Detecting the widths of shock fronts preceding coronal mass ejections

Abstract

The perturbed zones and shocks preceding coronal mass ejections (CMEs) are studied using the data of the Mark 4, LASCO C2, and LASCO C3 coronagraphs. Detection of the perturbed zone indicating the presence or absence of the shock is most reliable in a frame moving with the frontal structure of the CME. The ability to correctly measure the width δF of the shock front using the Mark 4 and LASCO C2 data is established. The front width δF observed along the streamer belt at distances R < 5 R⊙ from the center of the Sun is of the order of the mean free path of protons. This means that the energy dissipation in the shock front is collisional at such distances. At distances R ≥ (10 − 15) R⊙, a new discontinuity with a front δ*F ≪ δF is formed in the leading portion of the front. Within the errors, δ*F ≈ (0.1–0.2) R⊙ is independent of the distance R and is determined by the LASCO C3 spatial resolution. Initially, the discontinuity on the scale δ*F is weak and coexists with the front with width δF. The relative amplitude of this discontinuity increases and the brightness profile behind it flattens as long as the distance R increases. This transformation of the brightness profile from a front of width δF to a discontinuity of width δ*F ≪ δF is explained as a transition from a collisional to a collisionless shock.