Properties of magnetic null points associated with X-class flares during solar cycle 24

Abstract

ABSTRACT Since the launch of the Solar Dynamics Observatory (SDO) in 2010 and throughout the solar cycle 24, the Sun has produced few tens of X-class flares, which are the most energetic solar events. Those flares are produced in regions where the magnetic flux/energy is large and the magnetic configurations are complex. To provide more insights into the flaring process, we investigate the properties of magnetic null points (MNPs) and their correlation with the energy release sites. During solar cycle 24, we identify 17 X-class flares satisfying selection criteria. From SDO/HMI magnetograms, we perform potential extrapolations around the peak time of the flare to access the 3D coronal magnetic field and thus investigate the existence of coronal MNPs. We then correlate the flaring sites with the existing MNPs using SDO/AIA 171 Å; EUV observations, and deduce their properties (sign, spine, and fan). Six active regions out of 10 possess at least one MNP which is stable and with large magnetic field gradients: this implies that 35 per cent of X-class flares are associated with an MNP; of which 87.5 per cent of MNPs are of positive type. The MNPs associated with the flare sites are predominantly located at a height between 0.5 and 2 Mm, and with a vertical/radial spine field line. We also find a slight correlation between the MNPs not associated with a flare and negative-type MNPs (55 per cent) within the active region. Regarding the physics of flares, the association between the enhanced intensity at the flaring site and an MNP represents about a third of the possible scenarios for triggering X-class flares.