Abstract
Solar tornados are dynamical, conspicuously helical magnetic structures mainly observed as a prominence activity. We investigate and propose a triggering mechanism for the solar tornado observed in a prominence cavity by SDO/AIA on September 25, 2011. High-cadence EUV images from the SDO/AIA and the Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the neighbouring active-region (NOAA 11303), and their EUV waves, with the dynamical developments of the tornado. The timings of the flares and EUV waves observed on-disk in 195\AA\ are analyzed in relation to the tornado activities observed at the limb in 171\AA. Each of the three flares and its related EUV wave occurred within 10 hours of the onset of the tornado. They have an observed causal relationship with the commencement of activity in the prominence where the tornado develops. Tornado-like rotations along the side of the prominence start after the second flare. The prominence cavity expands with acceleration of tornado motion after the third flare. Flares in the neighbouring active region may have affected the cavity prominence system and triggered the solar tornado. A plausible mechanism is that the active-region coronal field contracted by the `Hudson effect' due to the loss of magnetic energy as flares. Subsequently the cavity expanded by its magnetic pressure to fill the surrounding low corona. We suggest that the tornado is the dynamical response of the helical prominence field to the cavity expansion.
Highlights
Prominences consist of relatively extended, cool, and overdense plasma seen in the lower corona above the solar limb (Martin 1998; Tandberg-Hanssen 1995; Mackay et al 2010)
We show with Solar TErrestrial RElations Observatory (STEREO) Extreme UltraViolet Imager (EUVI) observations that three strong flares in a neighbouring active region coincided with phases of the tornado activation
A solar tornado was observed by the Atmospheric Imaging Assembly (AIA) and EUVI-A on 25 September 2011
Summary
Prominences consist of relatively extended, cool, and overdense plasma seen in the lower corona above the solar limb (Martin 1998; Tandberg-Hanssen 1995; Mackay et al 2010). Their structure and composition are exceedingly complicated. Quiescent prominences mostly appear as curtains of vertical thread-like structures (Berger et al 2008) They look like tornados with rotations along, or of, their magnetic structures (Pettit 1943; Liggett & Zirin 1984). 2. The evolution of the tornado, its relationship to the solar flares and the EUV waves are described in Sect.
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