Browsing by Author "Lu, Gang"
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Item Auroral electrodynamics from simultaneous measurements at high and low altitudes(1991) Lu, Gang; Reiff, Patricia H.Two auroras sampled nearly simultaneously at high and low altitudes along a field line by the Dynamics Explorer (DE) spacecraft have been used to study auroral electrodynamics. Electric fields plotted as a function of invariant latitude show that the large-scale features are essentially the same at high and low altitudes outside the auroral acceleration region. Parallel electric fields associated with parallel currents are such as to filter out the small-scale structure in the high-altitude electric field pattern. From the magnetic field measurements, we find that there is a return current region embedded between two auroral arc structures. The latitude shift between the high-altitude and low-altitude return current regions indicates that the auroral arcs are moving equatorward with a velocity of about 250 m/s. Collisionless plasma kinetic theory (Knight, 1973) has been used to predict the relationship between the upward parallel current and the parallel potential drop. The DE 1/DE 2 pair offers a unique Opportunity to test this relationship because the DE 1 spacecraft can measure high altitude plasma parameters without contamination from auroral heating. Using measured values of J$\sb{\Vert}$ (mapped to the surface) and $\Phi\sb{\Vert}$, the ratio of J$\sb{\Vert}$/e$\Phi\sb{\Vert}$ varies considerably but with a mean value about 0.5$\sim$2.2 $\times$ 10$\sp{-9}$ mho/m$\sp2$. Suprathermal electron bursts are also observed in the diffuse aurora at the same invariant latitudes, both at high and at low altitudes. Thus we suggest that these "bursts" are more properly described as a spatial rather than temporal phenomenon. Observations of upflowing ionospheric ions are obtained by both DE 1 and DE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ion number flux is of the order of 10$\sp9$ cm$\sp{-2}$ s$\sp{-1}$. The ionosphere is predominantly O$\sp+$, and the ions with energies greater than 5 eV are a only very small fraction (less than 1%) of the total ion population. At high altitudes, the upflowing ions are accelerated and heated (with characteristic energies of hundreds of eV). Comparing upflowing fluxes at high and low altitudes yields an estimated height of the bottom of the auroral acceleration region of 1100-1400 km.Item RCM-E and AMIE studies of the Harang reversal formation during a steady magnetospheric convection event(Wiley, 2014) Yang, Jian; Toffoletto, Frank; Lu, Gang; Wiltberger, MichaelThis paper presents the results of a modeling study on the formation of the Harang reversal (HR) during a steady magnetospheric convection event. The Harang reversal is identified as the boundary of the northward and southward electric field in the nightside auroral zone using the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. We simulate the event with the Rice Convection Model-Equilibrium (RCM-E) by adjusting its boundary conditions to approximately match Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES observations in the nightside magnetosphere. Our results show that the HR is collocated with an upward region 1 field-aligned current, where converging ionospheric currents cause a southward/northward electric field on the poleward/equatorward side of the HR. Our results also indicate that the electric field reversal is slightly poleward of the ionospheric east–west current reversal and is to the northeast of the ground magnetic reversal, which is consistent with previous observations. We also test the sensitivity of the HR formation to a variety of parameters in the RCM-E simulations. We find that (1) the reduction of the flux tube entropy parameter PV5/3 near the midnight sector plays a major role in the formation of the HR; (2) a run carried out assuming uniform conductance produced the same major features as the run with more realistic precipitation-enhanced conductance; and (3) the detailed pattern of the polar cap potential distribution plays a minor role, but its dawn-dusk asymmetry significantly controls the location of the HR with respect to midnight. The RCM-E simulations also predict PV5/3 and flow distributions associated with the magnetospheric source of the HR in the plasma sheet, which can be further tested against observations.