FIRST MEASUREMENTS OF THE MASS OF CORONAL MASS EJECTIONS FROM THE EUV DIMMING OBSERVED WITH STEREO EUVI A+B SPACECRAFT

The masses of coronal mass ejections (CMEs) have traditionally been determined from white-light coronagraphs (based on Thomson scattering of electrons), as well as from extreme ultraviolet (EUV) dimming observed with one spacecraft. Here we develop an improved method of measuring CME masses based on EUV dimming observed with the dual STEREO/EUVI spacecraft in multiple temperature filters that includes three-dimensional volume and density modeling in the dimming region and background corona. As a test, we investigate eight CME events with previous mass determinations from STEREO/COR2, of which six cases are reliably detected with the Extreme Ultraviolet Imager (EUVI) using our automated multi-wavelength detection code. We find CME masses in the range of m CME = (2-7) x 1015 g. The agreement between the two EUVI/A and B spacecraft is mA /mB = 1.3 ± 0.6 and the consistency with white-light measurements by COR2 is mEUVI/mCOR2 = 1.1 ± 0.3. The consistency between EUVI and COR2 implies no significant mass backflows (or inflows) at r < 4 Rsun and adequate temperature coverage for the bulk of the CME mass in the range of T ∼ 0.5-3.0 MK. The temporal evolution of the EUV dimming allows us to also model the evolution of the CME density ne (t), volume V(t), height-time h(t), and propagation speed v(t) in terms of an adiabatically expanding self-similar geometry. We determine e-folding EUV dimming times of tD = 1.3 ± 1.4 hr. We test the adiabatic expansion model in terms of the predicted detection delay (Δt ∼ 0.7 hr) between EUVI and COR2 for the fastest CME event (2008 March 25) and find good agreement with the observed delay (Δt ∼ 0.8 hr).