GP33A-01 INVITED 13:40h
Examining Geomagnetic Jerks Using Comprehensive Magnetic Field Models
The latest comprehensive magnetic field model, CM4, spans 1960-2002, and as a result, traverses several instances of a phenomenon known as a geomagnetic jerk. These are generally accepted as worldwide events of internal origin, which have been theorized by some to be jumps in acceleration of the fluid motion at the core-mantle boundary or created by torsional oscillations in the core. They are usually manifested in the magnetic record as sudden changes in slope of otherwise linear segments in the secular variation of the field. It is speculated, however, that certain jerks may not be detected over the entire globe and may even be influenced by external signals. This is intriguing: CM4 provides not only a global description of the evolution of the field through time, but also attempts to separate the internal from the external signals, even the induced from the core, during quiet-time conditions. This suggests that the model is well suited to test such claims, and this talk will report on progress towards this end.
GP33A-02 INVITED 13:55h
Quiet Time Contributions to the Geomagnetic Field as Seen in CHAMP, Oersted and SAC-C Satellite Magnetic Measurements
The continuing satellite magnetic missions CHAMP, {\O}rsted and SAC-C enable us to model the internal geomagnetic field, its secular variation and acceleration to steadily increasing spherical harmonic degrees. Without regularization, the present limits stand at degree 80 for the static and degree 12 for the temporal derivatives. In order to reach and extend these limits, even minor contributions to the magnetic field have to be taken into account. These contributions from the ocean tides, ionospheric currents, ambient plasma, and the magnetosphere are interesting as such and will be discussed and illustrated here. For the latest NGDC/GFZ main field models, CHAMP data are first corrected for a time varying mis-alignment of the vector magnetometer orientation. Accounting for the magnetic pressure of the ambient plasma, the diamagnetic effect is estimated from the electron density and temperature given by CHAMP's Langmuir Probe. Ocean tidal fields are accounted for using the predictions by Kuvshinov of the induced electromagnetic signal, based on the TPXO tidal ocean flow model. External and induced ring current fields are represented by the new Est/Ist index pair. These indices result from splitting the magnetic disturbance field index Dst into an external and internal part for a 1D conductivity model of the Earth. The final model is estimated in two steps: First, a stable ring current field in SM and a degree-2 magnetospheric field in GSM coordinates are determined from a combined satellite data set with 24h local time coverage. After subtracting this external field, the internal field and its temporal derivatives are estimated from night side data only.
GP33A-03 INVITED 14:10h
Multi-satellite Observations of Ionospheric and Field-aligned Currents in the Polar Ionosphere.
Traditionally field-aligned currents in the polar region have been studied using data from only a single satellite but on a statistical basis. Some aspects can however only be investigated by the multi-satellite approach. We review recent work on multi-satellite observations of ionospheric and field-aligned currents in the polar region, discussing the advantages of the multi-satellite approach over a single satellite investigation. The validity of the assumptions normally applied in the single satellite approach is investigated, as well as the possibility to separate the effect of field-aligned and horizontal ionospheric currents in the satellite data.
GP33A-04 INVITED 14:25h
Swarm- The Earth's Magnetic Field and Environment Explorers
Swarm is the fifth Earth Explorer mission in ESA's Living Planet Programme. The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth's interior and climate. The mission is scheduled for launch in 2009. After release from a single launcher, a side-by-side flying lower pair of satellites at an initial altitude of 450 km and a single higher satellite at 530 km will form the Swarm constellation. High-precision and high-resolution measurements of the strength, direction and variation of the magnetic field, complemented by precise navigation, accelerometer and electric field measurements, will provide the necessary observations that are required to separate and model various sources of the geomagnetic field. The constellation design and performance analysis will get special attention during the presentation. Swarm analysis results in a unique "view" inside the Earth from space to study the composition and processes in the interior. It also allows analysing the Sun's influence within the Earth system. In addition practical applications in many different areas, such as space weather, radiation hazards, navigation and resource exploration, benefit from the Swarm concept.
http://www.esa.int/esaLP/swarm.html
GP33A-05 14:40h
Using the TIEGCM to Examine the Effects of Gravity and Plasma Pressure on Magnetic Perturbations at low Latitudes
We use the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General-Circulation Model (TIEGCM) to calculate the electric currents driven by gravity and plasma pressure gradients. Using spherical-harmonic analysis of the three-dimensional current system we determine the magnetic perturbations at the ground, within and above the ionosphere. In regions of enhanced plasma density like the equatorial anomaly the influence of currents driven by gravity and plasma pressure gradients on the magnetic perturbations is most pronounced. Using measurements from the CHAMP satellite L\"uhr et al. [2003] were able to quantify for the first time the size of the diamagnetic effect associated with plasma pressure on the magnetic perturbations, which can be up to 5 nanoteslas We find that the gravity driven current can cause magnetic perturbations comparable in size with those from the diamagnetic effect. Depending on the altitude and location the magnetic perturbations due to plasma pressure gradients and gravity can sometimes tend to cancel. We will examine the strength of the two contributions with altitude but also study the longitudinal and local time variability. In addition we expect changes over the solar cycle in the magnetic perturbations due the two current sources.
GP33A-06 14:55h
Toward remote sensing of ocean variability using ocean generated magnetic fields
The ocean is an electrically conducting fluid moving through the earth's background magnetic field. Through magnetohydrodynamic interaction between the flow and the magnetic field, secondary electric and magnetic fields are generated which reach far outside of the ocean. The secondary magnetic fields can be used in principle to remotely monitor ocean flow, temperature, and salinity variations. Here we review recent developments in assessing this potential. Theory and simulations show that the ocean generated magnetic fields at low satellite altitudes can be easily converted into information about large-scale flow transport variability which is very important in ocean and climate studies. The primary challenge is in extracting the relatively small (typically less than 10 nT) ocean signals from the magnetic field records which are also influenced by a variety of other sources. We show examples (both in principle and practice) where oceanic magnetic signals are extracted from the magnetic records by using statistical constraints imposed by the behavior of the flow sources. Finally, we discuss paths toward assessing the practical potential of this remote sensing method in light of present and up-coming magnetic surveys such as CHAMP, OERSTED, SAC-C, and SWARM.
GP33A-07 15:10h
Experimental and theoretical evaluation of magnetic field variations caused by ocean currents
The last few years have seen a greatly increased exploratory activity on possible use of magnetic field variations, observed at sea surface and at satellite altitude, as a source of information on ocean currents. Recent theoretical studies reported values up to 4 nanotesla (nT) for the field magnitude at the sea surface, which translates into 1 to 2 nT at the satellite altitudes 400 to 500 km. We now demonstrate that a more accurate mathematical formulation and numerical solution of the problem yields values three times as high as previously thought. This signal is strong enough to be detectable at either sea surface or satellite altitude by existing magnetometers. However, our analysis of data from island magnetometer observatories and from the Champ satellite indicates that the ocean-induced signal is not easy to discern from a noisy background of other factors (such as ionospheric noise, etc.). The only ocean area where the ocean-induced variations appear to be detectable in observations (using relatively simple means of data analysis) is an ACC region between approximately 90 and 190 deg E. Our results are presented along with the novel data analysis techniques developed for this study.
GP33A-08 15:25h
On the Resolution of the Magnetic Field at the Core-Mantle Boundary
A fundamental difficulty in mapping the magnetic field at the core-mantle boundary is the uncertain spatial resolution of the field there from surface and satellite data. Since Magsat in 1980, and quite possibly for earlier times too, spatial resolution has been limited not by data quality but instead by the presence of the crustal field which obscures the field from the core above some spherical harmonic degree. Unfortunately, where this switch-over occurs is poorly understood, but is most probably somewhere in the range from degree 12 to 16. Magnetic field maps with power only to degree 12 look very different from maps with power to degree 16. Here we discuss possible approaches to understanding where this switch-over occurs. Two possible approaches are first to attempt to understand the crustal field by forward modelling; a second is to test the consistency of core field maps with a simple, physical model of core dynamics. Using the second approach we argue that maps of the field at the core-mantle boundary produced over the last two decades are overly conservative, and that it is probable that we can resolve more detail in the core field than previously supposed.