过去五个太阳黑子周期中 $m=1$ 高纬度惯性模式的多普勒速度

Zhi-Chao Liang, Laurent Gizon
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摘要

在已确定的太阳惯性模式中,方位角阶$m=1$的高纬度模式(HL1)具有最大的振幅,并在塑造太阳的差转剖面中发挥作用。我们利用威尔逊山天文台(MWO)、GONG 和 HMI 的多普勒图,研究了 HL1 模式参数的演变,这些多普勒图涵盖了自 1967 年以来的五个太阳周期。我们计算了视距多普勒信号在经度上的平均值,并用相对于中心经线的经度正弦加权,作为地表带状速度的代用指标。我们根据这些高纬度地带性速度,在三年的滑动时间窗口中测量了模式的功率和频率。我们发现,HL1 模式的振幅变化非常大,在太阳周期 21、22 和 25 的开始阶段以及周期 23 和 24 的上升阶段达到最大值。该模式振幅与太阳黑子数(corr=$-0.50$)反相关,但与极场强度无关。在 1983-2022 年期间,模式振幅与纬度$60^\circ$处的自转速率密切反相关(corr=$-0.82$),即与模式临界纬度附近的自转速率密切反相关。模式频率变化很小,在噪声水平($\sim \pm 3$~nHz)之上没有显示出明显的太阳周期周期性。自 1990 年以来,模式频率以每年 0.25 nHz 的速度下降,这与纬度为 60^/circ$ 的角速度的长期下降是一致的。我们预计,模式特性的这些很长的时间序列将成为约束模式的关键,并揭示高纬度模式、自转和磁场之间的动力学相互作用。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Doppler velocity of $m=1$ high-latitude inertial mode over the last five sunspot cycles
Among the identified solar inertial modes, the high-latitude mode with azimuthal order $m=1$ (HL1) has the largest amplitude and plays a role in shaping the Sun's differential rotation profile. We aim to study the evolution of the HL1 mode parameters, utilizing Dopplergrams from the Mount Wilson Observatory (MWO), GONG, and HMI, covering together five solar cycles since 1967. We calculated the averages of line-of-sight Doppler signals over longitude, weighted by the sine of longitude with respect to the central meridian, as a proxy for zonal velocity at the surface. We measured the mode's power and frequency from these zonal velocities at high latitudes in sliding time windows of three years. We find that the amplitude of the HL1 mode undergoes very large variations, taking maximum values at the start of solar cycles 21, 22 and 25, and during the rising phases of cycles 23 and 24. The mode amplitude is anticorrelated with the sunspot number (corr=$-0.50$) but not correlated with the polar field strength. Over the period 1983-2022 the mode amplitude is strongly anticorrelated with the rotation rate at latitude $60^\circ$ (corr=$-0.82$), i.e., with the rotation rate near the mode's critical latitude. The mode frequency variations are small and display no clear solar cycle periodicity above the noise level ($\sim \pm 3$~nHz). Since about 1990, the mode frequency follows an overall decrease of $\sim 0.25$ nHz/year, consistent with the long-term decrease of the angular velocity at $60^\circ$ latitude. We expect that these very long time series of the mode properties will be key to constrain models and reveal the dynamical interactions between the high-latitude modes, rotation, and the magnetic field.
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