Estimating the cross-sectional sizes of potential solar triggers of near-Sun magnetic switchbacks

Context. The discovery of near-Sun magnetic switchbacks (SBs) represents a key result of the Parker Solar Probe (PSP) mission. Aims. Several theories and models suggest that near-Sun SBs may be triggered by small-scale transient phenomena occurring in the low solar atmosphere. Therefore, estimating the cross-sectional sizes of potential triggers that are consistent with near-Sun SB observations could place tighter constraints on related theories and models. Methods. We propose a simple method based on the conservation of magnetic flux in flux tubes connecting near-Sun SBs with their potential triggers in the low solar atmosphere. This method combines inferences of SB diameters and their magnetic fields from PSP in situ observations and estimates of the low-solar atmosphere magnetic fields, calculated at heights pertinent to proposed solar triggers of near-Sun SBs, from magnetic field extrapolations in areas on the Sun that are magnetically connected with PSP. The application of our method provides estimates of the radii of the cross-sections of potential near-Sun SB triggers taking place in the low solar atmosphere. Results. We applied our method to the SBs observed during the first solar encounter of PSP, when the spacecraft was connected to a small equatorial coronal hole. The inferred radii of the cross-sections of potential triggers of near-Sun SBs in the low solar atmosphere take values in the range ≈10–26 000 km; a more compressed range of ≈125–3500 km corresponds to the most representative SB diameters values. Conclusions. The latter range of potential solar near-Sun SB trigger radii is fully accessible by contemporary extreme UV (EUV) and UV instrumentation, with its lowest end of around 125 km being reachable by the Extreme Ultraviolet Imager on board the Solar Orbiter mission. This range is also consistent with the spatial scales of proposed near-Sun SB triggers in the low solar atmosphere. The smallest SB diameters we considered give rise to potential near-Sun SB solar trigger radii below ≈100 km, which are inaccessible with current EUV and UV instrumentation. Given that the inferred cross-sections sizes of the potential solar triggers of near-Sun SBs are significantly smaller than the respective errors in establishing their actual locations via magnetic connectivity tools, it is more appropriate to compare statistics of near-Sun SBs and their potential solar triggers on scales commensurate with the cross-sections inferred here, rather than with the entire connected region for encounter 1. Our simple method, paired with magnetic connectivity studies, is a step towards more refined assessments of potential solar triggers of near-Sun SBs.