Disturbed Zone and Piston Shock Ahead of Coronal Mass Ejection

Abstract

The 2010 June 13 coronal mass ejection (CME) propagating toward the position angle P.A. ≈ 245° (measured counterclockwise from the Sun's north pole) was studied from the SDO/AIA and SOHO/LASCO C2, C3 data. We show that ahead of the CME frontal structure, as a result of its interaction with the undisturbed solar wind, a disturbed region (with an increased and disturbed plasma density), whose size increases as the CME travels away from the Sun, emerges gradually. Discontinuity formation at the disturbed zone front is observed in the narrow P.A. ≈ 245°-250° range. Its characteristics satisfy the properties of a piston collision shock. In the other directions relative to the CME motion axis (P.A. > 250° and P.A. < 245°), there exists only the disturbed zone, whose density gradually decreases with distance. The discontinuity that was always observed at all distances where measurements were made is absent. The analysis of this CME and several other limb CMEs with different velocities from the MK4, LASCO C2, C3, and STEREO/COR2 data confirmed the previously established laws of piston shock formation ahead of a CME, which are as follows: (1) Shock formation ahead of a CME in a vicinity along its propagation axis may occur at various distances R = R_u from the Sun's center. Its formation is determined by fulfilling a local inequality u(R) > V_A (R), where u(R) is a CME velocity relative to the surrounding solar wind and V_A (R) is a local Alfvén velocity that is approximately equal to the velocity of fast magnetic sound in the solar corona. (2) At R < 6 R _⊙, the shock front width δ _F is on the order of the proton mean free path λ _p , and the mechanism for energy dissipation at the front is, apparently, collisional. (3) At R >~ 10-15 R _⊙, one observes the formation of a new discontinuity δ^sstarf _F Lt λ _p wide at the head of the collision front. Within limits of error, δ^sstarf _F does not depend on R and is determined to be δ^sstarf _F ≈ 0.1-0.2 R _⊙ by the LASCO C3 spatial resolution and δ^sstarf _F ≈ 0.03 R _⊙ for COR2. This discontinuity is identified with a collisionless shock.