We present an analysis of the fast coronal mass ejection (CME) of 2012 March 7, which was imaged by both STEREO spacecraft and observed in situ by MESSENGER, Venus Express, Wind and Mars Express. Based on detected arrivals at four different positions
in interplanetary space, it was possible to strongly constrain the kinematics and the shape of the ejection. Using the white-light heliospheric imagery from STEREO-A and B, we derived two different kinematical profiles for the CME by applying the novel constrained self-similar expansion method. In addition, we used a drag-based model to investigate the influence of the ambient solar wind on the CMEs propagation. We found that two preceding CMEs heading in different directions disturbed the overall shape of the CME and influenced its propagation behavior. While the Venus-directed segment underwent a gradual deceleration (from ~2700 km/s at 15 R_sun to ~1500 km/s at 154 R_sun), the Earth-directed part showed an abrupt retardation below 35 R_sun (from ~1700 to ~900 km/s). After that, it was propagating with a quasi-constant speed in the wake of a preceding event. Our results highlight the importance of studies concerning the unequal evolution of CMEs. Forecasting can only be improved if conditions in the solar wind are properly taken into account and if attention is also paid to large events preceding the one being studied.
In this study we use a numerical simulation of an artificial coronal mass ejection (CME) to validate a method for calculating propagation directions and kinematical profiles of interplanetary CMEs (ICMEs). In this method observations from heliospheri
c images are constrained with in-situ plasma and field data at 1 AU. These data are used to convert measured ICME elongations into distance by applying the Harmonic Mean approach that assumes a spherical shape of the ICME front. We use synthetic white-light images, similar as observed by STEREO-A/HI, for three different separation angles between remote and in-situ spacecraft, of 30{deg}, 60{deg}, and 90{deg}. To validate the results of the method they are compared to the apex speed profile of the modeled ICME, as obtained from a top view. This profile reflects the true apex kinematics since it is not affected by scattering or projection effects. In this way it is possible to determine the accuracy of the method for revealing ICME propagation directions and kinematics. We found that the direction obtained by the constrained Harmonic Mean method is not very sensitive to the separation angle (30{deg} sep: phi = W7; 60{deg} sep: phi = W12; 90{deg} sep: phi = W15; true dir.: E0/W0). For all three cases the derived kinematics are in a relatively good agreement with the real kinematics. The best consistency is obtained for the 30{deg} case, while with growing separation angle the ICME speed at 1 AU is increasingly overestimated (30{deg} sep: Delta V_arr ~-50 km/s, 60{deg} sep: Delta V_arr ~+75 km/s, 90{deg} sep: Delta V_arr ~+125 km/s). Especially for future L4/L5 missions the 60{deg} separation case is highly interesting in order to improve space weather forecasts.
