On acoustic and gravity waves in the solar photosphere and their energy transport
On acoustic and gravity waves in the solar photosphere and their energy transport
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关于太阳光球层中的声波和重力波及其能量传输
DOI：
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发表时间：
2011
期刊：
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通讯作者：
N. B. González
中科院分区：
文献类型：
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作者：
F. Kneer;N. B. González
目标。我们研究了安静太阳中的声波和大气重力波，以估计它们向色球层的能量传输。方法.本文用Fe i 5576 A和Fe i 5434 A（Lande因子g = 0）的同步光谱观测，分析了宁静太阳盘中心的二维时间序列。我们计算了两条谱线的速度响应函数和波的大气传输。为此，NLTE线形成颗粒和颗粒间模型大气的数值模拟进行。为了解释观测到的波和能量通量的估计，我们假设在等温模式大气中的平面波的绝热传播。进行了强度和速度波动的傅立叶分析。它们产生功率、相位和相干性作为频率ν的函数（从时间傅里叶变换）和在kh −ν平面中的函数（从三维变换）。功率谱与速度形成高度处的质量密度一起给出能量通量。结果在声波和重力波域中发现的均方根速度比早期工作中的低1.5倍。因此，我们承认一个系数为2的估计通量的向上校正。对于声波，我们发现：1）太阳上存在向上传播的波，频率高达14 - 15 mHz（2）等温大气中的平面绝热波近似对于估算能量通量是足够的; 3）声能通量与我们早期从地基二维光谱学中发现的范围相同，1500−3100 W m −2（在大气高度为380 km时）和1300−2700 W m −2（在570 km时）。重力波携带的能量通量很难确定。我们发现：1）连续谱和速度涨落的相位谱和相干谱表明对流超调和重力波是叠加的。我们占对流使用这些相干谱。2)在低频，垂直波长Λz可以很短（约300 km），对大气传输产生很大的修正（因子>100）。因此，我们从通量估计中排除了以下波：|KZ|> 20 Mm −1，垂直群速度为10 gr，z < 0。3k m s −1。它们的振幅很可能因辐射阻尼而大大降低。3)有了这些警告，重力波携带的能量通量在380公里处为4000 - 8200 W m-2，在570公里处为700 - 1400 W m-2。重力波因此也有助于能量传输到色球层。
Aims. We study acoustic and atmospheric gravity waves in the quiet Sun to estimate their energy transport to the chromosphere. Methods. A two-dimensional time sequence from quiet Sun disc centre was analysed with simultaneous spectroscopic observations in Fe i 5576 A and Fe i 5434 A (both with Lande factor g = 0). We calculated response functions of the velocities for the line minimum shifts and atmospheric transmissions of waves for the two lines. For this, NLTE line formation in granular and intergranular model atmospheres from numerical simulations were performed. For the interpretation of the observed waves and for the estimates of energy fluxes, we assumed adiabatic propagation of plane waves in an isothermal model atmosphere. Fourier analyses of intensity and velocity fluctuations were carried out. They yield power, phase, and coherence as functions of frequency ν (from temporal Fourier transforms) and in thekh−ν plane (from three-dimensional transforms). The power spectra, together with the mass densities at velocity formation heights, give then the energy fluxes. Results. The rms velocities found here in the acoustic and gravity wave domains are lower by a factor ∼1.5 as in earlier work. We therefore admit a factor of 2 for an upward correction of the estimated fluxes. For acoustic waves we find: 1) upward propagating waves are present on the Sun with frequencies up to 14−15 mHz (periods U ≈ 70 s); 2) the approximation of plane adiabatic waves in an isothermal atmosphere appears adequate for estimating the energy fluxes; 3) the acoustic energy fluxes are in the same range as found in our earlier work from ground-based, two-dimensional spectroscopy, 1500−3100 W m −2 at an atmospheric height of ∼380 km and 1300−2700 W m −2 at 570 km. The energy flux carried by gravity waves is difficult to determine. We find: 1) phase and coherence spectra between continuum and velocity fluctuations show that convective overshoot and gravity waves are superimposed. We account for the convective flows using these coherence spectra. 2) At low frequencies, the vertical wavelength Λz can be short (� 300 km), yielding large corrections for atmospheric transmissions (factors >100). We thus exclude from the flux estimates waves with |kz| > 20 Mm −1 and with vertical group velocities υgr,z < 0. 3k m s −1 . They are likely to be strongly reduced in amplitude by radiative damping. 3) With these caveats, the energy fluxes carried by gravity waves are found in the range of 4000−8200 W m −2 at 380 km and 700−1400 W m −2 at 570 km. Gravity waves thus also contribute to the energy transport into the chromosphere.