  The twin Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)
COR2 coronagraphs of the Solar Terrestrial Relations Observatory (STEREO)
provide images of the solar corona from two view points in the solar system.
Since their launch in late 2006, the STEREO Ahead (A) and Behind (B)
spacecrafts have been slowly separating from Earth at a rate of 22.5 degrees
per year. By the end of 2007, the two spacecraft were separated by more than 40
degrees from each other. At this time, we began to see large-scale differences
in the morphology and total intensity between coronal mass ejections (CMEs)
observed with SECCHI-COR2 on STEREO-A and B. Due to the effects of the Thomson
scattering geometry, the intensity of an observed CME is dependent on the angle
it makes with the observed plane of the sky. From the intensity images, we can
calculate the integrated line of sight electron density and mass. We
demonstrate that is is possible to simultaneously derive the direction and true
total mass of the CME if we make the simple assumption that the same mass
should be observed in COR2-A and B.
