Foreshock Transients: Their Characteristics and Effects on Particle Acceleration
- Author(s): Liu, Zixu
- Advisor(s): Angelopoulos, Vassilis
- et al.
Earth’s foreshock is filled with particles that have been reflected at the bow shock and are streaming away from it. Interaction of these particles with solar wind particles and discontinuities within this region can cause foreshock transients to form. Two types of foreshock transients, foreshock bubbles (FBs) and hot flow anomalies (HFAs), are especially important because they encompass a sizeable (one to several earth radii) core of hot ions which briefly repel the solar wind and can cause intense disturbances in the magnetosphere-ionosphere system. In the highly dynamic environment they create, particles can be potentially accelerated to high energies. In this thesis I investigate the formation, properties, and potential for acceleration of these two types of foreshock transients. As foreshock bubbles have only recently been discovered and their generation mechanism has not been fully explored I first examine their drivers. I demonstrate that in addition to solar wind rotational discontinuities, tangential discontinuities can also drive foreshock bubbles; therefore, the conditions for their formation are more common than previously thought. I show that fast solar wind and weak interplanetary magnetic field conditions statistically favor formation of foreshock bubbles and hot flow anomalies. Next, using multi-point observations I elucidate the spatial structure and evolution of FBs. I also provide definitive evidence that FBs can form their own foreshock, when their sunward expansion speed is fast enough, creating the conditions for further particle acceleration there. To investigate the role of the two most important types of foreshock transients (FBs and HFAs) for particle acceleration, I first employ statistical studies. I show that these types of foreshock transients almost always accelerate electrons, and often also accelerate ions. The solar wind speed is positively correlated with all measures of particle acceleration, thus plays an important role. To further understand the nature of this acceleration, I use case studies along with test particle simulations and hybrid simulations. I show that Fermi acceleration and betatron acceleration are two important acceleration mechanisms for electrons. Ions, on the other hand, can be accelerated by reflection at the earthward-moving boundary of the foreshock transients and can leak into the ambient foreshock region upstream due to their large gyroradii, thus obscuring detection of ion energization in the core when compared to its surroundings. Foreshock transients therefore play an important role in particle acceleration in shock environments. They accelerate particles in their cores, and provide an energetic particle source for parent shock acceleration, thereby increasing the acceleration efficiency of quasi-parallel shocks.