SM43A-01 INVITED
The Physics of the Foreshock: An Introduction
The foreshock is the region upstream from a collisionless shock in which energetic charged particles are flowing away from the shock and therefore streaming with respect to the solar wind. These particles can arise because they are "reflected" by the shock and because they are thermalized and scattered downstream of the shock but have sufficient thermal speed to move upstream. This latter process occurs above a critical Mach number so we refer to subcritical and supercritical shocks. Similarly ion reflection produces upstream beams only below theta BN angles of 39 deg. Thus we divide the shock into quasiparallel and quasiperpendicular regimes. Backstreaming electrons and ions create a plethora of waves from a panoply of instabilities and greatly modify the incoming flow. However, there is a second source of upstream waves: production at the shock itself. These whistler-mode waves generally affect only the region in and near the shock ramp and are only a factor at low Mach numbers, but these waves can exceed the solar wind velocity and propagate far upstream.
SM43A-02 INVITED
Ion properties at Earth's foreshock, recent achievements
Different types of backstreaming ion distributions have been reported in the ion foreshock: field-aligned beams (FABs), gyrating ions and diffuse ions distributions. Contrary to the first type, the two others are always associated with low frequency (ULF) waves. Multi-spacecraft observations by Cluster and the high quality 3-dimensional data obtained by Cluster/CIS has permitted to address specific quantitative questions that could not be resolved previously. Gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi-monochromatic waves. These waves reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. It has been shown recently from a Cluster data set that the gyrophase-bunched ion distributions are mainly produced by such a process from cyclotron-resonant FABs observed just at the edge of both the gyrating ions region and the boundary of ULF waves. New results have also been obtained on the FABs and gyrating ions exhibiting sometimes a high-energy tail likely produced by a local process at the shock which may be micro-turbulence. It has been found that the characteristics of both parallel and perpendicular particle distribution function profiles of field-aligned beams (FABs) are geometry-dependent. Other recent results deal with the properties of ion distributions associated with ULF waves with left-hand polarisation the existence of which has been definitively and unambiguously proven from the four-spacecraft analysis by Cluster. These recent achievements in the properties of ion distributions in the terrestrial ion foreshock will be discussed in the light of theoretical works and recent numerical simulations.
SM43A-03 INVITED
In-situ observations of the cross shock electric field at Earth's bow shock and implications for foreshock dynamics
Of all the ways in which a magnetized collisionless plasma shock differs from its neutral fluid counterpart, one of the most striking is the fact that at a collisionless shock, particles are observed to stream from the shock back into the upstream region. The combination of the backstreaming particles together with the upstream core plasma distribution is subject to a number of instabilities that lead to the generation of waves and subsequent wave-particle interactions, all forming an integral part of the shock. A key part of this process is the formation of the reflected ion population, which strongly depends on the electric field structure of the shock. The Cluster mission is a constellation of four identical spacecraft in polar orbit around the Earth and is designed to resolve spatial and temporal variations in space plasmas, in particular at the Earth's bow shock. We have used its multi-spacecraft capability to determine the structure of the current inside the shock, and investigate the out of plane magnetic field. New observations of the electric field are used to investigate the structure of the shock on ion scales; in particular the contribution of the Hall term in the generalized Ohm's law. These observations have implications for the dynamics of ion reflection and beam formation, which are discussed.
SM43A-04 INVITED
On Field-aligned Ion Beams in the Earth's Foreshock Region
The foreshock global morphology is organized by the interplanetary magnetic field direction, the bow shock shape, and, crucially, by particle dynamics. All physical processes at and around the Earth's bow are collisionless and the ion dynamic is determined by the wave-particle interactions. Particles are reflected and energized at the shock and some with high speeds are able to propagate upstream against the solar wind. Therefore, the generation of gyrating ions by reflection as well as the formation of field-aligned ion beams are essential parts of this dynamic. First, field-aligned ion beams, predominately found at the quasi-perpendicular bow shock, propagate into the distant foreshock region and may create wave activity. Secondly, ion reflection is most likely the first interaction of ions with the shock before they undergo the downstream thermalization or reflected upstream. These processes are fundamental and may be found at other astrophysical settings such as the termination shock. Recent observations at the Earth's bow shock with Cluster spacecraft provide new insights into time the evolution of particle distributions and the reflection properties of the oblique Earth's bow shock. Field-aligned ion beams appear to emerge from the gyrating ions in the shock ramp created by the same reflection process. In fact, effective scattering in pitch angle within the shock ramp during the reflection seems to be the basic production mechanism of field-aligned ion beams. We revisit ion reflection, the source and basic production mechanism of field-aligned ion beams and their dynamics in the foreshock region, by using multi- spacecraft measurements, and contrast these observations with existing theories.
SM43A-05 INVITED
REFORMATION OF PERPENDICULAR SHOCKS: 1-D VS 2-D SIMULATIONS
Perpendicular shocks simulated using 1-D both hybrid and PIC codes are strongly non-stationary (exhibit self-reformation process) for a wide range of upstream parameters (low proton beta and/or high Mach numbers). We present new 2-D simulation results using both hybrid and PIC codes of the perpendicular shocks and compare them with the corresponding 1-D simulations. One striking feature observed in the simulations is that shocks which are self-reforming in 1-D reveal to be almost stationary in 2-D and their foot region is dominated by whistler waves propagating obliquely with respect to the shock normal and to the magnetic field. These simulations indicate that the emitted whistler waves inhibit the self-reformation process.
SM43A-06 INVITED
Discovery of Alfven vortices in the plasma turbulence of the Earth's magnetosheath
The Earth's magnetosheath represents one of the natural laboratories of the collisionless plasma turbulence. The presence of the boundaries, the bow shock and the magnetopause, can play a crucial role in the turbulence nature. Here, as in the solar wind, the spectrum of magnetic fluctuations can be described by two well defined power laws separated by a spectral break, which is observed in the vicinity of the proton cyclotron frequency fcp. In the low frequency part, the magnetosheath turbulent spectrum follows a ~ f-1 law and it is ~ f-3 in the high frequency part. The low frequency part of the spectrum thus does not follow the Kolmogorov's f-5/3 law, as it is the case in the solar wind. But another important difference is that in the magnetosheath the spectral break is frequently accompanied by a large maximum. This maximum is usually interpreted by Alfvén Ion Cyclotron waves generated by the anisotropic ion distribution downstream of quasi- perpendicular bow shocks. We have established that this maximum corresponds to space-localized coherent magnetic structures in the form of Alfvén vortices [Alexandrova et al., JGR, 2006]. The Alfvén vortex is a non-linear cylindrical Alfvén wave, with an axis quasi- parallel to the mean magnetic field B0 and which is propagating in a plane perpendicular to B0. The frequent observation of such structures is an indication of their stability in the magnetosheath plasma. We discuss the role of the bow shock in the generation of the Alfvén vortex structures in the magnetosheath turbulence.