Abstract

The thin wisps of reddish gas observed peeking out above an eclipsed Sun are prominences, known as filaments when observed on the solar disc. These structures contain large amounts of neutral hydrogen (hence their reddish colour) and helium, and in recent decades, we have come to understand that they are often the important dense core of a coronal mass ejection. CMEs are violent eruptions of magnetically confined and driven plasma that are flung into the solar system, with sometimes very disruptive consequences at Earth.

On 7th June 2011, one such eruption captured the attention of solar physicists because – unusually – much of the material inside the erupting filament cascaded back towards the Sun. In attempting to understand why this should be the case, we developed a method for measuring the density of hydrogen in the ejecta, using state-of-the-art multi-wavelength imaging data. The superior time resolution, sensitivity and near-synchronicity of data from the Solar Dynamics Observatory’s extreme UV cameras allow us to combine these two techniques using photo-ionisation continuum opacity to determine the spatial distribution of hydrogen in filament material. We present our results from this study, and speculate on why this spectacular eruption was so unusual, and what the technique offers for the future of filament studies.