Ryan French

The Sun’s atmosphere (known as the corona) is dominated by magnetic fields. Unlike the Earth’s magnetic field, which can be approximated as a single north and south pole, the Sun’s surface is littered by regions of opposite fields. Areas of concentrated magnetic fields are called ‘active regions’, which are the primary source of eruptions on the Sun. In ordinary solar environments, magnetic field lines cannot pass through one another. Therefore, as active regions move and produce new magnetic fields, the coronal magnetic fields above them become twisted and tangled, resulting in a build-up of potential energy. If this potential energy becomes too great, magnetic fields are compressed together and a ‘current sheet’ forms between them. At this current sheet, magnetic fields are able to ‘reconfigure’, rearranging the magnetic fields into a lower energy configuration. The free energy is then released as radiation (from radio to gamma waves, across the light spectrum), particle acceleration and heating, in a process known as a solar flare. The reconfiguration of field lines in this way is known as magnetic reconnection. Some bundles of magnetic field (and the solar material within it) can be ejected into space as a result of this, known as a coronal mass ejection. //

The process of magnetic reconnection is not yet fully understood. Multiple models of reconnection exist, but their complexity makes it difficult to account for all of the observed features in a solar flare. Sweet-Parker reconnection is the earliest proposed model (1957), in which magnetic reconnection occurs along the full length of the current sheet simultaneously. However, it has been shown that this model cannot release energy as fast as that observed in solar flares. This energy release problem can be solved by the presence of the ‘tearing mode’ instability. In this model, if the current sheet length greatly exceeds its width, it will collapse and reconnect at multiple points along the current sheet to form ‘plasmoids’ or ‘magnetic islands’. //

On the 10th September 2017, a large solar flare erupted from the edge of the Sun. Due to the flare’s orientation, telescopes at Earth saw a ‘side-on’ view of the flare, observing a clear plasma sheet (heated material around the current sheet) emanating above the Sun’s surface (Figure 1). Such a perspective matches closely what we expect from a 2D cartoon of a solar eruption (Figure 1). Due to the unique perspective of the plasma sheet, observations provide the opportunity to probe the nature of magnetic reconnection within this event, to determine whether or not ‘plasmoid reconnection’ could be occurring. //

In this study, we use data from the Coronal Multi-channel Polarimeter (CoMP) telescope in Hawaii. CoMP is a coronagraph, blocking out light from the Sun’s surface in order to measure fainter light in the corona. Light in the corona can undergo ‘linear polarisation’, meaning the light waves are more inclined to oscillate in a particular direction. This level of linear polarisation is related to both local density and magnetic field direction in the corona. In the 10th September 2017 flare, we measure a very low linear polarisation in the plasma sheet (Figure 3). By ruling out other potential causes, we show that this low polarisation must be due to variation of magnetic field direction on scales smaller than one pixel (Figure 4). This result is consistent with the presence of small-scale magnetic structures (e.g. magnetic islands) within the plasma sheet, as predicted by theory.