Properties and formation mechanisms of foreshock transients

Abstract

Interactions between bow shock-reflected (foreshock) ions with solar wind particles can lead to the formation of foreshock transients, frequently observed upstream of the bow shock. Foreshock transients, such as foreshock bubbles, hot flow anomalies, and spontaneous hot flow anomalies, display heated, tenuous cores with large flow deflections bounded by compressional boundaries or shocks. They aid in particle acceleration at the bow shock and their significant dynamic pressure depletions can disturb the magnetosphere-ionosphere system. Thus, studying foreshock transients will expand our understanding of shocks and of the solar wind-planetary magnetosphere coupling throughout the universe. The dissertation presents an observational statistical study and numerical simulations of foreshock transients to investigate their solar wind conditions, properties, and formation mechanisms. The statistical study shows that occurrences of foreshock transients are higher for lower magnetic field strengths and higher solar wind speeds and Alfvén Mach numbers. A geometrical approach with a model bow shock reveals that they typically span up to 3 Earth radii along and extend up to 6 Earth radii from the bow shock. The study also finds that foreshock transients with local density enhancements (substructures), in otherwise density-depleted cores, are often larger in size than those without substructures, and provide variances to the dynamic pressure profiles that could further deform the bow shock surface and lead to more complicated geoeffects. In addition, foreshock transients with and without substructures occur in the same solar wind conditions, thereby implying that substructures are an inherent property of foreshock transients during their evolution. In numerical simulations where injected foreshock ions perform partial gyrations around a discontinuity to generate Hall currents that change the magnetic field topology, foreshock bubbles form from thin discontinuities, and hot flow anomalies form from thick discontinuities. The simulations prove that the foreshock ion current configuration, controlled by the magnetic field change the foreshock ions experience within their gyromotions, determines the magnetic field profile of the structure. Furthermore, the parameter scan results show that the initial foreshock ion distribution types determine how easily foreshock ions can cross the discontinuity and thus how strong the structure forms; parameters that increase their densities or speeds perpendicular to the magnetic field across the discontinuity leads to stronger Hall currents and more significant magnetic field variations. We also find that the initial foreshock ion densities, thermal speeds, and beam speeds all positively and linearly correlate with the expansion speeds and the density compression ratios of the formed structures. This, thereby, provides a possibility to quantify the role of these parameters in the formation and expansion model of foreshock transients and to forecast their aforementioned particle acceleration and geoeffects.