Statistics of coronal dimmings associated with coronal mass ejections. I. Characteristic dimming properties and flare association

Coronal dimmings, localized regions of reduced emission in the EUV and soft X-rays, are interpreted as density depletions due to mass loss during the CME expansion. They contain crucial information on the early evolution of CMEs low in the corona. For 62 dimming events, characteristic parameters are derived, statistically analyzed and compared with basic flare quantities. On average, coronal dimmings have a size of $2.15\times10^{10}$ km$^{2}$, contain a total unsigned magnetic flux of $1.75\times10^{21}$ Mx, and show a total brightness decrease of $-1.91\times10^{6}$ DN, which results in a relative decrease of $\sim$60% compared to the pre-eruption intensity level. Their main evacuation phase lasts for $\sim$50 minutes. The dimming area, the total dimming brightness, and the total unsigned magnetic flux show the highest correlation with the flare SXR fluence ($c\gtrsim0.7$). Their corresponding time derivatives, describing the dimming dynamics, strongly correlate with the GOES flare class ($c\gtrsim 0.6$). For 60% of the events we identified core dimmings, i.e. signatures of an erupting flux rope. They contain 20% of the magnetic flux covering only 5% of the total dimming area. Secondary dimmings map overlying fields that are stretched during the eruption and closed down by magnetic reconnection, thus adding flux to the erupting flux rope via magnetic reconnection. This interpretation is supported by the strong correlation between the magnetic fluxes of secondary dimmings and flare reconnection fluxes ($c=0.63\pm0.08$), the balance between positive and negative magnetic fluxes ($c=0.83\pm0.04$) within the total dimmings and the fact that for strong flares ($>$M1.0) the reconnection and secondary dimming fluxes are roughly equal.