Magnetic perturbations seen by CHAMP and evaluated using the TIE-GCM

International audienceThe Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) is a self-consistent, global, atmospheric model that can be used to estimate magnetic perturbations at satellite altitude. These computed perturbations can then be compared with the magnetic vector data provided by low-earth orbiting satellites. In this initial study, the quietest day of each month from 2001?2005 was selected for comparison. CHAMP magnetic vector residuals were computed for these intervals using the CHAOS model to remove core and crustal geomagnetic contributions. Under various input parameters, the TIE-GCM predictions were compared with the CHAMP residuals on an orbit by orbit basis. Initial results demonstrate a reasonable agreement between the TIE-GCM estimates and the CHAMP residuals in non-polar, dayside regions (±50° magnetic latitude) where both are able to resolve the Equatorial Electro-Jet (EEJ) and solar quiet (Sq) current systems. Although no clear component or temporal correlation was discerned, evidence showing the decrease in residual comparisons presents the possibility of using the TIE-GCM to pre-process geomagnetic data for main field modeling purposes

Contribution to solving the orientation problem for an automatic magnetic observatory

The problem of the absolute calibration of a vectorial (tri-axial) magnetometer is addressed with the objective that the apparatus, once calibrated, gives afterwards, for a few years, the absolute values of the three components of the geomagnetic field (say the Northern geographical component, Eastern component and vertical component) with an accuracy on the order of 1 nT. The calibration procedure comes down to measure the orientation in space of the three physical axes of the sensor or, in other words, the entries of the transfer matrix from the local geographical axes to these physical axes. Absolute calibration follows indeed an internal calibration which provides accurate values of the three scale factors corresponding to the three axes – and in addition their relative angles. The absolute calibration can be achieved through classical absolute measurements made with an independent equipment. It is shown – after an error analysis which is not trivial – that, while it is not possible to get the axes absolute orientations with a high accuracy, the assigned objective (absolute values of the Northern geographical component, Eastern component and vertical component, with an accuracy of the order of 1 nT) is nevertheless reachable; this is because in the time interval of interest the field to measure is not far from the field prevailing during the calibration process

Wavelet-based directional analysis of the gravity field: evidence for large-scale undulations

International audienceIn the eighties, the analysis of satellite altimetry data leads to the major discovery of gravity lineations in the oceans, with wavelengths between 200 and 1400 km. While the existence of the 200 km scale undulations is widely accepted, undulations at scales larger than 400 km are still a matter of debate. In this paper, we revisit the topic of the large-scale geoid undulations over the oceans in the light of the satellite gravity data provided by the GRACE mission, considerably more precise than the altimetry data at wavelengths larger than 400 km. First, we develop a dedicated method of directional Poisson wavelet analysis on the sphere with significance testing, in order to detect and characterize directional structures in geophys-ical data on the sphere at different spatial scales. This method is particularly well suited for potential field analysis. We validate it on a series of synthetic tests, and then apply it to analyze recent gravity models, as well as a bathymetry data set independent from gravity. Our analysis confirms the existence of gravity undulations at large scale in the oceans, with characteristic scales between 600 and 2000 km. Their direction correlates well with present-day plate motion over the Pacific ocean, where they are particularly clear, and associated with a conjugate direction at 1500 km scale. A major finding is that the 2000 km scale geoid undulations dominate and had never been so clearly observed previously. This is due to the great precision of GRACE data at those wavelengths. Given the large scale of these undulations, they are most likely related to mantle processes. Taking into account observations and models from other geophysical information, as seismological tomography, convection and geochemical models and electrical conductivity in the mantle, we conceive that all these inputs indicate a directional fabric of the mantle flows at depth, reflecting how the history of subduction influences the organization of lower mantle upwellings

IAGA: a major role in understanding our magnetic planet

Throughout the International Union of Geodesy and Geophysics's (IUGG's) centennial anniversary, the International Association of Geomagnetism and Aeronomy is holding a series of activities to underline the ground-breaking facts in the area of geomagnetism and aeronomy. Over 100 years, the history of these research fields is rich, and here we present a short tour through some of the International Association of Geomagnetism and Aeronomy's (IAGA's) major achievements. Starting with the scientific landscape before IAGA, through its foundation until the present, we review the research and achievements considering its complexity and variability, from geodynamo up to the Sun and outer space. While a number of the achievements were accomplished with direct IAGA involvement, the others represent the most important benchmarks of geomagnetism and aeronomy studies. In summary, IAGA is an important and active association with a long and rich history and prospective future.</p

Geomagnetic jerks characterization via spectral analysis

In this study we have applied spectral techniques to analyze geomagnetic field time-series provided by observatories, and compared the results with those obtained from analogous analyses of synthetic data estimated from models. Then, an algorithm is here proposed to detect the geomagnetic jerks in time-series, mainly occurring in the eastern component of the geomagnetic field. Applying such analysis to time-series generated from global models has allowed us to depict the most important space-time features of the geomagnetic jerks all over the globe, since the beginning of XXth century. Finally, the spherical harmonic power spectrum of the third derivative of the main geomagnetic field has been computed from 1960 to 2002.5, bringing new insights to understand the spatial evolution of these rapid changes of the geomagnetic field

ESD Ideas: A 6-year oscillation in the whole Earth system?

An oscillation of about 6 years has been reported in Earth&rsquo;s fluid core motions, magnetic field, rotation, and crustal deformations. Recently, a 6-year cycle has also been detected in several climatic parameters (e.g., sea level, surface temperature, precipitation, land ice, land hydrology, and atmospheric angular momentum). Here we suggest that the 6-year oscillations detected in the Earth&rsquo;s deep interior, mantle rotation, and atmosphere are linked together, and that the core processes previously proposed as drivers of the 6-year cycle in the Earth&rsquo;s rotation, cause in addition the atmosphere to oscillate together with the mantle, inducing fluctuations in the climate system with similar periodicities.</p

From pre-storm activity to magnetic storms: a transition described in terms of fractal dynamics

International audienceWe show that distinct changes in scaling parameters of the Dst index time series occur as an intense magnetic storm approaches, revealing a gradual reduction in complexity. The remarkable acceleration of energy release ? manifested in the increase in susceptibility ? couples to the transition from anti-persistent (negative feedback) to persistent (positive feedback) behavior and indicates that the occurence of an intense magnetic storm is imminent. The main driver of the Dst index, the VBSouth electric field component, does not reveal a similar transition to persistency prior to the storm. This indicates that while the magnetosphere is mostly driven by the solar wind the critical feature of persistency in the magnetosphere is the result of a combination of solar wind and internal magnetospheric activity rather than solar wind variations alone. Our results suggest that the development of an intense magnetic storm can be studied in terms of "intermittent criticality" that is of a more general character than the classical self-organized criticality phenomena, implying the predictability of the magnetosphere

Searching for possible seismogenic signatures in ionosphere by an entropy-based analysis of magnetic satellite data: A case study.

The importance of detecting possible electromagnetic signatures due to large earthquakes is self-evident, signatures which can be either anticipating, simultaneous or subsequent with respect to the main shock. Taking advantage of the present low Earth’s orbiting CHAMP satellite, we apply an “ad hoc” technique based on the Information Theory, to the satellite magnetic data with the aim at extracting eventual time anomalies. This technique has small time-space resolution using a preliminary wavelet analysis in order to detect shorter-wavelength anomalies. Some examples are given for magnetic satellite data taken over periods including the times of two large earthquakes, one being the Sumatra region event on 26 December 2004 (M=9.1)

Entropy based analysis of satellite magnetic data for searching possible electromagnetic signatures due to big earthquakes

The importance of detecting possible electromagnetic signatures due to big earthquakes is self-evident, signatures which can be either anticipating, simultaneous or subsequent with respect to the main shock. Taking advantage of the present low Earth orbiting CHAMP satellite, we apply two “ad hoc” techniques both based on the Information Theory (after the seminal monograph by Shannon [1]) to the satellite magnetic data with the aim at extracting eventual time anomalies. These techniques have different time-space resolutions: the first technique requires a preliminary spherical harmonic analysis of daily magnetic data and, potentially, detects long-wavelength variations, while the second uses a preliminary wavelet analysis and can detect shorter-wavelength anomalies. Some examples are given for magnetic satellite data taken in correspondence with the two big earthquakes occurred in the Sumatra region on 26 December 2004 (M = 9.1) and 28 March 2005 (M = 8.6)

The SGR 1806-20 magnetar signature on the Earth's magnetic field

SGRs denote ``soft $\gamma$-ray repeaters'', a small class of slowly spinning neutron stars with strong magnetic fields. On 27 December 2004, a giant flare was detected from magnetar SGR 1806-20. The initial spike was followed by a hard-X-ray tail persisting for 380 s with a modulation period of 7.56 s. This event has received considerable attention, particularly in the astrophysics area. Its relevance to the geophysics community lies in the importance of investigating the effects of such an event on the near-earth electromagnetic environment. However, the signature of a magnetar flare on the geomagnetic field has not previously been investigated. Here, by applying wavelet analysis to the high-resolution magnetic data provided by the CHAMP satellite, a modulated signal with a period of 7.5 s over the duration of the giant flare appears in the observed data. Moreover, this event was detected by the energetic ion counters onboard the DEMETER satellite.Comment: Science Editors' Choice: http://www.sciencemag.org/content/vol314/issue5798/twil.dt