SM53B-01
Power in the Magnetic Field Fluctuations and Diffuse Energetic Particle Density: Dependence on Distance From the Quasi-Parallel Bow Shock
Cluster magnetic field and energetic particle data have been used in order to determine the relation between the power in the low frequency magnetic field fluctuations and the energetic ion density upstream of the quasi-parallel bow shock. In particular, we have determined the distance dependence of (1) the power in the frequency range where the waves are resonant with field-aligned beam (FAB) particles, (2) of the power in transverse fluctuations in the frequency range where the waves are resonant with diffuse energetic ions and (3) of the power in compressive fluctuations in the frequency range resonant with the diffuse ions. The power in the transverse fluctuations is about constant as a function of distance along the magnetic field from the spacecraft location to the magnetic field - bow shock intersection point, whereas the power in compressive fluctuations increases exponentially with decreasing distance. The power in fluctuations resonant with FAB particles is constant as a function of distance from the bow shock. It is suggested that the power in transverse fluctuations over a wide frequency range is due to FAB instabilities in the region closer to the foreshock boundary. These waves are convected by the solar wind into the quasi-parallel regime where they are responsible for scattering of diffuse ions. As these transverse fluctuations are convected into a regime with increasing diffuse ion density they are converted partly into compressive fluctuations.
SM53B-02 INVITED
Solar Wind Processing by the Bow Shock: Implications for Magnetopause Dynamics
In addition to deceleration and heating of the solar wind, the bow shock impacts the solar wind through generation of ULF waves in the foreshock and magnetosheath. Also, the interaction of solar wind discontinuities with the bow shock lead to a variety of kinetic phenomena some of which are associated with drastic changes in plasma and fields in the magnetosheath, such as hot flow anomalies. In this presentation, we use results from global hybrid (kinetic ions, fluid electrons) simulations to illustrate a number of ways in which solar wind properties are modified by the bow shock. We then concentrate on the generation and evolution of ULF waves associated with the bow shock and how they interact with the magnetopause. The nature of this interaction is tied to the direction of the interplanetary magnetic field (IMF) and the angle (cone) it makes with solar wind flow direction. In the limit of radial IMF, the dayside bow shock corresponds to quasi- parallel regime with foreshock generated ULF waves modifying the solar wind and the magnetosheath properties. In the limit of large cone angles, the dayside bow shock is in the quasi-perpendicular regime leading to the generation of ion cyclotron and mirror mode waves in the magnetosheath. Relative to the quasi-parallel geometry, the magnetosheath plasma and fields are less turbulent during large cone angles and the influence of the bow shock on magnetopause dynamics may be though to be minimal. As we show however, the presence of ULF waves in the quasi-perpendicular magnetosheath has a significant impact on magnetopause by enhancing the time dependency of reconnection. Specifically, we show that the presence of ULF waves increases the rate of FTE formation by more than 50%.
SM53B-03 INVITED
Anomalous Flow Deflection at the Low Alfven Mach-Number Bow Shock and its Effect on the Magnetosphere
Earth's magnetosphere is an obstacle to the supersonic solar wind and the bow shock is formed in the front side of it. In ordinary hydrodynamics, the flow decelerated at the shock is diverted around the obstacle symmetrically about the Earth-Sun line, which is indeed observed in the magnetosheath most of the time. Here we show a case under a very low-density solar wind in which duskward flow was observed in the dawnside magnetosheath. A Rankine-Hugoniot test shows that the magnetic effect is crucial for this "wrong flow" to appear under low Alfven Mach number (MA). A full three-dimensional magnetohydrodynamics (MHD) simulation of the situation confirming this interpretation and earlier simulations is also performed. It is illustrated that in addition to the "wrong flow" feature, various peculiar characteristics appear in the global picture of the MHD flow interaction with the obstacle. One of these characteristics found in our simulation is skewing of the bow shock and the magnetosphere, which results in broadening of the dusk-tail magnetosheath and thickening of the dawn-tail magnetosheath. Evidence of magnetosphere skewing was detected in another low-MA solar wind event, where magnetic reconnection at the tail-flank magnetopause was significantly enhanced. The low-MA solar wind condition not only causes dawn-dusk asymmetry in shape but also possibly changes plasma transport and heating around/inside the magnetosphere.
SM53B-04 INVITED
Magnetosheath Turbulence
The Earth's magnetosheath is a transition region between the solar wind and the magnetosphere. An important question is how does the solar wind turbulence change across the bow-shock? The bow shock introduces an important temperature anisotropy. Relaxation of this anisotropy is accompanied by the generation Alfvén Ion Cyclotron (AIC) and mirror linear waves at frequencies below the ions cyclotron frequency fci. The superposition of these waves gives the spectrum of magnetic fluctuations ~ f- 1 in the vicinity of the bow-shock. In the flanks, however, we observe, for the first time, the Kolmorogov's ~ f-5/3 spectrum. At higher frequencies, f>fci, as in the solar wind, turbulent spectrum follows a ~ f-3 power law. At the difference with the solar wind turbulence, the transition between one inertial range to another is often accompanied by the presence of Alfvén vortices at the spectral break scales. Recent observations in the Kronian magnetosheath show the presence of such vortices at the same scales, revealing the generality of the phenomena. Alfvén vortex is a non-linear Alfvén wave with wavevectors k mainly perpendicular to a mean field. We show that this anisotropy of a turbulent energy distribution in k--space, i.e. k⊥≫ k|, is observed not only within the vortices, but for the whole range of small scales above the spectral break. This is observed for different plasma beta β in [1,10] and independently on the presence of the AIC or mirror waves at larger scales. We discuss possible influence of the magnetosheath turbulence features on the magnetopause reconnection.
SM53B-05 INVITED
Solar wind plasma processing in the magnetosheath at low Mach numbers
We illustrate alterations of the magnetosheath properties that occur during low Mach number solar wind. The key properties that we will discuss include: (1) the bow shock compression ratio and downstream magnetosheath plasma Beta, (2) changes in magnetosheath flows with asymmetric patterns (and substantial accelerations), and (2) the resulting asymmetric magnetopause and magnetotail shapes. We illustrate these phenomena primarily by use of global magneto-hydrodynamic simulations and discuss the mechanisms that determine these properties. We further emphasize the fact that these and other related effects tend to occur simultaneously, and typically during CME-driven storms at Earth, so as to render the solar wind – magnetosphere interaction drastically different from the more typical high Mach number case.
SM53B-06
Orientation and Structure of Reconnection Layer from Simulated Spacecraft Data
We test methods for determining the orientation and structure of quasi-two-dimensional reconnection regions using data from four simulated spacecraft (representing CLUSTER or MMS) flying in a tetrahedral formation through MHD and fully kinetic magnetic reconnection simulations. (The MHD simulations are 3D, but the large-scale structures are approximately two dimensional.) Methods tested for the determination of the orientation include minimization of the axial (quasi-out-of-plane) direction electric field [Sonnerup and Hasegawa, JGR A06208, 2005], and methods using the gradient of various quantities, including the methods of Shi et al. based on the gradient of the vector magnetic field [GRL L12105, 2005, GRL L08109, 2006]. For the MHD simulations, the method based on the electric field fails because the axial component of the electric field is far larger than the other components; the method minimizes the variation by selecting another component other than the true axial component. The method of Shi et al., however, works quite well, and has the advantage that it yields a point by point (at each time) determination of the orientation. Results for two- dimensional reconstruction of the magnetic field using the techniques of Sonnerup et al. [JGR A09204, 2006, JGR A05202] will also be shown. For the MHD simulation, the island structure is fairly well reconstructed, but the X-point is not (at least in part because the simulation has resistivity and the reconstruction method is currently valid only for ideal MHD). Results will also be shown for a simulated pass of spacecraft through a two-dimensional fully kinetic simulation.
SM53B-07
Asymmetric plasma mixing via symmetrically developed vortices at the Earth's magnetopause
Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. KHI is also excited in the tail-flanks of the magnetosphere where the flowing solar wind (SW) and the stagnant magnetospheric plasma sit adjacent to each other. It is considered to play an important role in the plasma mixing around the boundary under northward SW magnetic field when the SW entry into the magnetosphere is most enhanced. While KHI vortices have been detected, the observations have been performed only one side at a time and the questions related to dawn-dusk asymmetry have not been addressed. Here we show simultaneous detection in both flanks of KHI vortices under northward magnetic field condition. Visualization of the flow pattern indicates that the vortices grow quite symmetrically despite all the factors that may have broken the symmetry. Yet, the resultant plasma mixings show remarkable asymmetry in the ion energy distributions. Our results suggest that plasma mixing depends on the vorticity of the KHI vortices and may be related to microscale physics.
SM53B-08
Magnetic Island Formation Between Large-Scale Flow Vortices at an Undulating Post- noon Magnetopause for Northward IMF
THEMIS multi-spacecraft observations are presented for a ~2 hour-long post-noon magnetopause event on 8 June 2007 that for the first time indicate that the trailing (sunward) edges of Kelvin-Helmholtz (KH) waves are commonly related to small-scale <0.5 RE magnetic islands or flux transfer events (FTE) during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar BN structure with enhanced total pressure at their center. Most of the small-scale FTEs are not related to any major plasma acceleration. TH-A observations of one small FTE at a transition from the low-latitude boundary layer (LLBL) into a magnetosheath plasma depletion layer were reconstructed using two separate techniques that together confirm the presence of a magnetic island within the LLBL adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large-scale (~1 RE long and 2000 km wide) plasma vortices. We propose that the island may have been generated from a time-varying reconnection process at the sunward edge of the growing KH waves where the local magnetopause current sheet could be compressed by the action of the large-scale plasma vortices as suggested by numerical simulations of the KH instability.