Nils Olsen

We identify a previously unrecognized population of rapid geomagnetic variations associated with cusp dynamics. These variations produce distinct summertime signatures in the high–latitude ground magnetometer observations near the cusp. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of rapid geomagnetic variations in the ultra low frequency (ULF) regime as a function of season, magnetic latitude, magnetic local time, and interplanetary magnetic field parameters. The magnetic perturbations are characterised using a broadband metric that integrates variability in the 10–600 s period range. This metric captures geomagnetic responses to transient, impulsive, and wave-like magnetospheric phenomena as well as ULF wave activity. We find that rapid geomagnetic variability at the highest latitudes, in the cusp and beyond, is sensitive to seasonal change, indicating that the ionospheric currents generating the observed ground magnetic signatures depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, was investigated, and a clear cusp-related population during summer was found. In winter, this population weakens owing to a lack of sunlight and merges with other signals such as ULF waves associated with Alfvénic interhemispheric bouncing further south, and, thus, becomes indiscernible. Furthermore, we discuss other aspects of our statistical analysis and briefly address relationships between our detected signals and other known cusp phenomena.

Magnetic measurements from low earth orbiting (LEO) satellites have revolutionised our ability to globally map Earth’s magnetic field. This paper reviews the processing and calibration of satellite magnetometer data from both dedicated magnetic field missions and satellites carrying platform magnetometers. It describes the successive steps of vector magnetometer calibration, characterisation of spacecraft disturbance fields and alignment of the corrected magnetic field vectors into an Earth-fixed reference frame. Different parametrisations for these steps are discussed, including approaches to account for time- and temperature-dependent effects and non-linear response of the magnetometer. Dedicated missions such as Ørsted, CHAMP, Swarm and MSS-1, equipped with absolute scalar magnetometers, allow separate and highly accurate calibration steps; whereas, platform magnetometers on missions such as CryoSat-2, GRACE and GRACE-FO require joint estimation procedures using geomagnetic field models as reference. Despite their limitations, platform magnetometer data are a valuable supplement to dedicated missions, offering improved space–time coverage for modelling ionospheric, magnetospheric and induced magnetic field contributions. Recent advances in automated calibration are highlighted, underscoring the growing role of non-dedicated satellites in geomagnetic research. Graphical Abstract

Launched by the European Space Agency (ESA) in November 2013, the three-satellite Swarm constellation continues to deliver high-quality measurements of Earth’s magnetic field and the surrounding plasma environment. After more than 12 years in orbit, the mission has achieved remarkable scientific results, deepening our understanding of geomagnetic field dynamics and supporting applications that go well beyond the mission’s original goals.Equipped with 7 complementary instruments each spacecraft — including a Vector Field and an Absolute Scalar Magnetometer (VFM and ASM); star trackers (STR); a dual-frequency GPS receiver (GPSR); an accelerometer (ACC); an Electric Field Instrument (EFI), composed of two Langmuir Probes (LPs) and two Thermal Ion Imagers (TIIs) — Swarm has become a pivotal reference for geophysical research, supporting advances in areas such as core dynamics, ionospheric and magnetospheric processes, space weather monitoring, and the characterization of electric currents throughout the Geospace environment.This paper presents a comprehensive overview of the current status of the Swarm mission and constellation, with particular focus on the long-term performance, stability, and calibration of its instruments. The discussion highlights how the constellation’s unique configuration and consistently high data quality have ensured the continuity and reliability of key geophysical observations for more than a decade, with a look at plans for the next future of the mission.Furthermore, we outline the significant enhancements introduced with the latest Swarm data-processing baseline, which improves the accuracy, consistency, and overall usability of the mission’s data products.

ESA Earth Explorer Swarm mission, launched in November 2013 with the purpose of exploring and understanding the Earth’s interior and its environment, provided significant achievements in the observation of the geomagnetic field, the ionosphere, and electric currents. And it continues contributing to geomagnetism and ionospheric science fields.Each of the three satellites of the Swarm constellations carries onboard a set of instruments to achieve the mission objectives: a Vector Field and an Absolute Scalar Magnetometer (VFM and ASM); three star trackers (STR) for accurate attitude determination and, recently, for energetic particle detection; a dual-frequency GPS receiver (GPSR); an accelerometer (ACC); an Electric Field Instrument (EFI), composed of two Langmuir Probes (LPs) and two Thermal Ion Imagers (TIIs), dedicated to electric field and plasma measurements. The products derived from EFI instruments represent the focus of this work.A defining feature of the Swarm mission is its commitment to continuous improvement. Since its launch, advancements in data processing algorithms have been continuously applied: these updates have not only maintained the good quality of Swarm's measurements but have also allowed the mission to evolve and continue meeting the needs of the scientific community. These refinements served the development of novel Swarm-based data products and services, further broadening the mission’s impact, and allowing it to overcome the initial objectives and go beyond its original scope, such as in the Space Weather field.In December 2025, the most recent baseline has been transferred to operations, delivering updated datasets and evolved products. These algorithm updates greatly impacted the EFI LP products. This work will provide an overview of the improvements applied on Swarm plasma data products: detailed analyses are presented, dedicated to new plasma densities and temperatures parameters, new flags, and other upgrades; comparisons with other L1B and L2 Swarm products are performed; case studies in correspondence of recent main Space Weather events are also displayed, to highlight the innovative application of Swarm to this field.

Continuous monitoring of electric current systems in Earth’s environment, including the ionosphere and magnetosphere, is essential for characterising geospace. Such observations are required, for example, to determine the energy input into the upper atmosphere and to monitor disturbances in the space environment and associated hazards. This monitoring can be achieved through magnetic field measurements acquired both on the ground and in space.The spatio-temporal sampling provided by dedicated magnetic satellite missions such as Swarm and MSS can be significantly enhanced by incorporating platform magnetometer data from non-dedicated missions, including CryoSat-2, GRACE, and GRACE-FO.This presentation reports recent achievements in the use of platform magnetometer data within a study funded by ESA’s General Support Technology Programme (GSTP). In particular, it highlights the calibration and provision of magnetic field data from ESA’s Aeolus satellite.

High-latitude field-aligned currents (FACs) reflect, in steady-state, the force balance between magnetospheric plasma dynamics and the collisional coupling of plasma to the neutral atmosphere in the ionosphere. Assessing the impact of high-latitude FACs at low latitudes is difficult for at least two reasons. First, FACs are primarily inferred from magnetometer measurements in low-Earth orbit by estimating the radial current using horizontal magnetic field perturbations and converting it to a FAC using a geometric factor. While this yields a locally correct estimate of the FAC density, the magnetic field generated by a radial current system differs from that generated by the corresponding FAC system when field lines are not radial. As a result, the magnetic field of the horizontal component of FACs, including their remote magnetic field observed at low latitudes, are neglected. Second, in many numerical simulations, FACs are coupled to the ionosphere only at high latitudes, while boundary conditions are imposed at lower latitudes, arguably making it difficult, from a fundamental physics perspective, to trace how high-latitude forcing influences low latitudes.Here we use AMPERE estimates of high-latitude FACs at 10-min resolution derived from magnetometer measurements on the Iridium satellite constellation to quantify their low-latitude impact. FACs in both polar regions are used to calculate the remote magnetic field using the integration method of Engels and Olsen (1998, https://doi.org/10.1016/S1364-6826(98)00094-7). A recently developed magnetosphere-ionosphere coupling model (Laundal et al. 2025, https://doi.org/10.5194/angeo-43-803-2025) is used to compute the associated penetration electric field. The resulting magnetic and electric fields are compared with observations at low latitudes.

Ultra-low frequency (ULF) waves propagate through the cusp and generate distinct summertime signatures in high latitude ground-based magnetometer measurements. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of ULF signal distribution as a function of season, magnetic latitude, magnetic local time, and interplanetary magnetic field parameters. We find that ULF signals at the highest latitudes, in the cusp and beyond, are sensitive to seasonal change, indicating that the ionospheric currents that generate the signal depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, is investigated, and a clear cusp-related ULF signal population during summer was found. In winter, this population merges with other ULF signals associated with Alfvénic interhemispheric bouncing further south and thus disappears. Earlier studies, which have mainly been performed during winter conditions, failed to unambiguously identify cusp ULF signals. Furthermore, we discuss other aspects of our statistical analysis and briefly address implications for other known cusp phenomena.

This paper presents the second generation of the China Seismo-Electromagnetic Satellite (CSES) Global Geomagnetic Field Model (CGGM-2), specifically designed to derive candidate models for the fourteenth generation of the International Geomagnetic Reference Field (IGRF-14). To build this CGGM-2 model, we utilize more than six years of geomagnetic field data collected by the high-precision magnetometer (HPM) onboard CSES. The model is parametrized by expanding the internal field to spherical harmonic degree and order 45, now incorporating lithospheric contributions. This was not the case for the first generation CGGM model, which was limited to spherical harmonic degrees up to 15. We now also implement order six B-splines in time with one year knot spacing for degrees 1 to 13 to capture non-linear temporal variations in the core field, superseding the linear approximation used in CGGM. Despite a number of challenges, such as boom deformation on which the HPM is located (away from the star imager that provides attitude information) and high-latitude magnetic field platform disturbances, the CGGM-2 model succeeds at capturing the non-linear temporal variations of the geomagnetic field. In particular, the SV-2025-2030 candidate model derived from CGGM-2 performs particularly well among all 18 candidate models for IGRF-14. Candidate models derived from the CGGM-2 are also the only IGRF-14 candidate models using CSES data only and not directly relying on any ESA’s Swarm satellite data, thus providing useful candidate models to compare against all other IGRF candidate models. Our findings underscore the potential of CSES data for geomagnetic field modeling and the likely benefits of CSES’s revisiting capability for accurately capturing the main field's variations. It is expected that future data from the forthcoming companion CSES-02 mission will help improve the temporal–spatial resolution of the model and mitigate high-latitude magnetic platform disturbances. Graphical Abstract

Ultra-low frequency (ULF) waves propagate through the cusp and generate distinct summertime signatures in high latitude ground-based magnetometer measurements. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of ULF signal distribution as a function of season, magnetic latitude, magnetic local time, and interplanetary magnetic field parameters. We find that ULF signals at the highest latitudes, in the cusp and beyond, are sensitive to seasonal change, indicating that the ionospheric currents that generate the signal depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, is investigated, and a clear cusp-related ULF signal population during summer was found. In winter, this population merges with other ULF signals associated with Alfvénic interhemispheric bouncing further south and thus disappears. Earlier studies, which have mainly been performed during winter conditions, failed to unambiguously identify cusp ULF signals. Furthermore, we discuss other aspects of our statistical analysis and briefly address implications for other known cusp phenomena.