Title: Multi-spacecraft Measurements of the Evolving Geometry of the Solar Alfvén Surface Over Half a Solar Cycle Show Chinese title

Title: 太阳阿尔芬表面在半个太阳周期内演变几何结构的多航天器测量

Abstract: The geometry of a star's Alfv\'en surface determines stellar angular momentum loss, separates a causally distinct 'corona' and stellar wind, and potentially affects exoplanetary habitability. The solar Alfv\'en surface is the only such structure that is directly measurable and since 2021, has been routinely measured in situ by NASA's Parker Solar Probe (Parker). We use these unique measurements in concert with Solar Orbiter and L1 in situ data spanning the first half of the Solar Cycle 25 in time and from 0.045 - 1 au in heliocentric distance to develop a radial scaling technique to estimate the morphology of the Alfv\'en surface from measurements of the solar wind speed and local Alfv\'en speed. We show that accounting for solar wind acceleration and mass flux is necessary to achieve reasonable agreement between the scaled location of the Alfv\'en surface and the locations of direct crossings measured by Parker. We produce continuous 2D equatorial cuts of the Alfv\'en surface over half a Solar Cycle (ascending phase and maximum). Parker's earliest crossings clipped outward extrusions, many of which are likely transient related, while more recently Parker has unambiguously sampled deep sub-Alfv\'enic flows. We analyze the average altitude, departure from spherical symmetry, and surface roughness, finding that all are positively correlated to solar activity. For the current modest Solar Cycle, the height varies up to 30\% which corresponds to a near-doubling in angular momentum loss per unit mass loss. Show Chinese abstract

Abstract: 恒星的阿尔芬表面几何形状决定了恒星角动量损失，区分了因果上不同的“日冕”和恒星风，并可能影响系外行星的可居住性。 太阳的阿尔芬表面是唯一可以直接测量的此类结构，自2021年以来，已由NASA的帕克太阳探测器（Parker）进行常规的原位测量。 我们结合这些独特的测量数据，以及太阳轨道器和L1点的原位数据，时间覆盖太阳活动周期25的第一阶段，距离太阳的距离从0.045至1天文单位，开发了一种径向缩放技术，以根据太阳风速度和局部阿尔芬速度的测量值估计阿尔芬表面的形态。 我们证明，为了使缩放后的阿尔芬表面位置与帕克探测器直接穿越的位置达成合理的吻合，必须考虑太阳风加速和质量通量。 我们生成了半个太阳活动周期（上升阶段和峰值）内的连续二维赤道切片。 帕克探测器最早的穿越截断了向外的突出部分，其中许多可能是暂时性的，而最近帕克探测器已经明确地采样了深层亚阿尔芬流。 我们分析了平均高度、偏离球对称性的程度以及表面粗糙度，发现它们都与太阳活动呈正相关。 对于当前较弱的太阳活动周期，高度变化可达30%，这相当于单位质量损失的角动量损失几乎翻倍。