Global 30-240 keV proton precipitation in the 17-18 April 2002 geomagnetic storms: 2. Conductances and beam spreading

We investigate the global Pedersen and Hall conductance distribution during the 17-18 April 2002 geomagnetic storms by combining NOAA/POES 30-240 keV proton precipitation measurements with three-dimensional ( 3-D) particle transport modeling. Significant conductances are generated due to energetic proton precipitation in the selected storms, with peak values up to similar to 10 S in the dusk sector. The effect of horizontal beam spread on the conductances, which is a unique feature of ion precipitation, is first assessed in this paper. It is found that with the neglect of the beam spread, 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 similar to 50% or worse) is induced. Moreover, 3-D scattering extends the equatorward edge of the high conductance region down to around 55 degrees magnetic latitude in the storms. A significance domain ( up to 4.4 degrees) exists equatorward of the peak proton precipitation region, in which significant conductances would be seriously underestimated without the beam spreading effect. It is also found that 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. The findings emphasize the role of proton precipitation as well as its associated beam spreading, especially at the equatorward edge of the precipitation zone in the evening sector, where proton precipitation-produced conductivity can influence storm-time phenomena, such as subauroral polarization streams, ionospheric storm enhanced density structures, and inner magnetospheric plasma dynamics ( i.e., the ring current and plasmasphere).