轴向偶极和赤道偶极冕洞：全等的磁几何，不同的演化
Axial-Dipole and Equatorial-Dipole Coronal Holes: Congruent Magnetic Geometry, Divergent Evolution
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中文总结 AI 辅助
研究太阳轴向偶极和赤道偶极冕洞的磁几何与演化差异，发现二者磁几何全等但演化不同，磁几何无法解释EDCHs较慢的太阳风加速，可能归因于反射驱动的阿尔文波湍流加热。
中文摘要 AI 辅助
当太阳极场反转时，轴向偶极较弱，低纬度冕洞常见。赤道偶极冕洞（EDCHs）在赤道偶极较强时出现，形成于其磁极处，类似于极小期的轴向偶极冕洞（ADCHs）。并非所有低纬度冕洞都符合条件：2024年极大期由四极主导。帕克太阳探测器发现，来自一个EDCH的快速太阳风在$9.9\\,R_\odot$处深度亚阿尔芬，其渐近速度仅为匹配极风的$60\\%$，而极风通常被认为加速更快。我们提出疑问：冕洞结构是否能预测这一点，使用合成场和观测场在三个太阳活动周相位的势场模型。我们证明纯轴向偶极和赤道偶极完全全等，具有相同的开放面积和核心膨胀因子，因此任何差异必然源于相对于较差自转模式的取向或周围多极子的影响。我们发现ADCHs是静止的，由汇聚的子午流维持，而EDCHs依赖于赤道偶极，该偶极由活动区浮现构建、通量输运瓦解：在18天序列中，极小期轴向偶极强度稳定在$1\\%$，而极大期赤道偶极变化$21\\%$，下降相变化$8\\%$。EDCHs中的大多数开放通量比ADCHs膨胀更少，但其边界以伪流带为主，具有非单调膨胀。尽管存在这些差异，仅磁几何并不能预测EDCHs中较慢的加速；我们认为任何真实差异都指向反射驱动的阿尔文波湍流引起的延伸加热，或许在这些边界处有不同的供给，而势场模型无法捕捉这一点。
英文摘要
While the Sun's polar fields reverse, the axial dipole is weak and low-latitude coronal holes are common. Equatorial-dipole coronal holes (EDCHs) appear when the equatorial dipole is strong, forming at its magnetic poles, similar to the axial-dipole coronal holes (ADCHs) of minimum. Not every low-latitude hole qualifies: the 2024 maximum was quadrupole-led. Parker Solar Probe found fast wind from an EDCH deeply sub-Alfvénic at $9.9\,R_\odot$, at only $60\%$ of an asymptotic speed matching polar wind, which is usually thought to accelerate faster. We ask whether coronal structure predicts this, using potential-field models of synthetic and observed fields at three cycle phases. We show that pure axial and equatorial dipoles are exactly congruent, with the same open areas and core expansion factors, so any difference must arise from orientation relative to the differential-rotation pattern or from the surrounding multipoles. We find that ADCHs are stationary, sustained by converging meridional flow, whereas EDCHs depend on an equatorial dipole that active-region emergence builds and flux transport dismantles: over $18$-day sequences the axial-dipole strength at minimum is steady to $1\%$, while the equatorial dipole varies by $21\%$ at maximum and $8\%$ in the declining phase. Most open flux in EDCHs expands less than in ADCHs, but their boundaries are pseudostreamer-dominated, with non-monotonic expansion. Despite these differences, magnetic geometry alone does not predict slower acceleration in EDCHs; we argue that any real difference points to extended heating by reflection-driven Alfvén-wave turbulence, perhaps fed differently at those boundaries, which potential-field models cannot capture.
发表机构
- Advanced Heliophysics Inc.（高级日球物理公司）
- US Naval Research Laboratory（美国海军研究实验室）
- Center for Astrophysics, Harvard & Smithsonian（哈佛-史密森天体物理中心）
- European Space Agency, ESTEC（欧洲空间局，欧洲空间研究与技术中心）
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