1. On the February 14-15, 2011 CME-CME interaction event and consequences for Space Weather Manuela Temmer(1), Astrid Veronig(1), Vanessa Peinhart(1), Bojan Vrsnak(2) (1)IGAM-Kanzelhoehe Observatory, University of Graz, Graz, Austria (2)Hvar Obs, University of Zagreb, Zagreb, Croatia

2. Intensification of geoeffectiveness Successive CMEs (similar directions) may merge and form complex ejecta of single fronts (e.g., Gopalswamy et al. 2001; Burlaga et al. 2002, 2003; Wang et al. 2002; Wu et al., 2007 ). Radio enhancements, SEP events accelerated at the at the shock front(s) (e.g., Gopalswamy et al. 2001; 2002; Hillaris et al., 2011; Kahler & Vourlidas 2014 ) Foreacasting is tricky: arrival time and geo- effectivity varies depending on interaction>> Gopalswamy et al. 2001 Hillaris et al. 2011 extended periods of negative Bz (e.g. Wang et al. 2003; Farrugia et al. 2006 ) intense magnetic storms may often be the result of two closely-spaced moderate storms ( Burlaga et al. 1987; Kamide et al. 1997; Farrugia et al. 2006a,b ) simulations (see e.g., Lugaz et al. 2008; Xiong et al. 2009 )

3. February 14-15, 2011 CME-flare events Source region of CME1/CME2 is ~E12/ ~W5. CME2 has similar mass as CME1 (projected from ST-A data). CME-CME interaction studied by Maricic et al., 2014 and Temmer et al., 2014. Associated flare events (AR 11158): Feb 14@17:20UT M2.2 S20/W04 (m~4.7x10 15 g) Feb 15@01:44UT X2.2 S20/W10 (m~6.4x10 15 g) SDO Feb 15, 2011 ESA Proba2/SWA P Dimming evolution

4. Interaction in STEREO HI1-A FoV HI1-A difference image data showing the time range of interaction between CME1 and CME2. Labeled arrows mark the fronts of CME1 and CME2. CME2 forms a bulk in the southern region where no interaction takes place. Note: Southern polar CH From WL observations: deformation of frontal structure, changes in intensity (compression) Kinematics of the fronts of CME1 and CME2 (+trailing edge) is derived

5. 3D properties and orientation of FR Combining observational data with flux rope model ( Thernisien et al., 2006 ) 3D forward fitting reveals similar directions for both CME events > interacting ejecta Flux ropes/CME bodies are of different size and have different orientation

6. Lateral asymmetry in interaction Flux rope of CME1 is shown as yellow mesh and of CME2 as green mesh. Direction 80 – 100° strongest interaction between the two ejecta. 125° no interaction. Measurements over entire latitudinal extent along different PAs (spherical deprojection method for HI1-A data). Efficiency of the interaction process (in terms of acceleration/deceleration) may be related to the location of the magnetic flux ropes.

7. Change in CME propagation for interacting parts PA 70° almost no change from constant speed. Lateral expansion may prevail, hence compression is lower for radial direction PA 100° largest variation in speed – major interaction PA 125° largest speed – no interaction (influenced by a polar coronal hole)

8. Structuring of solar wind in IP space Inelastic collision? Maybe... Front of CME2 may influence rear of CME1 and hence, frontal part, much earlier due to momentum transfer (see also Maricic et al., 2013 ). >> Observational restrictions. Southern part of the CME (PA-125) might have been swept radially forward by the fast solar wind coming from the coronal hole in the south (cf. pancaking; e.g., Manchester IV et al., 2004 ). See posters by M. Reiss and A. Veronig!

9. Right panel: base difference images (ST-A and -B) Type III bursts (i.e. particle injection) stem from the same AR as both CMEs. Radio enhancement due to interaction (CME-CME, shock-CS)...? Type III burst stops at magnetic barrier of CME2, or is enhanced when entering the compressed plasma region between CME1 and CME2...? Interaction process observed w/ radio

10. Maricic et al., 2014 In-situ signatures 1: shock-sheath 2: ejection signature 3: reconnection–outflow exhaust? 5: magnetic cloud High magnetic field due to compression – consequence: stronger Space Weather effects especially for strong negative Bz components see also extreme cases: Dst = –1000nT ( Baker et al., 2013; Russell et al., 2013 )

11. Observational data reveal only the consequences of CME–CME interaction. How to develop advanced tracking methods (see e.g., Byrne et al., 2013 ) for better determining CME kinematics of sub-structures (front, back-end, shock-sheath region) - are we able to distinguish between sub-structures? Radial and lateral evolution, both give important hints on the interaction process. CME2 runs into high density, slow speed, high magnetic tension – we need to better define the ambient environment for CMEs. There might be a number of problems in determining the type of collision between two CMEs among others due to changes in the mass of the CMEs during evolution (see also Bein et al. 2013 ) as well as ongoing perturbation hours after the collision (see Lugaz et al. (2013) ). How are magnetic or thermal energies converted into kinetic energy? Extreme events in terms of Space Weather (intensification of magnetic field). Conclusion and open questions...