Geoeffectiveness of interplanetary shocks controlled by impact angles: A review

Abstract

The high variability of the Sun's magnetic field is responsible for the generation of perturbations that propagate throughout the heliosphere. Such disturbances often drive interplanetary shocks in front of their leading regions. Strong shocks transfer momentum and energy into the solar wind ahead of them which in turn enhance the solar wind interaction with magnetic fields in its way. Shocks then eventually strike the Earth's magnetosphere and trigger a myriad of geomagnetic effects observed not only by spacecraft in space, but also by magnetometers on the ground. Recently, it has been revealed that shocks can show different geoeffectiveness depending closely on the angle of impact. Generally, frontal shocks are more geoeffective than inclined shocks, even if the former are comparatively weaker than the latter. This review is focused on results obtained from modeling and experimental efforts in the last 15 years. Some theoretical and observational background are also provided.

</ce:displayed-quote>The possibility of the existence of shocks in the interplanetary space was then accepted. Parker (1961) suggested a model for shock propagation in the interplanetary space. His idea was further extended by Hundhausen and Gentry (1969). However, the first evidences of collisionless shocks in nature were observed in the interplanetary space. As seen above, the existence of the magnetosphere suggested the formation of a stationary collisionless shock at the front of the magnetosphere. Curiously, the existence of a stationary shock, i.e., a shock at rest in the Earth's reference frame, was suggested in the same edition of the Journal of Geophysical Research by Axford (1962) and Kellogg (1962). The bow shock was first observed by the Mariner 2 spacecraft as irreversible changes in the solar wind and IMF which were interpreted as hydromagnetic shocks (Ness et al., 1964; Sonett et al., 1964). The bow shock is formed if a supersonic flow interacts with an obstacle, and it separates the pristine solar wind from the subsonic plasma flow in the magnetosheath. Another magnetospheric boundary is the magnetopause, a current layer whose position is determined by the pressure balance condition between the solar wind dynamic pressure and the magnetospheric magnetic pressure, first observed by Heppner et al. (1963) and Cahill and Amazeen (1963). The region between the bow shock and the magnetopause is called the magnetosheath. The magnetosheath in the subsolar region for typical solar wind conditions is located approximately at a distance between 10R_E and 13R_E (Earth radius of ∼ 6400 km) from the Earth (Cahill and Amazeen, 1963; Heppner et al., 1963; Russell, 1984). This region is highly turbulent because it is formed by the shocked solar wind (Russell, 1984; Paschmann, 2005). The bow shock, magnetopause, and magnetosheath are shown in Fig. 2, as seen in Eastwood et al. (2014). The numerical gas dynamic model of Spreiter et al. (1966) reasonably well predicted the bow shock formation and magnetosheath flow between the bow shock and magnetopause obstacle. For a broader perspective on the discovery of the magnetosphere, see, e.g., Russell (1984) and Gillmor and Spreiter (1997).</ce:para>