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Mercury is the smallest and innermost planet of our solar system and has a dipole‐dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset toward north. Through the interaction with the solar wind, a magnetosphere is created. 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 further a semi‐empiric model of the magnetospheric magnetic field, which can explain the observations and help to improve the mission planning for the BepiColombo mission en‐route to Mercury. We present this semi‐empiric KTH22‐model, a modular model to calculate the magnetic field inside the Hermean magnetosphere. Korth et al. (2015, https://doi.org/10.1002/2015JA021022, 2017, https://doi.org/10.1002/2017gl074699) published a model, which is the basis for the KTH22‐model. In this new version, the representation of the neutral sheet current magnetic field is more realistic, because it is now based on observations rather than ad‐hoc assumptions. Furthermore, a new module is added to depict the eastward ring shaped current magnetic field. These enhancements offer the possibility to improve the main field determination. In addition, analyzing the magnetic field residuals allows us to investigate the field‐aligned currents and their possible dependencies on external drivers. We see increasing currents under more disturbed conditions inside the magnetosphere, but no clear dependence on the z‐component of the interplanetary magnetic field nor on the magnetosheath plasma β. Key Points: We present a revised model of Mercury's magnetospheric magnetic fieldThe model now includes an eastward ring shaped current and the neutral sheet current is calculated more precisely with Biot Savart's lawThe strength of the field‐aligned currents increases with higher magnetic activity [ABSTRACT FROM AUTHOR]

Context. Magnetic holes are ubiquitous structures in the solar wind and in planetary magnetosheaths. They consist of a strong depression of the magnetic field strength, most likely in pressure balance through increased plasma pressure, which is convected with the plasma flow. These structures are created through a plasma temperature anisotropy, where the perpendicular temperature (with respect to the magnetic field) is greater than the parallel temperature. The occurrence rate of these magnetic holes between Earth and Mercury can give us information about how the solar wind conditions develop on their way from the Sun to the outer Solar System. They also give information about basic plasma processes such as diffusion of magnetic structures. Aims. In this study we investigate the occurrence, size, and depth of magnetic holes during the cruise phase of BepiColombo and compare them with earlier studies. Methods. The BepiColombo magnetometer data were used to find the magnetic holes. We determined the size in seconds, the depth with respect to the background field, and the rotation angle of the background field across the structure. Minimum variance analysis delivers the polarization state of the magnetic holes. A direct comparison is made to the results obtained from the MESSENGER cruise phase. Results. We find an almost constant occurrence rate for magnetic holes between Mercury and Earth. The size of the holes is determined by the plasma conditions at the location where they are created and they grow in size, due to diffusion, as they move outwards in the Solar System. The greater the rotation of the background magnetic field across the structure, the larger the minimum size of the magnetic hole is. [ABSTRACT FROM AUTHOR]

External solar wind variability causes motion of the magnetopause and changes of this boundary's current structure, and the resulting inductive processes, may be exploited to determine the interior structure of magnetized planets. In preparation for the arrival of the BepiColombo spacecraft at Mercury, we here assess solar wind ram pressure forcing in this planet's environment, through analysis of data acquired by the Helios spacecraft, and the impact on the magnetopause's inducing field. These measurements suggest that BepiColombo will see highly unpredictable solar wind conditions and that the inducing field generated in response to variable solar wind ram pressure is non‐uniform across the planet's surface. The inducing magnetic field spectrum, with frequencies in the range of ∼5.5×10−5–1.5×10−2Hz $\sim 5.5\times {10}^{-5}\mbox{--}1.5\times {10}^{-2}\mathrm{H}\mathrm{z}$, suggests that the transfer functions derived from the two BepiColombo spacecraft could allow us to obtain a profile of conductivity through Mercury's crust and mantle. Plain Language Summary: In order to develop our understanding of the formation and evolution of Mercury, and hence, the mechanisms involved in the formation of our solar system, we need to precisely determine the interior composition of the terrestrial planet. Due to Mercury's magnetic field, that similar to Earth is produced in its liquid iron core, a layer of electric current shields the planet from the stream of charged particles ejected from the Sun, known as the solar wind. As Mercury is very close to the Sun, this current layer is strongly under the influence of the variable solar wind and the magnetic field of the Sun embedded in it. Here, we assess how changes in the solar wind impact this layer of current and how we could use this process as a natural metal detector to probe planet's interior when the BepiColombo mission arrives at Mercury in 2025. Key Points: Inductive processes due to forcing of Mercury's magnetosphere by solar wind can be used by BepiColombo to probe the planet's interiorVariable solar wind seen by Helios results in a non‐uniform inducing field at Mercury's surface similar to that BepiColombo will seeFrequencies in derived inducing field spectra could be used to obtain a conductivity profile through to Mercury's mantle with BepiColombo [ABSTRACT FROM AUTHOR]

Mercury's magnetosphere is a unique and dynamic system, primarily due to the proximity of the planet to the Sun and its small size. Interactions between solar wind and embedded interplanetary magnetic field (IMF) and the dayside Hermean magnetosphere drive an electric current on the system's magnetopause boundary. So far, electromagnetic induction due to magnetopause motion in response to changing external pressure has been used to constrain Mercury's iron core size. Here we assess the impact a changing IMF direction has on the Hermean magnetopause currents, and the resulting inducing magnetic field. Observations made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during dayside magnetopause boundary crossings in the first "hot season," are used to demonstrate the importance of the IMF direction to Mercury's magnetopause currents. Our 16 boundary crossings show that introduction of external IMFs change the magnetopause current direction by 10° to 100°, compared to the case where only the internal planetary field is considered. Analytical modeling was used to fill in the bigger picture and suggests for an east‐west reversal of the IMF, typical of the heliospheric current sheet sweeping over Mercury's magnetosphere, the inducing field at Mercury's surface caused by the resulting magnetopause current dynamics is on the order of 30% of the global planetary field. These results suggest that IMF variability alone has an appreciable effect on Mercury's magnetopause current and generates a significant inducing magnetic field around the planet. The arrival of the BepiColombo mission will allow this response to be further explored as a method of probing Mercury's interior. Plain Language Summary: Mercury has a large iron core, in which a magnetic field is produced. Determining the precise size of this core and the composition of Mercury's interior are key to developing our understanding of the terrestrial planet's formation and evolution and, therefore, the mechanisms involved in the formation of our solar system. Changes to the sheet of electric current that shields Mercury from the stream of charged particles ejected from the Sun can be used as a natural metal detector to reveal properties of Mercury's subsurface. Through observational data and modeling of the interactions between Mercury's magnetic field and the external magnetic field generated by the Sun, we assess a way in which this electric current sheet can be altered. Our results showed that variability in the orientation of the external magnetic field from the Sun has an appreciable impact on Mercury's shielding electric current sheet. The arrival of the BepiColombo mission in 2025 will allow this response to be further explored as a method of probing Mercury's interior. Key Points: We assess the impact of a changing interplanetary magnetic field (IMF) direction on Mercury's magnetopause current and resulting inducing fieldWe show that variability of the IMF direction alone has an appreciable effect on Mercury's magnetopause currentsThis response, and BepiColombo's arrival, will allow inducing fields to be further explored as a method of probing Mercury's interior [ABSTRACT FROM AUTHOR]

We present a comprehensive statistical study of magnetic holes, defined as localized decreases of the magnetic field strength of at least 50%, in the solar wind near Mercury, using MESSENGER orbital data. We investigate the distributions of several properties of the magnetic holes, such as scale size, depth, and associated magnetic field rotation. We show that the distributions are very similar for linear magnetic holes (with a magnetic field rotation across the magnetic holes of less than 25°) and rotational holes (rotations >25°), except for magnetic holes with very large rotations (≳140°). Solar wind magnetic hole scale sizes follow a log‐normal distribution, which we discuss in terms of multiplicative growth. We also investigate the background magnetic field strength of the solar wind surrounding the magnetic holes, and conclude that it is lower than the average solar wind magnetic field strength. This is consistent with finding solar wind magnetic holes in high‐β regions, as expected if magnetic holes have a connection to magnetic mirror mode structures. We also present, for the first time, comprehensive statistics of isolated magnetic holes in a planetary magnetosheath. The properties of the magnetosheath magnetic holes are very similar to the solar wind counterparts, and we argue that the most likely interpretation is that the magnetosheath magnetic holes have a solar wind origin, rather than being generated locally in the magnetosheath. Key Points: We present a comprehensive statistical study of magnetic holes in the near‐Mercury solar wind and magnetosheathWe find that magnetic hole scale sizes are log‐normally distributedWe suggest that a majority of the magnetosheath magnetic holes have a solar wind origin [ABSTRACT FROM AUTHOR]

Various possibilities are currently under discussion to explain the observed weakness of the intrinsic magnetic field of planet Mercury. One of the possible dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its weak magnetic field, Mercury exhibits a small magnetosphere whose subsolar magnetopause distance is only about 1.7 Hermean radii. We consider the magnetic field due to magnetopause currents in the dynamo region. Since the external field of magnetospheric origin is antiparallel to the dipole component of the dynamo field, a negative feedback results. For an αΩ-dynamo, two stationary solutions of such a feedback dynamo emerge: one with a weak and the other with a strong magnetic field. The question, however, is how these solutions can be realized. To address this problem, we discuss various scenarios for a simple dynamo model and the conditions under which a steady weak magnetic field can be reached. We find that the feedback mechanism quenches the overall field to a low value of about 100-150 nT if the dynamo is not driven too strongly. [ABSTRACT FROM AUTHOR]

Remotely sensed interplanetary scintillation (IPS) data from the Institute for Space-Earth Environmental Research (ISEE), Japan, allows a determination of solar-wind parameters throughout the inner heliosphere. We show the 3D analysis technique developed for these data sets that forecast plasma velocity, density, and component magnetic fields at Earth, as well at the other inner heliospheric planets and spacecraft. One excellent coronal mass ejection (CME) example that occurred on the 10 March 2022 was viewed not only in the ISEE IPS analyses, but also by the spacecraft near Earth that measured the CME arrival at one AU. Solar Orbiter, that was nearly aligned along the Earth radial at 0.45 AU, also measured the CME in plasma density, velocity, and magnetic field. BepiColombo at 0.42 AU was also aligned with the STEREO A spacecraft, and viewed this CME. The instruments used here from BepiColombo include: 1) the European-Space-Agency Mercury-Planetary-Orbiter magnetic field measurements; 2) the Japan Aerospace Exploration Agency Mio spacecraft Solar Particle Monitor that viewed the CME Forbush decrease, and the Mercury Plasma Experiment/Mercury Electron Analyzer instruments that measured particles and solar-wind density from below the spacecraft protective sunshield covering. This article summarizes the analysis using ISEE, Japan real-time data for these forecasts: it provides a synopsis of the results and confirmation of the CME event morphology after its arrival, and discusses how future IPS analyses can augment these results., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2023.)

The observed weakness of Mercury's magnetic field poses a long-standing puzzle to dynamo theory. Using numerical dynamo simulations, we show that it could be explained by a negative feedback between the magnetospheric and the internal magnetic fields. Without feedback, a small internal field was amplified by the dynamo process up to Earth-like values. With feedback, the field strength saturated at a much lower level, compatible with the observations at Mercury. The classical saturation mechanism via the Lorentz force was replaced by the external field impact. The resulting surface field was dominated by uneven harmonic components. This will allow the feedback model to be distinguished from other models once a more accurate field model is constructed from MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and BepiColombo data.