Case study of the forward-reverse interplanetary shock wave pair in May 2024, detected by Solar Orbiter

     Interplanetary (IP) shock waves are one of the direct manifestations of solar activity impacting the normal state of the solar wind as it moves through the heliosphere. In the heliosphere, shocks can occur individually or in linked pairs. Linked shock pairs manifest as two successive compression fronts in the plasma, typically travelling in the same direction, but originating from different sources. One such source of paired shocks are corotating interaction regions (CIRs), where the fast solar wind from the coronal hole overtakes the slower solar wind. When that happens, the interaction creates compression regions at the boundaries of the solar wind flux, leading to sudden changes in its parameters and resulting in the formation of forward and reverse shocks.

     Another source of the formation of paired shock waves are coronal mass ejections (CMEs) that occur in various active regions (ARs), or sequential CMEs from the same ARs. Paired shock waves are an effective accelerator of energetic charged particles, which are fundamental to heliospheric dynamics. They also play a key role in modulating cosmic rays and triggering geomagnetic disturbances in the near-Earth space.

     Our study examines the main characteristics of the forward and reverse shock pair detected on May 21, 2024, when Solar Orbiter flew at a distance of 0.79 AU from the Sun, and about 170 degrees west of the Earth-Sun line. We discuss CMEs as sources of the shock pair, and present the main parameters of the forward and reverse shocks in the interaction region. The study is based on experimental data regarding the kinetic parameters of the solar wind and characteristics of the interplanetary magnetic field, as derived from instruments on the Solar Orbiter mission. We also discuss an abrupt increase in energetic ion fluxes within the interaction region of both shocks, as recorded by the Energetic Particle Detector (EPD) onboard the Solar Orbiter mission.

     This work is supported by the “Long-term program of support of the Ukrainian research teams at the Polish Academy of Sciences carried out in collaboration with the U.S. National Academy of Sciences with the financial support of external partners”.