The corona, or the solar atmosphere, is a hot, magnetised plasma. At a million degree or higher, it is very hot – hot enough to almost completely ionise the gas so that it is composed of separate protons and electrons instead of neutral atoms. It is a very different environment to our atmosphere on Earth. Its structure and time evolution is dominated by the solar magnetic field. The plasma is constantly outflowing outwards into interplanetary space, and as it does so, it drags the magnetic field with it. This is called the solar wind, which flows past all the planets out to the very edge of the heliosphere at several hundred miles per second. The Earth therefore is orbiting within the extended atmosphere of the Sun.
In certain regions near the Sun, the magnetic field of the corona can become twisted and highly complex due to movement of the plasma at the Sun's surface. As the field becomes more and more twisted, or otherwise forced into a complex configuration, magnetic energy is stored. This is not a stable configuration! This stored energy can often erupt into huge and spectacular explosions, as seen in figure 1. A huge, complex and energetic cloud of magnetic plasma is ejected into the solar wind. These are called Coronal Mass Ejections (CMEs). Some of these will hit Earth and, if conditions permit, they can interact with the Earth's magnetic field, even forcing Earth's field to 'break', or to reconnect with the CME's magnetic field. In normal conditions, the Earth's magnetic field acts as a shield to prevent damage from CMEs. Society, however, depends on satellites and large CMEs under certain conditions may cause widespread damage. There is also evidence that solar storms, or space weather, can affect Earth's climate.
Figure 1 - Left: A huge eruption from the Sun's corona during 2012/08/31, as observed by the AIA instrument aboard the SDO spacecraft. Right: the subsequent CME observed travelling away from the Sun into interplanetary space, as observed by the LASCO C2 coronagraph aboard the SOHO spacecraft.
In order to predict the impact of a CME on the Earth, connections must be made between the Sun and heliosphere. This can only be achieved by the provision of accurate 3D information on both CMEs and the medium through which they propagate. Such information is currently based on sophisticated models which are limited by a lack of observational constraints. This project will give unique 3D information on the structure of the solar wind and CMEs over several years or decades of data. Many empirical constraints from the coronal base out to Earth orbit will be gained from novel analysis methods. Such information will lead to greater understanding of the system and more accurate forecasting of space weather events. An understanding of the background solar wind is important to understand the evolution of CMEs as they erupt from the Sun and travel towards Earth.
The project is split into two connected parts – development of certain data analysis tools for CMEs, and a complimentary development of analysis tools for the background solar wind. My goal is to produce a comprehensive atlas of the solar wind and CMEs over the last decade or so, and to keep updating the atlas in realtime. Linked to this is the provision of a realtime alert service, which can warn interested parties of an incoming CME.
In the first part of the project, a method to accurately constrain 3D positions of CMEs in the inner corona will be developed, made possible by advanced processing which isolates CME signal in coronagraph images. Once the CME signal is identified, triangulation allows a robust estimate of CME position. Figure 2 shows preliminary results. The method will be applied to data 2008–realtime and 3D information catalogued (position, true 3D velocity, true 3D mass) into an online database open to the research community. Additionally, and more importantly from a practical standpoint, an accurate realtime Earth-bound CME warning will be provided.
Figure 2 - Left: the probable 3D position of a CME observed by the LASCO C2 (middle) and STEREO A COR 2 (right) coronagraphs during 2009/01/14.
The goal of the second part of the project is to create tomography maps of the corona over a long time period, and to use this information to create maps of the solar wind structure out to Earth orbit. The Sun rotates approximately once every 27 days as observed from Earth. The corona rotates at a similar rate, therefore observations over half this period (~13 days) gives a complete 360 degree view of the corona. This makes tomography of the corona possible – transforming the set of 2D images into a 3D map of the corona structure. This is similar to medical tomography, where X-Ray images are taken through a patient at many different angles. Preliminary results are shown in figure 3.
Figure 3 - Left: the coronal electron density at a height of 5.0 solar radii, gained from tomography of coronagraph observations. Middle and right: heliospheric density and solar wind velocity from the corona to past 1AU in the equatorial plane, gained from extrapolation of the tomography map. Earth's position is the white diamond.
Dr Huw Morgan