SM21B-01 INVITED 08:00h
Global Hybrid Simulations of Solar Wind and Bow Shock Related Discontinuities Interacting with the Magnetosphere
Global hybrid (PIC ions, fluid electrons) simulations of the magnetosphere are used to investigate the interaction of intrinsic solar wind and bow shock-related discontinuities with the dayside magnetopause. Although ion kinetic processes often dominate these interactions, the consequences of the interactions are global. Hybrid simulations provide a unique map which can be used to select the proper spacecraft locations for multipoint studies. In general, the solar wind provides a variety of discontinuities. Some of these interact with the bow shock to form other structures and discontinuities such as hot flow anomalies (HFAs) in the foreshock. These secondary discontinuities can also interact with the magnetopause leading to perturbations in the magnetosphere. Even in the absence of solar wind discontinuities, dissipation processes related to the bow shock can introduce large scale structures, such as foreshock cavities, into the solar wind. These structures result in large variations in the properties of the magnetosheath impinging on the magnetopause. The results presented will address these various possibilities and their impacts on the dayside magnetopause.
SM21B-02 08:20h
Simultaneous Geotail and Wind Detections of Reconnection at the Subsolar and Flank Magnetopause
We report a Geotail-Wind conjunction at the magnetopause when both spacecraft observed the occurrence of reconnection at the same time at the subsolar and flank magnetopause during stable By dominated IMF. Wind, located at the dawn flank and 6.6 RE below the magnetic equator, observed plasma jets consistent with the presence of a tilted reconnection X-line poleward of the spacecraft. At around the same time the Geotail spacecraft encountered the magnetopause multiple times in the subsolar region. During the multiple magnetopause crossings, Geotail observed jet reversals and the reversal of the polarity of the normal magnetic field associated with flux transfer events which indicate the presence of an X-line in the vicinity of the spacecraft (and of the subsolar region). Taken together, the Geotail and Wind observations are consistent with the component merging model which predicts a tilted X-line hinged at the subsolar point during By dominated IMF.
SM21B-03 08:35h
Coordinated Polar Spacecraft, Geosynchronous Spacecraft, and Ground-based Observations of Magnetopause Processes and Their Coupling to the Ionosphere
In this paper, we present in situ observations of surface waves at the magnetopause and oscillatory magnetospheric field lines, and coordinated observations Pc5 waves at geosynchronous orbit by the GOES spacecraft, and on the ground by CANOPUS and 210? Magnetic Meridian (210MM) magnetometer arrays. On February 7, 2002 during a high-speed solar wind stream, the Polar spacecraft was skimming the magnetopause in a post-noon meridian plane for ~ 3 hours. During this interval, it made two short excursions and a few partial crossings into the magnetosheath and observed quasi-periodic cold ion bursts in the region adjacent to the magnetopause current layer. The multiple magnetopause crossings as well as the velocity of the cold ion bursts indicate that the magnetopause was oscillating with about 6 minute period. Simultaneous observations of Pc5 waves at geosynchronous orbit by the GOES spacecraft and on the ground by the CANOPUS magnetometer array reveal that these magnetospheric pulsations were forced oscillations of magnetic field lines directly driven by the magnetopause oscillations. The magnetospheric pulsations occurred only in a limited longitudinal region in the post-noon dayside sector, and were not a global phenomenon as one would expect for global field line resonance. Thus, the magnetopause oscillations at the source were also limited to a localized region spanning about 4 hours in local time. These observations suggest that it is unlikely that the Kelvin-Helmholz instability and/or fluctuations in the solar wind dynamic pressure were the direct driving mechanisms for the observed boundary oscillations. Instead, the likely mechanism for the localized boundary oscillations was pulsed reconnection at the magnetopause occurring along the X-line extending over the same 4-hour region. The Pc5 band pressure fluctuations commonly seen in high-speed solar wind streams may modulate the reconnection rate as an indirect cause of the observed Pc5 pulsations.
SM21B-04 INVITED 08:50h
Tests of substorm scenarios, based on multipoint measurements.
There are essentially two types of models for substorms: - Substorms are triggered by the spontaneous development of one (or several)X-line(s), in the mid-tail (20-30 Re), which leads to fast flows. Earthward of the reconnection site (X-line) flow bursts are directed earthward. While approaching the dipolar region the speed of these flows is reduced (flow braking), which can result in a dipolarization. Substorms are triggered by current disruption via an instability, which produces the observed dipolarization. The dipolarization starts in the NEPS, and propagates radially outward, thereby causing the reduction of the tail current. In this type of model the formation of X-line is the consequence of the dipolarization. Both models have several common features, but in a different order. In an attempt to establish the sequence of these events we use Cluster multipoint observations (hopefully combined with DSP), to determine the direction of propagation of the perturbation associated with early substorm development. As a second criterion we investigate the spatial distribution of the currents, which is expected to be different in the two models.
SM21B-05 09:10h
Transition from Substorm Growth to Substorm Expansion Phase as Observed with a Radial Configuration of ISTP and Cluster Spacecraft
Transition from the growth phase to the substorm expansion during a well-isolated substorm with a strong growth phase is investigated using a unique radial (THEMIS-like)spacecraft constellation near midnight, including the probing of the tail current at ~16 Re with Cluster, of the transition region at ~9 Re with Geotail and Polar spacecraft, and of the inner region at 6.6 Re with two LANL spacecraft. The activity development in both global scale and near the spacecraft footpoints was controlled with global auroral images (from the IMAGE spacecraft) and the ground network. The magnetospheric models tuned using in situ observations indicated a strong tail stretching and plasma sheet thinning, which included the growth of the near-Earth current (approaching 30 nA/m2) and possible formation of the local B minimum in the neutral sheet (~5nT) at ~10-12 Re near the substorm onset. However, the time delays around the distinct substorm onset show the propagation from the midtail to the inner magnetosphere suggesting that this strongly enhanced current was not spontaneously disrupted. The earliest perturbations near the onset observed at Cluster just outside the thinned plasma sheet are interpreted as the being due to localized Earthward-contracting newly-reconnected plasma tubes produced by the impulsive reconnection in the midtail plasma sheet. A peculiar feature of this substorm was that 12 min prior to this distinct onset, a clear soft plasma injection to the GEO orbit was recorded which has little associated effects both in the ionosphere and in the transition region at ~9Re. This pseudobreakup was probably due to either localized ballooning-type activity or due to the braking of very narrow BBF whose signatures were also recorded by the Cluster. This event manifested the (previously unknown) phenomenon, a strong tail overloading (excessive storage of magnetic energy) contrasted to the modest energy dissipation and plasma acceleration, which both are discussed and interpreted as the consequences of cold/dense and thick pre-substorm plasma sheet which often occurs after the long quiet period. The lessons of using the radial spacecraft configurations in substorm onset studies are also discussed.
SM21B-06 09:25h
Geosynchronous Response to Fast Plasma Flows in the Plasmasheet: Geotail-GOES Coordinated Observations
In this study we examine how geosynchronous magnetic configuration changes in response to the first plasma flow in the plasmasheet. One idea of the substorm trigger (the pile-up/braking model) proposes that the fast earthward flow in the plasmasheet, which is created by near-Earth reconnection, reduces the tail current intensity as the flow is decelerated and changes the plasma distribution and magnetic field configuration in the near-Earth region. Although occasional observations of subsequent occurrence of a fast plasmasheet flow and geosynchronous dipolarization are consistent with (but are not necessarily the proof of) this idea, the generality of such observations still remains to be understood. In fact, for a pseudobreakup event we examined previously [Ohtani, Yamaguchi et al., GRL, doi:10.1029/2001GL013785, 2002], one GOES satellite observed that geosynchronous magnetic field actually became more stretched after Geotail observed a fast flow in the plasmasheet, even though the two spacecraft were aligned in the X direction and were separated by only 4 RE. A preliminary study of similar Geotail-GOES coordinated observations suggests that such events are not uncommon. We will address how often and under what conditions geosynchronous magnetic field dipolarizes following the fast plasmasheet flow.