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| Spectral scaling in the turbulent Earth's plasma sheet revisited |
| Bursty bulk flow associated magnetic fluctuations exhibit at least three spectral scaling ranges in the Earth's plasma sheet. Two of the three scaling ranges can be associated with multi-scale magnetohydrodynamic turbulence between the spatial scales from ~100 km to several RE (RE is the Earth's radius). These scales include the inertial range and below ~0.5 RE a steepened scaling range, theoretically not fully understood yet. It is shown that, in the near-Earth plasma sheet, the inertial range can be robustly identified only if multi-scale quasi stationary (MSQS) data intervals are selected. Multiple bursty flow associated magnetic fluctuations, however, exhibit 1/f type scaling indicating that large-scale fluctuations are controlled by multiple uncorrelated driving sources of the bulk flows (e.g. magnetic reconnection, instabilities). |
| Publication date: 24 Sep 2007 |
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| Plasma sheet fine structure at high latitude |
| High-resolution ion observations made in recent years, by the TIMAS instrument on the Polar satellite and other instruments, reveal a dynamic and finely structured plasma sheet, at least at high latitude. This study invokes multipoint Cluster observations with the CIS CODIF instruments (ion composition and distribution function) to determine whether transverse density gradients can be of the order of keV proton gyroradii scale size, as suggested by the TIMAS observations. It is shown that the plasma sheet is indeed prominently filamentary and that the proton density with 40 eV <= E <= 40 keV can vary by Delta n = 0.4 cm-3 across less than five average proton gyroradii at R ~ 5 RE (average E ~ 7.5 keV at the time). This compares favorably with typical 10-km-size (or less) auroral structures when projected earthward. |
| Publication date: 21 Sep 2007 |
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| Electron dynamics and cross-shock potential at the quasi-perpendicular Earth's bow shock |
| The evolution of the electron distribution function through quasi-perpendicular collisionless shocks is believed to be dominated by the electron dynamics in the large-scale coherent and quasi-stationary magnetic and electric fields. We investigate the electron distributions measured on board Cluster by the Plasma Electron and Current Experiment (PEACE) instrument during three quasi-perpendicular bow shock crossings. Observed distributions are compared with those predicted by electron dynamics resulting from conservation of the first adiabatic invariant and energy in the de Hoffmann-Teller frame, for all pitch angles and all types of trajectories (passing and, for the first time, reflected or trapped). The predicted downstream velocity distributions are mapped from upstream measurements using an improved Liouville mapping technique taking into account the overshoots. Furthermore, for one of these crossings we could take advantage of the configuration of the Cluster quartet to compare mapped upstream velocity distributions with those simultaneously measured at a relatively well magnetically connected downstream location. Consequences of energy and adiabatic invariant conservation are found to be compatible with the observed electron distributions, confirming the validity of electron "heating" theories based on DC fields as zeroth-order approximations, but some systematic deviations are found between the dynamics of low- and high-adiabatic invariant electrons. Our approach also provides a way to estimate the cross-shock electric potential profile making full use of the electron measurements, and the results are compared to other estimates relying on the steady state dissipationless electron fluid equations. - Remainder of abstract truncated - |
| Publication date: 18 Sep 2007 |
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| Electron acceleration in the Van Allen radiation belts by fast magnetosonic waves |
| Local acceleration is required to explain electron flux increases in the outer Van Allen radiation belt during magnetic storms. Here we show that fast magnetosonic waves, detected by Cluster 3, can accelerate electrons between ~10 keV and a few MeV inside the outer radiation belt. Acceleration occurs via electron Landau resonance, and not Doppler shifted cyclotron resonance, due to wave propagation almost perpendicular to the ambient magnetic field. Using quasi-linear theory, pitch angle and energy diffusion rates are comparable to those for whistler mode chorus, suggesting that these waves are very important for local electron acceleration. Since pitch angle diffusion does not extend into the loss cone, these waves, on their own, are not important for loss to the atmosphere. We suggest that magnetosonic waves, which are generated by unstable proton ring distributions, are an important energy transfer process from the ring current to the Van Allen radiation belts. |
| Publication date: 15 Sep 2007 |
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| Contributions to the cross shock electric field at a quasiperpendicular collisionless shock |
| The normal electric field structure of a supercritical (Mms = 5.2), quasiperpendicular (tetaBn = 70°) collisionless shock is examined using Cluster four-spacecraft observations of the terrestrial bow shock. Comparing the observed electric field with magnetic field and plasma observations, two different techniques find that the J x B/ne term in the generalized Ohm's law accounts for a majority of the large-scale normal electric field and potential drop encountered by the ions - the solar wind ion deceleration is in good empirical agreement with the observed potential drop, confirming earlier work. Large amplitude electric field fluctuations on shorter timescales, corresponding to fine scale structure, are not observed to contribute to the ion energization. |
| Publication date: 13 Sep 2007 |
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| Electrostatic structure around spacecraft in tenuous plasmas |
| Most satellite-based in situ plasma experiments are affected in some manner by the electrostatic structure surrounding the spacecraft. In order to better understand this structure, we have developed a fully three-dimensional self-consistent model that can accept realistic spacecraft geometry, including both thin (~10-4 m) wires and long (~10² m) booms, with open boundary conditions. The model uses an integral formulation incorporating boundary element, multigrid and fast multipole methods to overcome problems associated with the large range in scale sizes and inherently three-dimensional structure. By applying the model to the Cluster spacecraft, we show that the electric potential structure is dominated by the charge on the wire booms, with the spacecraft body contributing at small distances. Consequently, the potential near the EFW (Electric Fields and Waves experiment) probes at the end of the wire booms is typically significantly above the true plasma potential. For the Cluster spacecraft, we show that this effect causes a 19% underestimation of the spacecraft potential and 13% underestimation of the ambient electric field. We further assess the electric field due to the sunward-oriented photoelectron cloud, showing that the cloud contributes little to the observed spurious sunward field in the EFW data. |
| Publication date: 13 Sep 2007 |
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| Cluster observations in the inner magnetosphere during the 18 April 2002 sawtooth event: Dipolarization and injection at r = 4.6 RE |
| The present study examines a sawtooth injection event that took place around 0800 UT on 18 April 2002 when the Cluster spacecraft were located in the inner magnetosphere in the premidnight sector. In association with this injection, Cluster, at a radial distance of 4.6 RE , observed that the local magnetic field became more dipolar and that both ion and electron fluxes increased without notable energy dispersion. These features were accompanied by intensifications of the equatorward component of a double-oval structure and also by an enhancement of the ring-current oxygen ENA flux. The event was also accompanied by large magnetic field (a few tens of nT) and electric field (a few tens of mV/m) fluctuations with characteristic timescales of a few tens of seconds. These observations strongly suggest that this sawtooth injection extended not only widely in local time but also deeply into the inner magnetosphere. Interestingly, Cluster repeatedly observed dipolarization-like signatures afterward, which, however, were not associated with enhancements of local energetic ion flux or with geosynchronous dipolarization or injection signatures. Instead, these magnetic signatures were accompanied by oscillatory plasma motion in the radial direction with a characteristic timescale of about 10 min, which appears to be related to the westward propagation of a spatially periodic auroral structure. The associated azimuthal electric field component was well correlated with the time derivative of the north-south magnetic field component, suggesting that the observed electric field is inductive. These findings suggest that electromagnetic processes far inside geosynchronous orbit play an important role in energization of energetic ions and auroral dynamics during magnetospheric storms. |
| Publication date: 31 Aug 2007 |
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| Near-simultaneous magnetotail flux rope observations with Cluster and Double Star |
| We present observations of three magnetic flux ropes in the tail of the Earth's magnetosphere on 7 August 2004 by the Cluster and Double Star TC-1 spacecraft. The first two flux rope signatures were observed, near-simultaneously, by Cluster and TC-1, which were located at (-16.3, -8.7, 0.10) RE GSM and (-10.3, -7.11, 0.81) RE GSM, respectively, a separation of 6.3 RE. A third signature was observed some four minutes later by two of the four Cluster spacecraft, while the other two spacecraft observed a feature resembling a Travelling Compression Region (TCR). These observations are interpreted as three individual flux ropes existing in the magnetotail, the first two, at least, simultaneously. The formation mechanism of the flux ropes and the consequences of their presence for the structure of the magnetotail on this day are discussed in the context of multiple X-point reconnection.
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| Publication date: 29 Aug 2007 |
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| Magnetosheath plasma expansion: Hybrid simulations |
| We investigate the effect of slow expansion on a magnetosheath plasma and low-frequency waves using a two-dimensional hybrid expanding box simulation. We start our simulation with a homogeneous high beta plasma, which is marginally stable to the mirror and proton cyclotron instabilities. The expansion is imposed as an external force: the physical size of the simulation box increases in two dimensions: one parallel and one perpendicular with respect to the ambient magnetic field. This expansion leads to a continuous decrease of proton beta and drives an increase of the proton temperature anisotropy. In the early stages of the simulation, both mirror and proton cyclotron waves appear. The system establishes a marginally stable state with respect to both mirror and proton cyclotron instabilities. Initially, the mirror waves dominate the proton cyclotron waves, even when the system is below the linear mirror threshold, but as time increases the proton cyclotron waves become dominant in the low beta region. We also include an initial comparison of the simulated data with Cluster observations. |
| Publication date: 07 Aug 2007 |
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| High-latitude plasma convection from Cluster EDI: variances and solar wind correlations |
| Based on drift velocity measurements of the EDI instruments on Cluster during the years 2001-2006, we have constructed a database of high-latitude ionospheric convection velocities and associated solar wind and magnetospheric activity parameters. In an earlier paper (Haaland et al., 2007), we have described the method, consisting of an improved technique for calculating the propagation delay between the chosen solar wind monitor (ACE) and Earth's magnetosphere, filtering the data for periods of sufficiently stable IMF orientations, and mapping the EDI measurements from their high-altitude positions to ionospheric altitudes. The present paper extends this study, by looking at the spatial pattern of the variances of the convection velocities as a function of IMF orientation, and by performing sortings of the data according to the IMF magnitude in the GSM y-z plane, |ByzIMF|, the estimated reconnection electric field, Er,sw, the solar wind dynamic pressure, Pdyn, the season, and indices characterizing the ring current (Dst) and tail activity (ASYM-H). The variability of the high-latitude convection shows characteristic spatial patterns, which are mirror symmetric between the Northern and Southern Hemispheres with respect to the IMF By component. The latitude range of the highest variability zone varies with IMF Bz similar to the auroral oval extent. The magnitude of convection standard deviations is of the same order as, or even larger than, the convection magnitude itself. Positive correlations of polar cap activity are found with |ByzIMF| and with Er,sw, in particular. The strict linear increase for small magnitudes of Er,sw starts to deviate toward a flattened increase above about 2 mV/m. - Remainder of abstract truncated - |
| Publication date: 30 Jul 2007 |
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| Motion of flux transfer events: a test of the Cooling model |
| The simple model of reconnected field line motion developed by Cooling et al. (2001) has been used in several recent case studies to explain the motion of flux transfer events across the magnetopause. We examine 213 FTEs observed by all four Cluster spacecraft under a variety of IMF conditions between November 2002 and June 2003, when the spacecraft tetrahedron separation was ~5000 km. Observed velocities were calculated from multi-spacecraft timing analysis, and compared with the velocities predicted by the Cooling model in order to check the validity of the model. After excluding three categories of FTEs (events with poorly defined velocities, a significant velocity component out of the magnetopause surface, or a scale size of less than 5000 km), we were left with a sample of 118 events. 78% of these events were consistent in both direction of motion and speed with one of the two model de Hoffmann-Teller (dHT) velocities calculated from the Cooling model (to within 30° and a factor of two in the speed). We also examined the plasma signatures of several magnetosheath FTEs; the electron signatures confirm the hemisphere of connection indicated by the model in most cases. This indicates that although the model is a simple one, it is a useful tool for identifying the source regions of FTEs. |
| Publication date: 30 Jul 2007 |
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| Multi-scale observations of magnetotail flux transport during IMF-northward non-substorm intervals |
| Simultaneous observations by the Cluster spacecraft and SuperDARN radars are presented of magnetotail flux transport during northward, but BY-dominated IMF. Two events are discussed, which occurred on 14 August 2004 and 17 September 2005, during intervals of negative and positive IMF BY, respectively. During both intervals the Cluster spacecraft observed isolated bursts of Earthward plasma convection in the central plasma sheet. During the first event, the flows observed by Cluster also had a significant Vperp.Y component in the duskward direction, consistent with westward azimuthal flows observed in the midnight sector by the Northern Hemisphere SuperDARN radars. During the second event, Cluster 4 observed a significant dawnward Vperp.Y component, again consistent with the Northern Hemisphere SuperDARN observations which revealed eastward azimuthal flow. In this instance, however, Cluster 3 observed a duskward Vperp.Y component which was more consistent with the duskward sense of the convection observed by the Southern Hemisphere SuperDARN radars. This implies that Cluster 3 and Cluster 4 were located on different field lines which experienced opposite net azimuthal forces and hence observed oppositely directed convection. These observations are consistent with previous SuperDARN studies of nightside flows under northward IMF and, more importantly, provide the first simultaneous in-situ evidence for a mode of tail reconnection occurring during non-substorm intervals in an asymmetric tail. |
| Publication date: 30 Jul 2007 |
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| Cluster Observations of the Magnetospheric Low-Latitude Boundary Layer and Cusp during Extreme Solar Wind and Interplanetary Magnetic Field Conditions: II. 7 November 2004 ICME and Statistical Survey |
| We present a study of the plasma properties inside and dynamics of the low-latitude boundary layer (LLBL)/cusp during the ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predominantly strongly northward, up to 50 nT, with some short-duration rotations. The observed LLBL/cusp is very thick (~6 - 7° invariant latitude (ILAT)) and migrates equatorward with rates of 0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming from multiple reconnection sites. During a period of extremely northward IMF, signatures of pulsed dual reconnection inside the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been suggested previously. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on 28 - 29 October 2003 and 7 - 10 November 2004 is performed.
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| Publication date: 21 Jul 2007 |
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| Strong bulk plasma acceleration in Earth's magnetosheath: A magnetic slingshot effect? |
| In the near-Earth environment, strong bulk plasma accelerations are frequently taken to be the diagnostic of the occurrence of magnetic reconnection. In this letter, we report new and unambiguous spacecraft observations and corresponding magnetohydrodynamic (MHD) simulation of strong bulk plasma acceleration in the terrestrial magnetosheath during low Alfvén Mach number solar wind conditions, which is demonstrably not associated with magnetic reconnection. We illustrate this effect with Cluster spacecraft data that show plasma accelerations up to speeds of 1040 km/s, while the ambient solar wind speed is only 650 km/s (i.e., in excess by 60%). Based on a comparison with global MHD simulations of the magnetosphere, we show that the acceleration results from enhanced magnetic forces exerted on the plasma by "stiff" magnetic flux tubes in a low-Beta magnetosheath that result from the low Alfvén Mach number solar wind. The MHD simulations demonstrate that the acceleration is asymmetric, as well as the magnetopause shape, and is the result of both magnetic pressure gradient and tension forces, showing that this effect is not a simple analogy to a "slingshot effect" for which magnetic tension would dominate. Like magnetic reconnection, this mechanism is capable of producing strong plasma acceleration in the near-Earth's environment. The low Alfvén Mach number solar wind condition leading to this mechanism is often characteristic of coronal mass ejections (CMEs). |
| Publication date: 18 Jul 2007 |
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| Dipolarization observed by TC1 and ClusterII during substorm in Sep. 14, 2004 |
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| Publication date: 15 Jul 2007 |
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| Dissipation in Turbulent plasma due to reconnection in thin current sheets |
| We present in situ measurements in a space plasma showing that thin current sheets the size of an ion inertial length exist and are abundant in strong and intermittent plasma turbulence. Many of these current sheets exhibit the microphysical signatures of reconnection. The spatial scale where intermittency occurs corresponds to the observed structures. The reconnecting current sheets represent a type of dissipation mechanism, with observed dissipation rates comparable to or even dominating over collisionless damping rates of waves at ion inertial length scales (x100), and can have far reaching implications for small-scale dissipation in all turbulent plasmas. |
| Publication date: 13 Jul 2007 |
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| Multi-scale structure of the electron diffusion region |
| Kinetic simulations of magnetic reconnection indicate that the electron diffusion region (EDR) can elongate into a highly stretched current layer with a width on the electron scale and a length that exceeds tens of ion inertial lengths. The resulting structure has no fluid analogue and consists of two regions in the exhaust direction. The inner region is characterized by the locale where electrons reach a peak outflow speed near the electron Alfvén velocity. Ions also approach ~80% of their peak velocity in this inner region but remain sub-Alfvénic. There exists a large electrostatic potential that can temporarily trap electrons within this inner region. The electron frozen-in condition is violated over a wider outer region characterized by highly collimated electron jets that are gradually decelerated and thermalized. Reconnection proceeds continuously but the rate is modulated in time as the EDR elongates into an extended layer. The elongation of the EDR is controlled by the competition between the outward convection of magnetic flux and the non-ideal term involving the divergence of the electron pressure tensor. The occasional balance between these two terms leads to periods of quasi-steady reconnection. However, over longer time scales, a natural feature of the reconnection process appears to be frequent formation of plasmoids due to the instability of the elongated EDR which leads to larger variations in the reconnection rate. These new findings provide testable predictions and indicate the need to reconsider the diagnostics for identification of the diffusion region and interpretation of observational data. |
| Publication date: 11 Jul 2007 |
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| Characteristics of terrestrial foreshock ULF waves: Cluster observations |
| ULF waves in the terrestrial foreshock observed simultaneously by the four Cluster satellites were analyzed to identify the plasma wave modes and to study the effect of plasma beta on the intrinsic wave properties. The wave properties in the spacecraft and solar wind frames, such as the wave frequency, total wave number, phase speed, and wave polarization, are experimentally derived using the minimum variance analysis (MVA) for the case study and the phase differencing (MVA-free) technique for the statistical study. Both studies indicate that the waves with a 30 s period propagate in the upstream direction at a finite angle with respect to the background magnetic field in the plasma rest frame but are then convected downstream in the spacecraft frame. It is shown that these waves propagate in the fast magnetosonic mode. A similar analysis of the 3 s period waves shows them to be propagating in the upstream direction in the Alfvén/ion cyclotron mode. The measured wave properties in the plasma rest frame are in good agreement with theoretical kinetic dispersion relation with a different plasma beta, which has rather significant deviation from fluid model especially for the high plasma beta. In conclusion it is found that the experimentally derived foreshock ULF wave properties are basically in good agreement with previous results but the effect of plasma beta is indispensable to choose the correct wave mode branch especially for the high plasma beta condition. |
| Publication date: 07 Jul 2007 |
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| A comparison of Cluster magnetic data with the Tsyganenko 2001 model |
| As part of an investigation of the magnetic effects of external currents in the magnetosphere, we have compared two years of perigee Cluster data to the Tsyganenko 2001 (T01) field model. Cluster data are not included in the T01 database and therefore can be used to independently verify the model. The model performs very well in a global sense; nevertheless, absolute residuals between the data and the model can reach ~20 nT near perigee. These deviations take two forms: a sharp, bipolar signature and well-defined trends over a larger spatial region. The bipolar signatures in the residuals are moderately stable, repeating on the phase period of the Cluster orbit. The bipolar nature of the signatures reflects variations in the Cluster data, therefore indicating that the spacecraft may be observing a field-aligned current. Although the size of the magnetic field perturbation in this region is not well determined by T01, the location of the observed field-aligned current system is accurately predicted. The bipolar signatures are observed in close proximity to the edge of the ring current, estimated from Cluster energetic electron spectrograms, indicating that they are associated with region 2 field-aligned currents. Longer-duration trends in the residuals indicate a slight difference between the model predictions and the Cluster data for various locations and seasons. For example, throughout most of 2003 and the first half of 2004, there is a residual in the total magnetic field for an hour centered on perigee, of ~20 nT. |
| Publication date: 29 Jun 2007 |
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| Dynamics of thin current sheets: Cluster observations |
| The paper tries to sort out the specific signatures of the Near Earth Neutral Line (NENL) and the Current Disruption (CD) models, and looks for these signatures in Cluster data from two events. For both events transient magnetic signatures are observed, together with fast ion flows. In the simplest form of NENL scenario, with a large-scale two-dimensional reconnection site, quasi-invariance along Y is expected. Thus the magnetic signatures in the S/C frame are interpreted as relative motions, along the X or Z direction, of a quasi-steady X-line, with respect to the S/C. In the simplest form of CD scenario an azimuthal modulation is expected. Hence the signatures in the S/C frame are interpreted as signatures of azimuthally (along Y) moving current system associated with low frequency fluctuations of Jy and the corresponding field-aligned currents (Jx). Event 1 covers a pseudo-breakup, developing only at high latitudes. First, a thin (H~2000 km~2 rhoi, with rhoi the ion gyroradius) Current Sheet (CS) is found to be quiet. A slightly thinner CS (H~1000-2000 km~1-2 rhoi), crossed about 30 min later, is found to be active, with fast earthward ion flow bursts (300-600 km/s) and simultaneous large amplitude fluctuations (deltaB/B~1). In the quiet CS the current density Jy is carried by ions. Conversely, in the active CS ions are moving eastward; the westward current is carried by electrons that move eastward, faster than ions. Similarly, the velocity of earthward flows (300-600 km/s), observed during the active period, maximizes near or at the CS center.
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| Publication date: 29 Jun 2007 |
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