SM51A-1606
Comparison of Magnetic Field-Based Methods for Solar Wind Propagation Time Delay from Single-Spacecraft Measurements at ACE and WIND Satellite
Interaction between the solar wind and the Earth's magnetic field influences several phenomena on Earth
and as a result is the subject of many studies. These studies often require accurate, "raw" solar wind
parameters just before the solar wind interacts with Earth's magnetosphere. One widely used technique for
determining interplanetary magnetic field (IMF) and solar wind parameters shifts the measurements at some
distant point on the Earth-Sun line. The Advanced Composite Explorer (ACE) orbiting the L1 Lagrange point
is the primary resource for collecting these measurements. The time delay for the shift is calculated using a
simple kinematic approach: the time is equal to the distance from the satellite divided by the measured
velocity of the solar wind. This approach can be relatively accurate due to the short distance to the L1 point
compared to the distance to the Sun and in many situations it is acceptable based on the assumption that
solar wind parameters do not change significantly before reaching the magnetosphere. However,
measurements provided by satellites such as ACE are never taken exactly on the line connecting the Earth
and the Sun. Because IMF is not homogeneous in any direction, the solar wind particles that actually reach
the Earth are different from those that pass the satellite. As a result, IMF and solar wind measurements near
the L1 point are usually different from IMF at the Earth.
Theoretical considerations as well as simultaneous observations from different satellites show that IMF
consists of current layers and wave fronts along which changes in magnetic induction are minimal at such a
short scale as a diameter of ACE orbit. This work compares different methods for time delay calculation
based on determination of orientation of such planar stationary structures in the solar wind by measurements
of IMF vector at ACE satellite using 1 seconds magnetic field data and compares them with WIND 3 seconds
magnetic field observations. This includes a determination of different IMF and solar wind parameters that
influence each method. Each method is tuned using free parameters presented in the algorithm. Parameters
that influence the quality of forecast for the methods are investigated. The tuning procedure uses sets of
continuous data 1--15 days in length from different time periods between 1998 and 2001. Analysis of
applicability of different methods has been made. Computations needed for analysis of these methods and
data were performed on high performance computers at the Arctic Region Supercomputing Center.
This research was initiated in response to the need for better IMF prediction in real-time during the
application of the University of Alaska Fairbanks Eulerian Polar Ionosphere Model (UAF EPPIM).
http://spaceweather.arsc.edu
SM51A-1607
Solar wind disturbance changes between L1 and Earth's magnetosphere: Modeled series at L1 and real events
While modeling the solar wind interaction with the Earth's magnetic dipole, most of the existing MHD codes
set initial boundary conditions at ~35 Earth radii upstream. These modeling results are often compared with
measurements taken by various spacecraft located at the Lagrangian L1 point and flown within the Earth's
magnetosphere at high and low orbits, and the ground-based observations. At the same time, a number of
experimental magnetospheric and space weather models utilize the L1 observations as input parameters.
These discrepancies in the outer boundary conditions may cause misinterpretation of the MHD &
experimental modeling results and real observations across various domains. Although numerous techniques
were developed for processing the solar wind plasma over ~200 Re between L1 and Earth (from simple
ballistics to minimum-variance methods), none of these techniques help when the slower and faster plasma
packets intersperse, causing various SW and IMF discontinuities heavily interact with each other on their way
to Earth. In this study, we attempted to understand through the MHD modeling how the pulsing (i.e.,
slowing/accelerating) solar wind flow changes along the way from L1 to Earth, and compare these modeling
results with a few real events measured by the ACE-WIND spacecraft pair during the intervals of minimal
spacecraft transverse separation. Our results might help in better understanding how the L1 solar wind data
should be used in the current techniques for space weather specification and forecasting.
http://mist.nianet.org/limie.html
SM51A-1608
Evidence for Nonlinear Langmuir Wave Behavior in the Earth's foreshock: Cluster Observations and Instrumental Concerns
Langmuir wave characteristics in the Earth's foreshock were examined to identify possible nonlinear wave behavior for two case studies. The occurrence rates of four types of power spectra near the foreshock edge were determined: (1) spectra with power at the local plasma frequency fpe only, (2) spectra with power at fpe and 2fpe, (3) spectra with double peaks near fpe, and (4) spectra with double peaks near fpe and peaks at low frequencies indicative of ion acoustic waves. The wave amplitudes from the Cluster WBD Plasma Wave Receiver and PEACE electron distributions were more consistent with three-wave electrostatic decay processes than modulational instabilities. However, only a few spectra had both the double peaks near fpe and ion acoustic waves needed for these three-wave interactions. According to theoretical studies, transverse electromagnetic waves at roughly twice the local plasma frequency 2fpe can be generated if beam-driven Langmuir waves coalesce with backscattered Langmuir waves that result from electrostatic decay processes. However, we found that in the Cluster data set, most harmonics are caused by instrumental effects. This suggests that past studies of harmonic generation based on observations of weak harmonics may be questionable.
SM51A-1609
THEMIS Observations of Electrostatic Waves in Context with Ion Foreshock Plasma Structures
The terrestrial foreshock region plays an essential role in preprocessing the undisturbed solar wind en route to Earth's bow shock and magnetopause. Such preprocessing involves a rich array of plasma structures observed within the terrestrial ion foreshock, including short-duration large-amplitude magnetic structures (SLAMS), hot flow anomalies (HFAs), foreshock cavities and recently examined density holes. Much work has been done to characterize the macroscopic structure of these objects, as revealed in plasma and DC fields data, and to study higher frequency waves using wave spectra. Little has been done to date to examine high-frequency waveform data in the foreshock, and to place such measurements into context of the sub-structure of the features observed there. Here we present case studies of foreshock electrostatic waves observed by THEMIS, from a few tens to few thousand Hz. The THEMIS/EFI, FGM and SCM instruments provide long duration three-axis measurements of electric and magnetic field waveforms from DC to 8000 Hz, allowing us to assess how these waves are organized within the substructure of various foreshock phenomena. Preliminary analysis indicates large (20-100 mV/m) amplitude oscillatory electrostatic waves from a few tens of Hz to few thousand Hz. Such waves are often but not always observed in association with fine-scale currents embedded within foreshock structures, which is suggestive of different generation mechanisms. These oscillatory waves appear to be consistent with short-scale, ion-acoustic like waves observed in the foreshock reported in the literature. Notably, we also see large amplitude, solitary-like electrostatic waveforms embedded within ion acoustic turbulence, which is suggestive of counter-streaming particles. We discuss the characteristics of the electrostatic waves, such as orientation with respect to magnetic field, wavelength, and relation to fine structure in the magnetic fields and plasma density.
SM51A-1610
Foreshock density cavitons
From global hybrid simulations (kinetic ions, fluid electrons) we have found the existence of foreshock density cavitons immersed in regions permeated by ULF waves (Omidi, 2007, Blanco-Cano et al., 2008). These cavitons show large depressions in magnetic field magnitude and density, and are bounded by regions with enhanced field and density. Foreshock cavitons share some characteristics with previously reported isolated foreshock cavities, but in contrats to the cavities, are always found surrounded by a sea of ULF waves. Recently, Sibeck et al. (2008) compared observations of isolated foreshock cavities with hybrid simulation results for a day with small cone angle and concluded that isolated foreshock cavities are associated with changes in the IMF orientation which causes the spacecraft to go in and out of the foreshock. In other words, isolated foreshock cavities should be interpreted in terms of transient encounters with the foreshock compressional boundary and not as distinct regions inside the foreshock. In this work we study the charactericstics of observed foreshock cavitons using Cluster magnetic field and plasma data and compare them with observed crossings of the foreshock compressional boundary for various IMF orientations.
SM51A-1611
Statistical analysis of the properties of foreshock density cavitons
Global hybrid simulations (kinetic ions, fluid electrons) have shown the existence of foreshock density cavitons immersed in regions permeated by ULF waves (Omidi, 2007, Blanco-Cano et al., 2008). These cavitons are characterized by large depressions in magnetic field magnitude and density, and are bounded by regions with enhanced field and density. In this work we study statistical properties of foreshock cavitons observed by Cluster spacecraft between the years 2001 and 2005. We have identified approximately 90 foreshock cavitons and use magnetic field and plasma data to analyze their durations, sizes, amplitude, and orientation. We compare caviton B and n values with ambient values. We also study the foreshock conditions in which the cavitons are detected, i.e. θBV, the angle between the incoming solar wind flow and the IMF, and Mach number, among others. We also determine the characteristics of the waves that surround the cavitons or even appear within them. We find that the foreshock cavitons can be observed in various ways - some are found as single cavitons immersed in ULF waves, others appear in groups, separated temporally only by a few minutes. In some cases we find two or three cavitons that are in the process of merging into a larger structure, and still developing.
SM51A-1612
Bow Shock Configurations as Affected by Planetary Dipole Tilt under Intermediate MHD Solar Wind Conditions
Previous numerical MHD studies have shown that MHD shocks formed from the interaction of a planetary magnetosphere with the solar wind under intermediate MHD shock conditions show a marked north-south asymmetry in shock characteristics (Cable, S., Lin, Y., and Holloway, J.L. (2007), JGR, 112, A09202). This asymmetry is the result of very different magnetic reconnection patterns in the northern and southern hemispheres. Specifically, under intermediate shock conditions, the solar wind magnetic field is very nearly sunward or anti-sunward and will, therefore, be nearly parallel to the planetary field at one pole, producing little magnetic reconnection there, and nearly anti-parallel to the planetary field at the opposite pole, where substantial magnetic reconnection can take place. Intermediate bow shocks are therefore quite novel compared to standard MHD fast shocks in that disturbances generated down stream at the poles can propagate upstream to the shock and affect the overall shock configuration. This immediately raises the question of how the tilt of the dipole of the planetary magnetic field might affect the configuration of a bow shock under intermediate shock conditions. We use MHD numerical studies to examine how the bow shock configuration is affected over a wide range of dipole tilt angles. We have employed two codes in this investigation: our own code that was used in the original examination of intermediate shocks with planetary magnetospheres, and the OpenGGCM, developed by Joachim Raeder and Timothy Fuller-Rowell, and available in the Community Coordinated Modeling Center "Runs on Request" system (http://ccmc.gsfc.nasa.gov). Though differences are visible in the results of the two codes, both codes show that dipole tilt has significant affects on the bow shock configuration. Similarities and differences in the results will be discussed.
SM51A-1613
Cluster Observations of the Local Dynamics of the Quasi-parallel Bow Shock
The four Cluster spacecraft were flying very close to the nose of the magnetosphere on 17 March, 2007. The spacecraft crossed the magnetopause several times during a 3 hour period from 17:00 to 20:00 UT. Some of the outward crossings were followed by short bow shock crossings immediately afterwards. Consequently, the thickness of the magnetosheath between these successive crossings was observed to be very small, only about 1.4 Re. During the event, all four of the spacecraft returned magnetic field data (FGM), and two of them plasma data (CIS-HIA instruments). The purpose of our study is to investigate how the quasi-parallel bow shock was modified allowing it to be this close to the magnetopause. We are also using upstream data from the L1 point to investigate the properties of the solar wind.
SM51A-1614
Statistical Study of Bow Shock Energetic Ions
It is generally believed that Fermi acceleration frequently takes place at the Earth's quasi-parallel bow shock and foreshock regions. Ions of solar wind energy undergo energy and spatial diffusion in this process and get accelerated to hundreds of keV by upstream turbulence and waves. It has been suggested that this acceleration process is highly associated with the bow shock magnetic geometry. With RAPID direct event data from the CLUSTER spacecraft, high-energy resolution ion composition data are extracted to determine characteristics of bow shock accelerated ions. By examining the characteristic energy of bow shock diffuse ions and shock geometry, we can then test the above hypothesis and further quantify their relationship. In this presentation, we will report our findings from a statistical study of RAPID data.
SM51A-1615
Variability of the Earth's Quasi-perpendicular Bow Shock: Temporal and Spatial Scales
Using 41 crossings of the Earth's quasi-perpendicular bow shock by the four Cluster satellites, we investigate the variability of the shock, seeking to discover if this variability has characteristic temporal and spatial scales. At each shock, and for each spacecraft, magnetic field measurements, centred on the shock encounter time, are used to create a shock variability profile. This is defined as the difference between a spacecraft's individual shock profile and the shock profile averaged over all four spacecraft. Cross- correlating these variability profiles between pairs of spacecraft, we investigate the dependence of the maximum cross-correlation coefficient on the temporal and spatial separation of the spacecraft. These are defined respectively as the time between shock encounters and the separation along the shock front in the Normal Incidence Frame, found using upstream solar wind velocity measurements. We demonstrate that the profiles decorrelate in both space and time, and investigate how the scales of this decorrelation vary with shock control parameters such as MA and θBn. To investigate the physical mechanism associated with the variability, this analysis is also performed normalising the temporal and spatial separations by various time and length scales previously associated with the shock, such as the ion gyroperiod and ion inertial length.
SM51A-1616
Nonstationarity of two-dimensional supercrititical perpendicular shocks: evidence of competing mechanims
Two-dimensional (2-D) full particle electromagnetic simulations are used for analysing in detail different nonstationary behaviors of perpendicular supercritical shocks. A recent study (Hellinger et al., 2007) has evidenced that the shock front is dominated by the emission of coherent large amplitude whistler waves for some plasma conditions and shock regimes. These whistler waves are emitted in two-dimensional perpendicular shocks and inhibit the self-reformation driven by the accumulation of reflected ions: then, the shock front appears almost "quasi-stationary", a result which could seem in apparent contradiction with previous results. The present study allows to clarify the situation by bringing new complementary results: (i) there exists a transition regime around a critical Mach number threshold Mwwe, within which both self- reformation and whistler waves emission can co-exist. (ii) Below (above) this threshold regime, the self- reformation (whistler waves emission) is fully retrieved and becomes dominant. (iii) As MA is larger than Mwwe, , this shock front looks "quasi-stationary" in 1-D y-averaged fields profiles, but in fact is nonstationary in full 2-D profiles and over a smaller time scale (lower than one ion gyroperiod). Moreover, this nonstationarity is characterized by a quasi-periodic reinforcement of nonlinear waves emission from the ramp. This effect results from the fact that the emission of nonlinear whistler waves varies in time according to the local need for balancing the nonlinear effects at the shock ramp (steepening). (iv) These results are observed for a strictly perpendicular shock, as B0 is within the simulation plane; in contrast, as B0 is perpendicular to the simulation plane, no whistler waves emission is evidenced even for large Mach number; only self-reformation is observed. Present results, even if unexpected, are shown to be not in disagreement with previous 2-D PIC and 2-D hybrid simulations these are compared with.
SM51A-1617
Reformation of the Quasi-Perpendicular Bow Shock: A Comparison of Full Particle Simulations With Cluster observations
Based on Cluster magnetic field and particle data obtained during a quasi-perpendicular bow shock crossing on 24 January 2001 Lobzin et al. (GRL 34, 2007) have concluded that the shock is highly nonstationary. We have performed one-dimensional full particle simulations for parameters appropriate to the 24 January 2001 shock crossing with a realistic ion to electron mass ratio. The shock exhibits large amplitude whistler waves with downstream directed phase velocities and with a frequency close to 2Hz. The Poynting flux is directed into the upstream direction. Such high frequency fluctuations have also been observed in the Cluster magnetic field data. However, in the simulations we do not obtain lower frequency waves characteristic of a nonlinear whistler train embedded in the shock front which eventually results in shock reformation, i.e., the shock is steady and non-reforming. When putting an artificial spacecraft into the simulation which moves through the shock with a constant velocity we obtain magnetic field profiles similar to the observed ones with multiple peaks in the shock transition region. However these peaks are not related to a nonlinear whistler but are due to slight back and forth motions of the ramp/overshoot. It is concluded that either (1) the reformation process during this crossing is due to a two-dimensional effect not adequately described by the 1-D simulation or (2) this shock was not reforming and the different profiles are due to small scale motions of the shock relative to the spacecraft.
SM51A-1618
Evidence of the self-reformation of the quasi-perpendicular terrestrial shock front from CLUSTER data
Several mechanisms issued from simulation and theoretical studies are proposed to account for the nonstationarity of quasiperpendicular supercritical shocks. One process –the so-called self-reformation - driven by the accumulation of reflected ions at a foot distance from the ramp has been intensively analyzed with simulations. Present results based on experimental CLUSTER mission clearly evidence signatures of this self-reformation process for the terrestrial bow shock. The study based on magnetic field measurements includes two parts: (i) a detailed analysis of two typical shock crossings for almost perpendicular shock directions where the risk of pollution by other nonstationarity mechanisms is minimum. A special attention is drawn on non appropriate treatment of data which could lead to wrong data interpretation. One key signature of this self-reformation is that the ramp width can reach a very narrow value covering a few electron inertial lengths only; (ii) a statistical analysis based on 24 crossings made by CLUSTER (i.e. 96 shock crossings by individual satellites) evidences the signatures of this nonstationarity versus different plasma conditions and shock regimes. Present results are compared with previous works.
SM51A-1619
Electron Cyclotron Waves in the Foot of a Supercritical Shock: Parametric Analysis
Recent simulations of supercritical perpendicular shock [1] have evidenced an electron cyclotron drift instability (ECDI) within the shock's foot, which is excited by the relative drift between the beam of reflected ions and the incoming electrons. Herein that work is extended to a more realistic parameter regime (higher mass ratio and higher plasma to cyclotron frequency ratio) by means of 1D PIC simulations restricted to the foot. The much higher spatial resolution afforded by the present simulations enables us to study in exquisite detail the development of the instability. Main features are: (i) Cyclotron harmonics with high k ρe values and frequencies near the upper hybrid are excited in the linear stage, in good agreement with linear dispersion properties. (ii) As the orbits of the beam's ions in Cerenkov resonance with the waves become nonlinear, the high k modes are quenched letting lower harmonics grow. (iii) The resulting ion phase mixing, whereby the same ions resonate with different wavelengths, leads to a spectral shift from high to low k modes in a timescale on the order of the trapping time. That dynamics (which is distinct from vortex coalescence related to the inverse cascade process) leads to an accumulation of spectral power near the lowest cyclotron harmonics and a significant heating of the electrons. The dynamics is analysed for different ion beam drifts that represents different supercritical yet moderate Mach number regimes of shocks. [1] Muschietti L. and B. Lembege, Adv. Space Res. 37, 483 (2006)
SM51A-1620
Statistical Properties and Stability of Electrostatic Solitary Waves Observed at Shock Crossings by the CLUSTER and CASSINI Spacecraft
Nonlinear Electrostatic Solitary Waves (ESWs) are kinetic-scale structures which have been frequently observed in the bow shock transition region of Earth by the GEOTAIL and WIND spacecraft, as well as more recently by the four CLUSTER spacecraft. In addition, ESWs have also been reported to be present at the bow shock of Saturn, as well as at a crossing of an Interplanetary Shock, through observations by the CASSINI spacecraft. The characterization of these ESWs through statistics provides an insight into the state of the plasma in this region and a standard by which to compare the characteristics of ESWs observed through models and simulations. We provide the statistics of ESW amplitude and time duration for several quasi-perpendicular and quasi-parallel bow shock crossings by the CLUSTER spacecraft and compare these to the same parameters for the Saturn Bow Shock and Interplanetary Shock crossings. We also evaluate the antenna angle of detection with respect to the background magnetic field for these same crossings in order to further shed light on whether electron or ion dynamics are likely to dominate the process of ESW generation. We end our discussion of ESW characteristics by investigating the stability of ESWs through their propagation from one CLUSTER spacecraft to another.
SM51A-1621
Mirror-Mode Instability versus L-Mode Electromagnetic Ion Cyclotron Instability: Comparison of 2-D and 3-D Simulations
Spacecraft observations show that the mirror instability dominates over the L-mode electromagnetic ion cyclotron (EMIC) instability in the terrestrial magnetosheath, although the theoretical linear growth rate of the L-mode EMIC wave is higher than that of the mirror mode waves. This has been a long-standing puzzle. To analyze the competing processes between the L-mode instability and the mirror instability, we have performed multi-dimensional hybrid simulations, assuming anisotropic energetic ions. In the 2D model, the energy of the L-mode wave is higher at the initial stage in a good agreement with the linear theory. However, in the 3D simulation, the mirror mode wave can gain much higher amplitude than the L-mode EMIC wave in both linear and nonlinear stages. Results issued from the 3D model evidence a torus-shape emission in the energy spectra (not accessible in the 2D model). The growth of the L-mode EMIC waves declines earlier in the 3D model than that in the 2D model, due to efficient proton scattering by the mirror mode waves. To understand the nonlinear processes, we have performed 2D and 3D hybrid simulations with higher spatial resolutions. The L-mode EMIC waves are subject to inverse-cascade in the 2D model, while this is not the case in the 3D model, so hence it vanishes. We analyzed the inverse-cascade of the L-mode EMIC waves. We also find that the nonlinear evolution of the mirror waves in the 3D model is significantly different from that in the 2D model. Although coalescence of the mirror mode structures takes place in both models, coalescence in the 3D case is much more rapid than 2D. The transfer of energy between electromagnetic fields and ions (heating) is analyzed both in linear and nonlinear stages.
SM51A-1622
Mirror Instability: Quasi-linear Effects
Nonlinear properties of the mirror instability are investigated by direct integration of the quasi-linear diffusion equation [Shapiro and Shevchenko, 1964] near threshold. The simulation results are compared to the results of standard hybrid simulations [Califano et al., 2008] and discussed in the context of the nonlinear dynamical model by Kuznetsov et al. [2007]. References: Califano, F., P. Hellinger, E. Kuznetsov, T. Passot, P. L. Sulem, and P. M. Travnicek (2008), Nonlinear mirror mode dynamics: Simulations and modeling, J. Geophys. Res., 113, A08219, doi:10.1029/2007JA012898. Kuznetsov, E., T. Passot and P. L. Sulem (2007), Dynamical model for nonlinear mirror modes near threshold, Phys. Rev. Lett., 98, 235003 . Shapiro, V. D., and V. I. Shevchenko (1964), Quasilinear theory of instability of a plasma with an anisotropic ion velocity distribution, Sov. JETP, 18, 1109.
SM51A-1623
Simulations of Solar Ion Motion in the Magnetosheath
We investigate the transport of representative solar-wind ions through the Earth's magnetosheath by tracing their trajectories. We use an analytical model, based on the draping of plasma streamlines and magnetic field lines around a conducting magnetopause that consists of a prolate ellipsoid, extending ~ 11 RE upstream and ~ 65 RE downstream from Earth, matched to a cylinder of radius ~ 28.4 RE. For a uniform but arbitrarily directed interplanetary magnetic field (IMF), the magnetosheath's magnetic field can be obtained by superposition of results for special cases in which the IMF is respectively parallel to and perpendicular to the solar-wind velocity. For the case in which the IMF is parallel to the solar-wind velocity, the magnetosheath's magnetic field is derivable from Euler potentials that can be expressed analytically as a function of ellipsoidal or cylindrical coordinates that scale inversely with the sixth root of solar-wind pressure. When the IMF is perpendicular to the solar-wind velocity, we can calculate the magnetosheath flow velocity from Bernoulli's equation and compute the time delay for the magnetosheath plasma to have traveled to any point of interest from the bow shock. Points along a magnetosheath field line are points of equal time delay from an upstream field line. Using the particle trajectories, we map phase space distributions in the magnetosheath from an upstream distribution. Taking moments of the phase space distributions, we compute simulated ion density, bulk velocity and temperature at various locations within the magnetosheath for simple IMF configurations.
SM51A-1624
Properties of high-energy electrons at dawnside magnetosheath: Cassini observations during the earth swing-by 1999
We report on the energetic electrons of the magnetospheric origin at the night side magnetosheath using the Low-Energy Magnetospheric Measurement System (LEMMS) aboard the Cassini spacecraft during the Earth swing-by maneuver on 17 August 1999. LEMMS is able to identify the energy and incidence direction of energetic ions and electrons with energies of a few tens of keV to several tens of MeV. The spacecraft was traveling with a velocity of 9 Earth radii (RE) per hour and made rapid transversals of the terrestrial magnetosphere. The spacecraft was outbound over the dawn magnetopause at approximately 10:50 UT at the last time, investigating through the magnetosheath afterward. LEMMS identified 6 electron bursts ranging at least up to 100 keV from X = -70 to -95 RE. These electron streams were away from the general magnetopause position along the magnetic field line. Those features were consistent with former works on the energetic particles in the magnetosheath [e.g. Sarafopoulos et al., 1999]. In this study, taking advantage of the uniqueness of the position in the distant dawnside magnetosheath and the fast snapshot during the swing-by, we discuss the source of these electrons and probability of the event compared to the ion bursts.
SM51A-1625
Lion roar emissions observed by the CLUSTER and THEMIS spacecraft
Lion roars are intense, narrow-band whistler-mode emissions sporadically
occurring in the Earth's magnetosheath. We present a statistical study
based on the CLUSTER data recorded during years 2001 and 2005. We have
used the spectral analyzer STAFF-SA which is processing signals from
three magnetic and two electric sensors. Frequency and power
properties of lion roars have been investigated and generally
correspond to previous results. We have also studied their spatial
distribution. We have compared the field-aligned component of the
Poynting flux and the local magnetic field in order to predict the
source region. Our result shows that lion roars are mainly propagating
toward the Earth. We thus expect that the source region is more often
close to the bow shock. The SCM and EFI instruments on-board THEMIS
provide measurements of all six components of the magnetic and
electric field. It allows us to investigate properties of lion roar
emissions. We present case studies of selected events.
http://os.matfyz.cz/papers/agu2008/
SM51A-1626
How common is reconnection in the magnetosheath?
Downstream a parallel bow-shock the magnetosheath is one of the most turbulent regions in the magnetosphere. In this environment thin current sheets are formed, and Retinó et al. (2007) found evidence that magnetic reconnection occurs in these layers. The width of a typical current sheet is of the order of an ion gyroradius but the reconnection signatures are the same as for large-scale reconnection at the magnetopause or in the solar wind. We perform a small statistical study to determine the probability that reconnection actually takes place given a thin current sheet in a turbulent plasma. If reconnection turns out to be common in the magnetosheath, we expect it to be a common process in turbulent plasmas throughout the universe.
SM51A-1627
Magnetosheath Plasma Heating near the Earth's Magnetopause
With the help of the four CLUSTER spacecraft we investigate isotropization and heating mechanism of plasma confined inside magnetic holes in the magnetosheath near the magnetopause current sheet. We show that the isotropization can be caused by scattering on plasma waves inside the holes. We also identify a transverse MHD Alfvén wave enveloping the holes. We revealed that the holes were shrinking and expanding with the periodicity of the Alfvén wave. We suggest that the slow shrinking-expansion of the holes together with the continuous fast plasma isotropization inside them resembles a similar mechanism to the collisional heating of confined plasma by oscillating electromagnetic fields.
SM51A-1628
Gyrokinetic Particle Simulation of Nonlinear Saturation of Mirror Instability
Low frequency compressible electromagnetic mirror modes driven by temperature anisotropy in high-beta
plasmas have been observed by satellites in space plasmas, such as planetary and cometary
magnetosheaths. In this work, mechanism of the nonlinear saturation of the mirror instability is studied using
the gyrokinetic particle simulation. Phase-space particle trapping due to the nonlinear mirror force is found to
be the dominant saturation mechanism in the simulation of a single mirror mode with relatively weak drive
[Nonlinear Saturation of Mirror Instability, H. Qu, Z. Lin, and L. Chen, Geophy. Res. Lett. 35, L10108 (2008)].
At the nonlinear saturation, the phase-space island of the distribution function is formed. The oscillation
frequency of the saturated perturbation amplitude is close to the bounce frequency of the trapped particles,
which is comparable to the linear growth rate of the mirror mode. Scaling of the saturation amplitude is
consistent with the onset of the particle trapping. With strong instability drive, relaxation toward marginal
stability dominates the nonlinear saturation of the mirror instability. Phase-space trapping, however, persists
after the saturation and continues to regulate the nonlinear evolution of the mirror mode. Applications of the
gyrokinetic particle simulation for the studies of nonlinear kinetic processes in space plasmas, such as solar
wind heating by Alfvenic turbulence and excitation of low frequency drift compressible modes, will also be
discussed. This work is supported by an NSF CAREER Award.
http://gk.ps.uci.edu/zlin/bib/qu08b.pdf
SM51A-1629
Thoughts on Three Dimensional Asymmetric Model of the Magnetopause
Considering the asymmetries of the global structure of the magnetopause and indentations nearby the cusps, a new three asymmetric global model of the magnetopause is presented in this talk. Based on 1249 global magnetopause crossings from Geotail, IMP8, Interball, TC1, THEMIS, Wind, Cluster, Polar, LANL and GOES, and corresponding upstream solar wind conditions from ACE or WIND with 5-minutes, this new model is parameterized by solar wind dynamic pressure and magnetic pressure, interplanetary magnetic field (IMF) BZ, and Earth's dipole tilt. These control parameters are selected on the basis of the decrease of the standard deviation by nonlinear multi-parameter fitting over different regions: the subsolar region, low-latitude region, middle-high-latitude region, region for the asymmetries fitting, region for the indentations fitting, and the global region. It is demonstrated that this model can be used to quantitatively study how DP+BP compresses the magnetopause, how IMF BZ erodes the magnetopause, how large the asymmetries of the magnetopause are, and how the Earth's dipole tilt influences the indentations and the asymmetries of the global magnetopause. In addition, the extrapolation for the distant tail magnetopause and for the extreme solar wind conditions is also considered for modeling magnetopause size and shape. Comparison with previous models implies that the standard deviation and the absolute deviation of the new model are obviously decreased. It is shown that this new model is not only appropriate to predict the magnetopause size and shape for the various solar wind conditions, but also can provide a reasonable estimation of the distant tail magnetopause shape.
SM51A-1630
The Sheath Transport Observer for the Redistribution of Mass (STORM) Imager
All of the solar wind energy that powers magnetospheric processes passes through the magnetosheath and magnetopause. Global images of the magnetosheath and magnetopause boundary layers will resolve longstanding controversies surrounding fundamental phenomena that occur at the magnetopause and provide information needed to improve operational space weather models. Recent developments showing that soft X-rays (0.15-1 keV) result from high charge state solar wind ions undergoing charge exchange recombination through collisions with exospheric neutral atoms has led to the realization that soft X-ray imaging can provide global maps of the high-density shocked solar wind within the magnetosheath and cusps, regions lying between the lower density solar wind and magnetosphere. We discuss an instrument concept called the Sheath Transport Observer for the Redistribution of Mass (STORM), an X-ray imager suitable for simultaneously imaging the dayside magnetosheath, the magnetopause boundary layers, and the cusps.
SM51A-1631
FTE signatures at the dayside magnetopause
Statistical studies reveal that FTEs observed on the flanks of the magnetosphere are equally common during periods of northward and southward IMF orientation. Since the events are attributed to reconnection along extended tilted lines passing through the subsolar magnetopause, a question arises as to why FTEs are so infrequently observed during periods of northward IMF orientation on the dayside magnetopause. We present an analytical model for the magnetic field perturbations generated by FTEs with elliptical cross sections moving slowly relative to flow velocities in the surrounding magnetosheath and magnetospheric media. Model results indicate that FTE perturbations in both regions diminish rapidly as the shear between the magnetosheath and magnetospheric magnetic fields decreases, and that the ratio of event amplitudes in the magnetosheath to those in the magnetosphere diminishes rapidly as the ratio of magnetospheric to magnetosheath magnetic field strengths increases. Consequently, magnetosheath events occurring for southward IMF orientations should dominate statistical surveys of the dayside magnetopause.
SM51A-1632
Flux transfer events probed by multiple THEMIS spacecraft
Transient bipolar perturbations of the magnetic field normal component are frequently encountered on the dayside magnetopause. Often the field magnitude increases to a peak at the center of the bipolar normal signatures. In the literature such signatures are identified as ¡®flux transfer events' (FTEs). In some cases the field magnitude increase shows a crater-like dimple in the center and is referred to as a ¡®crater FTE'. Behind the phenomenological definitions, there lie two physical interpretations: a typical FTE is a flux rope with strong core field compressed by the curvature force of surrounding twisted fields; a crater FTE represents a flux rope with a weak core field that through the presence of enhanced plasma pressure at the core of the flux rope balances the inward forces deriving from the magnetic tension of the surrounding twisted fields. It has been suggested that the signature of a crater FTE, identified in single spacecraft data, could be mimicked if the spacecraft trajectory passed through a depression on the magnetopause moving from the magnetosphere to the magnetosheath and then back to the magnetosphere. Only observations from multiple spacecraft with appropriately distributed impact parameters can distinguish a passage through a crater FTE from a passage through a perturbed magnetopause. In this study, we survey all the data recorded by multiple THEMIS spacecraft near the dayside magnetopause from May to October in 2007. We find that typical FTEs with central peaks are commonly embedded in depressions of the magnetopause. We also present several crater FTE events in which the flux rope structures with weak core fields are confirmed by multiple THEMIS spacecraft observations. We notice that, in the time interval examined, the crater FTEs are observed much less frequently than typical FTEs. The infrequent encounters with crater FTEs can be understood if they are finite in the axial direction and if non-uniform curvature stress generates field-aligned pressure gradients and associated flow along the axis and out of the flux rope. With this interpretation, the crater FTE would not be a steady structure and it would eventually evolve into a typical one.
SM51A-1633
Multi-Satellite Observations at Different Scales of Magnetic Reconnection at the Magnetopause.
Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small regions. We report on detailed observations at the magnetopause of reconnection separatrix regions at electron and ion scales. Important features include currents, strong electric fields, potential drops and associated particle acceleration. The features at small scales lead to effects at larger (fluid) scales, including energy conversion from magnetic to particle energy. Preliminary results from the four Cluster spacecraft at different simultaneous separations (about 40 km and 8000 km, i. e. the electron/ion and fluid scales, respectively) will be presented.