Using the "Ghost Front" to Predict the Arrival Time and Speed of CMEs at Venus and Earth

Using in situ measurements and remote-sensing observations, we study two coronal mass ejections (CMEs) that left the Sun on 2012 June 13-14 and impacted both Venus and Earth while the planets were in close radial alignment. The two CMEs generate multiple fronts in Solar Terrestrial Relations Observatory (STEREO)/Heliospheric Imager (HI) images, which can also be observed in the "J-map" as bifurcated features. We present the "ghost front" model to combine remote observations from STEREO/SECCHI and in situ observations from the Wind and Venus Express (VEX) spacecraft, and to derive the kinematics and propagation directions of the CMEs. By fitting the observations of multiple fronts to a kinematically evolving flux rope model and assuming the CMEs undergo deceleration through frictional drag with a steady-state solar wind, we confirm that the outer and inner fronts of the CMEs as detected in HI images are consistent with peaks in Thomson scattered light returned from the flank and nose of a single front for each CME. An interaction takes place between CME-1 and CME-2 that can be observed in the HI-1 field of view (FOV) before CME-1 encounters Venus. The multipoint in situ observations of the shock-CME interaction event serve as further evidence of the interaction between CMEs. The arrival times calculated from the ghost front model are within 2.5 hr of those observed at VEX and Wind. Our analysis indicates that ghost fronts could provide information about the longitudinally extended shape of the CME in the FOV of HI-1, which can be used to improve the forecast of interplanetary CME arrival time at Earth.