arXiv 2409.035742024-09-05astro-ph.SRphysics.space-ph

声学截止频率以上太阳振荡频率变化的纬度依赖性

Latitudinal dependence of variations in the frequencies of solar oscillations above the acoustic cut-off

Laura Jade Millson, Anne-Marie Broomhall, Tishtrya Mehta

AI总结：

本文利用近22年GONG数据研究太阳伪模频率偏移的纬度依赖性，发现不同纬度偏移形态存在差异且均与磁活动强反相关，并识别出四年的周期性。

摘要：

在声学截止频率以上的高频区域，太阳功率谱中可见一种峰状结构。这些被称为伪模的结构，其频率已被证明与太阳磁活动呈反相变化。在本工作中，我们确定了这些频率在太阳圆盘上的时间变化，目的是识别伪模频率偏移是否存在任何潜在的纬度依赖性。我们利用了近22年空间分辨的GONG数据，涵盖所有方位角阶数$\textit{m}$，谐波阶数范围为0 $\leq$ $\textit{l}$ $\leq$ 200，并使用重采样周期图法确定频率偏移。周期图实现由时间上重叠、连续的216天长的分段生成，并裁剪至5600-6800$μ$Hz。随后在这些实现之间反复生成互相关函数，以识别频率的任何变化及其不确定性。我们根据每个模的纬度敏感度对其进行分类，并利用该分类生成不同纬度带（大小为15$^\circ$和5$^\circ$）的平均频率偏移，将其与磁活动代理指标——$F_{\mathrm{10.7}}$指数和GONG综合磁图进行比较。我们发现不同纬度之间伪模偏移存在形态学差异，这种差异在太阳极大期上升阶段最为显著，此时偏移达到其最小值。在所有纬度上，偏移行为均与活动代理指标呈强反相关。此外，还观测到短于11年周期的周期性。利用小波分析，在所有纬度上识别出一个四年的周期性。

英文摘要：

At high frequencies beyond the acoustic cut-off, a peak-like structure is visible in the solar power spectrum. Known as the pseudo-modes, their frequencies have been shown to vary in anti-phase with solar magnetic activity. In this work, we determined temporal variations in these frequencies across the solar disc, with the aim of identifying any potential latitudinal dependence of pseudo-mode frequency shifts. We utilised nearly 22 years of spatially resolved GONG data for all azimuthal orders, $\textit{m}$, for harmonic degrees 0 $\leq$ $\textit{l}$ $\leq$ 200, and determined shifts using the resampled periodogram method. Periodogram realisations were created from overlapping, successive 216d-long segments in time, and cropped to 5600-6800$μ$Hz. Cross-correlation functions were then repeatedly generated between these realisations to identify any variation in frequency and the uncertainty. We categorised each mode by its latitudinal sensitivity and used this categorisation to produce average frequency shifts for different latitude bands (15$^\circ$ and 5$^\circ$ in size) which were compared to magnetic proxies, the $F_{\mathrm{10.7}}$ index and GONG synoptic maps. Morphological differences in the pseudo-mode shifts between different latitudes were found, which were most pronounced during the rise to solar maximum where shifts reach their minimum values. At all latitudes, shift behaviour was strongly in anti-correlation with the activity proxy. Additionally, periodicities shorter than the 11-year cycle were observed. Wavelet analysis was used to identify a periodicity of four years at all latitudes.

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