Research


The Sun provides a unique opportunity to study up close the fundamental physical processes that control how a star functions. But we have learned that living in close proximity to a star has can have a major impact on the Earth’s atmosphere and magnetic field, and therefore on our lives. Many of the Sun’s effects go unnoticed but some, such as the northern lights, are enjoyed by many.

Lucie’s research looks at the evolution of the Sun’s magnetic field and how our star can drive what is known as ‘space weather’ here on Earth. A major component of space weather originates in the coronal mass ejections that the Sun produces; these are huge eruptions of magnetic field and electrically charged gas that somehow break free from the Sun and speed into the Solar System at speeds of 100s to 1000s km/s. Coronal mass ejections are seen as clouds of outward moving material when the Sun is eclipsed. Some ejections can also be seen as so-called filament eruptions from just above the visible solar surface through specialist amateur telescopes. Lucie studies the magnetic source regions of coronal mass ejections with a view to understanding the changes in the Sun’s magnetic field that trigger them.

Over the last few years she has been interested in the particular configuration of these eruptive magnetic fields and has been testing theoretical ideas that predict that twisted magnetic fields (known as flux ropes) might be the progenitor of coronal mass ejections. Her observational studies have shown that flux ropes can indeed be found on the Sun before an eruption occurs. Studies of these flux ropes have enabled scientists to further understand their magnetic structure, which is needed in order to understand the physical processes that trigger and drive the ejections. The movie below shows a flux rope that was found in a reconstruction of the solar magnetic field carried out by Alex James, just before its eruption. The twisted magnetic field lines of the flux rope are clearly seen. The flux rope erupted shortly after this snapshot in time and more information can be found in James et al. (2017) and James et. al (2018).

 

The outer-most magnetic field lines of a flux rope looks like an S when view from above, if the rope is only weakly twisted. The video below shows one of the S-shaped regions studied by Lucie that was observed with the X-ray telescope onboard the Japanese Hinode satellite.It has been known for may years that such S-shaped structures are eruptive, but Green and Kliem (2009) provided evidence that the S shapes are twisted bundles of magnetic fields and have a rope structure and Green, Kliem and Wallace (2011) studied how much magnetic flux is contained in such a rope. They found a much higher value than models predicted, which started to be reconciled in Savcheva, Green et al. (2012).

Origin of a coronal mass ejection at the Sun from luciegreen on Vimeo.

Lucie’s work on flux ropes was extended through a collaboration to analyse and model the Sun’s seismic activity. Over the last decade, it has become well established that beams of particles accelerated during a solar flare can travel into the Sun and produce a sunquake (a short burst of seismic activity analagous to an earthquake). This collaboration found that sunquakes can also be triggered during the eruption of magnetic flux ropes. The research showed that as a magnetic flux rope accelerates away from the Sun, the rapidly changing magnetic field anchored in the solar surface may play an important role in the onset of the sunquake. These results highlight the importance of studying the magnetic field in order to understand how sunquakes are formed.

More recent modelling work has shown that the specific configuration of the magnetic field of the flux rope can be responsible for channelling the beam of accelerated particles, so that their deposition in the lower atmosphere is targeted onto a small area where the sunquake is then generated. This magnetic field configuration can be thought of as a magnetic lens or funnel that controls the placement of the particles. You can read more in the UK Solar Physics website science nugget on this work.

The view of the Sun given to us by space-age technology shows that the space around our star is far from empty. As well as blasting out coronal mass ejections, the Sun’s atmosphere itself extends billions of km into space – drawn out by a solar wind. The Voyager 1 space probe became the first human-made object to directly locate the edge of the Sun’s atmosphere when it passed into interstellar space in 2012 after having travelled 18 billion km from us. The vast size of the Sun’s atmosphere means that the Earth is sitting directly in this wind, which provides energy to drive space weather at Earth. The solar wind carries with it magnetic fields, which have their origin deep inside the Sun, as well as electrically charged particles out into the Solar System. The strength and complexity of the Sun’s magnetic field varies with the so-called solar cycle, which lasts roughly 11 years. We are currently in a solar minimum phase so the Sun has become very inactive and its magnetic field diminished. However, the Sun is still emitting the solar wind and coronal mass ejections still take place, so stormy space weather is still possible.