Abstract After surveying the spectra of 39 prominences observed by the Multi-channel Infrared Solar Spectrograph at Purple Mountain Observatory and the 25 cm coronagraph at the Norikura Solar Observatory, we found that about 28% of them show small spatial scale (6''--8'') and short time scale (tens of seconds to a few minutes), unusual large broadening and large shift velocities in spectral lines including Hα, Hβ, Hε, Ca II H, Ca II K, Ca II 8542Å, He I D3 and He I 10830Å. We present in detail two typical events observed respectively on 2002 May 27 and 1981 August 2. The full-width at half maximum of the widest profile of the 2001 prominence is 1.8Å for Hα and 2.9 Å for He I 10830Å, while that of the 1981 prominence is 5.3 Å for Hβ, 3.6 Å for Ca II K, 4.0 Å for Ca II H and 2.8 Å for He I D3. Such broadenings generally occur at a level of several-thousand kilometres above the chromosphere. Further, most of these prominences manifest a rotation of (0.4--1.35)×10-2 rads s-1 pointing to the Sun and large line-of-sight velocities of 20--200 km s-1. Some of these events correspond in time to an enhancement or a small peak in the GOES X-ray flux, indicating the existence of high energy process at work. These prominences generally display discernible changes in the Hα morphology around the time of large broadening, but do not show Hα brightening or overall eruption except for a few small surge-like events, hence, they are hardly observed in daily Hα patrols. According to the characteristics of their Hα structures and spectral properties, we infer these events are small-scale eruptions similar to nano-flares, which may contribute to the mass and energy transported into the corona. Large turbulent velocities of 25--120 km s-1 are responsible for the observed broadenings.

Keywords Sun: prominences --- line: profiles --- turbulence

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