Abstract. This paper presents results of a global MHD simulation of a substorm that occurred on December 10, 1996. We concentrate on the relationship between the simulation results and the magnetotail observations during the growth and expansion phases of the substorm. In general, we find excellent agreement between the single point observations made by various spacecraft in both the geosynchronous and mid-tail regions: the simulation accurately represented the energy loading (lobe field increase), small-scale activations (partial dipolarizations), and a global substorm onset (large dipolarizations and fast flows). The global view presented by the simulation shows complex series of discrete flow channels during the expansion phase prior to the onset of global reconnection. It is these flows channels that disrupt the thin current sheets present during the expansion phase of the substorm.

Three‐dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose location as and the relative subsolar magnetosheath thickness Δms/amp are strong functions of the IMF cone angle θ (between vsw and Bsw) and the Alfven Mach number MA For a given MA the shock is more distant for higher θ (restricted to the interval 0–90° by symmetries), while as/amp and Δms/amp increase with decreasing MA for θ ≳ 20° but decrease with decreasing MA for θ ∼ 0°. Large differences in Δms/amp are predicted between θ = 0° and 90° at low MA, with smaller differences remaining even at MA ∼ 10. The θ = 0° results confirm and extend the previous work of Spreiter and Rizzi [1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences on MA and θ into a sole dependence on Mms. Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] that as/amp and Δms/amp should depend strongly on θ and MA for MA ≲ 10 (as well as on other MHD variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for low MA and large θ ≳ 45° and closer locations found for θ ≲ 20° even at MA ∼ 8.

Abstract. The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event.The ground-based data show two substorm intensifications about 40 rain apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies.Both the observations and the simulation show two distinct regions of activity' a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.

Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solax wind momentum and energy to the magnetosphere. Both analytic modeling and analysis of geophysical data have shown that this coupling process should be a sensitive function of the clock angle of the IMF. In this letter we present results from a 3‐dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the reconnection voltage between the closed geomagnetic field and the IMF as a function of the IMF clock angle. These results are consistent with a sin (θ/2) functional behavior.

Plasmoids are thought to occur as a consequence of the formation of a near‐Earth neutral line during the evolution of a geomagnetic substorm. Using a 3D, global MHD simulation of the interaction of the Earth's magnetosphere with the solar wind, we initiate a substorm by a southward turning of the Interplanetary Magnetic Field (IMF) after a long period of steady northward field. A large plasmoid is formed and ejected. We show field line maps of its shape and relate its formation time to the progress of the substorm as indicated by the cross polar potential. Because of the large region of closed field in the magnetotail at the time of the substorm, this plasmoid is longer in axial dimension than is typically observed. We compare the simulation results with the type of satellite observations which have been used to argue for the existence of plasmoids or of traveling compression regions (TCRs) in the lobes or magnetosheath. The simulation predicts that plasmoid passage would result in a strong signal in the cross tail electric field.