SM53C-01 13:40h
Interball Statistics of Bow Shock Energetic Ions
Energetic ions observed by the Interball Tail spacecraft from August, 1995 through September, 2000 upstream from the Earth's bow shock are studied. Most of the time, energetic ion distribution is found to be nearly isotropic and less than 3% of the time for highly anisotropic distribution. We further investigated the dependence of ion flux and spectral slope on the solar wind density and velocity, $\theta_{Bn}$, and Kp index. Preliminary results reveal different patterns for the lower and higher energetic ions. For the diffuse events, the lower energetic component is better organized by the solar wind parameters and $\theta_{Bn}$. The spectral slope of ion energy spectrum shows some dependence on the solar wind velocity. These results are consistent with bow shock acceleration at the quasi-parallel shock for solar wind ions. However, the higher energetic ion flux is less dependent on the solar wind parameters and bow shock geometry. When energetic ion distribution is highly anisotropic, their flux becomes much better correlated with the Kp index. These results indicate that magnetospheric leakage dominates the higher energetic component. There are clearly indications that both shock acceleration and magnetospheric leakage contribute to the upstream energetic ions even for diffuse ion events. Their relative contribution is energy dependent.
SM53C-02 13:55h
Plasma Dynamics in the Vicinity of a Cusp-Like Magnetic Configuration
The magnetospheric cusps are an important structural element of the Earth's magnetosphere. Magnetic flux from the magnetospheric boundaries converges into the cusps and the magnetic field in the vicinity of the cusps rotates by 360 degrees. The magnetic field and plasma flow configuration allows for magnetic reconnection and Kelvin Helmholtz instabilities in the vicinity of the cusps. In addition plasma flow can become turbulent due to boundary layer separation as a result of the plasma flow over the cusp indentation. However, there are very few models based on first principles and with good numerical resolution which address the cusp physics. Here we will present results based on two and three dimensional MHD simulations with an initial condition which includes a cusp-like magnetic field indentation. The initial conditions particularly address cases with various magnetic field orientations relative to the magnetosheath plasma flow. The results consider a variation of the velocity on the magnetosheath side, the influence of viscosity on the the flow structure in and adjacent to the cusp, and the location of turbulent onset. Consideration will also be given to the possibility of standing waves within the cusp which is a topic of recent (contraversial) discussion in the field.
SM53C-03 14:10h
Causes of core ion upflow in the dayside cusp
We investigate the causes of low energy ($<20$ eV), or ``core'', ion upflows in Earth's cusp. The CUSP-2002 sounding rocket flew from Ny Alesund, Svalbard, on 14~December 2002, carrying a complement of instrumentation within the cusp to an altitude of 772~km. The Suprathermal Ion Imager (SII), a two-dimensional energy/arrival angle ion spectrograph, observed large $\rm{O}^+$ outflows within the cusp at altitudes as low as 500~km, while the ion kinetic temperature remained isotropic near 0.1--0.2 eV for the entire event. The upflow data exhibit clear transitions that correlate with the cusp boundary layer between closed and open magnetic field lines. We attempt to explain the time-evolution of these upflows by examining the effectiveness of the observed precipitating magnetosheath electron flux as a driver under the prevailing solar wind conditions. These observations provide quantitative information on the time-dependent response of polar ion upflow to soft electron precipitation, and offer an excellent opportunity for comparison with numerical models of ion outflow.
SM53C-04 14:25h
On the relationship between ion upflow events and cusp auroral transients
In this paper we present a one-to-one relationship between poleward moving auroral forms (PMAFs) in the cusp and ion upflows. Ion upflows have the same quasi-periodic nature as cusp auroral transients, which are believed to be a signature of flux-transfer events. This new observation corroborates the idea that low energy particle precipitation is the dominant source of energy for ion upflows. Some events were lacking elevated Ti which indicates that frictional heating is not required. The ion upflow events broadened with altitude in characteristic V-shape. We suggest that each PMAF was associated with an inverted V- potential structure moving over the radar.
SM53C-05 14:40h
A Survey of High-Latitude Flux Transfer Events Observed by Cluster
During Cluster's dayside magnetopause season (November - June), the four spacecraft cross the magnetopause near the magnetospheric cusps permitting high-latitude observations of Flux Transfer Events (FTEs), a signature of transient magnetopause reconnection. The combination of instruments available on Cluster, particularly the electron spectrometer PEACE and the Flux Gate Magnetometer, allow a range of FTE types to be identified. In some FTE observations, the spacecraft do not enter the reconnected flux tube and so observe no electron signature, but they do observe a magnetic field signature due to draping of the magnetosheath/magnetospheric magnetic field. In other cases, a magnetic field signature is either non-existent or difficult to resolve, but the electron data show exchange of plasma populations across the magnetopause and/or plasma acceleration. We will show examples of events where combining multiple instruments and four spacecraft at different separation scales helps to distinguish whether or not they are FTEs. We also present results of a compilation of a catalogue of FTEs observed by Cluster in the 2002/3 dayside season. Some statistical features observed are similar to the results of earlier surveys at lower latitudes. For example, the occurrence of FTEs exhibits a strong dependence on southward interplanetary magnetic field (IMF) with a broad peak in the dawnward-duskward component of the IMF. However, few FTEs are observed poleward of the cusps.
SM53C-06 14:55h
Tracing the location of the reconnection site from the northern and southern cusps
Full 3D plasma observations in the cusp have been successfully used to investigate the location of the reconnection site at the magnetopause. In this model low-velocity cutoffs in the precipitating and mirrored magnetosheath population on open cusp field lines are used to estimate the distance to the reconnection line. This distance is subsequently traced back along the model magnetic field lines to the magnetopause. Two cusp crossings on March 3, 2003, observed by the Cluster satellites in the northern hemisphere and the Polar spacecraft in the southern hemisphere during similar solar wind and southward IMF conditions, are used to trace the location of the reconnection site from two directions. Both estimates point to a reconnection site in the southern hemisphere close to the anti-parallel reconnection site.
SM53C-07 15:10h
Using global MHD simulations in conjunction with spacecraft observations to determine the large-scale topology of dayside merging
We report results from a study that employs three-dimensional magnetohydrodynamic (MHD) in conjunction with ion measurements from POLAR TIMAS and CLUSTER CIS experiments, and images of proton auroras from the IMAGE FUV instrument. Using plasma and magnetic field parameters measured upstream of the bow shock by the ACE spacecraft as input to the simulations, we consider the spacecraft observations in the context of the global topology of the merging as inferred from the simulations. Results indicate that the locations of the merging sites, though distorted, are mostly consistent with merging patterns predicted by the antiparallel merging model. The simulations also indicate that merging occurring at locations inconsistent with the model's predictions can often be explained by multiple merging processes. Such situations occur predominantly when the IMF is northward and has a significant By component. The simulations suggest that during these conditions, merging of the IMF with the Earth field can occur in several steps: first the merging of unconnected magnetosheath with closed field lines in regions expected from the model and then the merging of newly open field lines with either closed field lines or older open field lines in regions depending on the dynamics of the magnetosheath field. The results of the study emphasize the importance of the time evolution of the draping of the magnetosheath field in the global merging process.
SM53C-08 15:25h
Properties of magnetic merging at the magnetopause inferred from dayside 557.7-nm all-sky images
We combine {\it in situ} measurements from Cluster with high-resolution 557.7 nm all-sky images from South Pole to investigate the spatial and temporal evolution of merging on the dayside magnetopause. Variations of 557.7 nm emissions were observed at a 6 s cadence at South Pole on April 29, 2003, at a time when significant changes in the Interplanetary Magnetic Field (IMF) clock angle were reaching the magnetopause. Electrons energized at merging sites are plausible sources for 557.7 nm cusp emissions. At the same time Cluster was crossing the pre-noon cusp in the Northern Hemisphere. The combined observations confirm results of a previous study that high-latitude merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 30 s to 3 min. Under prevailing IMF $B_Y$ and $B_Z$ negative conditions, the optical patterns are consistent with a scenario in which merging occurs primarily at high latitudes at positions dictated by the IMF clock angle. The intensity of the emissions, and probably the merging rate, appear to vary with changes in both IMF $B_X$ and dynamic pressure of the solar wind. Significant $B_X$ affects the lag-times of merging in the two hemispheres. Cluster observations set the merging lag times for Northern Hemisphere sites while the post- noon optical emissions set times of Southern Hemisphere merging. Matching optical signatures with clock-angle, $B_X$, and dynamic pressure variations provides new insights about interplanetary control of temporal-spatial variability of dayside merging.