Brian J. Anderson

The magnetosphere can undergo periodic loading and unloading of magnetic energy, a mode of activity known as sawtooth oscillations. Whether individual oscillations represent intensified substorms or a distinct form of magnetospheric activity remains an open question. We investigate the magnetosphere–ionosphere (M–I) current systems during the sawtooth oscillations of the October 2024 geomagnetic storm to examine how the system reconfigures in space and time, using combined observations from SuperMAG ground magnetometers, AMPERE field‐aligned current maps, and GOES magnetic field measurements. Our results show that each oscillation is characterized by a sequential development of two distinct current wedges: a substorm current wedge (SCW) centered in the dusk‐to‐midnight sector, followed by a dawnward current wedge (DCW) centered at dawn. The SCW develops at the onset of each oscillation, spreads azimuthally, and extends ∼10 hr in MLT. The DCW forms during the decay of the SCW or after its disappearance, exhibits limited azimuthal motion, and maintains a relatively stationary morphology with a comparable MLT extent. While the SCW resembles typical substorms, albeit with larger MLT extents, the DCW is primarily observed for storm‐time substorms. Moreover, the DCW identified here differs from previously reported storm‐time DCW by exhibiting distinct onsets and nearly stationary MLT distributions without clear expansion from the midnight. These distinctions suggest that magnetotail configuration and dynamics during sawtooth oscillations differ systematically from those during non‐storm time substorms.

The global current systems of a planet represent electrodynamic interactions between its different parts, and between its magnetosphere and the ambient solar wind. Although some local currents of Mercury’s magnetosphere have already been revealed by previous observational studies, the global picture of current systems in Mercury’s magnetosphere remains unknown. Here we reconstruct the global current systems in the Mercury’s magnetosphere, using five years magnetic field and plasma measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. It reveals a complex picture: a magnetopause current, cross-tail current, inner and outer equatorial ring currents and an unexpected polar ring current. The equatorial ring currents and the polar ring current agree with the prediction of the plasma drifting model in a region with a pressure gradient. These ring currents could reshape Mercury’s magnetosphere and reveal new dynamics and energy transfer processes in Mercury.

We quantify the asymmetry between the Birkeland currents (also known as field‐aligned currents) in the Northern and Southern Hemispheres. We use data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) in 2010–2024 inclusive for both hemispheres, compute the observed asymmetry, and then subtract the modeled asymmetry. We use Block 1 data (based on the original Iridium constellation) and NEXT data (based on Iridium‐NEXT spacecraft), and compare the effect of changing the data processing pipeline on the latter (yielding NEXT v1 and NEXT v2 subdatasets). We find that the asymmetry is significantly smaller in NEXT v2 data than in Block 1 or NEXT v1 data . The previously reported asymmetry seen in Swarm is , indicating that the new data processing pipeline goes from approximately twice the Swarm asymmetry to approximately half. This may indicate that the new AMPERE processing pipeline overestimates the amount of current flowing in the Southern Hemisphere, whereas the old one underestimated this quantity. We use frequency analysis to find that the asymmetry varies with frequencies corresponding to seasonal effects, CME occurrence, Iridium constellation orbit precession, and geomagnetic storms. We find a correlation with IMF , indicating that more current flows in the Northern Hemisphere during positive and vice versa. We find that Block 1 data in 2010–2017 shows correlations with AL, AU, , PCN, and PCS index which indicate that the asymmetry is less pronounced when Dungey Cycle convection is stronger, but NEXT data in 2019–2024 do not show this.

Two geomagnetic storms occurred in October 2024 (Oct 6‐9 and 10–12), driven by the impact of a series of interplanetary coronal mass ejections on the magnetosphere. The first was a moderate storm, with peak Sym‐H near −150 nT, whereas the second was intense, Sym‐H reaching −340 nT. We compare and contrast the magnetospheric dynamics in each case, using observations of field‐aligned currents (FACs) from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) and ground magnetic perturbations observed by SuperMAG. The first storm responded linearly to solar wind driving, quantified by a dayside reconnection coupling function, and displayed typical substorm dynamics. The response during the second storm suggests that the cross‐polar cap potential saturated, and that the dynamics of the inner magnetosphere were complicated. Magnetospheric compression by high solar wind pressure during the passage of the sheath of the second storm produced elevated FAC magnitudes, indicating that both convection and compression control magnetosphere‐ionosphere coupling. We introduce a new FAC pattern complexity index which shows quantitively that the FAC pattern during the first storm largely retained the region 1 and 2 configuration associated with twin‐cell ionospheric convection, but that during the second storm the pattern became more highly structured. We conclude that storm intensity should not solely be quantified by Sym‐H but also by other aspects of the magnetospheric response to solar wind disturbances.

We present a robust method for estimating the auroral Joule heating in the northern hemisphere by combining SuperMAG, SuperDARN and AMPERE data products. This method is applied to 292 storms during 2010–2023. Temporal evolution and spatial distributions of the storm‐time Joule heating are studied using a superposed epoch analysis, with the storms divided into three categories based on the solar wind structures driving the storms' main phase: solar wind high speed streams and associated stream interaction regions (HSS/SIR), or sheath regions and magnetic clouds associated with interplanetary coronal mass ejections (ICME). It is found that the Joule heating has a fast increase at the beginning of the storm main phase when the storm is initiated by HSS/SIRs or by ICME sheath regions, while a more gradual and longer lasting increase is found in storms driven by magnetic clouds within the ICMEs. In all the driver categories the Joule heating during the main phase is concentrated on the morning and afternoon auroral ovals, with the largest hemispheric median heating of just over 240 GW seen at the end of the main phase in storms driven by sheaths and magnetic clouds. Joule heating is highly correlated with the SuperMAG SME index ( values between 0.72 and 0.76), while correlation with the Newell coupling function is moderate ( values between 0.52 and 0.56). Magnetic cloud driven storms have the highest geoefficiency in terms of solar wind energy input driving Joule heating.

Wo geomagnetic storms occurred in October 2024 (Oct 6-9 and 10-12), driven by the impact of a series of interplanetary coronal mass ejections (ICMEs) on the magnetosphere. The first was a moderate storm, with peak Sym-H near -150 nT, whereas the second was intense, Sym-H reaching -340 nT. We compare and contrast the magnetospheric dynamics in each case, using observations of field-aligned currents (FACs) from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) and ground magnetic perturbations observed by SuperMAG. The first storm responded linearly to solar wind driving, quantified by a dayside reconnection coupling function, and displayed typical substorm dynamics. The response during the second storm suggests that the cross-polar cap potential (CPCP) saturated, and that the dynamics of the inner magnetosphere were complicated. Magnetospheric compression by high solar wind pressure during the passage of the sheath of the second storm produced elevated FAC magnitudes, indicating that both convection and compression control magnetosphere-ionosphere coupling. We introduce a new FAC pattern complexity index which shows quantitively that the FAC pattern during the first storm largely retained the region 1 and 2 configuration associated with twin-cell ionospheric convection, but that during the second storm the pattern became more highly structured. We conclude that storm intensity should not solely be quantified by Sym-H but also by other aspects of the magnetospheric response to solar wind disturbances.

We present a robust method for estimating the auroral Joule heating in the northern hemisphere by combining SuperMAG, SuperDARN and AMPERE data products. This method is applied to 286 storms during 2010-2024. Temporal evolution and spatial distributions of the storm-time Joule heating are studied using a superposed epoch analysis, with the storms divided into three categories based on the solar wind structures driving the storms’ main phase: solar wind high speed streams and associated stream interactions regions (HSS/SIR), or sheath regions and magnetic clouds associated with interplanetary coronal mass ejections (ICME). It is found that the Joule heating has a fast increase at the beginning of the storm main phase when the storm is initiated by HSS/SIRs or by ICME sheath regions, while a more gradual and longer lasting increase is found in storms driven by magnetic clouds within the ICMEs. In all the driver categories the Joule heating during the main phase is concentrated on the morning and afternoon auroral ovals, with the largest hemispheric median heating of 240-250 GW seen at the end of the main phase in storms driven by sheaths and magnetic clouds. Joule heating is highly correlated with the SuperMAG SME index ($R^2$ values between 0.71-0.76), while correlation with the Newell coupling function is moderate ($R^2$ values between 0.52-0.56). Magnetic cloud driven storms have the highest geoefficiency in terms of solar wind energy input driving Joule heating.

Earth’s field-aligned currents (FAC) exhibit pronounced hemispheric and seasonal dependence, reflecting asymmetries between the hemispheres in magnetosphere-ionosphere coupling processes. These asymmetries are caused not only by ionospheric conductivity but also by other factors including the deviation of Earth’s magnetic field from a simple dipole, the offset of magnetic and geographic poles, Earth’s orbital inclination, and the Interplanetary Magnetic Field (IMF). The Iridium constellation has provided years’ worth of engineering-grade magnetometer data through the AMPERE program. This study uses AMPERE data from 2019 to 2022 to analyze FAC variability using Principal Component Analysis (PCA) implemented as a nonlinear sequential regression. To separate seasonal and hemispheric effects, PCA is conducted for each hemisphere and season. The analysis has identified that the mean and leading modes of FAC variability are similar between both hemispheres under comparable solar irradiance conditions. While the leading mode of FAC variability for local summer shows strong dayside variability, the local winter mode shows pronounced variability in the dusk and pre-midnight region. To further investigate these modes, we utilize Canonical Correlation Analysis (CCA), resulting in modes of variability expressed by a linear combination of Empirical Orthogonal Functions (EOFs) that maximize correlation between the hemispheres for a specific season. This approach allows us to transform the FAC interhemispheric morphology identified independently for each hemisphere and obtain grid-point-by-grid-point quantification of hemispheric asymmetries utilizing the Multivariate Alteration Detection (MAD) technique. This CCA-MAD technique reveals that hemispheric similarities are greatest in local summer, whereas the greatest hemispheric differences are observed during the equinox.

We study the morphology and magnitude of dayside field‐aligned currents (FACs) for northward interplanetary magnetic field (IMF) using observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). For near‐zero IMF clock angles the FACs form a quadrupolar pattern centered at local noon, which is associated with the reverse convection cells driven by lobe reconnection. The poleward pair of FACs, known as the northward‐ or NBZ FAC system, comprises upward and downward FAC pre‐ and post‐noon; and the equatorward pair have the opposite polarity. The magnitude of the NBZ FACs is modulated by dipole tilt and phase of the solar cycle, the former controlling the solar zenith angle in the vicinity of the FACs, and the latter controlling the solar radio flux at 10.7 cm (F10.7), both of which contribute to the conductance of the ionosphere. The NBZ FACs are also modulated by the magnitude of the IMF (or the Z‐component of the IMF for near‐zero clock angle), which we presume controls the lobe reconnection rate. The NBZ FACs do not respond to the X‐component of the IMF, solar wind speed, nor solar wind density, so we presume that these do not affect the lobe reconnection rate. High solar wind speed leads to the appearance of region 1 and 2 FACs at auroral latitudes, which we suggest are associated with a viscous interaction between the solar wind and the magnetosphere.