宁静日冕中的磁核点数目
The Number Of Magnetic Null Points In The Quiet Sun Corona
摘要：
在特定光球区域上方的日冕磁场会在一定数量的点处消失，这些点称为零点。这些点可以直接在位场外推中找到，或者可以从磁图的傅立叶谱估计它们的密度。谱估计，假设外推场是随机的，均匀的，并具有高斯统计，这里被发现是相对准确的安静的太阳磁图从SOHO的MDI。大多数零点出现在低海拔地区，它们的分布由傅立叶频谱中的高波数决定。这部分频谱受泊松噪声的影响，并且从直接外推法识别的多达六分之五的零点可以归因于噪声。发现1500公里以上的零分布取决于MDI在其低分辨率或高分辨率模式下可靠测量的波长。校正后的频谱，以消除白噪声和补偿调制传递函数，我们发现，位场外推包含，平均而言，一个磁零点，高度大于1.5毫米，以上每322平方毫米补丁安静的太阳。对最近两次太阳活动极小期的562个平静太阳磁图的分析表明，零点密度相对恒定，每天的变化约为10%。在1.5 μ m以上的高度，零点密度大约随着高度的倒数立方而减小。宁静太阳的光球场可以很好地近似为平均通量为1.0e ~（19）Mx，密度n=0.007/m2随机分布的离散单元的光球场。
英文摘要：
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points. These points can be found directly in a potential field extrapolation or their density can be estimated from Fourier spectrum of the magnetogram. The spectral estimate, which assumes that the extrapolated field is random, homogeneous and has Gaussian statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO's MDI. The majority of null points occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null point, with altitude greater than 1.5 Mm, above every 322 square Mm patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minimum shows that the null point density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete elements with mean flux 1.0e19 Mx distributed randomly with density n=0.007 per square Mm.
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