Magnetic clouds and their driven shocks/sheaths near Earth: geoeffective properties studied with a superposed epoch technique Dasso S.1,2, Masías-Meza J.J.1, Demoulin P.3, Rodriguez L.4 & Janvier M.5 1 Universidad de Buenos Aires, Buenos Aires, Argentina 2 CONICET – Universidad de Buenos Aires. Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina 3 Observatoire de Paris, Meudon Principal Cedex, France 4 Royal Observatory of Belgium, Brussels, Belgium 5 Institut d’Astrophysique Spatiale, Orsay Cedex, France Main aim of this work: To find common plasma and magnetic properties in MCs near Earth. How: Superposed epoch method to a large set of MCs observed in situ by the spacecraft ACE. Results and conclusions: (1) Slow MCs have more massive sheaths. We conclude that low speed of these events is mainly due to solar wind drag, and not due to initial conditions. (2) Slow MCs have more symmetric B profile and sheaths are in a self-similar expansion as MC. (3) Fast MCs have asymmetric B profile and sheath in compression. Main Conclusions: Typical profiles of sheath/MCs distinguishing slow/middle/fast structures, can be used for forecasting or modelling these events. Also useful for improving future operative space weather activities.

1. Data sample and examples b d a c d c a d c b b b -ACE: MAG and SWEPAM -Time cadence: 64 s -Range: 1998-2006 -Sample events: intersection of ICMEs from Richardrson & Cane MCs by Lepping -No multiple MCs -MCs having sheath & shock -44 events studied -Fluctuations Fig. 1: Examples of MCs. Four different regions are identified for each event: pre-shocked solar wind (a), sheath (b), magnetic flux rope (c) & post-flux rope wake (d).

Fig. 2: Superposed profiles (same quantities as Fig. 1). 2. Superposed epoch technique applied to 44 MCs Superposed method: (a) Time renorm before/after shock, (b) MC interface (sheath: 0 to 1 / MC: 1 to 4), (c) re-binning: 50 bins for full sheath and 50 for full MC. Superposed profile: mean and median (red) values. a d d Notice: Clear jump at shock: B, V, np, Tp Expansion (V slope) up to t~3.3 Piled-up mass and B (sheath) Low 𝛽 value inside flux rope Peak of rmsBoB and rmsB at the shock, rmsBoB a factor 3 lower in MC than in the ambient SW Faster plasma in the wake than in the pre-shocked solar wind. Strongly perturbed wake up to at least t~6 normalized units B similar at sheath and start MC. Same for V. a b c b c Fig. 2: Superposed profiles (same quantities as Fig. 1).

3. Splitting epoch by velocity (best parameter) Best ’order-parameter’: splitting with each ordering parameter: B, V, np, Tp, 𝛽, rmsBoB, and rmsB, largest separation between mean values is when V is used. -Slow events: massive sheaths. Selection effect: slow MCs slowed down as they encountered slow/dense plasma ambient during travel from Sun. -Slow events properties compatible with more relaxed configuration: expanding sheath at similar rate as MC (cumulated B in the sheath has time to reach a quasi- equilibrium with ambient). Nearly symmetric B profile inside the MC. -NOT sharp transition, but progressive sheath-MC transition (up to ~20% of MC). Present in 3 groups and individual cases (not smearing effect). Consequence of 3D magnetic reconnection and appear as ’diffusion’ between sheath and MC front. c d c a b a b c d a d b Fig. 3: Superposed epoch for three subgroups, according with the best ordering parameter: slow (VMC<450km s-1), mid (450km s-1<VMC<550km s-1), and fast (VMC>550km s-1).

Table: Mean values of all quantities according with the region (sheath/MC) and the bulk velocity. 4. Main Conclusions -Superposed epoch allowed us to identify several phenomena that are common to most of the events. -The results presented here improve the knowledge of MCs and their sheaths and their evolution in the solar wind. -These typical ’mean’ profiles will help to improve the geoeffectiveness, as for instance if the MC velocity can be estimated before its arrival to the terrestrial environment, some properties (as the non-symmetric shape of the Bprofile) can be known in advance. References: Masías Meza et al., 2016; A&A 592, A118 Richardson & Cane, 2010; Solar Physics 264, 189-237 Rodriguez et al., 2016; Solar Physics 291, 2145-2163