SM51C-0379 0800h
Alfven Vortex Filament-Like Structures Observed by Cluster Downstream the Bow-Shock
In the downstream region of the perpendicular bow-shock, localised coherent magnetic structures are observed. Using the four Cluster satellites we show that the structures are localized in time and space and that they are transported by the plasma flow. We identify these structures as field-aligned cylindrical currents almost in magnetostatic equilibrium but with a vortical structure, Alfv\'en vortex filaments. Analysing a set of such Alfv\'en filaments we find that they are distributed quasi-periodically in the plane perpendicular to an average magnetic field. Investigating several shock crossings, we show that the characteristic scale of a filament depends on the distance to the shock front.
SM51C-0380 0800h
X-Ray Emission from the Terrestrial Magnetosheath
X-rays are generated throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and geocoronal neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. A model has been created to simulate this X-ray radiation. Previously simulated images were created as seen from an observation point outside the geocorona. The locations of the bow shock and magnetopause were evident in these images. The cusps, however, were not taken into account in the model. We have now used dynamic three-dimensional simulations of the solar wind, magnetosheath and magnetosphere that were performed by the CCMC at Goddard Space Flight Center for the March 31st , 2001 geomagnetic storm. We have generated a sky map of the expected X-Ray emissions as would have been seen by an observer at the IMAGE space craft location at that time. We have also generated images as seen from an observation point well outside the geocorona. In both cases the presence of the cusps can clearly be observed.
SM51C-0381 0800h
Study of Cusp Structure and Wave Activity in Cusp Regions
Several cusp crossing events observed by Cluster spacecraft are analyzed to study the structure of the cusp region and wave activity in the region. Signatures of magnetic field and plasma parameters show that during a cusp crossing event, the spacecraft encounter plasmas from the following regions: (1) high latitude magnetopause boundary layer, which separate the magnetosheath and the magnetosphere; (2) lobe/cusp boundary layer; (3) the cusp proper; and (4) a closed field line region where keV electrons and enhanced energetic particles (30 - 1500 keV for ions and 20 - 400 keV for electrons) are observed. Along the above Cluster orbits, the fourth region is usually observed between the magnetopause boundary layer and the cusp. These observations provide an overall picture of the cusp structure along the Cluster trajectories. Magnetic field and plasma fluctuations (ULF and VLF waves) are mostly observed in the magnetopause boundary layer and in the cusp and lobe/cusp boundary layer, accompanying strong plasma flow/beam bursts. Calculation of the wave Poyting flux and kinetic energy flux will be presented to examined the energy flow in these regions. Preliminary results show the Poynting flux in the cusp is toward the ionosphere, indicating that ULF wave activity in the cusp is one of the sources of energy that transferred into the magnetosphere.
SM51C-0382 0800h
Ion Accelerating by Turbulent Electric Fields in High-Altitude Cusp
The high-altitude dayside cusp is an extremely dynamic region in geospace. The cusp energetic particles (CEP) have been observed there, showing orders of magnitude increase of ion intensities with energies from 20 keV up to 10 MeV. Associated with these charged particles are large diamagnetic cavities and strong electromagnetic turbulence. Turbulent cusp electrical field with an amplitude up to 120 mV/m has been detected. Dominated by its perpendicular components, the cusp electric field spectral density for fluctuations in the ultra-low frequency (ULF) range shows orders of magnitude increase. Both left- and right-hand polarizations have been measured. The ULF range covers the cusp ion gyrofrequencies. The cusp ions can be energized by both resonant and stochastic acceleration mechanisms.
SM51C-0383 0800h
Comparison between ionospheric convection vortices and the associated equivalent currents
The equivalent current pattern derived from CANOPUS, NRCAN/GSC and MACCS magnetometers has been compared with the ionospheric convection pattern observed by SuperDARN HF radars. The discrepancies between the equivalent convection (EQC) and the SuperDARN-observed convection (SDC) patterns are explained in terms of the effect of day-night photoionization conductance gradient and the coupling between field-aligned currents (FACs) and ionospheric conductances. In particular, the agreement between the EQC and SDC patterns is usually worse for a counterclockwise convection vortex than for a clockwise cell, but a consistent pattern of discrepancy for counterclockwise convection vortices has been found. We suggest that the discrepancies are due to a downward FAC-conductance coupling process. Since the counterclockwise vortices and clockwise vortices occur predominantly in the dawn and dusk sectors, respectively, in accordance with the usual 2-cell global convection pattern, the asymmetry between the EQC and SDC patterns for counterclockwise vortices and clockwise vortices would naturally lead to a dawn-dusk asymmetry as well. This is revealed by a global statistical study of the deviation of direction between the magnetic equivalent convection and the SuperDARN convection in different time sectors and latitudes. In the dawn sector, the statistical results reveal that, at lower latitudes, the EQC direction deviation is slightly counterclockwise with respect to the SDC direction, whereas the deviation is significantly clockwise at high latitudes. These deviations are consistent with the discrepancy pattern for counterclockwise convection vortices, as found in the individual vortex event studies.
SM51C-0384 0800h
Near Real Time Measurement of the Magnetospheric Reconnection Rate Using the Super Dual Auroral Radar Network
We present a technique to calculate the magnetic reconnection rate between the interplanetary magnetic field and the magnetosphere with a delay of approximately 10 minutes from real time. This technique is based on remote sensing of the F-region ionospheric plasma velocity using High Frequency (HF) radar. By using the Super Dual Auroral Radar Network (SuperDARN), this technique allows us to measure the reconnection rate along the entire dayside separatrix (0600 - 1800 MLT). The magnetic separatrix is identified as the ellipse that best separates high spectral width (>150 m/s) backscatter, indicative of open magnetic field lines, from low spectral width backscatter. A figure of merit indicates the confidence in this identification and the resulting reconnection rate measurement. The electric field on the separatrix is determined from the best fit of the line-of-sight F-region plasma velocity to an eighth-order spherical harmonic function of ionospheric electrical potential. The reconnection rate is determined from the electric field component along the separatrix in the separatrix rest frame. We also present examples of reconnection rate measurements using this technique.
SM51C-0385 0800h
Kinetic Aspects During a Quasi-Continuous Reconnection Event Tailward of the Cusp: Cluster CIS Observations.
During a period of mainly northward interplanetary magnetic field, a reconnection event tailward of the cusp, lasting almost continuously during four hours, was observed by CIS onboard three Cluster spacecraft. Tests on reconnection and considerations on the global reconnection configuration during this event have been the object of a previous study (Retin\`o et al., submitted to Ann. Geophys. 2004). In the present study kinetic aspects related to reconnection will be presented. The detailed distribution functions of protons and oxygen ions close to the magnetopause will be presented and their characteristics will be discussed.
SM51C-0386 0800h
Statistical Survey of the Properties of Gyrating Ions in the Earth's Foreshock
Gyrating backstreaming ions displaying gyrophase-bunching around the local magnetic field are frequently observed upstream of the Earth's bow shock. These ions are always associated with quasi-monochromatic right-hand mode low-frequency waves. Previously some case studies have been made to explain these ion features. The proposed mechanism is either a non-local production by a specular reflexion at the bow shock itself or a local nonlinear wave- particle interaction involving initially field-aligned beam ions and the waves. From a much larger data set now available, the physical properties of the gyrophase-bunched ions can be determined by sophisticated mathematical methods allowing to fit distributions without the maxwellian assumption. These derived properties are used to test and discriminate the two possible production mechanisms proposed. A strong emphasis is put on the association with field-aligned beams given from multi-spacecraft observations by Cluster.
SM51C-0387 0800h
Intermittent Reconnection, Flux Ropes and Vortices Generation at the Dayside Magnetopause
In the presence of a nonzero IMF $B_y$ component and of southward IMF $B_z$, magnetic neutral points are formed near the flanks of the magnetosphere. The relative role of almost anti-parallel merging near neutral points vs. component reconnection at the subsolar stagnation point is a matter of ongoing discussions. To address this problem we employ global MHD model BATSRUS with adaptive mesh refinement. We perform simulations of magnetopause dynamics after IMF turning from an initial northward orientation to IMF clock angles $90 < \theta < 180$. An analysis of the reconnected magnetic flux budget showed that reconnection occurs along the extended area at magnetopause surface. We found that magnetopause surface become unstable and demonstrated formation of pressure bubbles, flux ropes (FTEs), and traveling density depletion regions. Vortices generation in the vicinity on magnetic neutral points at the flanks is also demonstrated. We discuss how these results depend on grid resolution.
SM51C-0388 0800h
A Survey of the Motion of Flux Transfer Events Observed by Polar
We present in this paper a survey of the motion of flux transfer events using high resolution magnetic and electric field data from the Polar spacecraft. We have surveyed the magnetic field data during the three-year period from 2001 to 2003 when Polar skimmed the dayside magnetopause near its apogee and identified 67 encounters of the dayside magnetopause with evidence of flux transfer events (FTEs). For each FTE identified, we determine the local structure of the flux tube (including the helical magnetic structure, the axial orientation and the intensity of the 'core' field.) and its E $\times$ B convection velocity. We investigate how the orientation and motion of flux tubes depend on the IMF (especially the By component) and the location on the magnetopause. The results will provide information on the nature of time-varying reconnection that is responsible for the generation of FTEs, especially the location and orientation of the X-line. Our statistical results of FTEs will be discussed in terms of the above perspectives.
SM51C-0389 0800h
Electron Cyclotron Microinstability in the Foot of a Perpendicular Shock: a Self-consistent PIC Simulation
A few decades ago it was argued that the ions which are reflected in perpendicular supercritical shocks might excite various plasma microinstabilities within the shock's foot. Only now, thanks to advances in computational power, are we able to investigate them in a truly self-consistent way, where the instabilities are embedded within the larger frame of an evolving shock structure. With the help of a newly developed Darwin PIC code, we have identified an electron cyclotron microinstability that occurs during the reformation phase of strictly perpendicular shocks at low $\beta_i$. The instability involves the beam of reflected ions and the ambient electrons. It takes place within the foot itself, exhibits a rapid growth, and propagates along the shock normal towards upstream. Its frequency is comparable to the electron cyclotron frequency and its wavelength shorter than the electron inertia length. The instability basically results from the coupling of electron Bernstein waves with an ion beam mode carried by the reflected ions, whereby we identify it as a variety of the electron cyclotron drift instability [e.g. {\it Forslund et al.}, 1970]. A dispersion analysis is presented. We discuss the effects of varying parameters, in particular as the fake ion-to-electron mass ratio used in the simulations converges to more realistic values. The connection with the Buneman instability reported in the much higher Mach simulations of [{\it Shimada and Hoshino}, 2000] is outlined.
SM51C-0390 0800h
On the Generation of Enhanced Sunward Convection and Transpolar Aurora in the High-Latitude Ionosphere by Magnetic Merging
The IMAGE SI-12 instrument indicates a region of diffuse aurora poleward of the duskside Northern Hemisphere oval, while the IMAGE WIC instrument observes a transpolar auroral (TPA) feature at the polar cap boundary of the diffuse aurora during much of the 0110 UT to 0445 UT time interval on 16 December 2001. We here focus on the 0302 UT to 0312 UT period when the SuperDARN convection data display enhanced 800-1100 m/s sunward directed ionospheric flows near the TPA region in a four-degree wide region centered at 81$^{\circ}$ magnetic latitude between 14 MLT and 16 MLT. This flow channel seemingly drives a single dayside lobe cell with a convection reversal boundary near 84$^{\circ}$ and 14 MLT. At 0300 UT, the Cluster C1 S/C traverses the Northern Hemisphere duskside flank magnetopause at [X,Y,Z]=[-1.2,9.2,9.1] $R_{E}$ (GSM) where it encounters a localized density depletion region and a clear deflection of the bulk plasma velocity relative to the observed magnetosheath flow. The magnetic field and plasma velocity observed by C1 satisfy the Walen stress balance relation for a rotational discontinuity and the corresponding flow deflection dV suggests a merging site poleward and tailward of the spacecraft. The IMF magnitude as measured by ACE is 18 nT with a steady and favorable 45$^{\circ}$ clock angle (positive By and Bz) direction for merging in the vicinity of these duskside regions. The Tsyganenko T01 model maps the Cluster position to 75$^{\circ}$ and 14 MLT at 0130 UT or a few degrees equatorward of the ionospheric flow channel. Based on these data sets, we examine whether the active merging region on the magnetopause is consistent with the enhanced flow signatures observed by SuperDARN on 16 December 2001 and the IMAGE observations in these duskside high-latitude regions. An MHD simulation of this event puts these observations into a global context and is further used to map the Cluster location at the time of the plasma flow deflections to the ionosphere.
SM51C-0391 0800h
Computing the Reconnection Rate at the Earth's Magnetopause Using Two Spacecraft Observations
A new multi-spacecraft technique is introduced which, under some restrictive assumptions and conditions, provides a snapshot of the reconnection inflow velocity into the magnetosphere and an estimate of the distance from the spacecraft to the reconnection site. The two quantities are not obtained independent of one another and additional, independent information is needed to separate them. This new technique is applied to Cluster spacecraft observations at the Earth's magnetopause. Additional Cluster observations and observations from the IMAGE spacecraft are used as independent information to provide an estimate of the distance from the spacecraft to the reconnection site for the event. From this distance estimate and the new multi-spacecraft technique, it is concluded that component reconnection was probably occurring at the magnetopause and that the local inflow velocity was significantly less than 0.1 of the local Alfven speed.
SM51C-0392 0800h
Successive crossings of magnetic island structure at dusk side magnetopause observed by a single spacecraft
In a recent paper we reported the observational evidence for magnetic island structures at dawn and dusk side magnetopause based on the analysis of AMPTE/IRM plasma and magnetic field data and solving the Grad-Shafranov equation as a spatial initial value problem [Teh and Hau, EPS, 56, 681, 2004]. The dusk side event occurring on August 8, 1985 displays great similarities with the dawn side event of October 19, 1984 in that the reconstructed two-dimensional magnetic field maps involve a series of plasmoids with island width of about 500 km separated by the X lines. Both events are associated with multiple magnetopause crossings and occur when the in-and-out magnetopause motion finally subsides. We have recently found that one of the thirteen crossings prior to the reported dusk event also shows the unique feature of a series of pearl-like magnetic islands imbedded within the magnetopause current layer. The two successive crossings have nearly the same magneotopause normal and deHoffmann-Teller velocity and also with the same island width and aspect ratio.
SM51C-0393 0800h
Ionospheric Signatures of Plasma Injections in the Cusp Triggered by Solar Wind Pressure Pulses
During sporadic reconnection events (flux transfer events or FTEs) at the dayside magnetopause, magnetosheath plasma enters the magnetosphere along cusp field lines. It is expected that these enhanced parallel plasma flows occur in conjunction with enhanced ionospheric convection events driven by the magnetic tension at the reconnection site. Associated optical auroral emissions result from enhanced precipitation of the magnetosheath plasma. If such events have long been recognised as due to IMF variations, the triggering role of solar wind pressure pulses is not definitely established. We analyse coordinated observations made on July 14, 2001 simultaneously in the mid-altitude cusp by Cluster and at the ionospheric magnetic footprint by SuperDARN and IMAGE during a period of three successive solar wind dynamic pressure pulses. In association with each of these pulses, Cluster observes plasma injections while auroral images from the IMAGE spacecraft show enhanced precipitation in the cusp. Following these plasma injections, convection flow channels are observed in the ionosphere by the SuperDARN radars. Based on the spatial and temporal relation between these various signatures, a description of the response of the dayside magnetosphere to the pressure pulses is proposed. The main considerations involved in this description are: (1) the solar wind dynamic pressure pulses are the drivers of plasma injections from the magnetosheath. (2) The ionospheric convection bursts start shortly after the auroral intensifications and their duration is much longer (10 min as against 4 to 6 min for the auroral intensifications). (3) The convection bursts do not occur at the same latitude as the precipitation, but on the poleward side of the cusp precipitation. (4) Alfven waves are responsible of the transmission of the magnetic stress from the reconnection site to the ionosphere where they are strongly attenuated by reflection in the upper ionosphere. This set of observations demonstrates that the convection bursts are a "fossil" signature of the compression-injection process as it is also the case for reconnection at the dayside magnetopause driven by the IMF alone.
SM51C-0394 0800h
CLUSTER-II Observations of Mid-altitude Polar Cleft Turbulence
A study of mid-altitude polar cleft turbulence observed by CLUSTER-II spacecraft is presented. Using CLUSTER-II dayside perigee passes during summer 2001, the multi-instrument capabilities of the mission allows a detailed investigation of particles and waves commonly observed in regions adjacent to the polar cusp. It is found that in the ULF frequency range, kinetic Alfven waves associated with filamentary currents are identified in these regions. Their origin as well as their effect on the local plasma in terms of acceleration and/or heating of electrons and ions is investigated.
SM51C-0395 0800h
Fractal Reconnection at the Earth's Magnetopause and Associated Ionospheric Convection.
Large-scale properties of reconnection structures on the magnetopause can be explained successfully by simple models incorporating laminar magnetosheath flow with antiparallel reconnection. However, such models are inconsistent with the highly turbulent nature of the magnetosheath flow adjacent to the magnetopause. This presentation proposes a fractal reconnection model that resolves this contradiction by replacing the laminar magnetosheath flow with a turbulent flow which has realistic levels of fluctuation. The resultant fractal reconnection structures preserve the large-scale behaviour of simpler models, consistent with ground-based observations, but have small-scale fluctuations consistent with those observed in situ by spacecraft. We also present observations of evidence for scale-free fluctuations in ionospheric convection associated with dayside reconnection. This scale free behaviour may well arise as a consequence of the fractal reconnection model proposed.
SM51C-0396 0800h
Statistical Analysis of Flux Transfer Events Simultaneously Observed by ISEE-1 and ISEE-2
Flux transfer events (FTEs) are transient phenomena (duration: a few to several minutes) observed near the magnetopause, characterized by bipolar perturbations in the magnetic field component perpendicular to the magnetopause ($B_N$ below). FTEs are generally thought to be the results of transient reconnections at the magnetopause. There exist many reports in literature on FTEs. However, so far there exist only limited number of reports doing statistical analyses on the directions of motions of FTEs, because one satellite is not sufficient to clarify the spatial structures and the directions of motions of FTEs. To address the directions of motions of FTEs, in this paper we present a statistical analysis of FTEs simultaneously observed by two satellites ISEE-1 and ISEE-2, which made observations for continuous ten years (1978-1987). Two-point observation does not 100% resolve the direction of motion; we cover it by large-scale statistics ($>$600 events) of the ten-years data of ISEE. For each FTE we have determined the time lag from ISEE-1 to \mbox{ISEE-2} as that which maximizes the correlation function between $B_N$'s at ISEE-1 and ISEE-2. We have then examined the time lags as a function of the spatial distance from ISEE-1 to ISEE-2. The results include the following. The time lag tends to be proportional to the longitudinal distance, consistent with east-west motions away from the subsolar point. On the other hand, the time lag shows little correlation with the latitudinal distance. The significant east-west motion suggests that FTEs are unlikely to have a structure whose longitudinal scale is much longer than its latitudinal scale; a flux tube structure is more likely for FTEs than a long-X-line type structure.
SM51C-0397 0800h
Current Structure and Motion of a Northward IMF X-Line
On March 18, 2002, between 14:50 and 15:03 UT, Cluster passed from the tail lobe northward and sunward into the magnetosheath. The IMAGE FUV instrument observed a proton emission, northward of the auroral zone, that endured for a period of hours, including the time of the Cluster crossing. The location is consistent with the footprint of a northward IMF reconnection site (Frey, et. al.). Phan, et. al., mapped the location of cluster during its magnetopause crossing to the location of the ionsopheric footprint observed by IMAGE. We argue from Cluster PEACE electron and magnetic field data that the Cluster spacecraft passed very closely through an active reconnection site, with the x-line actually passing through the centroid of the Cluster. This proximity allows us to infer the current structure, velocity, orientation, and size of the x-line. The magnetic fields experience minima in the GSE x-z plane that are consistent with an x- line along the y-direction. Meanwhile, near these minima, the y-component of the magnetic field undergoes a large enhancement. The PEACE electron moments, distributions, and pitch angle spectra give evidence of magnetic shear current at the reconnection site, as well as perpendicular currents that bundle an enhanced magnetic field along the reconnection line. We find the spatial derivatives and curlometer shear current at the x-line through a least squares estimate at the centroid of the Cluster spacecraft. The x-line current from the curl of the Cluster measurements is consistent with that found from the PEACE moments on the four spacecraft, emphasizing its quasiuniformity on 40-50 km size scales. We map the position of the x-line at the points of closest approach to each spacecraft through inversion of a low-order expansion for the magnetic field near the x-line. From the estimated locations and the measured times, we develop a least squares fit to the x-line velocity. These estimates of x-line location also fit a functional form Bx(z) and Bz(x) for the x-line that allows us to measure its motion and location at times when Cluster is further away. The functional fit to the data also gives an estimate of the thickness of the shear current sheet. The x-line earth and IMF fields are orientated at a large angle (~65) from the GSE x-direction and enhanced in the By direction, consistent with antiparallel reconnection, but with a twist. We propose that the x-line, though it does waver across the spacecraft several times, is largely stable over the times of Cluster observations.