Diagnosing the Time-Dependence of Active Region Core Heating Using Emission Measures

It is widely believed that the cross-field spatial scale of coronal heating is small, so that the fundamental plasma structures (loop strands) are spatially unresolved. We therefore must appeal to diagnostic techniques that are not strongly affected by spatial averaging. One valuable observable is the emission measure distribution, EM(T), which indicates how much material is present at each temperature. The slope of the distribution coolward of its peak is related to the frequency of the presumed impulsive energy release. Nanoflares that have a long delay before repeating on the same loop strand give rise to shallow slopes, while nanoflares that repeat with a timescale shorter than a cooling time (or truly steady heating) give rise to steep slopes. Comparing recent Hinode observations with hydrodynamic loop simulations, we find that about 36% of active region cores are consistent with low-frequency nanoflares. The observational uncertainties are large, however, so as many as 77% or as few as none are consistent with low-frequency nanoflares when the uncertainties are taken into account. Constraining the time dependence of the heating is important for identifying the physical mechanism.