Magnetohydrodynamic Shocks in the Interplanetary Space: a Theoretical Review

Abstract

I discuss in this brief review some properties of magnetohydrodynamic (MHD) discontinuities in the interplanetary space. My emphasis is on a special case of MHD discontinuity, namely interplanetary (IP) shocks, and those that are found at 1 AU. I derive the Rankine-Hugoniot (RH) equations to evaluate plasma parameters in the downstream region (shocked plasma) in relation to the upstream region (unshocked plasma). These properties are used to classify IP shocks in terms of their geometry and their direction of propagation in relation to the Sun. The shock geometry is determined in terms of two angles: $\theta _{B_{n}}$ , the angle between the upstream magnetic field and the shock normal, and $\theta _{x_{n}}$ , the angle between the shock normal and the Sun-Earth line. Sources of IP shocks frequently found in the solar wind at Earth’s orbit are presented. Then the RH equations are solved for two categories of IP shocks in a special case: perpendicular shocks, when $\theta _{B_{n}}$ is 90 ∘, and oblique shocks, when that angle is 45 ∘. Finally, I highlight the importance of knowing the shock geometry, mainly the impact angle $\theta _{x_{n}}$ , specially whether the shock is frontal or inclined, for space weather-related investigations. IP shocks are known to be more geoeffective if they strike the Earth’s magnetosphere frontally, or with impact angle nearly null. These results have been reported both by modeling and experimental studies in the literature.