The high-quality IRIS observations reveal some phenomena that have not been reported, and we try to give reasonable explanations on them. In this work, we focus on the spectral evolution of an eruptive polar crown prominence (the prominence located at high latitude) on 2015 April 28th, which erupts successfully with a CME. studied an eruptive prominence in quiet region with radiative transfer computations they derived the electron densities of the prominence between 1.3 × 10 9 and 6.0 × 10 10 cm −3, the mean temperature around 1.1 × 10 4 K, and the total hydrogen mass between 1.3 × 10 14 and 3.2 × 10 14 g. Among the few works, reported an erupting prominence in active region using IRIS observations the authors found a faint component with a LOS velocity up to 460 km s −1, and revealed the unwinding motions during the prominence eruption. The IRIS has been widely used to study the dynamics of quiescent prominences, but the spectroscopic observations of eruptive prominences are still rare. So the IRIS is also suitable to observe prominences and filaments. The prominence core has a chromospheric temperature, and prominence also has a prominence-corona transition region (PCTR). The IRIS especially has an advantage of observing chromosphere and transition region with some strong resonance lines of Mg ii (temperature of formation of log T ∼ 4.0), C ii (log T ∼ 4.3), and Si iv (log T ∼ 4.8). It provides simultaneous high-resolution spectral and imaging data from the photosphere to the corona. The Interface Region Imaging Spectrograph (IRIS ), is a small explorer spacecraft launched in 2013 June. However, high-quality spectral data of eruptive prominences are rare due to limited field of view (FOV) of general spectroscopic observations and randomness of prominence eruptions. Spectroscopic observation is an effective way to reveal plasma properties and line-of-sight (LOS) motions.
Hence studying the triggering mechanism and evolution of prominence eruptions are important topics in solar physics.
Prominence eruptions have a close relationship with flares and coronal mass ejections (CMEs), and the latter two phenomena are main causes of the space weather storms. Solar prominences are composed of cold and dense plasma suspended in the hot corona. 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. The blue shifts in the third period are due to that the prominence erupts toward the observer. 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. We propose that the opposite Doppler shifts in the first period is a feature of the polar crown prominence that we studied. A faint region is also found in AIA 304 A ̊ images along the prominence spine, and the faint region gets darker during the expansion of the spine. During the second period, a faint component appears in Mg ii k window with a narrow line width and a large red shift. 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. 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. Spectroscopic observation is an effective way to explore the plasma properties, but the spectral observations of eruptive prominences are rare. Prominence eruption is closely related to coronal mass ejections and is an important topic in solar physics.