Browsing by Subject "Auroral physics"

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    The Flow of Poynting Flux into the Terrestrial Cusp and Auroral Zone and Its Role In Powering Energy Intensive Collisionless Acceleration Mechhanisms
    (2017-12) Tian, Sheng
    During geomagnetic active times, the earth’s magnetosphere experiences boost in the circulation of mass and energy. The ionosphere interacts with the magnetosphere mainly in the high latitude region through the connection of the magnetic field. The dayside cusp and the nightside aurora are the two main regions of ionosphere-magnetosphere coupling. The Poynting flux, carrying the electromagnetic energy, enhances in the two regions (Keiling et al., 2003). Poynting flux may be an important contributor to energy flow in stars and planets and drive collisionless acceleration mechanisms that generate intense particle energy flux along magnetic field lines. Utilizing the abundant in-situ measurements, we study the intense earthward Poynting flux in the earth high-latitude region. We found that in the dayside cusp, an unexpected energetic wind of ions is driven by the enhanced earthward Poynting flux. The ion outflow originated from a narrow region from the dayside cusp. Based on both case and statistical studies, we showed that the ion energization occurs in the mid-altitude cusp, well above the ionosphere where the ions are driven to upwell. Although many mechanisms on local heating and acceleration have been proposed, the energy circulation related to the energization process was rarely studied. Our findings show that the earthward Poynting flux is the only energy source for the ion energization. More than a quarter of the electromagnetic energy is converted to the kinetic energy of the ion outflow. The remaining Poynting flux continues to flow earthward and partially converted to accelerate electrons downward. The study reveals the existence of an efficiently accelerated wind of ions at the cost of electromagnetic energy, which may be applicable to the solar wind and the powerful winds in other astrophysical objects. The identification of the free energy source as Poynting flux would place several constraints on the possible mechanisms of ion energization. For the aurora in the nightside, people generally agree that the poleward portion of the aurora is powered by the earthward Poynting flux within the plasma sheet boundary layer (PSBL). The Alfvénic Poynting flux dissipates its electromagnetic energy at the “aurora acceleration region” to accelerate electrons, which in turn excite the aurora. In the second part of the thesis, we show a case study which, for the first time, demonstrates the direct correlation between the discrete aurora arc and the earthward Poynting flux. The electromagnetic energy carried by the Poynting flux is also partially converted to the simultaneously observed oxygen outflows originated from the nightside ionosphere. The Poynting flux is mainly earthward and the largest Poynting flux spikes are identified to be due to Alfvén waves. Different from the case of the cusp, most of the electromagnetic energy in the nightside is converted to accelerate electrons, only a small portion is converted to power the ion outflow. The event is also the first conjunction between the auroral westward traveling surge (WTS) and high-altitude spacecraft. Given the good correlation between the Poynting flux and aurora, it is likely that the WTS represents the perpendicular propagation of the MHD waves in the magnetosphere.