Absolute velocity measurements in sunspot umbrae
Absolute velocity measurements in sunspot umbrae
复制标题
DOI：
10.1051/0004-6361/201832886
复制
发表时间：
2018-04
影响因子：
6.5
通讯作者：
J. Löhner-Böttcher;W. Schmidt;R. Schlichenmaier;H. Doerr;T. Steinmetz;Ronald Holzwarth
中科院分区：
文献类型：
--
作者：
J. Löhner-Böttcher;W. Schmidt;R. Schlichenmaier;H. Doerr;T. Steinmetz;Ronald Holzwarth
语境。在太阳黑子本影中，对流在很大程度上受到强磁场的抑制。之前的测量报告显示本影核心中的对流可以忽略不计。基于此，大量研究以本影作为零参考来计算周围活动区的多普勒速度。我们的目标是通过以前所未有的准确度和精确度直接测量多普勒速度来阐明本影最暗部分的对流运动量。我们使用德国真空塔望远镜的激光绝对参考摄谱仪（LARS）对太阳黑子本影进行了光谱观测。激光频率梳能够校准高分辨率摄谱仪和 13 个观察序列的绝对波长位置。彻底的光谱校准（包括参考波长的测量）产生了光谱线 Ti I5713.9 Å 的多普勒频移，不确定度约为 5 m s−1。平分线分析给出了与深度相关的线不对称性。结果。测量的多普勒频移是本影对流和磁声波的组合。为了分析对流变化，我们对每个序列进行时间平均以减少叠加的波信号。与安静太阳中高达−350 m s−1 的对流蓝移相比，太阳黑子本影产生的对流蓝移大大减少在−30 m s−1 左右。我们发现太阳黑子本影的速度与磁场强度显着相关，而且还与定义 Ti I5713.9 Å 线深度的本影温度相关。垂直向上运动随着场强的增加而减小。将线性近似外推至零磁场可再现测量的安静太阳蓝移。以同样的方式，我们发现对流蓝移随着线深度的增加而减小。结论。简单地将太阳黑子本影作为计算光球多普勒图的零速度参考可能意味着系统速度误差达到 100 m s−1 或更高。建立垂直速度和磁场强度之间的关系为太阳分光偏振测量提供了一种补救措施。我们提出了一种新颖的方法，通过包含磁场信息来定义本影参考速度，从而显着提高太阳黑子区域多普勒速度的准确性。
Context. In sunspot umbrae, convection is largely suppressed by the strong magnetic field. Previous measurements reported on negligible convective flows in umbral cores. Based on this, numerous studies have taken the umbra as zero reference to calculate Doppler velocities of the ambient active region.Aims. We aim to clarify the amount of convective motion in the darkest part of umbrae, by directly measuring Doppler velocities with an unprecedented accuracy and precision.Methods. We performed spectroscopic observations of sunspot umbrae with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the high-resolution spectrograph and absolute wavelength positions for 13 observation sequences. A thorough spectral calibration, including the measurement of the reference wavelength, yielded Doppler shifts of the spectral line Ti I5713.9 Å with an uncertainty of around 5 m s−1. A bisector analysis gave the depth-dependent line asymmetry.Results. The measured Doppler shifts are a composition of umbral convection and magneto-acoustic waves. For the analysis of convective shifts, we temporally averaged each sequence to reduce the superimposed wave signal. Compared to convective blueshifts of up to −350 m s−1in the quiet Sun, sunspot umbrae yield strongly reduced convective blueshifts around −30 m s−1. We find that the velocity in a sunspot umbra correlates significantly with the magnetic field strength, but also with the umbral temperature defining the depth of the Ti I5713.9 Å line. The vertical upward motion decreases with increasing field strength. Extrapolating the linear approximation to zero magnetic field reproduces the measured quiet Sun blueshift. In the same manner, we find that the convective blueshift decreases as a function of increasing line depth.Conclusions. Simply taking the sunspot umbra as a zero velocity reference for the calculation of photospheric Dopplergrams can imply a systematic velocity error reaching 100 m s−1, or more. Setting up a relationship between vertical velocities and magnetic field strength provides a remedy for solar spectropolarimetry. We propose a novel approach of substantially increasing the accuracy of the Doppler velocities of a sunspot region by including the magnetic field information to define the umbral reference velocity.