Browsing by Author "Shen, Yangyang"

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    Microphysics of Ion and Electron Energization in the Topside Ionosphere
    (2019-10-30) Shen, Yangyang; Knudsen, David J.; Cully, Christopher M.; Jackel, Brian J.; Moazzen-Ahmadi, Nasser; O'Keefe, Kyle P. G.; Moore, Thomas E.
    Ionospheric ion and electron energization and field-aligned transport are critical processes of magnetosphere-ionosphere-thermosphere coupling. The Canadian Enhanced Polar Outflow Probe (e-POP) satellite carries particle and field instruments specifically designed to study micro-scale characteristics of ion energization and outflow processes in the topside (325-1,500 km) ionosphere. The Suprathermal Electron/Ion Imager (SEI) instrument onboard e-POP has the capability of resolving two-dimensional low-energy (from sub-eV to 325 eV) particle distributions at 10-ms time scale, or less than 100-m spatial scale. This dissertation presents techniques to derive and validate the ion bulk flow velocity and temperature from SEI and reports several discoveries resulting from direct measurements of particles, waves and magnetic fields from e-POP. First, to identify the dominant drivers of ion upflow at near 1000 km altitude, I present three cleft ion fountain events with intense (>1.6 km/s) ion upflow velocities during quiet periods. Conjunctional observations from multiple satellites and radars indicate that the observed ion upflows are primarily driven by ambipolar electric fields due to soft electron precipitation. Second, to test the statistical significance of wave-ion heating at low altitudes (325-730 km), we show that significant transverse O+ ion heating from broadband extremely low frequency (BBELF) waves is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Ion heating in association with ion downflows rather than upflows suggests an active ``pressure cooker'' in the low-altitude return current region. Third, using numerical test particle simulations that take into account ion-neutral collisions to explain BBELF-induced ion heating at low altitudes, we find that the most effective mechanism is through collisional cyclotron heating by short-scale electrostatic ion cyclotron (EIC) waves. The interplay between finite perpendicular wavelengths, wave amplitudes, and ion-neutral collision frequencies collectively determine the ionospheric ion heating limit, which has been derived both numerically and analytically. Finally, we present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to B in the topside (900-1,500 km) ionosphere. These are a counterpart to transverse ion acceleration which has been reported extensively since the 1970's.