SM33B-01
High-energy tail associated with backstreaming ions: A comprehensive study
Detailed particle distributions of backstreaming ions observed upstream of the Earth's bow shock by the Cluster Spacecraft are examined. A recent study found that the characteristics of both parallel and perpendicular particle distribution function profiles of field-aligned beams (FABs) are geometry-dependent [Meziane et al., 2007]. FABs observed at oblique shocks have reduced particle distribution functions that exhibit high-energy tails, in contrast to FABs observed at quasi-perpendicular shocks. Gyrating ion as well as diffuse ion populations also show evidence of high energy tails in the reduced distributions. Usually, these tails are satisfactorily fit with a stretched exponential function. It is not clear whether the tails associated with the different populations are related. The existence of these high energy tails raise interesting issues regarding their origin and how they fit with our current outstanding of of backstreaming ions. It is now accepted that the probability distribution functions tails are likely to arise from a development of nonlinear coherent structures. We have not found any evidence that these tails grow from upstream electromagnetic fluctuations, therefore favoring their production at the shock.
SM33B-02
Modeling pitch-angle and gyrophase restricted ~0.5~MeV foreshock ions
Ions with energies > 100~keV backstreaming from the bow shock have been well-studied, with most observed fitting into three catagories: those with isotropic distributions accelerated via diffusive processes, typically having energies no greater than 250 keV; those with pitch angle, and sometimes gyrophase, constrained distributions extending past 1 MeV in energy, accelerated during single encounters with the shock via the shock drift mechanism; and those of magnetospheric origin that `leak' across the magnetopasue and find their way to the upstream. In principle, some of these mechanisms could act in concert, although that has not been examined thoroughly. We have examined relatively infrequent Wind/3DP observations that appear to be of the second type (restricted in pitch angle and gyrophase), but which have no apparent source of seed particles needed to produce the observed fluxes using straightforward application of shock drift acceleration theory. Due to the different travel times required for ions of different pitch angles and energies to travel from the shock to the spacecraft, and gyrophase dependent trajectories for ions with gyroradii of the order of an Earth radius, a single distribution snapshot is a convolution of sources that can be well-distributed along the bow shock. In order to better understand our observations we have applied particle tracking methods to find how ions at different energies and arrival directions map back to the shock, which permits us to model point-by-point contributions to the distributions observed at the spacecraft. We present results from this investigation, and consider different hypotheses for producing the unobserved, but required, seed populations.
SM33B-03
The origin of Earth's foreshock waves
In this work we make use of kinetic theory to study the origin of low frequency compressive waves (shocklets) and high frequency waves (~ 1 Hz) observed within the Earth's foreshock using Cluster data. Empirical dispersion points obtained with the Wave Telescope Technique are presented and compared with kinetic dispersion relations for the ion/ion RH and LH resonant instabilities. We also performed a detailed characterization of the observed waves and present ion distributions associated with the waves. A very good agreement was found between the data and the theoretical curves for RH and LH observed shocklets, which probes that these RH (LH) compressive waves are generated by ion/ion RH (LH) kinetic instabilities. In contrast, 1 Hz waves' properties did not fit any instability due to local ions, this clearly supports previous hypothesis that upstream whistler modes are generated at planetary shocks.
SM33B-04
Cavities and ULF waves in Earth's foreshock
We use global hybrid simulations to study the formation and evolution of density cavities and ultra low frequency (ULF) waves in the foreshock. We also study the effects that these structures can have on processing the solar wind in the foreshock, on bow shock structure and ultimately on solar wind coupling with the magnetosphere. Global hybrid simulations give a collective picture of processes taking place in the foreshock, bow shock and magnetosheath. Because ions are treated as particles, these codes also give information on ion-scale microphysics allowing us to understand how kinetic phenomena modulate global characteristics. Simulation results are compared with observations.
SM33B-05
Stochastic Acceleration of particles in turbulent magnetic regions
The stochastic acceleration of particles is a recognized mechanism to energize particles in magnetospheres. In this work I have focused in develop an analytic model of the stochastic acceleration due to fluctuations of the magnetic field of small scale that shown to be extremely efficient accelerating. I show that with a stochastic force of 10e-15 N a proton can be accelerated from 0 to 10e3 eV in 8.49x10e-5 seconds, from 0 to 10e6 eV in 2.68 seconds, and from 0 to 10e8 eV in 40 minutes. Since the time in which this mechanism accelerates depends on the average force exerted by the fluctuations then it is possible that in very turbulent regions (e.g. shock waves, CMS, pulsars), larger energies can be reached in the same time.
SM33B-06
High-latitude Bow Shock: Tilt Angle Effects
The bow shock is created in front of an obstacle immersed into a supersonic flow and its location depends on the size and shape of the obstacle. It was found that the obstacle (magnetopause) is scaled with the solar wind dynamic pressure and changes its dimensions and location with dipole tilt angle and interplanetary magnetic field (IMF) orientation. The similar functional dependences would be used for prediction of the bow shock position. Many bow shock models have been developed for various conditions in course of years; however, none of these models consider the parametrization of bow shock properties with the tilt angle. The present study employs a set of about 3000 bow shock crossings registered during 1994-2002 by the different spacecraft (INTERBALL-1, IMP-8, GEOTAIL, MAGION-4, and CLUSTER-2) and presents an investigation of the tilt angle influence on the bow shock location. The study is based on a comparison of recent bow shock models with experimental data and with global MHD models.
SM33B-07
Large parallel and perpendicular electric fields on electron spatial scales in the terrestrial bow shock
Large (≤ 600 mV/m) perpendicular and parallel electric fields were measured in the Earth's bow shock for the first time by the vector electric field experiment on the Polar Satellite. These fields exist on spatial scales comparable to or less than the electron skin depth (a few kilometers) and they contain parallel potentials of tens of volts and perpendicular potentials up to a kilovolt. We discuss these results in the context of electron kinetics in the shock front.
SM33B-08
Interaction of Interplanetary Shocks With the Bow Shock and Their Propagation Within the Magnetosheath
Fast forward interplanetary (IP) shocks have been identified as a source of large geomagnetic disturbances. However, the shocks can evolve in the solar wind, they are modified by interaction with the bow shock and during their propagation through the magnetosheath. A few previous papers refer the inclination and deceleration of the IP shock front in this region. Our contribution continues this effort and presents the study of an IP shock interaction with the bow shock. Since the bow shock is a reversed fast shock, the interaction of the IP shock and bow shock is a problem of interaction of two fast MHD shocks. We compare profiles of magnetic field and plasma parameters observed by several spacecraft in the solar wind and magnetosheath with the profiles of the same parameters resulting from the MHD numerical model. The MHD model suggests that the interaction of an IP shock with the bow shock results in an inward bow shock displacement that is followed by its outward motion. Such motion will result in an indentation propagating along the bow shock surface. This scenario is confirmed by multipoint observations. Moreover, the model confirms also previous suggestions on the IP shock deceleration in the magnetosheath.