Prominence eruption is closely related to coronal mass ejections and is an important topic in solar physics. Spectroscopic observation is an effective way to explore the plasma properties, but the spectral observations of eruptive prominences are rare. In this paper we will introduce an eruptive polar crown prominence with spectral observations from the Interface Region Imaging Spectrograph (IRIS), and try to explain some phenomena that are rarely reported in previous works. The eruptive prominence experiences a slow-rise and fast-rise phase, while the line-of-sight motions of the prominence plasma could be divided into three periods: 2 hours before the fast-rise phase, opposite Doppler shifts are found at the two sides of the prominence axis; then, red shifts dominate the prominence gradually; in the fast-rise phase, the prominence gets to be blue-shifted. During the second period, a faint component appears in Mg ii k window with a narrow line width and a large red shift. A faint region is also found in AIA 304Å images along the prominence spine, and the faint region gets darker during the expansion of the spine. We propose that the opposite Doppler shifts in the first period is a feature of the polar crown prominence that we studied. The red shifts in the second period are possibly due to mass drainage during the elevation of the prominence spine, which could accelerate the eruption in return. The blue shifts in the third period are due to that the prominence erupts toward the observer. We suggest that the faint component appears due to the decreasing of the plasma density, and the latter results from the expansion of the prominence spine.

日珥爆发与日冕物质抛射密切相关，是太阳物理学的一个重要课题。光谱观测是探索等离子体性质的有效方法，但对喷发日珥的光谱观测很少见。在本文中，我们将通过界面区域成像光谱仪 (IRIS) 的光谱观测介绍喷发的极地冠突，并尝试解释一些以前工作中很少报道的现象。爆发的日珥经历慢升和快升阶段，而日珥等离子的视线运动可分为三个阶段：在快升阶段前2小时，在两个阶段发现相反的多普勒频移突出轴的两侧；然后，红移逐渐占据突出地位；在快速上升阶段，突出部分会发生蓝移。第二阶段，Mg中出现微弱成分iik 窗口具有较窄的线宽和较大的红移。在 AIA 中也发现了一个微弱的区域304一种̊沿突出脊柱的图像，并且在脊柱扩展过程中，微弱区域变暗。我们提出，第一周期中的相反多普勒频移是我们研究的极冠日珥的一个特征。第二阶段的红移可能是由于隆起过程中的大量排水，这可能会加速喷发。第三阶段的蓝移是由于日珥向观察者爆发。我们认为微弱成分的出现是由于血浆密度的降低，而后者是由突起脊柱的扩张引起的。