Comparison of Prominences in Hα and He II 304 Å

In this letter, we bring attention to prominences which show different morphology in Hα and He ii 304 Å, as observed simultaneously by BBSO and EIT on board SOHO. Those two lines have been thought to represent similar chromospheric structures although they are formed at significantly different temperatures. We give two examples representing two kinds of anomaly: (1) prominences showing strong Hα emissions in the lower part and strong He ii emissions in the upper part, and (2) erupting prominences showing extensive He ii emission, but nothing in Hα. Our results indicate that a part or the whole of a prominence may be too hot to emit Hα radiation, possibly due to heating or thermal instability. Please note that these are not just two isolated cases, many other prominences show the similar differences in Hα and He ii 304 Å.