高频振荡的吸收及其与发射率降低的关系
Absorption of high frequency oscillations and its relation to emissivity reduction
摘要：
已知太阳黑子是太阳振荡模态功率的强吸收体。证明这一点的最有说服力的方法是通过傅里叶-汉克尔分解（FHD）完成的，其中本地振荡场被分离为输入波和输出波，显示功率的降低。由于日震和磁成像仪的高节奏多普勒测量，可以在超过声学截止频率的频率下研究功率吸收。我们进行FHD的五个太阳黑子区域和两个安静的太阳控制区域，并研究由此产生的吸收光谱$α_\ell(ν)$，特别是在频率$ν$ > 5.3兆赫。我们观察到一个未报告的高频吸收功能，这只出现在太阳黑子的存在。这一特征仅限于起源与太阳黑子中心重合的单跳波的相速度，在这种情况下，$v_{ph}$ $v_{ph}$ = 85.7 km/s。通过在恒定的相速度下对吸收光谱进行拟合，我们发现峰值吸收强度$α_{max}$在约0.009（5%）的噪声水平下位于0.166和0.222之间。在较低频率下沿脊的公知吸收可达到$α_{max}\approx$ 0.5。因此，我们在吸收光谱中的发现较弱，但仍然很重要。从高频波能量收支的初步考虑来看，这一观测结果可能是由太阳黑子内发射率的降低来解释的。我们导出了发射率和吸收率之间的简单关系。我们的结论是，太阳黑子产生的波功率吸收签名（仅适用于某些相位速度），这可能有助于理解对流和源激发的强磁场的影响，并可能在了解一般的太阳黑子地下结构。
英文摘要：
Sunspots are known to be strong absorbers of solar oscillation modal power. The most convincing way to demonstrate this is done via Fourier-Hankel decomposition (FHD), where the local oscillation field is separated into in- and outgoing waves, showing the reduction in power. Due to the Helioseismic and Magnetic Imager's high-cadence Doppler measurements, power absorption can be investigated at frequencies beyond the acoustic cutoff frequency. We perform an FHD on five sunspot regions and two quiet-Sun control regions and study the resulting absorption spectra $α_\ell(ν)$, specifically at frequencies $ν$ > 5.3 mHz. We observe an unreported high-frequency absorption feature, which only appears in the presence of a sunspot. This feature is confined to phase speeds of one-skip waves whose origins coincide with the sunspot's center, with $v_{ph}$ = 85.7 km/s in this case. By employing a fit to the absorption spectra at a constant phase speed, we find that the peak absorption strength $α_{max}$ lies between 0.166 and 0.222 at a noise level of about 0.009 (5%). The well-known absorption along ridges at lower frequencies can reach up to $α_{max}\approx$ 0.5. Thus our finding in the absorption spectrum is weaker, but nevertheless significant. From first considerations regarding the energy budget of high-frequency waves, this observation can likely be explained by the reduction of emissivity within the sunspot. We derive a simple relation between emissivity and absorption. We conclude that sunspots yield a wave power absorption signature (for certain phase speeds only), which may help in understanding the effect of strong magnetic fields on convection and source excitation and potentially in understanding the general sunspot subsurface structure.
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