The physical relationship between flares and surges observed in the extreme ultraviolet

A search was made for EUV surges among the EUV flares recorded by the Harvard spectroheliometer on ATM. Out of a large set of partial observations of such flares, a subset of 24 complete events was chosen. More than 24 associated surges were found, many of them multiple events. The flare-surge correlation is therefore considerably higher in the EUV than in H, presumably because EUV surges generally appear in emission, and in high contrast compared to H. In over 70% of the cases, the surges were found to grow out of the flare structure. Making reasonable assumptions, it was possible to infer the magnitude of the gas pressure gradient from the flare core into the surge by using the EUV intensity gradient. The inferred pressure gradient appears sufficient to drive the surge, although higher resolution observations will be required to corroborate this, and rule out the importance of magnetic Lorentz force.