基于PSP观测校准EUHFORIA中的WSA模型
Calibrating the WSA model in EUHFORIA based on PSP observations
AI总结：
本研究利用PSP在2018-2021年太阳极小期的观测校准EUHFORIA中的WSA模型四个常数，发现近日最优配置与地球处最优预测配置不一致，指出需全局重构耦合的WSA关系式。
摘要：
我们利用帕克太阳探测器（Parker Solar Probe, PSP）在最近一次太阳活动极小期（2018-2021年）的观测数据，对欧洲空间天气预报工具EUHFORIA中使用的Wang-Sheeley-Arge（WSA）日冕模型版本进行校准。WSA模型提供0.1天文单位（au）处的一组边界条件，这是启动EUHFORIA日球层部分（即太阳阿尔文点以外的区域）所必需的。为校准WSA模型，我们基于PSP观测数据，通过观测约束了WSA半经验公式中的四个常数。我们展示了校准后的更新版WSA在0.1 au处的边界条件与相近距离处的PSP观测结果的对比情况，并进一步根据EUHFORIA的磁流体动力学（magnetohydrodynamic, MHD）方法，将这些条件在日球层中进行传播。我们基于动态时间规整（Dynamic Time Warping）技术评估了模型在地球位置的预测结果。研究结果表明，在研究的时段内，最贴合近日处PSP观测结果的WSA配置，与能在地球位置提供更优预测所需的配置并不一致。造成这一差异的一个原因是PSP记录到的快速太阳风速度数据稀缺。模型校准是基于意外偏慢的速度完成的，与此前的研究相比，这使得我们无法实现太阳风预测的整体全局提升。其他原因则包括EUHFORIA日球层部分缺失部分物理过程，此外，当前与日球层MHD区域耦合使用的WSA关系式，可能需要进行全局重构，而不仅仅是更新本研究中考虑的四个常数因子。
英文摘要：
We employ Parker Solar Probe (PSP) observations during the latest solar minimum period (years 2018 -2021) to calibrate the version of the Wang-Sheeley-Arge (WSA) coronal model used in the European space weather forecasting tool EUHFORIA. WSA provides a set of boundary conditions at 0.1 au necessary to initiate the heliospheric part of EUHFORIA, namely, the domain extending beyond the solar Alfvenic point. To calibrate WSA, we observationally constrain four constants in the WSA semi-empirical formula based on PSP observations. We show how the updated (after the calibration) WSA boundary conditions at 0.1 au are compared to PSP observations at similar distances, and we further propagate these conditions in the heliosphere according to EUHFORIAs magnetohydrodynamic (MHD) approach. We assess the predictions at Earth based on the Dynamic Time Warping technique. Our findings suggest that, for the period of interest, the WSA configurations which resembled optimally the PSP observations close to the Sun, were different than the ones needed to provide better predictions at Earth. One reason for this discrepancy can be attributed to the scarcity of fast solar wind velocities recorded by PSP. The calibration of the model was performed based on unexpectedly slow velocities that did not allow us to achieve generally and globally improved solar wind predictions, compared to older studies. Other reasons can be attributed to missing physical processes from the heliospheric part of EUHFORIA but also the fact that the currently employed WSA relationship, as coupled to the heliospheric MHD domain, may need a global reformulation beyond that of just updating the four constant factors that were taken into account in this study.
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