High-speed jets and related phenomena at Earth's bow shock and magnetosheath
Time: Wed 2022-11-23 13.00
Location: F3, Lindstedtsvägen 26 & 28, Stockholm
Subject area: Physics
Doctoral student: Savvas Raptis , Rymd- och plasmafysik
Opponent: Drew Turner, Johns Hopkins University Applied Physics Laboratory
Supervisor: Tomas Karlsson, Rymd- och plasmafysik
Magnetosheath high-speed jets are transient and localized dynamic pressure enhancements downstream of Earth’s bow shock. Their formation has been associated with several mechanisms, including solar transient events and the dynamical evolution of the bow shock. After their formation, jets interact with the background magnetosheath population, exciting various waves and accelerating particles. When they reach the magnetosphere, they can penetrate the magnetopause, drive surface waves, and cause magnetopause reconnection. Their effects to the inner geospace environment can be seen through substorm activity and ground magnetometer measurements. In this thesis, a series of papers on the formation, evolution and statistical properties of jets is presented. Most of the work is done using NASA’s Magnetosphere Multiscale (MMS) mission, while other missions like THEMIS and upstream solar wind monitors (e.g., ACE and Wind) are also used. For our analysis, we also make complementary use of neural networks and computer simulations. Our investigation initially showed the importance of classifying jets based on the shock orientation and interplanetary magnetic field (IMF), resulting in an open-access database of magnetosheath jets using MMS. This dataset was then used to derive statistical properties for each class of magnetosheath jets (Paper I). The jets were also classified using neural networks (Paper II), while a comparison between their statistical properties and computer simulated jets was performed (Paper III). Another aspect we investigated through multi-point measurements is the excitation of waves due to the interaction of jets with the magnetosheath (Paper IV). We then focused on the formation and evolution of jets close to the Earth’s bow shock. We showed direct in-situ evidence that shock reformation and the evolution of upstream waves can generate downstream high-speed jets (Paper V). By evaluating the properties of jets on a kinetic level, we demonstrated that jets exhibit complex velocity distribution functions (VDFs) throughout their lifetime. Deriving partial plasma moments to isolate the jet from the background population, we revealed the limitations of studying these phenomena from a single-fluid perspective and how the derived partial plasma moments are related to the upstream solar wind and its foreshock structures (Paper VI).