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Coronal mass ejections (CMEs) are energetic expulsions of mass from the Sun with typical velocities of several hundred up to >2000 km/s. CMEs propagating through the interplanetary (IP) space towards Earth may cause severe disturbances in the Earth's geophysical environment, our so-called “space weather”. Our society is more and more dependent on satellite systems for communication, navigation, and data transfer, and is thus increasingly affected by space weather disturbances. A basic requirement for space weather predictions is the accurate forecast of CME arrival times at Earth, which demands better knowledge of their causing/driving and propagation processes as well as the heliospheric conditions where they are embedded in. The main aim of the proposed project is to analyse the dynamics of CMEs from their eruption at the Sun up to their arrival at the Earth at 1 AU using observations from the new and unrivalled NASA STEREO mission, launched in October 2006. STEREO consists of two identical spacecrafts, STEREO A and B, with identical instruments aboard. Most important for the proposed project are the STEREO coronagraphs COR1, COR2 and Heliospheric Imager HI1+2. STEREO offers for the first time a stereoscopic view of the entire evolution of CMEs propagating from the Sun to the Earth and even beyond.
Short description of the task performed by Croatian partner
We will study the kinematics and dynamics of CMEs from their initiation and acceleration close to the Sun together with the variable solar wind conditions and their influence on the CME propagation in the IP space up to 1 AU. The main force acting on CMEs during their initiation phase is the Lorentz force, which determines the peak acceleration of the disturbance and is dependent on the associated magnetic energy release. As the CME propagates through the IP space, the main force acting is the aerodynamic drag force exerted by the ambient solar wind plasma. Owing to the interaction between the solar wind and the CME, the CME speed can decrease or increase depending on the velocity difference between CME and solar wind, until it finally becomes adjusted to the solar wind speed. This strongly alters the CMEs' travel time to the Earth as estimated from near-Sun observations, and hence severely affects the forecasting accuracy of such disturbances. Especially large and fast CMEs, which are known to be the most geoeffective events, are adjusted to the solar wind flow farther out, most probably outside the field of present coronagraphs such as SoHO/LASCO. STEREO now provides for the first time CME observations over the full distance range from the Sun to the Earth, from which we aim to gain better knowledge of the interrelation between the different forces acting on CMEs at different distances from the Sun. Based on the CME dynamics derived over the full Sun-Earth range together with an empirical relation obtained between the location and size of solar coronal holes, which are the main source of high speed solar wind streams, and in situ solar wind conditions at 1 AU, we will develop a semi-empirical model to improve the prediction of CME arrival times at the Earth.