Urs Ganse

Dipolarization fronts (DFs) are transient structures occurring in Earth's magnetotail, characterized in spacecraft observations by sudden enhancements of the dipolar magnetic field component. Often linked to transient channels of fast earthward plasma flow, DFs are responsible for energy conversion, particle acceleration and magnetic flux transport. We investigate a DF event identified in a global hybrid-Vlasov simulation and study how it converts energy from electromagnetic fields to plasma flow and heat. We analyze velocity distribution functions associated with the event and trace test particles sampled from them. Our results indicate that non-compressional effects can be important to DF dissipation processes. Test particle tracing reveals that the DF actively replaces the local plasma population by transporting new particles into the region and displacing the pre-existing plasma, leading to a decoupling between the plasma properties before and after the front. The results may explain atypical temperature decreases reported in spacecraft observations of DFs.

We investigate ion velocity‐space features within the exhaust of magnetopause reconnection using a global hybrid‐Vlasov simulation. The Hermite transform and Gaussian Mixture Model (GMM) are applied to quantify the complexity of velocity‐space structures that arise during the mixing of magnetospheric and magnetosheath ion populations. Using the Hermite transform, we calculated the enstrophy metric to quantify available free energy. From the GMM multi‐beam decomposition we obtained pseudothermal energy. We find that pseudothermal energy appeared due directed beam motion accounts for nearly half of the apparent thermal energy within the exhaust. The enstrophy evolves similarly to the pseudothermal energy estimate selected via the optimal number of Gaussian components. These results revise ion energy partitioning during magnetopause reconnection and suggest an approach for defining thermal energy in non‐Maxwellian plasmas.

We report on the relationship between flux transfer events (FTEs) at Earth's magnetopause and the geoelectric field that is induced near the FTEs' magnetic footpoints. We study this system using the global hybrid‐Vlasov code Vlasiator , which has recently been extended to model ionospheric physics. We also highlight the significance of 3D magnetic null points, which in our simulation can separate the FTEs into multiple flux ropes. Near the null points, the coiled FTE magnetic field lines are rerouted toward Earth, so that the magnetic footpoints are planted near the Region 1 ionospheric current system. The helicities of the flux ropes are organized by the ‐component (Geocentric Solar Ecliptic) of the magnetic field at the Earth's magnetopause. This occurs in our simulation due to the absence of a ‐component of the interplanetary magnetic field, which would normally supply the FTE guide field that determines the helicity. We observe Alfvénic, Earthward‐flowing field‐aligned currents generated near the magnetopause that correlate with the passage of FTEs nearby. These pulses of current coincide with the formation of rotational geoelectric field structures that appear near the noon meridian and propagate around the auroral oval toward the nightside.

The dynamics of the inner magnetosphere and magnetotail are determined by a number of factors such as magnetic reconnection, plasma instabilities, and large-scale plasma motion. We use the global hybrid-Vlasov simulation Vlasiator to study these dynamics as well as their signatures in the ionosphere. We observe magnetic reconnection, fast flows, and vorticity in the transition region between the Earth's dipolar field and the magnetotail. In our simulation, reconnection is first triggered at the dawn and dusk sides of the magnetotail current sheet. It then spreads across the current sheet. Concurrently, an azimuthally periodic, wave-like density structure develops in the transition region along with fast Earthward flows and enhanced vorticity patterns. The Earthward flows and vorticity induce field-aligned currents, which map onto the ionospheric simulation domain, creating a patchy current distribution. We find that the event is driven by the combination of reconnection-induced fast flows and the ballooning/interchange instability.

Vlasiator is a global hybrid-Vlasov space plasma simulation, modeling the velocity distribution of ions in a large region of near-Earth space. Due to the high memory and computation demands of the kinetic method as well as the large physical scale, optimisations are required to make simulation feasible. This presentation explores optimisations used in the spatial and velocity grids.We first consider the spatial dimension. For this, Vlasiator utilises cell-based octree adaptive mesh refinement (AMR). Essentially, each spatial cell may be split in all three spatial dimensions to create eight smaller children in order to improve simulation accuracy in relevant regions. This can be repeated if necessary, with runs typically using four levels of refinement. Refinement may be done statically at the start of the simulation, or dynamically based on the plasma parameters.Vlasiator uses a combination of several parameters for dynamic runtime refinement. These include scaled gradients of macroscopic variables to detect steep changes, the ratio of the current density to perpendicular magnetic field for current sheets and reconnection, as well as pressure anisotropy and vorticity for foreshock refinement.For the velocity grid we use a somewhat similar method of stretching. In order to simplify translation, the velocity grid is static and identical in each spatial cell. To eliminate splitting of acceleration pencils, the size of cells in each coordinate direction is a function of that coordinate. Thus if we consider a grid with higher resolution around some point, the grid appears stretched along the coordinate axes when moving away from that point. The main purpose of the stretched grid is to enable modeling of colder distributions requiring a higher resolution without increasing resolution for the entire velocity grid.Combining these optimisations enables simulation on modern supercomputers with scale and resolution which would be unfeasible without them. This is achieved by limiting resources expended on regions where they are less critical for simulation accuracy and the scientific focus of a given run, while allowing higher fidelity in more important regions. These methods are applicable to other kinetic simulations, as well as grid-based simulations in general.

We investigate ion velocity-space dynamics within the exhaust region of asymmetric magnetopause reconnection using global hybrid-Vlasov simulations. To quantify the complexity of velocity-space structures arising from the mixing of magnetospheric and magnetosheath ion populations, we employ the Hermite transform and Gaussian Mixture Model (GMM) analyses. In the Hermite representation, we use a fixed number of 22 harmonics to ensure computational feasibility. From this expansion, we compute a scalar measure of enstrophy—the total power contained in the non-zero Hermite modes—which characterizes the available free energy in the system. For the GMM approach, we test different numbers of ion populations and evaluate the corresponding multi-beam thermal energy for each decomposition. We further define the thermal energy drop as the relative difference between the thermal energy of an equivalent single-Maxwellian distribution and the total multi-beam thermal energy. Both enstrophy and thermal energy drop diagnostics (for any number of beams considered) exhibit consistent trends during the phase of plasma thermalization and anisotropic acceleration, demonstrating that the redistribution of thermal energy can be effectively captured even with a limited number of Hermite modes.

The foreshock is a large region of space upstream of a collisionless shock characterised by the presence of particles of solar wind origin that have been reflected at the shock. The interaction between these particles and the pristine solar wind can also generate ultra-low frequency (ULF) waves that are advected toward the quasi-parallel bow shock, that is, the part of the bow shock where the interplanetary magnetic field (IMF) and shock normal are almost parallel. The interaction between the ULF waves and the shock cause the quasi-parallel bow shock and the magnetosheath downstream of it to become turbulent and dynamic, which in turn can lead to the formation of transient structures such as magnetosheath jets. During intervals of quasi-radial IMF at Earth, the quasi-parallel bow shock is upstream of the dayside magnetosheath, and the dynamics of the quasi-parallel bow shock and magnetosheath have larger potential to have geoeffective consequences. Understanding the properties of the foreshock is therefore important, but studying the overall structure of this extended region using point measurements by spacecraft is difficult.In this study, we present the first ever global 6D (3D+3V) hybrid-Vlasov simulation of near-Earth space with quasi-radial IMF conditions, featuring a high-resolution foreshock. We introduce the new criterion that was used to identify the foreshock for the purpose of applying adaptive mesh refinement, and elaborate on some of the technical challenges that needed to be overcome to make the simulation possible. We investigate the effects of ULF waves on the velocity distributions in different parts of the foreshock. Finally, we probe the velocity distributions inside magnetosheath jets in order to study their kinetic nature.

Magnetic reconnection and plasma instabilities in Earth's magnetotail can lead to dipolarisation fronts (DFs), rapid enhancements of the magnetic field component aligned with Earth's magnetic dipole axis. DFs are often associated with channels of fast plasma flow, and they accelerate particles and transport magnetic flux to the inner magnetosphere. Satellite observations suggest that DFs propagate Earthward until they decelerate and either vanish, rebound or deflect as they reach the transition region between tail-like and dipole-like magnetic field configurations.We study the evolution and characteristics of DFs in a global magnetospheric simulation conducted using Vlasiator, a 3D hybrid-Vlasov code that solves ion dynamics by evolving the ion distribution functions explicitly. Events are identified using a magnetic field time derivative threshold, and individual fronts are tracked from their formation to their termination. Preliminary results show how magnetic forces affect the propagation of DFs and how energy is converted as the fronts develop. Tracking the evolution of DFs in global simulations offers a complementary point of view to satellite observations, where following individual fronts is often impossible.

Conductances are key properties of the ionospheric electrodynamics and the difficulty of measuring them directly is a significant limitation to the usefulness of many analysis techniques. We have utilized all available field-aligned observations from the EISCAT incoherent scatter ultra-high frequency (UHF) radar since 2001 and from the 42 m EISCAT Svalbard Radar (ESR) since 1998 to develop a new empirical model for estimating the high-latitude ionospheric Hall and Pedersen conductances. The solar radiation component of the model is parametrized with the solar zenith angle and the F10.7 index, and the auroral precipitation component is parametrized with the magnetic local time and the divergence-free part of the horizontal ionospheric current density, which is obtained from ground-based magnetic field observations. We have also derived a new technique based on spherical elementary current systems that can be used to solve for the ionospheric potential electric field and field-aligned current density from known ionospheric conductances and ground-based magnetic field observations, taking into account induction in the ionosphere and in the ground. The new empirical conductance model and solver were applied to IMAGE magnetometer network observations. Comparison of the results with Swarm and Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite observations showed reasonable agreement in the electric field profile and direction of the field-aligned current, but in the post-midnight sector the modelled amplitudes tended to be weaker than observations. The combination of the new conductance model and analysis technique allows estimating the key properties of ionospheric electrodynamics from ground-based magnetic field observations.