AI 中文总结
研究小行星发电机作用起始，通过模拟在不同参数下探究其起始情况，发现沿特定路径起始取决于磁场强弱，新临界值标准促进发电机活动，延长磁场寿命并允许在更小天体产生磁场。
AI 中文摘要
在早期太阳系中，已发现多颗陨石带有小行星核心产生的原生剩余磁化强度。热演化模型表明热对流可能在这些天体的早期演化中驱动了发电机作用。在如此小的核心中，磁雷诺数被认为接近发电机作用的阈值。然而，在发电机模拟中，磁雷诺数的临界值（即起始时的值）也受到的约束不足。我们进行发电机模拟，在准全球体中研究不同埃克曼数（$E$）和磁普朗特数（$Pm$）下发电机的起始情况。在$(E,Pm)$空间中的两条经验路径上，发电机作用的起始取决于初始磁场是强还是弱。当朝着更现实的参数值移动时，发电机的起始发生在更大的超临界性（瑞利数（$Ra$）与其临界值（$Ra_c$）之比）下。一旦外推到小行星核心条件，对于强初始磁场（即磁能与动能量级相同或更大），发电机起始的超临界性约为$10^3$。对于弱初始场，所需的$Ra/Ra_c$会更大。与先前估计相比，这个修订标准促进了发电机活动，因此与先前模型相比延长了磁场寿命，并允许在更小的小行星中产生磁场。
英文摘要
Several meteorites have been found to carry primary remanent magnetizations imparted by fields generated within planetesimal cores during the early solar system. Thermal evolution models have shown that thermal convection likely drove the dynamo in the early evolution of these bodies. In such small cores, the magnetic Reynolds number is thought to be close to the threshold for dynamo action. However, the critical value of the magnetic Reynolds number, meaning its value at the onset, is also poorly constrained in dynamo simulations. We perform dynamo simulations to investigate the onset of the dynamo at different Ekman ($E$) and magnetic Prandtl ($Pm$) numbers, in a quasi-full sphere. Along two empirical paths in this $(E,Pm)$ space, the onset of dynamo action depends on whether the magnetic field is initially strong or weak. The onset of the dynamo occurs at larger supercriticality (ratio between the Rayleigh number ($Ra$) and its critical value ($Ra_c$)) when moving toward more realistic parameter values. Once extrapolated to planetesimal core conditions, the supercriticality for the onset of the dynamo is about $10^3$ for a strong initial magnetic field (meaning magnetic energy is of the same order of magnitude or larger than the kinetic energy). For a weak initial field, the required $Ra/Ra_c$ would be larger. Compared to previous estimates, this revised criterion facilitates dynamo activity, and so extends the lifetime of the magnetic field compared to previous models and allows the generation of a magnetic field in smaller planetesimals.
