Conductances and the Beam Spreading Effect for Global 30 240 keV Proton Precipitation in the 17 18 April 2002 Geomagnetic Storms

We investigate the global Pedersen and Hall conductance distribution using in situ energetic proton precipitation measurements during the 17-18 April 2002 geomagnetic storms. Global 30-240 keV proton precipitation patterns are constructed using newly developed 3-hour NOAA/POES data products, and provide the energy input at the upper model boundary. A 3-D Monte Carlo ion transport model is employed to calculate the resulting ionization rates and therefore the conductance distribution. A unique feature of ion precipitation is the horizontal spreading effect, resulting from charge exchange and electron stripping collisions within an incident ion beam. The horizontal beam spreading effect is turned off or on in this study to assess the influence on the Pedersen and Hall conductances. This first assessment of the proton beam spreading on the conductances draws two important conclusions. First, with the neglect of the beam spreading effect, an overestimation (up to 10%) occurs at the center of the major proton precipitation region. At the edge (particularly at the equatorward edge), a severe underestimation (around -50% or worse) is induced. Both the overestimation and underestimation are physically meaningful, as they occur in regions of the conductances with a significant magnitude. This enhanced conductance equatorward of the main auroral oval will impact the development of the storm-time ring current (that is, the source of the precipitating ions), and this feedback is discussed. Second, the beam spreading effect is more significant for the Pedersen conductances than for the Hall conductances, because of the altitude dependence of both the conductivity profiles and the spreading. It is concluded that proton precipitation as well as its associated beam spreading are important for the ionospheric electrodynamics and the magnetosphere-ionosphere coupling, and need to be included in a global conductance model.