Investigation of ELMs on Alcator C-Mod*

C-Mod typically operates in regimes without large ELMs. Recently, discrete ELMs have been routinely produced by making plasmas with large lower triangularity (i.e. $>$0.75), compared to the more typical C-Mod values $<$0.6. The ELM character was substantially modified as the triangularity was reduced, changing from discrete ELMs of $\sim$60 $\mu$sec duration, to H-to-L mode back transitions, lasting $\sim$4msec. The discrete ELMs are most apparent when the density is just above the low-density H-mode threshold, $n_e\sim8x10^{19}m^{-3}$. Pedestal $T_e$ up to 1 keV was measured early in the H-mode phase. The spatial structure and propagation of the discrete ELMs are studied using fast-framing ($\sim$250 kHz) cameras and other high resolution optical diagnostics. The magnetics and the optical diagnostics show a rapidly-growing precursor oscillation (100-200 kHz just prior to the ELM crash) that is localized radially to around the top of the pedestal. Outside the LCFS the enhanced emission from the ELM propagates radially outward with a complicated spatial structure, similar in many respects to `blobs.'