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Moving from the lower corona to the interplanetary medium, shocks evolve rapidly since the sound speed drops as plasma density and magnetic field strength decline as ~1/r^2. Solar Orbiter’s coronagraphs will remotely identify shock front location, speed, and compression ratios through this critical region within ~10 RSun. Combining this information with local electron densities as well as coronal ion velocities given by Solar Orbiter radio and light polarization observations will provide critical constraints on shock evolution models in regions too close to the Sun for direct sampling. 

In the regions explored by Solar Orbiter close to the Sun, the IMF is almost radial with much less variation (uncertainty) in length than is the case at 1 AU, so the knowledge of the actual path length improves by a factor of 3-5 as the length shortens. Having observed the CMEs and their radio signatures in the corona and the X-ray signatures of the energetic particles near the Sun, Solar Orbiter will then determine the subsequent arrival time of the particles in situ that can be accurately compared to CME position. As the shock then rolls past the spacecraft, Solar Orbiter will measure the shock speed and strength as well as the associated plasma turbulence, electric, and magnetic field fluctuations. This will give a complete description of the acceleration parameters in the inner heliosphere where much of the particle acceleration takes place. Indirect evidence from 1 AU indicates that shock acceleration properties depend on the longitude of the shock compared to the observer; close to the Sun, Solar Orbiter can cleanly test this property since the IMF is nearly radial, the CME lift-off site is known, and the accelerated particles will have little chance to mix. In the high-latitude phase of the mission, Solar Orbiter will be able to look down on the longitudinal extent of CMEs in visible, UV, and hard X-rays, allowing first direct observations of the longitudinal size of the acceleration region. This will make it possible to test currently unconstrained acceleration and transport models by using measured CME size, speed, and shape to specify the accelerating shock. 




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