Early solar wind and dynamo magnetic field topology predictions for (16) Psyche and other asteroids

TL;DR

This work leverages adaptive-m mesh MHD simulations to predict how Psyche’s magnetic field could reflect three core magnetization histories: early solar wind remanence, core-dynamo remanence (either from an intact body or a parent-body fragment), and nebular-disk magnetization. By exploring a wide set of spin orientations and applying spin-averaging, the authors show that wind-induced fields tend to be dipolar and aligned with the spin axis for certain obliquities, while dynamo-derived fields can be tilted, multipolar, or weak depending on the geometry and source, with clear topological distinctions that can guide magnetometer observations. They also quantify observational caveats, such as solar wind perturbations and electromagnetic induction, and discuss how topological analysis (e.g., dipole vs quadrupole, axisymmetry, and wake-region signatures) can help attribute measured fields to remanent sources. The framework, including the Astrobear-based simulations and the moment-conversion methodology, offers a broadly applicable approach to interpreting magnetic signatures in other metal-rich asteroids and informs mission planning for Psyche’s magnetometer investigations. Overall, the paper provides actionable predictions for magnetic topology, strength, and orientation tied to formation history, emphasizing that topology and temporal variability are key to discriminating remanent sources in the presence of solar-wind and induction effects.

Abstract

Asteroid (16) Psyche is a metal-rich body that might record an ancient coherent magnetization if some relict crust or mantle is preserved. Herein, we use magnetohydrodynamic simulations to predict (16) Psyche's field topology for several distinct pathways, (i) an early solar wind-induced magnetization imparted after a larger body was impacted, forming the present-day asteroid, (ii) a core dynamo magnetization imparted in an asteroid that is either presently largely intact or was a rubble pile, and (iii) magnetization in the turbulent solar nebula disk. For pathway (i) we find the field to be predominantly dipolar and spin axis-aligned. For pathway (ii) we find the field to be either dipolar and spin axis-misaligned, or highly multipolar. We also find that (iii) a field produced earlier before the solar nebula cleared, would be highly multipolar. In cases (i) and (ii) we also place constraints on the field strength. Simple detection of a magnetic field without constraining its topology and temporal variability would be insufficient to confirm a remanent source, due to the influence of the present-day solar wind, electromagnetic induction, and (16) Psyche's high obliquity. For sufficiently strong fields however, the field topology and orientation may reveal key observable consequences of the nature and history of (16) Psyche. Our framework is also broadly applicable to the study of magnetic fields from other asteroids.

Figures (3)

• Figure 1: Our simulations are carried out in a frame centered on the ellipsoidal asteroid, but here we show all spin axis orientation cases that we consider in the frame of reference of the background solar wind/magnetic field. The asteroid configuration is represented by the colored solid ellipsoids with the shortest axis being the spin axis. The long arrows represent the spin axes for each particular simulation. The labels show the angle of the spin axis from the z-axis or magnetic field direction ($\theta$), and x-axis or wind propagation direction ($\varphi$). Panel (a) shows cases that generically span the positive octant of spin orientations on a unit sphere, from which all other analogous octants can be determined by symmetry. Panel (b) shows cases with obliquity $(\theta)=98\degree$ and $\varphi$ at $45\degree$ increments for averaging over the orbit (revolution) period of (16) Psyche.
• Figure 2: Synthetic remanent magnetic field lines colored by magnitude and surface remanent magnetization (grayscale) in the $y=0$ plane. The $z=0$ line highlights the North - South symmetry or asymmetry in some cases. Top and middle rows are the WIM cases after spin averaging with the spin axis oriented at (a) $\theta = 98 \degree, \ \varphi = 0 \degree$, (b) $\theta = 98 \degree, \ \varphi = 90 \degree$, (c) $\theta = 54.74 \degree, \ \varphi = 45 \degree$, (d) is further averaged over present day Psyche's orbit (8 cases with $\theta = 98 \degree$ as shown in Fig. \ref{['fig:setupB']}b. Panel (e) shows a core dynamo imparted field if Psyche were at the core mantle boundary of a larger parent body with the size and magnetic field of present day Mercury with its spin axis at $45 \degree$ from the core dynamos dipole. The surface remanent magnetization is not shown as the whole body is magnetized. Panel (f) is a case of a strong core dynamo ($10^{16} \mathrm{A \, m}^2$) inclined at $45 \degree$ in the $xz$ plane and slightly off-center of Psyche itself at (25, 0, 25) km (denoted by a blue star) and for which the crust cooled from the outside inward. Case (f) is multipolar and non-axisymmetrical, hence our analysis is only first order accurate. On the panels most relevant for (16) Psyche, and thus the Psyche spacecraft, we have shown dot-dashed violet ellipses representing one of several orbits the probe will take at $100 \ \mathrm{km}$ altitude from the asteroid's body. The labels in the bottom left of each subfigure correspond to the corresponding entries of Table \ref{['tab:all']}.
• Figure 3: Magnetic field lines colored by the (logscale) amplification relative to the solar wind field (100 nT) that arise from simulating the early solar wind overrunning the surface of one of our the (golden colored) bi-axial ellipsoid cases in Fig. \ref{['fig:setupB']}a, G3(55,45). The asteroid's surface is shown in translucent gray.