Lauren Doyle

Solar flares are extremely powerful events and are observed across the entire electromagnetic spectrum, possessing energy outputs of up to 1032 erg (Fletcher et al. 2011). Stellar flares have been observed on stars similar to and less massive than the Sun over many decades with energies exceeding 1033 erg (e.g. Schaefer et al. 2000). Known as `superflares' these large outbursts can have severe consequences for any orbiting planets atmosphere. The lack of detailed spatial and temporal stellar flare observations poses an issue in the community. Therefore, we should look to the Sun to provide these details.
In solar physics, there is a well-established correlation between solar flares and sunspots on the Sun which dates back to the 1930’s. It is generally accepted that these phenomena are closely related. However, the stellar flare starspot connection tells a different story. Lightcurves of stars show rotational modulation, similar to a sinusoidal pattern, is produced due to the presence of a starspot on the disk of the star moving in and out of view. In many stars it has been noticed that flares are present throughout the lightcurve and are not solely seen when the spot is most visible. This comes as a surprise as it contradicts what we believe is true from observations of the Sun and suggests these flares originate through a different mechanism.
In order to investigate this further, studies of both a solar flare and stellar flares were carried out. Firstly, TESS 2-min cadence lightcurves were used to conduct a statistical analysis into the relationship between flares and starspots. In total, 209 solar-type stars between F7 and K2 spectral types were selected with 1980 flares ranging in energy from 1031 – 1036 erg. Overall, no correlation between flares and starspots was found, suggesting the flares are randomly distributed and do not coincide with starspot producing modulation. This comes as a surprise as it goes against what is observed on the Sun.
Next, high cadence, Hα observations of a filament eruption and jet made using the CRISP spectro-polarimeter mounted on the SST were utilised and allowed for the construction of velocity maps of the event, providing details on the kinematics. The observations were contrasted with a 3D MHD simulation of a breakout jet in a closed-field background (similar to Wyper et al. 2017) and a close qualitative agreement was found. Overall, it was concluded the breakout model can not only be applied to CMEs and jets but flares also. This model has the potential to be used to investigate the stellar flare origins in more detail.
Finally, a comparison between the solar and stellar flare studies is possible with the key being the solar 3D MHD simulation. The 3D MHD simulation was scaled up to see how it would produce flares of greater energies, like the ones observed in solar-type stars. It was found, to produce a flare of energy 1034 erg would require a polarity of size 200Mm and field strength 2kG. In terms of the Sun the field strength of 2kG would be possible as sunspots tend to be in the region of 1kG - 4kG, however, a sunspot which is a third of the solar disk is extremely unlikely. From this it is clear there are other things to be considered with regards to these stars such as age and rotation period. This shows there needs to be more collaborations between solar and stellar models, similar to this study, to explain the origin of superflares in more detail.