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arXiv 1203.4091astro-ph.SR
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从30.4nm附近的窄带图像诊断日冕

Coronal Diagnostics from Narrowband Images around 30.4 nm

V. Andretta, D. Telloni, G. Del Zanna

摘要:

在以30.4 nm为中心的波段拍摄的图像通常用于绘制太阳盘面上He II Ly α线的辐射。该谱线是在太阳光谱中观察到的极紫外线中最强的谱线之一,也是该波长范围内提供有关上色球层或下过渡区信息的少数谱线之一。然而,当观察离边日冕时,来自附近Si XI 30.3 nm线的贡献可能变得显着。在这项工作中,我们的目标是估计这两条线在太阳活动最小的太阳日冕的相对贡献。我们将2008年8月CDS的测量结果与日冕半经验模型的温度和密度分布相结合,计算了两条线以及其他代表性日冕线(例如,Mg X 62.5nm,Si XII 52.1nm)。考虑到两个诊断量从线比(温度和密度)和线辐射的绝对单位,我们得到一个很好的整体观测和模型之间的匹配。我们发现,在流光中,Si XI谱线从临边上方一直到~2 RSun,对He II谱线的贡献占主导地位,而在~2 - 3RSun以外的日冕中,Si XI谱线对30.4nm波段窄带成像的贡献将变小,甚至可以忽略不计,其精确值强烈地依赖于日冕温度廓线。

英文摘要:

Images taken in the band centered at 30.4 nm are routinely used to map the radiance of the He II Ly alpha line on the solar disk. That line is one of the strongest, if not the strongest, line in the EUV observed in the solar spectrum, and one of the few lines in that wavelength range providing information on the upper chromosphere or lower transition region. However, when observing the off-limb corona the contribution from the nearby Si XI 30.3 nm line can become significant. In this work we aim at estimating the relative contribution of those two lines in the solar corona around the minimum of solar activity. We combine measurements from CDS taken in August 2008 with temperature and density profiles from semiempirical models of the corona to compute the radiances of the two lines, and of other representative coronal lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed quantities from line ratios (temperatures and densities) and line radiances in absolute units, we obtain a good overall match between observations and models. We find that the Si XI line dominates the He II line from just above the limb up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in the 30.4 nm band is expected to become smaller, even negligible in the corona beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal temperature profile.

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