The Magnetic Field of Mercury: On the Influence of the Magnetosphere on the Dynamo Within the Planetary Interior

Als Ursache des ungewöhnlich schwachen planetaren Magnetfelds wird ein Dynamoprozess im Inneren angenommen. Die Magnetosphäre kann diesen Dynamoprozess dämpfen. In dieser Arbeit wird das Feedbackdynamomodell für den Merkur diskutiert. Die Wechselwirkung des Sonnenwindes mit einem merkurähnlichen Dipolfeld, wird anhand einer Reihe Magnetosphärenmodelle untersucht. Dazu wird das interne Dipolmoment variiert und das sich ergebende externe Feld bestimmt. Dieses Feld besteht im wesentlichen aus dem Magnetopausenfeld. Die Amplitude dieses Feldes wird nur vom Sonnenwinddruck bestimmt. Dies bestätigt sich in einer realitätsnahen Sonnenwindsimulation. Das parametrisierte Magnetopausenfeld wird auf ein vereinfachtes kinematisches Dynamomodell angewendet. Es existiert ein Lösungsbereich gibt, bei dem der Dynamo durch den Feedback frühzeitig stabilisiert wird. Der Dynamo darf nicht zu stark angetrieben und das Saatfeld nicht zu stark sein, damit des externe Feld einen dämpfenden Effekt haben kann. Die weitere Analyse ergibt, dass stabile, schwache Dynamos mit zunehmenden externen Feld existieren. In der Frühzeit des Sonnensystems herrschte ein wesentlich stärkerer Druck, was den Effekt des externen Feldes auf den Merkurdynamo verstärkt. Mittels einer realitätsnahen Dynamosimulation können die Ergebnisse der kinematischen Modellierung für den heutigen Merkur bestätigt werden. Zusätzlich ergibt sich das äußere Spektrum eines planetaren Dynamos unter dem dämpfenden Einfluss der Magnetosphäre. Dieser unterbindet den Regimewechsel von der Geostrophie zur Magnetostrophie. Das äußere Magnetfeld ist durch einen schwachen Dipol dominiert,was die Eignung des Feedbackeffektes zur Erklärung des schwachen Dipolmoments demonstriert. Das Spektrum jenseits der Dipolmode wird mit den aktuellen Analyseergebnissen der MESSENGER-Mission verglichen. Das gemessene Spektrum kann bisher durch keines der vorgeschlagenen Dynamomodelle, einschließlich des Feedbackdynamos, allein erklärt werden.A dynamo process is assumed to be the cause of the unusually weak planetary magnetic field. The magnetosphere can attenuate this dynamo process. In this work, the feedback dynamo model for Mercury is discussed. The interaction of the solar wind with a dipole field similar to the Hermean is examined through a series of magnetospheric models. For this purpose, the internal dipole moment is varied and the resulting external field is determined. This field consists essentially of the magnetopause field. The amplitude of this field is determined only by the solar wind pressure. This is confirmed in a realistic solar wind simulation. The parameterized magnetopause field is applied to a simplified kinematic dynamo model. There is a solution area is where the dynamo is stabilized by the early feedback. The dynamo may not be too strongly driven and the seed field may not be too strong, so that the external field can have a dampening effect. Further analysis shows that stable, weak dynamos exist with increasing external field. In the early days of the solar system, there was a much stronger pressure, which increases the effect of the external field on the Hermean dynamo. By means of a realistic dynamo simulation, the results of kinematic modeling for present-day Mercury are confirmed. In addition, the spectrum of a planetary dynamo under the dampening effect of the magnetosphere is calculated. The feedback prevents the regime change of geostrophy to magnetostrophy. The external magnetic field is dominated by a weak dipole, which demonstrates the suitability of the feedback effect to explain the weak dipole moment . The spectrum beyond the dipolar mode is compared with the current analysis results of the MESSENGER mission. None of the proposed dynamo models including the feedback dynamo can explain the magnetic spectrum

Error Propagation of Capon’s Minimum Variance Estimator

The error propagation of Capon’s minimum variance estimator resulting from measurement errors and position errors is derived within a linear approximation. It turns out, that Capon’s estimator provides the same error propagation as the conventionally used least square fit method. The shape matrix which describes the location depence of the measurement positions is the key parameter for the error propagation, since the condition number of the shape matrix determines how the errors are amplified. Furthermore, the error resulting from a finite number of data samples is derived by regarding Capon’s estimator as a special case of the maximum likelihood estimator

On the magnetic characteristics of magnetic holes in the solar wind between Mercury and Venus

The occurrence rate of linear and pseudo magnetic holes has been determined during MESSENGER's cruise phase starting from Venus (2007) and arriving at Mercury (2011). It is shown that the occurrence rate of linear magnetic holes, defined as a maximum of 10∘ rotation of the magnetic field over the hole, slowly decreases from Mercury to Venus. The pseudo magnetic holes, defined as a rotation between 10 and 45∘ over the hole, have mostly a constant occurrence rate

Estimation of a planetary magnetic field using a reduced magnetohydrodynamic model

Knowledge of planetary magnetic fields provides deep insights into the structure and dynamics of planets. Due to the interaction of a planet with the solar wind plasma, a rather complex magnetic environment is generated. The situation at planet Mercury is an example of the complexities occurring as this planet’s field is rather weak and the magnetosphere rather small. New methods are presented to separate interior and exterior magnetic field contributions which are based on a dynamic inversion approach using a reduced magnetohydrodynamic (MHD) model and time-varying spacecraft observations. The methods select different data such as bow shock location information or magnetosheath magnetic field data. Our investigations are carried out in preparation for the upcoming dual-spacecraft BepiColombo mission set out to precisely estimate Mercury’s intrinsic magnetic field. To validate our new approaches, we use THEMIS magnetosheath observations to estimate the known terrestrial dipole moment. The terrestrial magnetosheath provides observations from a strongly disturbed magnetic environment, comparable to the situation at Mercury. Statistical and systematic errors are considered and their dependence on the selected data sets are examined. Including time-dependent upstream solar wind variations rather than averaged conditions significantly reduces the statistical error of the estimation. Taking the entire magnetosheath data along the spacecraft’s trajectory instead of only the bow shock location into account further improves accuracy of the estimated dipole moment

Improved modelling of SEP event onset within the WSA-Enlil-SEPMOD framework

Multi-spacecraft observations of solar energetic particle (SEP) events not only enable a deeper understanding and development of particle acceleration and transport theories, but also provide important constraints for model validation efforts. However, because of computational limitations, a given physics-based SEP model is usually best-suited to capture a particular phase of an SEP event, rather than its whole development from onset through decay. For example, magnetohydrodynamic (MHD) models of the heliosphere often incorporate solar transients only at the outer boundary of their so-called coronal domain -- usually set at a heliocentric distance of 20-30 $R_{\odot}$. This means that particle acceleration at CME-driven shocks is also computed from this boundary onwards, leading to simulated SEP event onsets that can be many hours later than observed, since shock waves can form much lower in the solar corona. In this work, we aim to improve the modelled onset of SEP events by inserting a "fixed source" of particle injection at the outer boundary of the coronal domain of the coupled WSA-Enlil 3D MHD model of the heliosphere. The SEP model that we employ for this effort is SEPMOD, a physics-based test-particle code based on a field line tracer and adiabatic invariant conservation. We apply our initial tests and results of SEPMOD's fixed-source option to the 2021 October 9 SEP event, which was detected at five well-separated locations in the inner heliosphere -- Parker Solar Probe, STEREO-A, Solar Orbiter, BepiColombo, and near-Earth spacecraft.Comment: 31 pages, 8 figures, 4 tables, accepted for publication in Journal of Space Weather and Space Climat

Influence of Large-scale Interplanetary Structures on the Propagation of Solar Energetic Particles: The Multispacecraft Event on 2021 October 9

An intense solar energetic particle (SEP) event was observed on 2021 October 9 by multiple spacecraft distributed near the ecliptic plane at heliocentric radial distances R ≲ 1 au and within a narrow range of heliolongitudes. A stream interaction region (SIR), sequentially observed by Parker Solar Probe (PSP) at R = 0.76 au and 48° east from Earth (ϕ = E48°), STEREO-A (at R = 0.96 au, ϕ = E39°), Solar Orbiter (SolO; at R = 0.68 au, ϕ = E15°), BepiColombo (at R = 0.33 au, ϕ = W02°), and near-Earth spacecraft, regulated the observed intensity-time profiles and the anisotropic character of the SEP event. PSP, STEREO-A, and SolO detected strong anisotropies at the onset of the SEP event, which resulted from the fact that PSP and STEREO-A were in the declining-speed region of the solar wind stream responsible for the SIR and from the passage of a steady magnetic field structure by SolO during the onset of the event. By contrast, the intensity-time profiles observed near Earth displayed a delayed onset at proton energies ≳13 MeV and an accumulation of ≲5 MeV protons between the SIR and the shock driven by the parent coronal mass ejection (CME). Even though BepiColombo, STEREO-A, and SolO were nominally connected to the same region of the Sun, the intensity-time profiles at BepiColombo resemble those observed near Earth, with the bulk of low-energy ions also confined between the SIR and the CME-driven shock. This event exemplifies the impact that intervening large-scale interplanetary structures, such as corotating SIRs, have in shaping the properties of SEP events

MESSENGER observations of induced magnetic fields in Mercury's core

Orbital data from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft allow investigation of magnetic fields induced at the top of Mercury's core by time‐varying magnetospheric fields. We used 15 Mercury years of observations of the magnetopause position as well as the magnetic field inside the magnetosphere to establish the presence and magnitude of an annual induction signal. Our results indicate an annual change in the internal axial dipole term, g10, of 7.5 to 9.5 nT. For negligible mantle conductivity, the average annual induction signal provides an estimate of Mercury's core radius to within ±90 km, independent of geodetic results. Larger induction signals during extreme events are expected but are challenging to identify because of reconnection‐driven erosion. Our results indicate that the magnetopause reaches the dayside planetary surface 1.5–4% of the time

Rationale for BepiColombo Studies of Mercury's Surface and Composition

BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury's surface than the suite carried by NASA's MESSENGER spacecraft. Moreover, BepiColombo's data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER's remarkable achievements. Furthermore, the geometry of BepiColombo's orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping. We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury's surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER's more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution. We anticipate that the insights gained into Mercury's geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury's origin including the nature and original heliocentric distance of the material from which it formed.Peer reviewe

The dynamic evolution of multipoint interplanetary coronal mass ejections observed with BepiColombo, Tianwen-1, and MAVEN

We present two multipoint interplanetary coronal mass ejections (ICMEs) detected by the Tianwen-1 and Mars Atmosphere and Volatile Evolution spacecraft at Mars and the BepiColombo (0.56 au ∼0.67 au) upstream of Mars from 2021 December 5 to 31. This is the first time that BepiColombo is used as an upstream solar wind monitor ahead of Mars and that Tianwen-1 is used to investigate the magnetic field characteristics of ICMEs at Mars. The Heliospheric Upwind Extrapolation time model was used to connect the multiple in situ observations and the coronagraph observations from STEREO/SECCHI and SOHO/LASCO. The first fast coronal mass ejection event (∼761.2 km s−1), which erupted on December 4, impacted Mars centrally and grazed BepiColombo by its western flank. The ambient slow solar wind decelerated the west flank of the ICME, implying that the ICME event was significantly distorted by the solar wind structure. The second slow ICME event (∼390.7 km s−1) underwent an acceleration from its eruption to a distance within 0.69 au and then traveled with the constant velocity of the ambient solar wind. These findings highlight the importance of background solar wind in determining the interplanetary evolution and global morphology of ICMEs up to Mars distance. Observations from multiple locations are invaluable for space weather studies at Mars and merit more exploration in the future