Magnetosheath jets throughout the solar cycle

The Sun constantly emits a stream of charged particles, called the solar wind. The Earths geomagnetic field constitutes an obstacle to the solar wind. Hence, the solar wind has to slow down ahead of the region where the geomagnetic field dominates, called the magnetosphere. This happens at the bow shock. The region between the bow shock and the magnetosphere is called magnetosheath. The proposed study is concerned with particle jets inside this magnetosheath region. As these jets are able to propagate all the way from the bow shock to the outer shell of the magnetosphere, the magnetopause, they are considered as important coupling elements between the solar wind and the magnetosphere. Other phenomena that severely influence the magnetosphere are extreme solar wind events, like large scale expulsions of plasma and magnetic field from the Suns surface or interaction regions between solar wind streams of different velocities. Extreme solar wind events are strongly connected to the 11-year solar activity cycle. So far, nothing is known about how jets relate to solar cycle phases and extreme solar wind events. The proposed study is intended to shed some light on these relations. The goals of the study are: (1) to reveal under which conditions and by which mechanisms jets are generated during extreme solar wind events, and (2) to discover the properties of these jets and assess their impact on the magnetosphere throughout the solar cycle. To achieve these goals, first, data sets of all relevant quantities (jets, extreme solar wind events, solar and geomagnetic activity phases, etc.) have to be created or enhanced, based on observations of the Sun, the solar wind, the magnetosheath, and the magnetosphere. Second, a number of extreme solar wind events have to be analyzed in detail to understand how and how often jets are generated during them, and which properties and impacts they have. Third, these detailed case study results have to be incorporated into a large-scale statistical analysis of jets encompassing at least one entire solar cycle. Thereby, our understanding of jets and of their role in coupling the solar wind in all its forms to the magnetosphere will be significantly advanced. This study is only possible as an interdisciplinary effort, bringing together experts in jet and solar research at the Space Research Institute of the Austrian Academy of Sciences and the Institute of Physics of the University of Graz.

The terrestrial geomagnetic field is protecting the Earth's atmosphere from being stripped away by the solar wind, a constant flow of plasma from the Sun. The outer dayside compressed boundary of the geomagnetic field is the magnetopause (MP) which diverts the plasma flow. Before reaching the MP the solar wind plasma is supersonic, therefore a shock wave is formed already upstream in the solar wind, which decelerates the flow, forming a turbulent and compressed boundary layer called the magnetosheath. In this project, various aspects of the interaction between the structured and time-varying solar wind and magnetic field with the shock and magnetosheath are studied. The focus is on the so-called magnetosheath jets, which are mesoscale structures of enhanced dynamic pressure pulses, generated by different physical mechanisms upstream of or at the shock. The jets propagate through the magnetosheath, and some of them reach the magnetopause, leading to space weather events. Based on an extensive database of jets, it was shown that the jets have a higher probability of hitting the magnetopause, potentially generating surface waves, when the magnetic field is aligned with the flow direction and the flow reaches higher velocities. An evidence was found that high speed jets in the magnetosheath are generated by a combined effect of shock reformation and evolution of upstream waves. On the basis of numerical simulations it was also shown that shock front corrugations can be influenced by turbulence. It was suggested that to understand the multi-scale fluctuations that might also play a role in jet formation, the energy budget, including electromagnetic and pressure-strain terms, must be estimated. These specific conditions for jet generations are partially determined by large-scale solar wind structures. The first-ever statistical study conducted within the framework of the project has shown that: a.) occurrences of jets decrease when coronal mass ejecta, representing massive expulsions of plasma and magnetic field from the solar corona, hit the bow shock; b.) occurrences of jets increase when high speed streams emanating from coronal holes or stream interaction regions hit the bow shock. High-speed solar wind plasma was observed to strongly influence high-energy ion fluxes in the magnetosheath as well. A statistical comparison of jet occurrences during two solar cycles, from1996 to 2019, indicated that jet formation does not strongly depend on the solar cycle. To facilitate further international collaboration on shock-plasma flow-magnetic field interactions and jets, including both statistical and event studies, proper, freely accessible databases have been created. The project results are important for a better understanding of solar-terrestrial relations, but can also be applicable to planetary and astrophysical systems where collisionless plasmas are present.