Wednesday

Observations from MESSENGER have demonstrated that bursty dayside reconnection plays a proportionally larger role in the driving of Mercury's magnetosphere than it does at Earth. Studies using MESSENGER data indicate that individual bursts of dayside reconnection, called flux transfer events or FTEs, open magnetic flux equivalent to up to 5% of Mercury’s polar cap, whereas equivalent spacecraft studies at Earth find that individual FTEs typically open 0.1-1% of the terrestrial polar cap. Coupled with the much higher repetition rate of FTEs at Mercury, this has led to the suggestion that bursty reconnection is the dominant driver of Mercury's magnetosphere, whereas quasi-steady reconnection predominates at Earth. However, our recent work suggests that the contribution of FTEs at Earth may have been severely underestimated. If correct, similar arguments could apply at Mercury, and the significance of FTEs there could be underestimated too. Therefore, we extend our earlier work and consider upper limits for the contribution of individual FTEs at Mercury. We find that it could be reasonable for an individual FTE to open 20-30% of Mercury's polar cap - in other words, in cases of extreme driving, only a handful of FTEs (with corresponding magnetotail reconnection) might be needed to refresh Mercury's polar cap entirely.