Coronal Mass Ejection (CME)-induced shock formation, propagation and some temporally and spatially developing shock parameters relevant to particle energization

Abstract

Interplanetary shocks accelerate solar energetic particles (SEPs) from the point of shock formation in the lower corona, and continuously as the shock propagates outward to 1 AU and beyond. In this study, the formation properties of a CME-induced shock and propagation characteristics are studied from the inner corona to 1 AU. We use a 2D, three-component (i.e., 2.5D), time-dependent MHD code in our model. A well-studied CME event (the 1997 January 6−12 Sun-Earth Connection Event) is used as a baseline for this study. The solar wind conditions measured at 1 AU (WIND data) are used to motivate our effort to model the CME driven shock. It is found that the fast forward shock forms originally at ∼3.2 Rs (solar radii) from the solar surface in the ecliptic plane for the assumed CME and background solar wind parameters. In our model, this occurs ∼2 hrs after CME initiation. The shock formation at higher (∼30 ◦ ) latitudes measured from the ecliptic plane is further from the Sun (∼3.6 Rs) because of higher local magnetosonic speeds that must be exceeded by the original disturbance for shock formation. Finally, the shock becomes symmetric at 16 Rs. In the ecliptic plane at 16 Rs the fast shock Mach number (Mf) is ∼3.5, and at 30 ◦ latitude, Mf ∼ 1.7, considerably weaker. A maximum in the fast shock Mach number of 4 is reached at 130 Rs in the ecliptic plane. The Mf decreases to 3.5 by 1 AU. Other properties of the shock, as well as its relationship to the local interplanetary properties through which it passes, are discussed. The interplanetary counterpart, ICME, of the coronal CME, is also discussed. These shock properties, we believe, are relevant to the shock's ability to accelerate particles to energies as high as 100 MeV. The actual physical process, however, is not discussed in this paper.