Ionospheric and Solar Plasmas in Geospace Storms

We consider the formation of ring current plasmas in the inner magnetosphere in moderately active conditions that precondition the plasma sheet and ring current-like region for full fledged geospace storms. We seek to better understand recent IMAGE energetic neutral atom observations of the ring current, showing that proton injection is relatively smooth and continuous, while O+ injection is episodic in close association with multiple substorms. We use a modeling framework of collisionless test particle motions in magnetospheric fields from a magnetohydrodynamic simulation. The simulation is used to generate bulk properties and detailed velocity distributions at key locations, for comparison with observations. Particles are initiated in regions representative of the solar wind proton source upstream of the bow shock, the polar wind proton source, and the auroral zone enhanced outflows of O+, or AƒÆ'A+â_TAƒâ_sA,AøAƒÆ'A,AøAƒAøAøâ_sA¬A.A¡Aƒâ_sA,A¬AƒÆ'Aøâ,¬A¦AƒAøAøâ_sA¬A.â_oauroral windAƒÆ'A+â_TAƒâ_sA,AøAƒÆ'A,AøAƒAøAøâ_sA¬A.A¡Aƒâ_sA,A¬AƒÆ'A,A_Aƒâ_sA,A¿Aƒâ_sA,A½. Trajectories are run up to 24 hours to assure a complete circumnavigation of the Earth. Results reflect steady growth phase conditions after 45 minutes of southward interplanetary field, Bz = -5 nT (By=0). Solar wind protons enter the ring current principally through the dawn low latitude boundary layer, while polar wind protons and auroral wind O+ enter the ring current through the midnight plasma sheet. Thus, solar wind and ionospheric plasmas take very different transport paths to the ring current region. Accordingly, they should be expected to respond differently to substorm dynamics of the magnetotail, as recently observed. Polar wind protons make a minor contribution to ring current pressure under steady conditions, but auroral wind O+ has the potential to dominate the ring current, when dayside outflow is strongly enhanced as it is observed to be during periods of enhanced solar wind dynamic pressure fluctuations.