Abstract
The Heliospheric Imagers on board NASAs twin STEREO spacecraft show that
coronal mass ejections (CMEs) can be visually complex structures. To
explore this complexity, we created a web-based citizen science project
in collaboration with the UK Science Museum, in which participants were
shown pairs of differenced CME images, taken by the inner cameras of the
Heliospheric Imagers (HI-1) on board the twin NASA STEREO spacecraft
between 2008 and 2016. Participants were asked to decide which image in
each pair appeared the most complicated. 4,028 volunteers conducted
246,692 comparisons of 20,190 image pairs, with each pair being
classified by 12 independent users. A Bradley-Terry model was then
applied to these data to rank the CMEs by their visual complexity. This
complexity ranking revealed that the annual average visual complexity
values follow the solar activity cycle, with a higher level of
complexity being observed at the peak of the cycle, and the average
complexity of CMEs observed by HI1-A was significantly higher than the
complexity of CMEs observed by HI1-B. Visual complexity was found to be
associated with CME size and brightness, but the differences between
HI1-A and HI1-B images suggest that complexity may be influenced by the
scale-sizes of structure in the CMEs. Whilst it might not be surprising
that the complexity observed in these CME images follows the trend
observed in sunspots and the solar cycle; these results demonstrate that
there is a quantifiable change in the structure of CMEs seen in the
inner heliosphere.