To prepare for the arrival of the JUICE (JUpiter ICy moons Explorer) mission, developing a comprehensive and physically grounded magnetic field model is essential to understand the interactions with charged particles and the complicated plasma dynamics. Existing models range from simplified representations, such as the model only include dipole field and induced ocean, to computationally intensive magnetohydrodynamic (MHD) or hybrid simulation codes, yet a critical gap remains for a model balancing physical fidelity with computational efficiency. We aim to present the development of a semi-empirical magnetic field model of Ganymede, based on the frameworks used for Mercury and Earth. Our superposition model integrates observational constraints with theoretical insights to accurately represent the Jovian background field and Alfvén wing, Ganymede’s internal field, the ocean-induced field, and the field generated from different currents in Ganymede’s magnetosphere. The model enables rapid, scalable simulations of particle access across the magnetosphere. It will support mission planning, enhance the interpretation of JUICE data, and provide key predictions for particle-driven surface processes on magnetized icy moons. Ultimately, this work enhances our understanding of moon-magnetosphere interactions and the dynamic interplay between internal fields, induced currents, and external plasma environments.

The solar wind interaction with Mercury’s magnetic field generates a bow shock in front of the planet. As at Earth, the region upstream of the shock that is magnetically connected to it, and known as foreshock, is permeated by a variety of waves. The characteristic frequencies and wave properties so far reported are (i) high frequency 2 Hz whistler waves (similar to the 1 Hz waves at Earth), (ii) intermediate frequency of 0.8 Hz, whose properties and formation mechanism remains unknown and (iii) lower frequency compressive waves in the 0.3 Hz range (corresponding to the large amplitude 30-s waves observed at Earth’s foreshock). The existence of ultra-low frequency waves indicates that backstreaming ions are able to drive instabilities as in the terrestrial case. However, simultaneous occurrence of different modes at Earth is not often observed. In this work we use Messenger magnetic field data to study some examples of extended regions at Mercury’s foreshock where multiple wave modes at frequencies 2, 0.8 and 0.3 Hz co-exist. The waves can maintain coherence over long intervals of time which may be related to the fact that the shock is weaker with Mach numbers in the range 2-5, and so that less backstreaming ions and density gradients are expected. Future work using plasma data from the BepiColombo mission are needed to understand in more detail wave generation and evolution in Mercury’s environment.

Mercury hosts a highly dynamic magnetosphere in which energy and momentum can be transported between different regions by field-aligned currents (FACs). The region 1 FAC are generated near the dawnside magnetopause, propagate along magnetic field lines toward the planet, and return toward the dusk magnetospheric flank. In-situ observations have established FACs as a ubiquitous feature of the Hermean magnetosphere. However, in the absence of a substantial ionosphere, the mechanisms by which these currents close remain poorly understood. In this study, we present the analysis results from in-situ MESSENGER magnetic field observations and apply the magnetometric resistivity inversion technique to infer FAC closure pathways above and within the planetary surface and within Mercury’s interior without any pre-assumptions about the conductivity structure. We further show the influence of different inversion side constraints, including solenoidal current continuity and minimum-norm regularization, on the inferred current systems.

We examine the first three BepiColombo Mercury flybys Using data from the Miniature Ion Precipitation Analyzer (MIPA), an ion mass analyzer in the Search for Exospheric Refilling and Natural Abundances package on the Mercury Planetary Orbiter designed to study magnetospheric dynamics. These flybys all passed from dusk to dawn through the nightside equatorial region but were noticeably different from each other. In the first flyby, we observe a low latitude boundary layer and 1 keV ions near closest approach. For flybys 2 and 3 we see ions up to 14 keV in the same location, including freshly injected precipitating ions inside the loss cone. High time resolution data from flyby 3 show variations consistent with bursty bulk flows 10s long and occurring over s periods, the first such observation in this region. MIPA data demonstrate that high-energy injection processes are an important source of precipitation ions at Mercury.

Planetary magnetic fields act as protective barriers, shielding a planet’s atmosphere and surface from energetic charged particles originating from its parent star and outer space. During its fourth flyby of Mercury on 4 September 2024, the ESA/JAXA BepiColombo mission passed within just 165 km of the planet’s surface. This close approach coincided with the elevated fluxes of high-energy charged solar particles, providing an opportunity to study shielding processes of a planet in close proximity to its star. Here, utilizing observations from the Solar Intensity X-Ray and Particle Spectrometer (SIXS) onboard BepiColombo, we investigate the penetration of high energy particles from the Sun into Mercury’s magnetosphere. The variations in energetic particle fluxes depend on the particle type, direction and energy. Our analysis reveals that the disappearance of protons is related to magnetic shadowing by the planet, while electrons dropouts occur due to a wide magnetic loss cone. These findings show that sustained high-energy particle precipitation can occur during solar energetic particle events on weakly magnetized rocky planets orbiting close to their parent star. Such processes probably play a significant role in regolith alteration and exospheric variability, with broader implications for surface geochemistry and potential habitability. BepiColombo’s flyby at just 165 km from Mercury observed sustained precipitation of energetic solar protons and electrons onto the planet’s surface, with potentially long-term impact on its weathering and mineralogy.

Planet Mercury, with its weak internal magnetic field, is continuously exposed to an intense solar wind. This interaction becomes particularly dynamic during coronal mass ejections, resulting in a strong compression of the magnetosphere. Such events drive electrical currents within the planet, which, depending on the planetary conductivity structure, lead to secondary magnetic fields detectable outside. Analysis of these induced fields provides insights into Mercury’s interior structure.Here, we utilized data from the Helios-1 probe, recorded during a CME at 0.31 AU, to evaluate changes in solar wind conditions and their impact on Mercury's magnetosphere. We applied a semi-empirical model to estimate the external field variations and employed endmembers of radial symmetric conductivity models to calculate the range of induced magnetic fields. Our analysis highlights the influence of these variations on Mercury's upper mantle layers, taking into account both dipolar and quadrupolar components of the magnetic field. Eventually, we predict the potential induced magnetic fields at the future location of the BepiColombo spacecraft, currently en route to Mercury.

Modeling the plasma and magnetic field state in Mercury's magnetosheath is one of the most urgent tasks in Mercury science in view of the upcoming BepiColombo mission. By considering the steady-state and constructing the Laplace equation for the scalar magnetic potential in the magnetosheath (eliminating the interplanetary magnetic field in the magnetosphere and vice versa), the plasma and magnetic field state is obtained as a function of the solar wind condition and the spatial coordinates of the magnetosphere. We make extensive use of the exact solution of the Laplace equation for the parabolically shaped magnetosheath, and map the solution onto the realistic shape of magnetosheath by assuming the magnetosheath thickness is scalable between the parabolic shape and the realistic shape along the magnetopause-normal direction. The quality of the constructed model can successfully be tested against the global hybrid simulation of Mercury's magnetosheath, promising that the model serves as a useful tool for BepiColombo's detailed magnetosheath studies at Mercury.

Mercury is known to possess a Magnetosphere that is highly responsive to the upstream Solar Wind conditions. Previous studies using MESSENGER data have contributed to understanding the dynamics of Mercury's respond to the upstream. However, the interactions between the Magnetospheric plasma and the Solar Wind is yet to be fully understood; and it is indeed one of the main focuses of the ESA/JAXA's current mission, BepiColombo. We report the observations of BepiColombo's flyby-3 at Mercury on 19th June 2023, using ion data from SERENA-PICAM and magnetic field data from MAG/MGF instruments. The preliminary analyses have given an insight into the rapidly changing plasma, at the inbound Magnetopause crossing. There is evidence that bursty reconnection could be the main contributor to such dynamic boundary.

Mercury, the smallest and innermost planet of our solar system, is exposed to a strong solar wind. The internal field is dipole-dominated, relatively weak, axisymmetric and significantly offset towards north. Through the interaction with the strong solar wind, this field leads to a comparatively small and dynamic magnetosphere.To first order the magnetopause completely separates the magnetosphere from the magnetosheath and thus no magnetic field may penetrate this boundary. In reality, the magnetosheath magnetic field may diffuse across the very thin boundary within a finite time. We first investigate how the magnetosheath magnetic field changes under different IMF conditions and directions. Second, we can investigate the penetration of the magnetic field from the magnetosheath through the magnetopause inside the magnetosphere and obtain the structure of the IMF influence on the Hermean magnetosphere.

<p>Mercury is the smallest an innermost planet of our solar system and has a dipole-dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset towards north. Through the interaction with the solar wind, this field leads to a magnetosphere. Compared to the magnetosphere of Earth, Mercury’s magnetosphere is smaller and more dynamic.</p> <p>A semi-empirical magnetospheric model can capture the large-scale magnetospheric structures. Using the residuals between in-situ data and the model prediction we further seek to improve our understanding of the Hermean magnetosphere. <br />To first order the magnetopause completely separates the magnetosphere from the magnetosheath and thus no magnetic field may penetrate this boundary. In reality, the magnetosheath field may diffuse across the very thin boundary within a finite time. </p> <p>Here, we investigate this penetration and compare the different interplanetary field (IMF) components by their ability to enter into Mercury’s Magnetosphere. For this, we use in-situ MESSENGER magnetic field data to estimate the IMF for the time frame with the probe located inside the magnetosphere. The amount of penetration is found by least-square fitting to magnetospheric model results.<br />First statistical results indicate that the penetration is stronger under southward IMF conditions.</p>

Mercury possesses a weak planetary dipole moment and is subject to a strong solar wind inflow. Thus, a small magnetosphere is formed. On the nightside, a neutral current sheet elongates the magnetic field lines to form a magnetotail. From hybrid simulations it is known that this current sheet reacts to changes in the interplanetary magnetic field (IMF). In order to understand the magnetospheric reaction to changes in the solar wind, it is essential to further assess the neutral current sheet movements. The strongly radial IMF at Mercury facilitates magnetopause reconnection in high latitudes which decreases the magnetic pressure in one of the magnetospheric lobes depending on the radial IMF polarity. This produces a northward (or southward) shift of the neutral sheet. Here, we present statistical results from in-situ MESSENGER magnetic field data analysis on the IMF direction as well as the neutral sheet displacement. MESSENGER was a single probe in orbit around Mercury and, as such, it was blind to the solar wind state after having entered the bow shock. Thus, we need to estimate the current IMF radial polarity for the time frame with the probe located inside the magnetosphere. For this, we evaluate different interpolation methods with an adapted bootstrap analysis method on data taken within the upstream solar wind at Mercury. Eventually, the outcome of the statistical analysis on the neutral sheet displacement is compared to the results from hybrid simulations done in the past.

Mercury is the smallest an innermost planet of our solar system and has a dipole-dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset towards north. Through the interaction with the solar wind, this field leads to a magnetosphere. Compared to the magnetosphere of Earth, Mercury’s magnetosphere is smaller and more dynamic. To understand the magnetospheric structures and processes we use in-situ MESSENGER data to develop a semi-empiric model, which can explain the observations and help to improve the mission planning for the BepiColombo mission en-route to Mercury. We will present this semi-empiric KTH-model, a modular model to calculate the magnetic field inside the Hermean magnetosphere. Korth et al. (2015 and 2017) published a model, which is the basis for the KTH-Model. In this new version, the calculation of the magnetic field for the neutral current sheet is restructured based on observations rather than ad-hoc assumptions so that the description is more realistic. Furthermore, a new model is added to depict the partial ring current. An analysis of the residuals shows a better visibility of the field-aligned currents. In addition, this model offers the possibility to improve the main field determination.

The internal magnetic field of Mercury is best described by a northward offset dipole with almost zero obliquity. Its offset, weakness, axisymmetry and lack of secular variation still poses a challenge to dynamo theory. After NASA’s Mariner 10 flybys in the 1970’s and MESSENGER’s orbital mission in 2011-2015, BepiColombo performed a flyby at Mercury in October 2021. For the first time, magnetic field measurements are obtained from the southern hemisphere by the fluxgate magnetometer MPO-MAG. We will present an overview of the flyby data and compare the new in-situ data to magnetospheric models obtained from the previous missions to the innermost terrestrial planet. Does the flyby data reveal any secular variation? Has the dipole offset changed? These are some of the questions we will discuss with this unprecedented magnetometer data. We will close with a discussion on what is to be expected from the orbital phase of BepiColombo.

Mercury is the smallest an innermost planet of our solar system and has a dipole-dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset towards north. Through the interaction with the solar wind, this field leads to a magnetosphere. Compared to the magnetosphere of Earth, Mercury’s magnetosphere is smaller and more dynamic. To understand the magnetospheric structures and processes we use in-situ MESSENGER data to develop a semi-empiric model, which can explain the observations and help to improve the mission planning for the BepiColombo mission en-route to Mercury. We will present this semi-empiric KTH-model, a modular model to calculate the magnetic field inside the Hermean magnetosphere. Korth et al. (2015 and 2017) published a model, which is the basis for the KTH-Model. In this new version, the calculation of the magnetic field for the neutral current sheet is restructured based on observations rather than ad-hoc assumptions so that the description is more realistic. Furthermore, a new model is added to depict the partial ring current. An analysis of the residuals shows a better visibility of the field-aligned currents. In addition, this model offers the possibility to improve the main field determination.