Isabelle Fratter

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 field.

SWARM Absolute Scalar Magnetometer accuracy: Analyses and measurement results

The different factors that affect the SWARM optically pumped Absolute Scalar Magnetometer (ASM) accuracy are reviewed and analyzed. An overall precision of less than 45 pT (1 σ) is reported, which is well under the 300 pT specified for global ASM accuracy.

Swarm's absolute magnetometer experimental vector mode, an innovative capability for space magnetometry

European Space Agency’s Swarm satellites carry a new generation of 4He absolute magnetometers (ASM), designed by CEA-Leti and developed in partnership with Centre National d’Etudes Spatiales. These instruments are the first ever spaceborne magnetometers to use a common sensor to simultaneously deliver 1 Hz independent absolute scalar and vector readings of the magnetic field. Since launch, these ASMs provided very high-accuracy scalar field data, as nominally required for the mission, together with experimental vector field data. Here we compare geomagnetic field models built from such ASM-only data with models built from the mission’s nominal 1 Hz data, combining ASM scalar data with independent fluxgate magnetometer vector data. The high level of agreement between these models demonstrates the potential of the ASM’s vector mode for data quality control and as a stand-alone magnetometer and illustrates the way the evolution of key field features can easily be monitored from space with such absolute vector magnetometers

In-flight performance of the Absolute Scalar Magnetometer vector mode on board the Swarm satellites

The role of the Absolute Scalar Magnetometer (ASM) in the European Space Agency (ESA) Swarm mission is to deliver absolute measurements of the magnetic field’s strength for science investigations and in-flight calibration of the Vector Field Magnetometer (VFM). However, the ASM instrument can also simultaneously deliver vector measurements with no impact on the magnetometer’s scalar performance, using a so-called vector mode. This vector mode has been continuously operated since the beginning of the mission, except for short periods of time during commissioning. Since both scalar and vector measurements are perfectly synchronous and spatially coherent, a direct assessment of the ASM vector performance can then be carried out at instrument level without need to correct for the various magnetic perturbations generated by the satellites. After a brief description of the instrument’s operating principles, a thorough analysis of the instrument’s behavior is presented, as well as a characterization of its environment in flight, using an alternative high sampling rate (burst) scalar mode that could be run a few days during commissioning. The ASM vector calibration process is next detailed, with some emphasis on its sensitivity to operational conditions. Finally, the evolution of the instrument’s performance during the first year of the mission is presented and discussed in view of the mission’s performance requirements for vector measurements.

A 2015 International Geomagnetic Reference Field (IGRF) candidate model based on Swarm's experimental absolute magnetometer vector mode data

Each of the three satellites of the European Space Agency Swarm mission carries an absolute scalar magnetometer (ASM) that provides the nominal 1-Hz scalar data of the mission for both science and calibration purposes. These ASM instruments, however, also deliver autonomous 1-Hz experimental vector data. Here, we report on how ASM-only scalar and vector data from the Alpha and Bravo satellites between November 29, 2013 (a week after launch) and September 25, 2014 (for on-time delivery of the model on October 1, 2014) could be used to build a very valuable candidate model for the 2015.0 International Geomagnetic Reference Field (IGRF). A parent model was first computed, describing the geomagnetic field of internal origin up to degree and order 40 in a spherical harmonic representation and including a constant secular variation up to degree and order 8. This model was next simply forwarded to epoch 2015.0 and truncated at degree and order 13. The resulting ASM-only 2015.0 IGRF candidate model is compared to analogous models derived from the mission’s nominal data and to the now-published final 2015.0 IGRF model. Differences among models mainly highlight uncertainties enhanced by the limited geographical distribution of the selected data set (essentially due to a lack of availability of data at high northern latitude satisfying nighttime conditions at the end of the time period considered). These appear to be comparable to differences classically observed among IGRF candidate models. These positive results led the ASM-only 2015.0 IGRF candidate model to contribute to the construction of the final 2015.0 IGRF model.

Development and space qualification of the swarm Absolute Scalar Magnetometer

The Absolute Scalar Magnetometer will be flown on the three Swarm satellites to be launched by ESA in 2012. It will offer the best resolution and absolute accuracy ever attained in space. Since this instrument is essential to fulfill the projects scientific objectives, its reliability and availability have to be guaranteed over the lifetime of the four year mission. To achieve these ambitious goals, the instrument is at the cutting edge of technology. It implements an innovative fiber laser used for the optical pumping of a helium 4 gas cell located within a non magnetic rotating sensor. This led us to use specific parts, materials and processes which had to be previously space qualified. The success of the Proto-Flight Model qualification was a major achievement this year. The testing of the flight models is now in progress. This paper illustrates the long way to reach the required Technology Readiness Level starting from a concept which had already been validated on a prototype.