SM23A-1681
2D Hybrid Simulations of Plasma Transport by Kelvin-Helmholtz Instability at the Magnetopause
In order to study the kinetic aspects of plasma mixing due to Kelvin Helmholtz instability at the magnetopause, we carry out two-dimensional hybrid simulations of this shear-driven instability. While MHD simulations have been popularly and successfully used to study Kelvin-Helmholtz instability as it is described by fluid theory, they cannot model certain important aspects of the instability behavior, such as mixing of magnetosheath and magnetosphere ions. We therefore use hybrid simulations which treat ions kinetically and electrons as a massless, charge-neutralizing fluid, to model the kinetic processes of interest. Our simulation is set up with velocity shear between the magnetosphere and magnetosheath plasma flows and B0 perpendicular to the plasma flow direction. Plasma density and magnetic field strength are non-uniform between the two sides, and their values are varied to see the effect on instability behavior. We look at different ways to quantify the plasma mixing and find that while the non-uniform density configuration yields smaller and less well-defined vortices than the uniform density, the number of simulation cells containing a mix of magnetosphere and magnetosheath plasma increases relatively linearly during the instability growth phase. In addition, we examine the velocity space distribution of the resultant ion populations in the interface region.
SM23A-1682
Sensitivity of Kelvin-Helmholtz instability to initial perturbations
The spatial degree of freedom in actual situations, for example, at the magnetopause, is much larger than box size of usual numerical simulations of the Kelvin-Helmholtz instability (KHI). If one allowed a larger simulation size, not only the fastest growing mode (the fundamental mode), but also the subharmonic modes start to grow. Eventually, the longest wave mode dominates the system and a large scale vortex appears [Wu, 1986; Miura, 1999]. In general, the subharmonic modes are initialized by seed perturbations with constant amplitudes and random phases among modes. However, 2-D simulations of the HD and MHD KHI showed that the growth of the subharmonic modes is sensitive to the initial perturbations: when the phases among modes are coherently set, the subharmonic mode saturates at larger amplitude [Patnaik et al., 1976; Baty et al., 2003]. In this study, we focus on sensitivities of the KHI to the initial perturbations in terms of phase and a spectrum power index. MHD simulations were conducted to examine how the initial perturbations determined the fate of the KHI through competitive processes of the subharmonic modes and the secondary instabilities. First, we examined two cases focusing on the initial phase difference of the perturbations. 3-D MHD simulations of the KHI show that when the phase difference between the first subharmonic (m=1) and the fastest growing (m=2) modes is zero, the energy of the mode m=2 is inversely cascaded to the mode m=1, resulting an emergence of a 2-D large scale vortex. The vortex paring inhibits the growth of the secondary MRI [Matsumoto and Seki, 2007]. When the phase of the m=1 is shifted by 0.5π, the secondary instability grows inside each KH vortex and the system undergoes a transition to a turbulent state even under the strong magnetic field (β=1.0). In addition to the phase difference, we have also examined the effect of a spectrum power index of initial perturbations. This has been examined by 2-D MHD simulations by comparing the growth rates of the subharmonic modes. Initially, we added 8 modes (m=4 is the fastest growing mode) whose amplitudes are related to the spectrum power index of α ranging from -1 to +1.5, while the phases are randomly shuffled. We found much faster appearance of the largest vortex (m=1) for the case α=1.5 than for α=-1 in which we observed growth of m=1 mode as expected from the linear theory. The maximum growth rate of the mode m=1 reached three times as large as the one from the linearly theory. In this presentation, we also show a possible mechanism of rapid formation of a broad mixing layer by a combination of the secondary R-T instability (direct cascade) [Matsumoto and Hoshino, 2006] and the fast appearance of the large scale vortex (inverse cascade).
SM23A-1683
The Interactions Between Two Neighboring Kelvin-Helmholtz Instabilities in the MHD Plasmas
The slow-mode and fast-mode Mach-cone-like plane waves are found in our two-dimensional magnetohydrodynamic (MHD) simulations of the Kelvin-Helmholtz (K-H) instability. The slow-mode and the fast-mode Mach-cone-like plane waves are generated by the constructive interferences of the slow-mode and the fast-mode waves emitted from the ridges of the surface waves. Unlike the nonlinear surface waves, these Mach-cone-like plane waves can expand away from the velocity shear layers. The development of the Mach-cone-like plane waves expanding into a neighboring velocity shear layer has been studied. From our simulation results, the Mach-cone-like plane waves will change into the turbulences after they propagate through the vortex structures generated by the neighboring velocity shear layer. Our simulation results also indicate that the Mach-cone-like plane waves can be a source to trigger a neighboring K-H instability. We found that the interactions between two neighboring K-H instabilities have dependence of the vorticities of the two neighboring velocity shears. Applications of our simulation results to the real space are also discussed in this study.
SM23A-1684
2-D MHD and Hall-MHD local simulations of the Kelvin-Helmholtz Instability at the Ionopause of Mars
Our previous simulation results and comparison with Cluster spacecraft data have shown that reconnection can be generated in thin current layers produced by the Kelvin-Helmholtz Instability (KHI) at the Earth's magnetosphere. Our simulation studies of the KHI for the Earth's magnetospheric parameters have shown that reconnection due to KHI can facilitate efficient plasma transport from the solar wind to the magnetosphere during northward IMF conditions producing diffusion coefficients of the order 109 m2/s. When the magnetic fields are initially parallel and the asymmetry is small across the initial shear flow layer the reconnection inside KH vortices occurs always first at the high-density spine of the vortex –leading to plasma transport from high-density magnetosheath to low density magnetosphere. We expect that in Mars and Venus this process may be reversed because the plasma inside the ionopause is more dense that in the solar wind. Although, Mars and Venus are un-magnetized the IMF will have a strong impact on the evolution of the KHI. We will present a systematic parameter study of the Kelvin-Helmholtz instability at the ionopause of Mars using 2-D local MHD and Hall-MHD approximations. The efficiency and direction of the plasma transport will be investigated for the typical plasma and magnetic field profiles observed in the vicinity of the Martian ionopause.
SM23A-1685
THEMIS Observations of the Wavy Variations in the Plasma Velocity at the inner edge of the Low-Latitude Boundary Layer
The THEMIS spacecraft provides opportunities to make unique simultaneous observations in the magnetosheath, the magnetospheric boundary layers and the magnetosphere by a string-of-pearls configuration. We have examined how the variations caused by the surface waves on the dayside magnetopause can propagate into the magnetosphere. On July 31, 2007, THEMIS-C and -D observed the wavy variations in the plasma velocity at the inner edge of Low-Latitude Boundary Layer (LLBL) under the northward Interplanetary Magnetic Field (IMF) and quiet solar wind conditions. These wavy velocity variations appeared in the Vx and Vy components. The associated magnetic field had microscopic wavy fluctuations, but no periodic variations were found in simultaneous plasma density and temperature. The hodograms of the velocities perpendicular to the magnetic field line had clear vortex-like structures. From these results, observed plasma velocity variations are Alfvén waves. On the other hand, THEMIS-E also observed the wavy velocity variations at the same region as well as THEMIS-C and -D, but the velocity hodogram did not present the vortex-like structures. Therefore, these wavy velocity variations are local phenomena. THEMIS-B simultaneously observed the diamagnetic-like waves due to the magnetopause undulations in the magnetosheath. This is because the correlation between the magnetic and plasma pressures was clearly out-of-phase. During the interval of these waves, the periodicities of these pressure variations were between 0.5 minute and 1 minute, and consistent with those of the wavy velocity variations. From these results, the fast-mode waves are induced by the magnetopause diamagnetic-like waves, and can propagate into the magnetosphere.
SM23A-1686
Formation of Structured Dayside Boundary Layers under Different Solar Wind Conditions: THEMIS Observations.
We have begun an investigation of the formation of the dayside low latitude boundary layer under different solar wind conditions using data from the THEMIS spacecraft. We present two cases of magnetopause/LLBL interface crossings made by the five spacecraft; one under long lasting northward IMF and a second for a period of southward IMF. All spacecraft during these observations traversed the day side magnetosphere in a string-of-pearls configuration with the farthest distance between spacecraft less than ~2 RE. The sequence of observations from spacecraft, as they crossed the magnetopause, shows the development of a highly structured boundary layer regardless of the polarity of the IMF. We discuss possible scenarios for the development of such structured boundary layers, including low latitude reconnection under northward IMF as well as double reconnection in opposite hemispheres.
SM23A-1687
Physics Mining of the CLUSTER Data Using a New Automated Technique: New List of Flux Transfer Events
A new data mining technique called MineTool-TS is used to develop a model for automated detection of flux transfer events (FTEs) at Earth's magnetopause in the Cluster spacecraft time series data. The model classifies a given time series into one of three categories of non-FTE, magnetosheath FTE, or magnetospheric FTE. One important feature of MineTool-TS is the ability to explore the importance of each variable or combination of variables as indicators of FTEs. FTEs have traditionally been identified based on their magnetic field signatures, but here we find that some plasma variables like the perpendicular temperature can be equally strong indicators of FTEs. For example, the perpendicular ion temperature yields a model accuracy of 96%. We also find that models using GSM coordinates yield comparable accuracy to those using boundary normal coordinates. This is useful since there are regions where magnetopause models are not accurate. Another surprising result is the finding that the algorithm can distinguish between magnetosheath and magnetospheric FTEs solely based on the magnetic field data, something that experts may not do so straightforwardly based on short time series intervals. The most accurate models use combination of plasma and magnetic field variables and achieve a very high accuracy of prediction of 97%. A list of FTEs, which contains both the expert labels and the automated detection results from Cluster data during years 2001 - 2003, can be obtained from the authors.
SM23A-1688
Pc 3-4 Pulsations Near the Cusp: Observations using Search Coil Magnetometers and HF Radars on Svalbard
Pc 3-4 pulsations (f ~10-100 mHz) originating in the ion foreshock upstream of Earth's bow shock due to the interaction between reflected ions and the solar wind frequently are observed in dayside ground magnetometer records at high latitudes. Many early studies noted increased Pc 3-4 wave power in the vicinity of the dayside cusp and inferred entry via the cusp, but later studies have revealed a more complex picture. In this study, we examined power near local noon at Pc 3-4 frequencies observed by search coil magnetometers at three closely-spaced stations on Svalbard (Ny Alesund, Longyearbyen, and Hornsund). Nineteen days with clear band-limited Pc 3-4 wave power near local noon, and thirteen days with only broadband wave power, were selected for which clear cusp signatures were available in data from the Finland SuperDARN radar, which was used to accurately determine the cusp latitude. 3-s resolution fixed- beam mode data from a radar beam viewing over Svalbard also made it possible to determine Pc 3-4 periodicity in regions of the cusp and auroral zone ionosphere that returned sufficient signal. Band-limited (Pc 3-4) and broadband ULF signals in near-noon magnetometer data showed a similar latitude distribution: wave power peaked about 4-5 degrees south of the cusp. Radar backscatter from the cusp showed no evidence of Pc 3-4 modulation in this region during any of these intervals, whereas two intervals of data at lower latitudes did show modulation at Pc 3-4 frequencies. These observations are consistent not with entry via the cusp proper but along field lines equatorward of the cusp which map to the low-latitude boundary layer or outer magnetosphere. Because this location is similar to that observed for the maximum power of traveling convection vortex events / magnetic impulse events (TCVs/MIEs), we suggest that similar physics may play a role in transmitting perturbations from the upstream solar wind into the dayside magnetosphere in both cases.
SM23A-1689
Pc 3-4 Pulsations Near the Cusp: Latitude dependence near the open-closed field line boundary
Dayside ground magnetometer records at high latitudes frequently show evidence of Pc 3-4 pulsations (f ~ 10-100 mHz) which originate in the ion foreshock upstream of the Earth's bow shock due to the interaction between reflected ions and the solar wind. Previous studies have noted increased Pc 3-4 wave power in the vicinity of the dayside cusp and inferred that the upstream waves gained entry via the cusp, although more recent studies have revealed a more complex picture. Here, we examine Pc3-4 wave power near local noon observed by search coil magnetometers at three closely-spaced stations on Svalbard. Three intervals are chosen when the upstream conditions are favourable for Pc3-4 generation, clear band-limited Pc3-4 wave power is observed near local noon, and an extended interval of HF radar backscatter indicative of the cusp is detected by the Hankasalmi SuperDARN radar. A stereo mode of radar operation is employed, such that 3 s time resolution is available on one radar beam, whilst the high latitude convection is revealed with 1 min. resolution. The location of the equatorward edge of the HF radar cusp may then be directly compared with the Pc3-4 wave power measured at three latitudes as the cusp migrates across the stations. The radar data show clear evidence of transient ionospheric flows and high spectral widths associated with field lines newly- opened by dayside reconnection processes, but no evidence of oscillations in the Pc3-4 frequency range. In the ground magnetic field a peak in Pc3-4 power is generally observed in the equatormost magnetometer, except when the cusp is significantly poleward of the stations, consistent with a peak in wave power ~4 degrees equatorward of the cusp, but suggesting a modest dependence of wave power with latitude on closed field lines When the cusp does move equatorward of the magnetometer stations the Pc3-4 power drops rapidly, and does so earliest at the most poleward magnetometer station, suggesting a sharp drop in wave power when the open field line region is reached. This behaviour is confirmed by comparison of Pc3-4 wave power recorded at Svalbard with that recorded at lower latitudes at Halley Bay in Antarctica. These observations are not consistent with entry via the cusp proper, but rather along closed field lines equatorward of the cusp which map to the low-latitude boundary layer or outer magnetosphere.
SM23A-1690
Undulations on the Equatorial Edge of the Auroral Oval
We present 5 events with observation of the evolution of large scale auroral undulations imaged by IMAGE FUV. Auroral undulations are large scale wave structures seen at the equatorial edge of the auroral oval during enhanced geomagnetic activity. They develop close to, or during the maximum of strength of the ring current. As the auroral undulations propagate westward along the duskside, their amplitude is growing. When they enter the plasmasphere further west, the auroral undulations are seen to reduce in amplitude, and vanish. DMSP F13 data shows velocity shears in the plasma close to the crest of the undulations, where the velocity shears are due to the establishment of SAPS channels. Observations of the precipitating energetic ion flux show an excellent latitudinal overlap with the location and extent of the auroral undulation. We suggest that the large scale auroral undulations are a consequence of the Kelvin-Helmholtz instability, and that energetic ions constitute the plasma population that is modulated.