mice of littermates ((+/+) mice) as a control were used. No homozygous (-/-) transgenic mutant PAC1-R-deficit mouse was bred in our colony. It was observed that; (1) changing a single alley in the multiple mazes interfered with retrieval of memory in both (-/+) mice and (+/+) mice, and they made considerable errors; (2) a rotation of the multiple mazes for 180 degrees in relation to the geomagnetic field made also considerable errors, but in only (+/+) mice and not in (-/+) mice; and (3) an exposure for perception of fear signals made also considerable errors, but in only (+/+) mice and not in (-/+) mice. It is inferred that PAC1-R may play a gating role in passage permitting through perception of geomagnetic orientation and fear signals, but not perception of visual orientation, and it regulates behavioral performance in retrieval of memory in mice.

Geomagnetic perturbations on stratospheric circulation in late winter and spring

This study investigates if the descent of odd nitrogen, generated in the thermosphere and the upper mesosphere by energetic particle precipitation (EPP‐NOx), has a detectable impact on stratospheric wind and temperature in late winter and spring presumably through the loss of ozone and reduction of absorption of solar UV. In both hemispheres, similar downward propagating geomagnetic signals in the extratropical stratosphere are found in spring for those years when no stratospheric sudden warming occurred in mid‐winter. Anomalous easterly winds and warmer polar regions are found when the 4‐month averaged winter Ap index (Ap) is high, and the signals become clearer when solar F10.7 is low. In May, significant geomagnetic signals are obtained in the Northern Hemisphere when the data are grouped according to the phase of the stratospheric equatorial QBO. The magnitudes of changes in spring stratospheric wind and temperatures associated with Ap signals are in the range of 10–20 m s−1 and 5–10 K, which are comparable with those of the 11‐yr SC signals typically found in late winter. The spring Ap signals show the opposite sign to that expected due to in situ cooling effects caused by catalytic destruction of stratospheric ozone by descending EPP‐NOx. Thus it is unlikely that the in situ chemical effect of descending EPP‐NOx on stratospheric ozone would have a dominant influence on stratospheric circulation. Instead, we suggest that the detected Ap signals in the extratropical spring stratosphere may be an indirect consequence of geomagnetic and solar activity, dynamically induced by changes in wave ducting conditions.

Are changes of the geomagnetic field intensity related to changes of the tropical Pacific sea-level pressure during the last 50 years?

[1] The influence of solar variability into the lower atmospheric regions has been suggested on different atmospheric parameters in different time scales. However, a plausible mechanism to explain these observations remains unclear. Although it is widely accepted that the climate change over the past 50 years is attributed to human influence, we present the case that local climate change in the tropical Pacific may be due to changes in the Earth's magnetic field strength. The changes in the tropical Pacific circulation have been observed during the last 50 years, and they are attributed to the increase of the global surface temperature. However, a geomagnetic modulation of the net radiative flux in the southern tropical Pacific was recently suggested. Moreover, comparisons of long-term reconstructions of the Northern Hemisphere surface temperature and solar activity proxies indicated that the existence of a geomagnetic signal in climate data would support a direct link between solar variability and their effects on climate. Here we show that in the tropical Pacific the sea-level pressure, which is a component of the Walker circulation, could be related to the magnetospheric, ionospheric, and upper-atmosphere processes which may propagate downward to the lower atmosphere. Furthermore, we show that the changes in sea-level pressure and the Walker circulation are correlated to the westward drift of the magnetic anomaly. We compare the region averaged monthly values of the sea-level pressure in the tropical Pacific with those of the magnetic field intensity near the surface for the last 50 years. We find that the sea-level pressure in the tropical Pacific is increasing as the magnetic field intensity is decreasing. The correlation coefficient of the sea-level pressure 36-month running means versus the magnetic field intensity is 0.96. We anticipate our investigation to be a starting point for a more sophisticated investigation of the coupling between the space weather processes and lower atmosphere and ocean dynamics.

Time variant analysis of geomagnetic signals describes the volcanic activity of Mt. Etna between early 2000 and late 2002

Volcanomagnetic anomalies have been mostly observed during strong eruptions. Our aim is to improve the geomagnetic data analysis to evidence the anomalies occurring in a larger time span, especially in the phases preceding the eruptive events. We developed a time variant statistical approach and applied it to the 2000–2002 Etna geomagnetic temporal series. It is based on an algorithm that statistically predicts the geomagnetic field at the station on the volcanic edifice by that recorded at the remote one. In such a way a number of significant changes in the time series (called statistical innovations), marking the local magnetic field change, were detected. The distribution of such statistical innovations accurately describes the Etna volcanic evolution: we note a progressive increase of the innovation occurrence as the eruptive cycles were approaching and only few and weak innovations at times between the various eruptive cycles. The significance of this analysis is further confirmed by the close agreement among the mean square prediction error, strain release and the volcanic activity behavior. On the contrary, the geomagnetic field at a single station or its difference at two stations do not have any clear correlation with other measured physical quantities. The complex pattern of the prediction error was also investigated by a multifractal analysis. We found that the Holder regularity increases with the intensification of the volcanic activity, implying that innovations tend to be less sporadic and correlated during the major volcanic phases.

Investigating non-uniform scaling behavior in Ultra Low Frequency (ULF) earthquake-related geomagnetic signals

The scaling properties of the time dynamics of 2000–2001 ULF geomagnetic data (in the frequency range between 1 mHz and 10 Hz) observed at Izu Peninsula in Japan were investigated. On the basis of detrended fluctuation analysis (DFA), which is a powerful method to detect scaling in non-stationary time series, deviations from uniform power-law scaling were identified and quantified. Our results suggest that a significant non-uniform scaling behavior in ULF geomagnetic data could be related to the occurrence of intense seismic clusters. These first results could be useful in the framework of short-term earthquake prediction.

Magnetorheological model of lithogenetic structures in the pelagic region of the Pacific Ocean: Rheology of the Tucker-Khramov zone

Based on fundamental laws of physicochemical mechanics of disperse structures, a magnetorheological model of lithogenetic structures in open (far from land) regions of the Pacific Ocean is constructed. Special attention is given to the magnetization mechanism of bottom sediments below the water-seafloor interface. This process is most active in the so-called Tucker-Khramov zone, where strength properties of a suspension are either unobservable or weak. A quantitative model describing both the filtering (smoothing) of a geomagnetic signal and its time lag in the rheological Tucker-Khramov zone is proposed for sedimentary rocks of various water contents. In the lower part of the zone, the sediment is a thixotropic solidifying structure in which the spatial orientation of magnetic dipoles is fixed due to an increase in the strength properties of the soil.

Non-uniform scaling behavior in ultra-low-frequency (ULF) earthquake-related geomagnetic signals

Investigating highly non-stationary time series, which typically exhibit long-range correlations, is a classic problem in physics. Here, we analyze the scaling properties of the dynamics of ultra-low-frequency (ULF) geomagnetic data (in the frequency range between 1 mHz and 10 Hz) observed at Izu Peninsula in Japan. On the basis of the detrended fluctuation analysis (DFA), deviations from uniform power-law scaling were quantified. Our findings point out to a significant non-uniform scaling behavior in ULF geomagnetic data in relationship with the occurrence of intense seismic clusters.

Disentangling Geomagnetic and Precipitation Signals in an 80-kyr Chinese Loess Record of10Be

The cosmogenic radionuclide10Be is produced by cosmic-ray spallation in Earth's atmosphere. Its production rate is regulated by the geomagnetic field intensity, so that its accumulation rate in aeolian sediments can, in principle, be used to derive high-resolution records of geomagnetic field changes. However,10Be atmospheric fallout rate also varies locally depending on rainfall rate. The accumulation rate of10Be in sediments is further complicated by overprinting of the geomagnetic and precipitation signals by10Be attached to remobilized dust, which fell from the atmosphere at some time in the past. Here, we demonstrate that these signals can be deconvoluted to derive both geomagnetic field intensity and paleoprecipitation records of Asian Monsoon intensity in an 80,000-yr-long10Be record from Chinese loess. The strong similarity between our derived paleomagnetic intensity record and the SINT 200 (Guyodo and Valet 1996) and NAPIS 75 (Laj et al. 2002) stacked-marine records suggests that this method might be used to produce multimillion-yr-long records of paleomagnetic intensity from loess. This technique also reveals a new method for extracting quantitative paleoprecipitation records from continental interior regions. Our derived precipitation record is broadly similar to the speleothem δ18O-based records of paleo-Asian Monsoon intensity from Dongge (Yuan et al. 2004) and Hulu (Wang et al. 2001) caves, and suggests that the paleo-Asian Monsoon intensity may be responding to a combination of both Northern and Southern Hemisphere insolation forcing.

Watch out for aurorae

From Alastair McBeath The articles on the aurorae in A&G at the start of this year made interesting reading for those of us with a long-time interest in the aurora, and observing it from the UK. My own first auroral sightings from Britain were made by chance in 1978, at the geographic latitude I still observe from, about halfway up the British Isles, around 55°N in south-east Northumberland. In the 28 full years since, 104 visible displays have come my way, with anything from zero to 14 nights of aurorae per calendar year, 102 of those since I first started making regular checks of the sky every clearer night, in 1982. Consequently, I was surprised by some of Jim Wild’s pessimistic comments (Wild 2006) regarding the supposedly poor prospects for UK auroral observers generally. My site is pretty average both for overnight weather, and its location within the UK. Yes, observers further north than me do see significantly more aurorae than I do, but with a long-term annual mean of 3.7 nights of aurorae here, this seems rather better than Wild’s comment would suggest, that “on average we can expect to see visible aurorae over the UK a couple of times per year”. In 2005, I spotted definite displays on five nights, but I achieved this with no technological prompting. As experience from Britain has demonstrated repeatedly over the decades, strong geomagnetic or radio auroral signals frequently do not translate into visible aurorae, nor do visible aurorae always give strong instrumental signatures. As an example to compare with Wild’s, the 21/22 January 2005 auroral event was at its very best for the entire night visually for me between 19.00 and 19.30 UT. At its strongest, at 19.19 h, the auroral light was casting shadows, and the display’s southern edge passed through the zenith. Between 19.20 h and 19.26 h, the southern edge was within 60° of the southern horizon, so the display covered two-thirds of the sky. As Jim Wild showed, geomagnetically, there was at the time only a moderate chance of UK activity as recorded by his AuroraWatch system. After 19.30 h, the aurora remained confined to my northern sky for the rest of the night through to 02.20 h, when clouds halted observing. There were a few brighter interludes, notably around 22.15 h very briefly, and more sustainedly from 22.25 h to 22.50 h (at 22.35 h the display reached the zenith again momentarily). This coincided with the AuroraWatch’s strongest response for the whole of 21 January, though visually the display was far less impressive than it had been soon after 19.00 h. By 23.00 h, only a feeble glow within 30° of my northern horizon remained, growing weaker and lower as the night continued, yet the Aurora Watch geomagnetic response was only marginally lower from 23.00–00.00 h than it had been from 19.00–20.00 h. While geomagnetic observations of the aurora are valuable in their own right, I would not recommend anyone should rely on them solely who genuinely wanted to observe the visible aurora. The best method for seeing auroral displays remains to check the northern sky regularly after dark, knowing what to look out for, and what that northern sky should look like under different conditions with no aurora present. There does seem to be a general lack of awareness of just how much detailed visual and photographic auroral observing and recording has been, and continues to be, carried out in the British Isles, especially since the International Geophysical Year in 1957–58, from the Society’s recent journals (witness also Harrison’s historical aurora comments from 1771–1805 [2005], who seemed to struggle to find suitable recent comparison data from the Stroud area, though there is no shortage). Consequently, it seems sensible to mention here that further observing advice for prospective UK aurora-watchers, and details of auroral events from recent decades, are available from the Aurora Sections of the Society for Popular Astronomy (www. popastro.com) and the British Astronomical Association (baaaurora.fsnet.co.uk). Alastair McBeath, Morpeth, Northumberland. meteor@popastro.com

An investigation of the 1/fα long-range fluctuations in short-term time variability of ULF geomagnetic data

Time-scaling properties of ULF geomagnetic signals are analysed by means of the power spectral density, Higuchi method and Hurst R/S analysis. We analyse the three ULF geomagnetic components (two horizontal and one vertical) measured by one station installed in Crete, Greece, one of the most seismically active areas of the Mediterranean Region. The range of time-scales investigated varies from minutes to weeks. It is shown that two time-scaling regimes mainly characterize the temporal fluctuations of the signals. Our results suggest the presence of fractal features expressing long-range time correlation with scaling coefficients, which are the clue of persistent mechanisms.

Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modelling

Many geomagnetic excursions have been documented during the Brunhes Chron. However, high-resolution paleomagnetic studies of sediments often provide evidence of few (if any) excursions. We have investigated the reasons for this observation by modelling the post-depositional remanent magnetization (PDRM) lock-in process. A cubic lock-in function produces more rapid lock-in compared to linear and exponential functions proposed in the literature. We therefore used a cubic function to model “best-case” scenarios for the quality of the paleomagnetic record when a high-frequency geomagnetic input signal is convolved with the sediment lock-in function for a wide range of sedimentation rates. Even for a lock-in depth of 10 cm, where 95% of the PDRM is locked in 5 cm below the surface mixed layer, an input signal containing abundant excursions (with 1-kyr duration) is smoothed so drastically at low sedimentation rates (1–3 cm/kyr) that no excursions are recorded. Excursions are further attenuated by increasing the lock-in depth, as well as by sampling at discrete stratigraphic intervals. The PDRM process acts as a low-pass filter for high-frequency secular variation signals even at moderate sedimentation rates (>10 cm/kyr). We have also modelled the effects of modulation of sedimentation rate by a climatic forcing function (e.g., insolation). Even though the average sedimentation rate might be substantial, PDRM recording becomes much less reliable in intervals where sedimentation rates are lowest. In these cases, some excursions are reliably recorded in detail, while others may not be recorded at all. Our modelling suggests that in order to consistently detect the presence of geomagnetic excursions, it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm/kyr. If the normal spectrum of geomagnetic field behaviour contains abundant excursions, failure to document excursions in many apparently high-resolution analyses probably results from PDRM lock-in within relatively slowly deposited sediments, or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates.

Some aspects of the geomagnetic response to solar wind pressure variations: a case study at low and middle latitudes

Abstract. We examined geomagnetic field observations at low and middle latitudes in the Northern Hemisphere during a 50-min interval (12 May 1999), characterized by a complex behaviour of the solar wind dynamic pressure. For the entire interval, the aspects of the geomagnetic response can be organized into four groups of events which show common characteristics for the H and D components, respectively. The correspondence between the magnetospheric field and the ground components reveals different aspects of the geomagnetic response in different magnetic local time (MLT) sectors. For the H component, the correspondence is highly significant in the dusk and night sectors; in the dawn and prenoon sectors it shows a dramatic change across a separation line that extends approximately between (6 MLT, 35°) and (13 MLT, 60°). For the D component, the correspondence has significant values in the dawn and prenoon regions. We propose a new approach to the experimental data analysis which reveals that, at each station, the magnetospheric field has a close correspondence with the geomagnetic field projection along an axis (M1) that progressively rotates from north/south (night events) to east/west orientation (dawn events). When projected along M1, the geomagnetic signals can be interpreted in terms of a one-dimensional pattern that mostly reflects the field behaviour observed at geostationary orbit. Several features appear more evident in this perspective, and the global geomagnetic response to the SW pressure variations appears much clearer than in other representations. In particular, the MLT dependence of the geomagnetic response is much smaller than that one estimated by previous investigations. A clear latitudinal dependence emerges in the dusk sector. The occurrence of low frequency waves at ~2.8mHz can be interpreted in terms of global magnetospheric modes driven by the SW pulse. This event occurred in the recovery phase after the day the SW almost disappeared (11 May 1999): in this sense our results suggest a rapid recovery of almost typical magnetospheric conditions soon after a huge expansion. Overshoot amplitudes, greater than in other cases, are consistent with a significant reduction of the ring current.

Assessing the dipolar signal in stacked paleointensity records using a statistical error model and geodynamo simulations

Stacks of globally distributed relative paleointensity records from sediment cores are used to study temporal variations in the strength of the geomagnetic dipole. We assess the intrinsic accuracy and resolution of such stacks, which may be limited by errors in paleointensity, non-dipole field contributions, and the age scales assigned to each sediment core. Our approach employs two types of simulations. Numerical geodynamo models generate accurate predictions of time series of magnetic variations anywhere in the world. The predicted variations are then degraded using an appropriate statistical model to simulate expected age and paleointensity errors. A series of experiments identify the major contributors to error and loss of resolution in the resulting stacks. The statistical model simulates rock magnetic and measurement errors in paleointensity, and age errors due to finite sampling and approximations inherent in interpolation, incomplete or inaccurate tie point information, and sedimentation rate variations. Data sampling and interpolation to a designated age scale cause substantial decorrelation, and control the maximum level of agreement attainable between completely accurate records. The particular method of interpolation appears to have little effect on the coherence between accurate records, but denser tie point data improve the agreement. Age errors decorrelate geomagnetic signals, usually at shorter periods, although they can destroy coherence over a broad range of periods. The poor correlation between neighboring paleomagnetic records often observed in real data can be accounted for by age errors of moderate magnitude. In a global dataset of 20 records, modeled after the SINT800 compilation and spanning 300 kyr, our results show that dipole variations with periods longer than about 20 kyr can be recovered by the stacking process. Reasonable contributions to error in the paleointensity itself have a modest influence on the result, as do non-dipole field contributions whose effect is minor at periods longer than 10 kyr. Modest errors in the ages of tie points probably account for most of the degradation in geomagnetic signal. Stacked sedimentary paleomagnetic records can be improved by denser temporal sampling and careful selection of independent high-quality tie points.

Spatiotemporal response of a conducting sphere under simulated geomagnetic storm conditions

A theoretical study of satellite-based geomagnetic induction signals from uniform and heterogeneous spheres is undertaken using existing analytic and semi-analytic harmonic solutions. Transient responses to ring current excitations are computed using inverse Fourier and Laplace methods. A wide range of simulated storm excitations from impulse to step-on are considered. The early-time radial and late-time polar responses are diagnostic of the electrical conductivity of the sphere. The early-time diagnostic is likely more useful because of better signal to noise characteristics. Induction signals from deeply buried heterogeneities are likely to be contained in the late-time responses and their extraction will require new statistical approaches that average over many storms and incorporate multi-satellite and ground-based observations. The spatiotemporal induction response shows an interesting crossover during which time the rapidly-evolving polar component starts to decays while the slowly-evolving radial component continues to grow. At the crossover time, the intensity of the field is independent of colatitude, an effect which could be sought in satellite induction data.

Geomagnetic episodes of the last 1.2 Myr recorded in Chinese loess

The Matuyama‐Brunhes (M/B) boundary, termination of Jaramillo normal subchron (J/M) as well as three young geomagnetic excursions has been revealed in the Weinan loess section [Zhu et al., 1994,1998,1999]. Recently, we carried out further paleomagnetic directional studies on the same section and recovered the onset of the Jaramillo normal subchron (M/J) and four geomagnetic excursions, two in the Brunhes and two in the late Matuyama. The fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short‐lived geomagnetic signals could be only recorded in loess units, and may have been fully smoothed or wiped out in paleosol units by strong pedogenesis. Constrained by a new astronomical timescale (HLD00), ages of the geomagnetic excursions and reversal boundaries have been estimated. We tentatively correlated these excursions with previous results, indicating that a large lock‐in model may not be correct for the whole loess profile, although some apparently big positive age discrepancies were observed for the main boundaries.

The bispectral analysis in geomagnetic micro pulsation signal

Based on the fact that the geomagnetic micro pulsation signal behave as a non Gaussian random process in most cases, a signal processing method using higher order statistics is proposed to analyze the geomagnetic micro pulsation signal. Deriving the bispectrum estimated via indirect method, then reconstructing the power spectrum and the signal itself based on it. Results by using an example show that comparing with the other traditional analytical ways, the bispectral analysis method is superior.

A metrologic method of anomaly field amplitude bottom reduction in undersampled geomagnetic marine surveys

We show the results obtained by means of a seabed reduction technique on the intensity of geomagnetic anomaly fields applied to a synthetic case and then to the real case of a geomagnetic survey of eastern Ligurian Sea (Italy). The eastern Ligurian Sea has very intense short waves anomaly fields and a sea bed that varies greatly in depth. As a result the geomagnetic space signal is characterized by a very large spectral content; in these conditions it is not possible to obtain a full sampled marine survey and vertical continuation analytic procedures and standard numerical bottom reduction based on a single vertical incremental parameter, whichever is applicable, fails to give accurate results. The present technique, which has been fine-tuned over 4 years of experimentation in environmental researchs, aims to provide a simple and efficient means to reduce the distortion of geomagnetic anomalies field caused by the variation of distance between survey plane and magnetic outcrop source position. The compensation procedure is based on evaluation, by comparison of two measurements carried out at different altitudes, of the mean vertical increment typical of each anomaly field principal frequency component bands. The component anomaly fields are then corrected by application of the corresponding vertical increments and lastly, the anomaly geomagnetic field reduced to the sea-bed is computed as Inverse Fourier Transform of a spectrum built as synthesis of the component anomaly fields' spectra. The results obtained have shown a notable increase in definition of anomaly field intensity without the production of appreciable distortions or false geomagnetic echoes.