Two sources of magnetosheath ions observed by Cluster in the mid-altitude polar cusp

Double cusps have been observed on a few occasions by polar orbiting spacecraft and ground-based observatories. The four Cluster spacecraft observed two distinct regions, showing characteristics of a double cusp, during a mid-altitude cusp pass on 7 August 2004. The Wind spacecraft detected a southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF–Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion precipitation around 1 keV on the equatorward side of the cusp and a dense ion population in the cusp centre. Cluster 1, entering the cusp around 1 min later, observed only a partial ion dispersion with a low energy cutoff reaching 100 eV, together with the dense ion population in the cusp centre. About 9 min later, Cluster 3 entered the cusp and observed full ion dispersion from a few keV down to around 50 eV, together with the dense ion population in the centre of the cusp. The ion flow was directed poleward and eastward in the step/dispersion, whereas in the centre of the cusp the flow was directed poleward and westward. In addition the altitude of the source region of ion injection in the step/dispersion was found 50% larger than in the cusp centre. This event could be explained by the onset of dayside reconnection when the IMF turned southward. The step would be the first signature of component reconnection near the subsolar point, and the injection in the centre of the cusp a result of anti-parallel reconnection in the northern dusk side of the cusp. A three-dimensional magnetohydrodynamic (MHD) simulation is used to display the topology of the magnetic field and locate the sources of the ions during the event.     

Dayside Proton Aurora: Comparisons between Global MHD Simulations and IMAGE Observations

The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12 and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the IMF changes in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree with the antiparallel merging model, though locally component merging might broaden the merging region, especially in the region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause, as well as the intricate connection between the bow shock and the auroral region.     

On the source region of flux transfer events

We have examined the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45° to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about ± 6 R_E wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about ± 6 R_E wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, our observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.     

Mission-oriented theory for ISTP

The goal of mission-oriented theory is to develop techniques and models which can be used by experimentalists and theorists to interpret spacecraft measurements, deducing from them the maximum amount of information about both local and large-scale dynamics. To be effective, theorists and experimentalists must express their results in a common format. A reasonable starting point is for mission theorists to adopt the format currently used by experimentalists. To this end we have developed new diagnostics for plasma kinetic simulations, which display the results in formats very similar to those commonly used to present satellite wave and particle measurements. We have used a simulation of broadband electrostatic noise to demonstrate how, by comparing simulation results with observations, we can infer quantities which cannot be measured, such as the wave mode. We are also developing the capability of creating data streams from virtual spacecraft located in the simulation region. For example, we used a kinetic magnetopause simulation to explore the ways in which simulations can assist in the interpretation of single and multiple satellite measurements in regions of strong spatial inhomogeneity. To address directly the mission objective of measuring global transport, global MHD models are employed. In order to facilitate the initial comparison with ISTP satellites, time histories of simulated generic states of the magnetosphere will be stored on optical disks; these will then be used to create dynamical displays of both local parameters and the global configuration. Finally we demonstrate the use of data based phenomenological magnetic field models in single particle trajectory calculations to describe large-scale kinetic properties of the magnetospheric plasma. We briefly discuss the success of large-scale kinetic calculations in delineating the structure of the plasma sheet, and present some possible ISTP research initiatives which can be used to determine the structure of the very distant tail and the entry of plasma into the tail.