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Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here, we explore the utility of the free-surface multiple of the P-wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs-RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust P-to-S (Ps)- and PPs-RFs with InSight recordings of five marsquakes. Our Ps-RF results verify the direct Ps converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps-RFs, our PPs-RFs lack an arrival at 7.2 s lag time. Whereas Ps-RFs on Mars could be equally well fit by a two- or three-layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three-layer crust, and is highly sensitive to velocity and thickness of intra-crustal layers. We show that a three-layer crust is also preferred by S-to-P (Sp)-RFs. While the deepest interface of the three-layer crust represents the crust-mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre-existing crustal materials. PPs-RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data-constrained circumstances such as planetary missions., Journal of Geophysical Research: Planets, 126 (11), ISSN:0148-0227, ISSN:2169-9097

Micromagnetic tomography aims at reconstructing large numbers of individual magnetizations of magnetic particles from combining high‐resolution magnetic scanning techniques with micro X‐ray computed tomography (microCT). Previous work demonstrated that dipole moments can be robustly inferred, and mathematical analysis showed that the potential field of each particle is uniquely determined. Here, we describe a mathematical procedure to recover higher orders of the magnetic potential of the individual magnetic particles in terms of their spherical harmonic expansions (SHE). We test this approach on data from scanning superconducting quantum interference device microscopy and microCT of a reference sample. For particles with high signal‐to‐noise ratio of the magnetic scan we demonstrate that SHE up to order n = 3 can be robustly recovered. This additional level of detail restricts the possible internal magnetization structures of the particles and provides valuable rock magnetic information with respect to their stability and reliability as paleomagnetic remanence carriers. Micromagnetic tomography therefore enables a new approach for detailed rock magnetic studies on large ensembles of individual particles., Key Points Micromagnetic Tomography uniquely recovers higher‐order multipole terms for several individual grains in a sampleHigher order multipole moments are an expression of the internal domain structure of magnetic grainsUltimately, this enables to select individual grains for rock‐ and paleomagnetic studies based on domain configuration

The cause for the damping of the Earth's free core nutation (FCN) and the free inner core nutation eigenmodes has been a matter of debate since the earliest reliable estimations from nutation observations were made available. Numerical studies are difficult given the extreme values of some of the parameters associated with the Earth's fluid outer core, where important energy dissipation mechanisms can take place. We present a fully 3D numerical model for the FCN capable of describing accurately viscous and Ohmic dissipation processes taking place in the bulk of the fluid core as well as in the boundary layers. We find an asymptotic regime, appropriate for Earth's parameters, where viscous and Ohmic processes contribute to the total damping, with the dissipation taking place almost exclusively in the boundary layers. By matching the observed nutational damping, we infer an enhanced effective viscosity matching and validating methods from previous studies. We suggest that turbulence caused by the Earth's precession can be a source for the enhanced viscosity., Key Points We establish asymptotic scaling laws for the free core nutation (FCN)'s Ohmic and viscous dissipation in the fluid coreMost of the energy dissipation associated with the FCN takes place in the boundary layersOur work validates inferences from previous studies that assumed the flow in the outer core as a uniform vorticity flow

Several hypotheses on the origin of the continental Moho are still debated and multiple mechanisms may contribute to its formation. Here, we present quantitative estimation of the seismic properties and anisotropy of the crust‐mantle transition in the Western Alps where an example of newly formed (proto)‐continental Moho is unusually shallow. We make use of teleseismic P‐to‐S converted‐waves recorded by stations deployed on top of the Ivrea Body (IB), a volume of possibly serpentinized mantle peridotite below exhumed (ultra‐)high pressure crustal rocks. The IB has been mapped by gravity, magnetic, active and passive seismic surveys suggesting an extremely shallow Moho. We demonstrate that the P‐to‐S converted waves propagating through this region display coupled features: (a) they record expected presence of strong seismic velocity contrast at shallow depth as due to the lower crustal and upper mantle transition; (b) they are decomposed due to anisotropic properties of rocks involved. The proto‐continental Moho is recognized as an increase in S‐wave velocity (∼0.4–1 km/s) at shallow depths of 5–10 km. The presence of anisotropy within the IB and overlying crustal rocks is evidenced by back‐azimuthal dependence of the amplitude of P‐to‐S phases. The strength of anisotropy is ∼−14% on average pointing out the presence of metamorphosed/hydrated material (e.g., serpentinite) below the Moho. Anisotropic directions are consistent across Moho in both crust and upper mantle. The similarity of the anisotropy parameters between crust and upper mantle suggests they have been shaped by the same deformation event., Key Points Receiver Functions and harmonics decompositionContinental Moho and Ivrea BodySeismic anisotropy

We test the use of magnetic measurements of evergreen needles as a proxy for particulate matter pollution in Salt Lake City, Utah. Measurements of saturation isothermal remanent magnetization indicate needle magnetization increases with increased air pollution. Needle magnetization shows a high degree of spatial variability with the largest increases in magnetization near roadways. Results from our magnetic measurements are corroborated by scanning electron microscopy of needle surfaces and by inductively coupled plasma mass spectrometry of metal concentrations in residues collected from sampled needles. Low‐temperature magnetic analysis suggests the presence of small (, Key Points Needle magnetization is highest proximal to major roadways and varies on a spatial scale of tens to hundreds of metersChanges in magnetization correlate with changes in concentration of several toxic metals measured from the surfaces of needlesEvergreen needles are capable of recording severe periods of degraded air quality that last days to weeks

The strength and structure of the Earth's magnetic field is gradually changing. During the next 50 years the dipole moment is predicted to decrease by ∼3.5%, with the South Atlantic Anomaly expanding, deepening, and continuing to move westward, while the magnetic dip poles move northwestward. We used simulations with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model to study how predicted changes in the magnetic field will affect the climate of the thermosphere‐ionosphere system from 2015 to 2065. The global mean neutral density in the thermosphere is expected to increase slightly, by up to 1% on average or up to 2% during geomagnetically disturbed conditions ( Kp≥4). This is due to an increase in Joule heating power, mainly in the Southern Hemisphere. Global mean changes in total electron content (TEC) range from −3% to +4%, depending on season and UT. However, regional changes can be much larger, up to about ±35% in the region of ∼45°S to 45°N and 110°W to 0°W during daytime. Changes in the vertical E→×B→ drift are the most important driver of changes in TEC, although other plasma transport processes also play a role. A reduction in the low‐latitude upward E→×B→ drift weakens the equatorial ionization anomaly in the longitude sector of ∼105–60°W, manifesting itself as a local increase in electron density over Jicamarca (12.0°S, 76.9°W). The predicted increase in neutral density associated with main magnetic field changes is very small compared to observed trends and other trend drivers, but the predicted changes in TEC could make a significant contribution to observationally detectable trends., Key Points Predicted magnetic field changes from 2015 to 2065 cause a very minor (up to 1–2%) increase in global mean thermosphere neutral densityEffects on total electron content are larger, up to ±35% in the American sector ( ∼45°S to 45°N, 110°W to 0°W)Reduced vertical E→×B→ drifts at Jicamarca and surrounding longitudes ( ∼105–60°W) cause a weakening of the equatorial ionization anomaly

Particulate matter (PM) emitted in the steel production and cement industry, as well as in the transport and energy production sector, possesses enhanced magnetic properties which enable to delineate the impact of airborne pollution once the particles have deposited onto the Earth’s surface. Topsoil magnetic susceptibility, for instance, has been shown a suitable parameter for airborne pollution monitoring in areas with homogeneous soil type and land use. However, when study areas compri se multiple types of land use , the distribution of magnetic particles down t he soil profile can vary strongly and im pede reliable investigation of pollution impact s. Here, we demonstrate how an adaptive approach involving depth-integrated magnetic susceptibility records can mediate adverse effects of varied land use on topsoil magnetic records for pollution studies (Declercq et al., 2020). Furthermore, in tandem with other receptors of magnetic PM (strawberry and grass leaves, plastic coated cardboards (PCCs) and wiped anthropogenic surfaces), long-term variations captured in soil magnetic records were compared to short-term pollution impacts. Although this approach enables to reliably discriminate spatiotemporal variations in airborne pollution impacts, a straightforward means to relate environmental magnetic signatures more directly to specific emission outputs, and pollutant sources - facilitating the development of control measure strategies - remains lacking. To this end, we characterised the physicochemical properties of PM generated by the largest emitter of magnetic PM in our study area, a steel mill. Particular PM loads involved in different steps of the steel production process (crude iron ore, sinter plant, blast furnace and steel mill), as well as ambient PM collected on PCCs nearby the factory, were magnetically, morphologically and chemically characterized and mutually compared. Source-specific fingerprints facilitated the interpretation of magnetic pollution impacts and revealed how emissions from the blast furnace and steel mill strongly affected the observed magnetic signatures in the environment. We show how the presented magnetic approach supports spatial investigation of source-specific airborne pollution impacts, both on the short and long term.

International audience; The geomagnetic field has been almost continuously monitored from LEO orbits since 1999, complementing ground-based observatory data by providing accurately calibrated scalar and vector measurements with global planetary coverage. The very successful three-satellite ESA Swarm constellation is planned to remain in operation up to hopefully 2024. Further monitoring the field from space with high-precision absolute magnetometry beyond that date is of critical importance for improving our understanding of the long and short-term dynamics of the multiple components of this field. Here, we will report on the fast developing status of the NanoMagSat constellation project, which aims at deploying and operating a constellation of identical 12 U nanosatellites on a combination of inclined (approximately 60°) and polar LEO orbits. The mission instruments would consist of an advanced Miniaturized Absolute scalar and self-calibrated vector Magnetometer (MAM) combined with a set of star trackers (STR) for very precise attitude restitution, a compact High-frequency Field Magnetometer (HFM, sharing subsystems with the MAM), and a multi-needle Langmuir Probe (m-NLP). The dual frequency GNSS receiver on the platform will further provide both precise orbit determination and Total Electron Content (TEC) measurements. The Level 1 data to be produced would include 1 Hz high-precision absolutely calibrated and accurately oriented magnetic vector field (using the MAM and STR), high frequency (at least 1 kHz) very low noise level magnetic scalar (using the MAM) and vector (using the HFM) field, high-frequency (at least 1 kHz) local electron density (using the m-NLP) as well as precise timing, location and TEC products derived from the GNSS receiver. Beyond describing the programmatic status of the project, this presentation will also discuss the science goals of the proposed constellation, highlighting its ability to take over and complement many of the science goals of the ongoing Swarm mission at a much lower cost and to form the basis of a permanent collaborative constellation of nanosatellites for long-term monitoring of the geomagnetic field and ionospheric environment from space.

The thermal demagnetization of pseudo‐single‐domain (PSD) magnetite (Fe3O4) particles, which govern the magnetic signal in many igneous rocks, is examined using off‐axis electron holography. Visualization of a vortex structure held by an individual Fe3O4 particle (~250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic‐induction maps. Stepwise demagnetization of the remanence‐induced Fe3O4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed that its vortex state remains stable under heating close to its unblocking temperature and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe3O4 particle exhibits thermomagnetic behavior comparable to a single‐domain carrier, and thus, vortex states are considered reliable magnetic recorders for paleomagnetic investigations., Key Points Thermal demagnetization of nonideal magnetite particlesVisualized magnetization of vortex state with temperatureVortex structures are reliable paleomagnetic signal recorders

International audience; The zone of interaction between the Cocos (CO), Caribbean (CA) and North America (NA) plates in Guatemala is defined by the sub-parallel Motagua and Polochic strike-slip faults, a series of north-south-trending extensional grabens immediately south of the Motagua Fault, the Middle America trench, and faults within the Middle America volcanic arc. Historical earthquakes associated with these faults include the destructive 1976 Mw 7.5 earthquake along the Motagua fault and the 2012 Mw 7.5 Champerico thrust earthquake. The latest published GPS-based present-day kinematic model of the region shows that about two-thirds of the strain accumulation from the NA/CA relative motion concentrates on the Motagua fault and one third across the Polochic fault, suggesting that slip varies with time as a result of mechanical interactions within the Motagua-Polochic fault system. As part of the efforts to quantify the present-day kinematics and slip behavior of these faults, we use interferometric synthetic aperture radar (InSAR) to measure the strain rates across faults in Guatemala and to constrain slip partitioning among them. We processed L-band ALOS-1 images spanning from 2006 to 2011, and C-band Sentinel-1 images spanning from 2015 to 2019, from ascending and descending tracks covering the Polochic and Motagua faults, the Ipala and Guatemala City grabens, and part of the volcanic arc to the south. We are using the New Small temporal and spatial baselines (NSBAS) workflow to compute the interferograms, make tropospheric and ionospheric corrections, and perform time-series analysis. We present the first InSAR-based maps of interseismic velocity for this region, which will contribute to the refinement of interseismic locking estimates across the Motagua-Polochic fault system, the subduction zone, and other nearby faults.

Observations of thermospheric infrared radiative cooling by carbon dioxide (CO2) and nitric oxide (NO) from 2002 to 2018 are presented. The time span covers more than 6,000 days including most of solar cycle (SC) 23 and the entirety of SC 24 to date. Maxima of infrared cooling rate profiles (nW/m3) are smaller during SC 24 than SC 23, indicating a cooler thermosphere. Rates of global infrared power (W) from CO2 are now at levels observed during the deep solar minimum of 2009. Rates of NO power are still larger than those observed during 2009 and are being maintained at an elevated level by geomagnetic activity. During SC 24 to date, the thermosphere has radiated 70% of the energy of the mean of the past five cycles and would require an additional 1,690 days at current infrared radiation rates to reach that amount., Key Points Global infrared power radiated by NO and CO2 from thermosphere during solar cycle 24 are, to date, only 50% and 73% of solar cycle 23SC 24 would have to last 1,690 more days (making it 1 year longer than SC 23) for its infrared power to equal mean of past five cyclesNO power levels currently are still 34% larger than at solar minimum conditions of 2009, due to higher geomagnetic activity in 2018

Sampling of sediment cores using plastic U‐channels has made possible the acquisition of detailed records of paleomagnetic secular variation, geomagnetic polarity, environmental magnetic studies, and relative paleointensity over the past several million years. U‐channel measurements provide the great advantage of rapid measurements of long sediment cores, but the signal resolution is attenuated by the response function of the magnetometer sensors, which therefore restrains the recovery of rapid and large‐amplitude field changes. Here we focus on the suitability of the dynamics of reversals and excursions derived from U‐channel measurements. We compare successive individual paleomagnetic directions of 1.5 cm × 1.5 cm × 1.5 cm cubic discrete samples with those of a 1.5‐m equivalent U‐channel sample train obtained by placing the samples adjacent to each other. We use varying excursion and transition lengths and generate transitional directions that resemble those of the most detailed paleomagnetic records. Excursions with opposite polarity directions recorded over less than 7.5 cm are barely detected in U‐channel measurements. Regarding reversals, U‐channel measurements smooth the signal of low‐resolution records and generate artificial transitional directions. Despite producing misleading similarities with the overall structure of transition records, longer transitional intervals fail also to reproduce the complexity of field changes. Finally, we test the convolution of magnetization by different response functions. The simulation reveals that even small response function changes can generate significant differences in results., Key Points U‐channel measurements fail to detect rapid excursions, except in the presence of high deposition ratesU‐channel fails to duplicate the original features of the field changes during polarity transitionsDiscrete samples of small size are thus required to fully reconstruct the detailed dynamics of reversals and excursions

Accurate knowledge of Mercury's magnetospheric magnetic field is required to understand the sources of the planet's internal field. We present the first model of Mercury's magnetospheric magnetic field confined within a magnetopause shape derived from Magnetometer observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. The field of internal origin is approximated by a dipole of magnitude 190 nT RM 3, where RM is Mercury's radius, offset northward by 479 km along the spin axis. External field sources include currents flowing on the magnetopause boundary and in the cross‐tail current sheet. The cross‐tail current is described by a disk‐shaped current near the planet and a sheet current at larger (≳ 5 RM) antisunward distances. The tail currents are constrained by minimizing the root‐mean‐square (RMS) residual between the model and the magnetic field observed within the magnetosphere. The magnetopause current contributions are derived by shielding the field of each module external to the magnetopause by minimizing the RMS normal component of the magnetic field at the magnetopause. The new model yields improvements over the previously developed paraboloid model in regions that are close to the magnetopause and the nightside magnetic equatorial plane. Magnetic field residuals remain that are distributed systematically over large areas and vary monotonically with magnetic activity. Further advances in empirical descriptions of Mercury's magnetospheric external field will need to account for the dependence of the tail and magnetopause currents on magnetic activity and additional sources within the magnetosphere associated with Birkeland currents and plasma distributions near the dayside magnetopause., Key Points First Mercury magnetic field model confined to observed magnetopauseModel includes internal dipole and field from magnetopause and tail currentsRemaining residuals due to unmodeled current systems

To validate the usage of global indices in studies of geomagnetic activity, we have examined the latitude dependence of geomagnetic variations in Fennoscandia and Svalbard from 1994 to 2010. Daily standard deviation (SD) values of the horizontal magnetic field have been used as a measure of the ground magnetic disturbance level. We found that the timing of the geomagnetic minimum depends on the latitude region: corresponding to the minimum of sunspot cycle 22 (in 1996), the geomagnetic minimum occurred between the geomagnetic latitudes 57–61° in 1996 and at the latitudes 64–67° in 1997, which are the average auroral oval latitudes. During sunspot cycle 23, all latitude regions experienced the minimum in 2009, a year after the sunspot minimum. These timing differences are due to the latitude dependence of the 10 s daily SD on the different solar wind drivers. In the latitude region of 64–67°, the impact of the high-speed solar wind streams (HSSs) on the geomagnetic activity is the most pronounced compared to the other latitude groups, while in the latitude region of 57–61°, the importance of the coronal mass ejections (CMEs) dominates. The geomagnetic activity maxima during ascending solar cycle phases are typically caused by CME activity and occur especially in the oval and sub-auroral regions. The strongest geomagnetic activity occurs during the descending solar cycle phases due to a mixture of CME and HSS activity. Closer to the solar minimum, less severe geomagnetic activity is driven by HSSs and mainly visible in the poleward part of the auroral region. According to our study, however, the timing of the geomagnetic activity minima (and maxima) in different latitude bands is different, due to the relative importance of different solar wind drivers at different latitudes.

In recent years, magnetic surveying has proven its efficiency to discover and characterize archaeological structures across large areas. Doing so, archeologists can have a global understanding on the general settlements of the studied site and decide their excavations with more efficiency.Since several years, the laboratory practices such king of magnetic cartography using three-component fluxgate magnetometers and got many results even for low magnetization contrasts corresponding to magnetic anomalies of only a few nannoteslas. The homemade system is based on light and low consuming magnetic sensors with their electronic device and associated with a DGNSS navigation allowing very precise positioning. The computed magnetic maps have an accuracy of about 1 nT.Located in the Eastern province of Saudi Arabia, 90 km west of the Gulf Coast and the modern port of Jubayl, Thaj is the largest pre-Islamic site known in Eastern Arabia. It is composed of a fortified city of 40 ha, surrounded by a massive fortification wall of trapezoidal shape; an extramural suburb covering ca 14 ha and a huge necropolis of about 500 burial mounds extending in to the West, South and East of the city.The data were collected in November 2016 and November 2017, a total of 37 ha have been mapped during these two surveys, each representing 10 days of work. All the data has been processed to obtain magnetic maps with 0,25 m node spacing and an accuracy of about 1 nT. Potential field transforms (reduction to the pole, derivatives, analytic signal, tilt-depth) are then applied to the map to better highlight the archeological structures producing the magnetic anomalies and, in particular, to characterize their depths.Thanks to those transforms, magnetic lineations are better characterized and generally, they are related to ancient streets. The interpretation has been checked using airborne imagery with good results and, at many places, magnetic lineations are used to extend the urban plan.

During burial of clay-rich sediments, magnetic minerals, just as other minerals, are continuously produced, altered, and dissolved. In shales, from early burial to anchi-metamorphism (Tburial< 250°C), magnetite is the main magnetic mineral. In slates, it is known that the dual breakdown of magnetite and pyrite trigger the formation of pyrrhotite for burial temperature close to 300°C (greenschist metamorphic grade). We propose to bracket the range of temperature of magnetite and pyrite breakdown using in combination magnetic studies and burial temperature obtained by Raman spectroscopy. Samples are late Cretaceous slates (burial temperature 250°-600°C) from Pyrenees. The identification of magnetite and pyrrhotite is firmly constrained by the identification of Verwey ( 120 K) and Besnus ( 32K) magnetic transition. Burial temperatures are estimated within ±30°C. Results show: 1) Magnetite is dominant for Tburial

Geomagnetic dipole moment variations associated with polarity reversals and excursions are expressed by large changes of the cosmogenic nuclide beryllium‐10 (10Be) production rates. Authigenic 10Be/9Be ratios (proxy of atmospheric 10Be production) from oceanic cores therefore complete the classical information derived from relative paleointensity (RPI) records. This study presents new authigenic 10Be/9Be ratio results obtained from cores MD05‐2920 and MD05‐2930 collected in the west equatorial Pacific Ocean. Be ratios from cores MD05‐2920, MD05‐2930 and MD90‐0961 have been stacked and averaged. Variations of the authigenic 10Be/9Be ratio are analyzed and compared with the geomagnetic dipole low series reported from global RPI stacks. The largest 10Be overproduction episodes are related to dipole field collapses (below a threshold of 2 × 1022 Am2) associated with the Brunhes/Matuyama reversal, the Laschamp (41 ka) excursion, and the Iceland Basin event (190 ka). Other significant 10Be production peaks are correlated to geomagnetic excursions reported in literature. The record was then calibrated by using absolute dipole moment values drawn from the Geomagia and Pint paleointensity value databases. The 10Be‐derived geomagnetic dipole moment record, independent from sedimentary paleomagnetic data, covers the Brunhes‐Matuyama transition and the whole Brunhes Chron. It provides new and complementary data on the amplitude and timing of millennial‐scale geomagnetic dipole moment variations and particularly on dipole moment collapses triggering polarity instabilities., Key Points Authigenic 10Be/9Be ratios from three equatorial ocean cores are stacked and averaged outlining atmospheric 10Be production changesThe 10Be cosmogenic nuclide overproduction episodes are systematically associated with geomagnetic dipole lows over the last 850 kaThe 10Be signatures of the Brunhes/Matuyama transition and Brunhes geomagnetic excursions yield distinct rates and durations

Polarity reversals of the geomagnetic field are a major feature of the Earth's dynamo. Questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition. A large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field. In addition, over the past two decades, numerical dynamo simulations have also provided insights into the reversal mechanism. Yet despite the large paleomagnetic database, controversial interpretations of records of the transitional field persist; they result from two characteristics inherent to all reversals, both of which are detrimental to an ambiguous analysis. On the one hand, the reversal process is rapid and requires adequate temporal resolution. On the other hand, weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records. This paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations. We discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process. Records from marine sediments dominate the database. They give rise to transitional field models that often lead to overinterpret the data. Consequently, we attempt to separate robust results (and their subsequent interpretations) from those that do not stand on a strong observational footing. Finally, we discuss new avenues that should favor progress to better characterize and understand transitional field behavior., Key Points We examine and assess an updated database of geomagnetic reversal recordsWe discuss robust and controversial features of the transitional fieldFuture work should involve millimeter‐size specimens and physics‐based models

International audience; The commissioning phase of Swarm satellites provided opportunities to acquire measurement during special acquisitions sessions. The Absolute Scalar Magnetometer (ASM) instrument of each satellite were operated in "burst mode": the recording frequency of the instruments were risen from the nominal 1 Hz to 250 Hz. A total of seven burst mode sessions, covering between 3 and 48 hours of continuous recording, were realized between December 2013 and February 2014. During that time, the local time of Swarm satellites orbits shifted from midnight-noon to post sunrise-post sunset. The last burst-mode session occurred during the recovery phase of a geomagnetic storm. Therefore ASM burst mode data allow to investigate both quiet and geomagnetic active periods. The analysis of burst mode data was focused on the phenomena that can be investigated only using this higher sampling rate, providing insight in the frequency band between few Hz to 125 Hz. From the analysis of the magnetic field data we could observe typical signals related to different geomagnetic phenomena, showing distinct features at low, mid and high latitude. We were able to detect fast magnetic field fluctuations with a typical pattern, associated with low-latitudes irregularities in the ionospheric plasma during the development of post-sunset plasma bubbles. We also present results of the analysis of short electromagnetic signals that were detected mostly in the mid-to-low latitudes evening sector, investigating their correlation with the occurrence of lightning on the ground. Over the high latitudes, we detected fast variations in the intensity of the magnetic field during the active periods and analyse their correlations with plasma irregularities.

International audience; Each of the three Swarm satellites embarks an Absolute Scalar Magnetometer (ASM) to provide absolute scalar measurements of the magnetic field with high accuracy and stability. Nominal data acquisition of these ASMs is 1 Hz. But they can also run in a so-called "burst mode" and provide data at 250 Hz. During the commissioning phase of the mission, seven burst mode acquisition campaigns have been run simultaneously for all satellites, obtaining a total of ten days of burst-mode data. These campaigns allowed the identification of issues related to the operations of the piezo-electric motor and the heaters connected to the ASM, that do not impact the nominal 1 Hz scalar data. We analyze the burst mode data to identify high frequency geomagnetic signals, focusing the analysis in two regions: the low latitudes, where we seek signatures of ionospheric irregularities, and the high latitudes, to identify high frequency signals related to polar region currents. Since these campaigns have been conducted during the initial months of the mission, the three satellites where still close to each other, allowing to analyze the spatial coherency of the signals.

The semiannual and annual lines in a long series of magnetic observatories daily values, as well as in the aa-activity index series, are investigated. For both periods, amplitudes and phases of the lines corresponding to the different series present grossly common variations on decadal time scales; relative phases and amplitude ratios between the observatories change with the same time constants. The results are briefly discussed with regards to commonly received theories of the semiannual variation of magnetic activity, and some possible mechanisms for the observed geographical variability are suggested.