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arXiv 0907.4954astro-ph.SR
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Fe XII辐射图上日冕亮点的发射高度

Emission heights of coronal bright points on Fe XII radiance map

H. Tian, C. -Y. Tu, J. -S. He, E. Marsch

摘要:

我们研究了与之相关的双极磁环中光球层上方日冕亮点的发射高度。由于BPs是在相对于圆盘的投影中看到的,因此它们的真实发射高度是未知的。详细研究了EIT Fe XII辐射图上BP位置与磁场特征(特别是中性线)的相关性。日冕磁场是由光球场外推到圆盘上方不同高度而确定的。发现大多数BPs位于光球中性线上或附近,但发射发生在大约5微米的高度。一些BPs,虽然在投影中看到,似乎仍然与中性线重合,尽管它们的排放发生在10微米以上的高度,但这种巧合在20微米以上的排放中几乎消失。我们还预测了3-D磁场线的上方不同高度,分别落在x-y平面上。将每个BP的形状与附近的相应场线段进行比较。这种比较表明,大多数日冕BPs实际上位于其相关磁环的顶部。最后,我们计算了每个选定的BP区域的Fe XII强度增强和外推的磁场矢量的水平分量之间的相关系数,分别在不同高度的平面中的相同的x-y位置。我们发现,对于几乎所有的BP区域,我们研究的相关系数,随着高度的增加,增加到一个最大值,然后再次下降。对应于该最大值的高度被定义为相关高度,对于大多数亮点,发现其范围低于20 μ m。

英文摘要:

We study the emission heights of the coronal bright points (BPs) above the photosphere in the bipolar magnetic loops that are apparently associated with them. As BPs are seen in projection against the disk their true emission heights are unknown. The correlation of the BP locations on the Fe XII radiance map from EIT with the magnetic field features (in particular neutral lines) was investigated in detail. The coronal magnetic field was determined by an extrapolation of the photospheric field to different altitudes above the disk. It was found that most BPs sit on or near a photospheric neutral line, but that the emission occurs at a height of about 5 Mm. Some BPs, while being seen in projection, still seem to coincide with neutral lines, although their emission takes place at heights of more than 10 Mm. Such coincidences almost disappear for emissions above 20 Mm. We also projected the upper segments of the 3-D magnetic field lines above different heights, respectively, on to the x-y plane. The shape of each BP was compared with the respective field-line segment nearby. This comparison suggests that most coronal BPs are actually located on the top of their associated magnetic loops. Finally, we calculated for each selected BP region the correlation coefficient between the Fe XII intensity enhancement and the horizontal component of the extrapolated magnetic field vector at the same x-y position in planes of different heights, respectively. We found that for almost all the BP regions we studied the correlation coefficient, with increasing height, increases to a maximal value and then decreases again. The height corresponding to this maximum was defined as the correlation height, which for most bright points was found to range below 20 Mm.

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