Magnetic field microscopy of rock samples using a giant magnetoresistance–based scanning magnetometer

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95412/1/ggge1634.pd

Evaluation of candidate geomagnetic field models for IGRF-12

Background: The 12th revision of the International Geomagnetic Reference Field (IGRF) was issued in December 2014 by the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group V-MOD (http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html). This revision comprises new spherical harmonic main field models for epochs 2010.0 (DGRF-2010) and 2015.0 (IGRF-2015) and predictive linear secular variation for the interval 2015.0-2020.0 (SV-2010-2015). Findings: The models were derived from weighted averages of candidate models submitted by ten international teams. Teams were led by the British Geological Survey (UK), DTU Space (Denmark), ISTerre (France), IZMIRAN (Russia), NOAA/NGDC (USA), GFZ Potsdam (Germany), NASA/GSFC (USA), IPGP (France), LPG Nantes (France), and ETH Zurich (Switzerland). Each candidate model was carefully evaluated and compared to all other models and a mean model using well-defined statistical criteria in the spectral domain and maps in the physical space. These analyses were made to pinpoint both troublesome coefficients and the geographical regions where the candidate models most significantly differ. Some models showed clear deviation from other candidate models. However, a majority of the task force members appointed by IAGA thought that the differences were not sufficient to exclude models that were well documented and based on different techniques. Conclusions: The task force thus voted for and applied an iterative robust estimation scheme in space. In this paper, we report on the evaluations of the candidate models and provide details of the algorithm that was used to derive the IGRF-12 produc

International Geomagnetic Reference Field: the 12th generation

The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic fiel

Pre-mission InSights on the Interior of Mars

Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans- port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars

Les champs magnétiques de la Terre et de Mars : apport des satellites Oersted et Mars Global Surveyor

The aim of this thesis is to chart and understand the multitude of magnetic fields encountered at the surface of the Earth, in near-Earth space, and in near-Mars space.We first introduce terrestrial magnetic field measurement techniques, and we describe in detail the validation and the processing of the Ørsted magnetic measurements. After introducing geomagnetic reference field models (with their limited resolution, due to the poor geographical data distribution), we show the major improvement that the Ørsted magnetic data has had in describing the Earth’s magnetic field. Our models are computed with a resolution similar to that of the MAGSAT (1979-1980) models. Comparisons of the new models with previous ones lead to a better understanding of the dynamics of the magnetic field at the core-mantle boundary. It also improves the description and the understanding of the magnetic field of lithospheric origin.We then introduce data from the Mars Global Surveyor satellite. These data allow the first model of the Martian magnetic field to be made. This field is a remanent one, frozen in the upper lithosphere. Correlations between the magnetic field and the avalaible topographic and gravimetric data are used to make inferences about the chronology of the martian lithosphere, and more generally about Mars’ evolution.Combined studies of the Earth’s and Mars’ magnetic fields will undoubtedly provide new constraints on the geodynamo and the now-extinct areodynamo.Le sujet de ce mémoire est l’utilisation des mesures magnétiques effectuées à bord de satellites pour mieux décrire et comprendre le champ magnétique de la Terre et celui de Mars.Dans une première partie, nous décrivons les techniques d’acquisition des mesures magnétiques terrestres, en insistant particulièrement sur la validation et le traitement des données du satellite Ørsted. Après avoir introduit les modèles magnétiques de référence, et leur limitation due à la mauvaise répartition des données terrestres, nous montrons l’apport essentiel des données Ørsted pour décrire le champ magnétique terrestre avec une résolution qui jusqu’ici n’avait été atteinte que pendant la période MAGSAT (1979-1980). Les nouveaux modèles du champ géomagnétique, et leur comparaison avec des modèles dérivés des données MAGSAT permettent de mieux appréhender la dynamique du champ magnétique à la surface du noyau, mais aussi de mieux décrire et interpréter (en termes de structures géologiques) le champ magnétique d’origine lithosphérique.Dans une deuxième partie, nous utilisons les données de la mission Mars Global Surveyor pour obtenir les premières descriptions du champ magnétique martien. Celui-ci, figé dans les couches superficielles, ne présente pas les mêmes caractéristiques que le champ lithosphérique terrestre. Nous discutons la corrélation du champ magnétique martien avec les données topographiques et gravimétriques disponibles, et nous énonçons des hypothèses quant aux séquences temporelles de la mise en place de la lithosphère, et plus généralement sur l’histoire de l’évolution de la planète Mars.L'étude combinée du champ magnétique terrestre et du champ magnétique martien, et leur comparaison apportera à terme des contraintes quant à la dynamo terrestre et l’ancienne dynamo martienne

Les champs magnétiques de la Terre et de Mars (apport des satellites Ørsted et Mars Global Surveyor)

Le sujet de cette thèse est l'ulilisation des mesures magnétiques effectuées à bord de satellites pour mieux décrire et comprendre le champ magnétique de la Terre et celui de Mars. Dans un premier temps, nous décrivons les techniques d'acquisition des mesures magnétiques terrestres, en insistant particulièrement sur la validation et le traitement des données du satellite Orsted. Après avoir introduit les modèles magnétiques de référence, et leur limitation due à la mauvaise répartition des données terrestres, nous montrons l'apport essentiel des données Orsted pour décrire le champs magnétique terrestre avec une résolution qui jusqu'ici n'avait été atteinte que pendant la période MAGSAT (1979-1980). Les nouveaux modèles du champ géomagnétique, et leur comparaison avec des modèles dérivés des données MAGSAT permettent de mieux appréhender la dynamique du champ magnétique à la surface du noyau, mais aussi de mieux décrire et interpréter (en termes de structures géologiques) le champ magnétique d'origine lithosphérique. Dans un deuxième temps, nous utilisons les données de la mission Mars Global Surveyor pour obtenir les premières description du champ magnétique martien. Celui-ci, figé dans les couches superficielles, ne présente pas les mêmes caractéristiques que le champ lithosphérique terrestre. Nous discutons la corrélation du champ magnétique martien avec les données topographiques et gravimétriques disponibles, et nous énonçons des hypothèses quant aux séquences temporelles de la mise en place de la lithosphère, et plus généralement sur l'histoire de l'évolution de la planète Mars. L'étude combinée du champ magnétique terrestre et du champ magnétique martien, et leur comparaison apportera à terme des contraintes quant à la dynamo terrestre et l'ancienne dynamo martiennePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Correlated Time‐Varying Magnetic Fields and the Core Size of Mercury

International audienceMercury is characterized by a very peculiar magnetic field, as it was revealed by the MESSENGER mission. Its internal component is highly axisymmetric, dominated by the dipole, and very weak. This in turns leads to a very dynamic magnetosphere. It is known that there exist relationships between the internally generated field and the external field, although their dynamics are complex. In this study we derive steady and time‐varying spherical harmonic models of Mercury's magnetic field using MESSENGER measurements and interpret these models both in terms of correlated features and of the internal structure of Mercury. The influence of the hemispheric data distribution of MESSENGER is evaluated to grant the robustness of our models. We find a quadrupole‐to‐dipole ratio of 0.27 for the steady magnetic field. The time‐varying models reveal periodic and highly correlated temporal variations of internal and external origins. This argues for externally inducing and internally induced sources. The main period is 88 days, the orbital period of Mercury around the Sun. There is no measurable time lag between variations of external and internal magnetic fields, which place an upper limit of 1 S/m for the mantle conductivity. Finally, the compared amplitudes of external and internal time‐varying field lead to an independent (from gravity studies) estimate of the conductive core radius, at 2,060 ± 22 km. These analyses will be further completed with the upcoming BepiColombo mission and its magnetic field experiment, but the presented results already lift the veil on some of the magnetic oddities at Mercury

The combined effects of escape and magnetic field histories at Mars

Mars is thought to have hosted large amounts of water and carbon dioxide at primitive epochs. The morphological analysis of the surface of Mars shows that large bodies of water were probably present in the North hemisphere at late Noachian (3.7–4 Gyr ago). Was this water solid or liquid? For maintaining liquid water at this time, when the Sun was (likely) less bright than now, a CO2 atmosphere of typically 2 bars is required. Can sputtering, still presently acting at the top of the Martian atmosphere, have removed such a dense atmosphere over the last 3.5–4 Gyr? What was the fate of the 100–200 m global equivalent layer of water present at late Noachian? When did Martian magnetic dynamo vanish, initiating a long period of intense escape by sputtering? Because sputtering efficiency is highly non-linear with solar EUV flux, with a logarithmic slope of ≈7:Φsput≈ΦEUV7, resulting in enhanced levels of escape at primitive epochs, when the sun was several times more luminous than now in the EUV, there is a large uncertainty on the cumulated amount of volatiles removed to space. This amount depends primarily on two factors: (i) the exact value of the non-linearity exponent (≈7 from existing models, but this value is rather uncertain), (ii) the exact time when the dynamo collapsed, activating sputtering at epochs when intense EUV flux and solar wind activity prevailed in the solar system. Both parameters are only crudely known at the present time, due the lack of direct observation of sputtering from Martian orbit, and to the incomplete and insufficiently spatially resolved map of the crustal magnetic field. Precise timing of the past Martian dynamo can be investigated through the demagnetisation signature associated with impact craters. A designated mission to Mars would help in answering this crucial question: was water liquid at the surface of Mars at late Noachian? Such a mission would consist of a low periapsis (≈100 km) orbiter, equipped with a boom-mounted magnetometer, for mapping the magnetic field, as well as adequate in situ mass and energy spectrometers, for a full characterization of escape and of its response to solar activity variations. Surface based observations of atmospheric noble gas isotopic ratios, which keep the signatures of past escape processes, including sputtering for the lightest of them (Ne, Ar), would bring a key constraint for escape models extrapolated back to the past

A Spherical Harmonic Model of Earth's Lithospheric Magnetic Field up to Degree 1050

International audienc