By Luke Barnard
Coronal mass ejections (CMEs) are eruptions of coronal plasma and magnetic flux from the Sun’s corona, out into interplanetary space. CMEs are widely recognised as being a main driver of space weather and those CMEs that travel on a trajectory that intersects Earth’s orbit can be highly “geo-effective”, potentially generating geomagnetic storms and affecting Earth’s radiation belts. The risks associated with CME-driven space weather hazards can, to some extent, be mitigated by accurately forecasting the time at which a CME will interact with the Earth system – more specifically, what time it will “hit” Earth’s magnetic field. Therefore, a major theme in space weather research is developing a better understanding of the physics of CMEs, especially the dynamics of CME propagation from the Sun to Earth.
We use the Heliospheric Imager (HI) instruments aboard the twin STEREO satellites to study the dynamics of CMEs. These are white-light cameras with a wide field-of-view that can image plasma motions such as CMEs all the way from the outer edge of the Sun’s corona to near-Earth space. The two STEREO satellites, each carrying a HI instrument, are in Earth like orbits, but one drifts ahead of Earth (STEREO-A) and one drifts behind (STEREO-B), separating from Earth by about 20 degrees per year. Therefore, HI images allow us to study CMEs travelling towards Earth from two different vantage points.
Figure 1: (A) An example of an image taken by the HI instrument aboard STEREO-A. Each HI consists of two separate cameras, HI1 with a 20 degree field-of-view (on the right), and HI2 with a 70 degree field-of-view (on the left). The cameras are aligned so that the ecliptic plane runs horizontally along the center; the Sun is located just outside the rightmost edge of the HI1 image, whilst Earth is located just off the leftmost edge of the HI2 image. (B) An image from HI1 that has been processed to remove the background stars and enhance the visibility of a CME, which can be observed on the right hand side as the higher contrast white and black regions.
However, there are challenges in using the HI data to study CMEs. Firstly, there is no absolute definition of what constitutes a CME and so their identification and characterisation is subjective. Secondly, the CME characterisation is typically done by manually analysing images, which is very time consuming. Finally, CMEs are sufficiently complex and variable that it is difficult to automate this analysis, which would reduce the subjectiveness and labour of our research.
Solar Stormwatch (www.solarstormwatch.com) is a citizen science project that solves many of these problems. The project consists of several activities, completed via a web interface, where anyone who is interested can identify and characterise CMEs visible in the HI images.
Figure 2: An example of the Solar Stormwatch web interface in which CMEs are identified in the HI1 cameras aboard STEREO-A and STEREO-B.
For example, in one activity participants are asked to view a movie of HI images and to record the time at which they can see a CME enter the HI field-of-view from either STEREO satellite. When many participants tell us they can see a CME entering the HI field-of-view, we can be confident they are probably correct. In a second task, we direct participants to images in which the profile of a CME should be visible, and ask them to locate the front of the CME in the image. When many participants characterise the same CME, we can average their individual estimates to produce a consensus profile of the CME. This consensus profile does not suffer from the subjectiveness of an individual expert’s identification, and the variability of this average gives us new quantitative information about how well defined the event is. You can see the results of this process in the animation in Figure 3, which shows the propagation of a CME through the HI1 field-of-view, upon which the CME front identified by the Solar Stormwatchers has been overlaid in red. The yellow lines mark the outer-limits of the HI1 field-of-view analysed by Solar Stormwatch.
Figure 3 (link to animated GIF): This movie shows a sequence of HI1 images, processed similarly to Figure 1B, in which a CME can be seen to enter and propagate across the HI1 field-of-view. The yellow lines mark the outer limits of the image region analysed. The red lines mark the location of the consensus profile of the CME front, calculated by averaging the observations of many Solar Stormwatchers.
Solar Stormwatch has now been running for approximately 4 years, with input from more than 16,000 citizen scientists, resulting in a data set in excess of 38,000 characterisations of CME trajectories. We have recently turned these observations into a catalogue of CMEs observed by the HI instruments. This is a new and unique catalogue, providing information about CMEs at distances away from the Sun not presently covered by other widely used CME catalogues. These data are all publicly available, and we hope they will aid new research into the dynamics of CMEs – some of which is already being done here in Reading. The next stage of Solar Stormwatch is to update it with new data, as presently only the HI images from 2007 to 2010 have been analysed – with 2010 to 2014 left to analyse there is much more data to process!
To take part: http://www.solarstormwatch.com
ACKNOWLEDGEMENTS Many thanks to Chris Scott for Figure 1A.