ESA Science & Technology - Publication Archive

The generation of anomalous collision rate and resistivity by lower-hybrid drift waves is investigated. A general expression involving particle density and current perturbations and electromagnetic field fluctuations is used to estimate the effective collision frequency. Results of Vlasov-code simulations and Cluster spacecraft measurements suggest that electrostatic and electromagnetic fluctuations provide approximately equal contributions to the electron collision rate, which is of the order of the lower-hybrid frequency. The anomalous resistivity based on such collision rate could be significant for the large-scale plasma dynamics in Earth's magnetosphere, such as magnetic reconnection.

The interaction between broadband drift mode turbulence and zonal flows has been studied through the wave-kinetic approach. Simulations have been conducted in which a particle-in-cell representation is used for the quasiparticles, while a fluid model is employed for the plasma. The interactions have been studied in a plasma edge configuration which has applications in both tokamak physics and magnetopause boundary layer studies. Simulation results show the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients.

We have studied the entry paths of solar wind plasma into the magnetosphere during an extended period of northward IMF using an OpenGGCM MHD simulation of the cold dense plasma sheet (CDPS) event observed on October 23, 2003 by the Cluster spacecraft. We find that high-latitude reconnection occurs tailward of both cusps between the IMF and geomagnetic field. The newly created closed magnetic flux tubes capture magnetosheath plasma, and subsequently sink and shrink into the magnetosphere, while convecting tailward. The plasma that enters near the reconnection site is driven sunward and toward the low latitude region initially; it then drifts to the flanks. The captured plasma is characterized by small flow velocity, and it is moderately heated in the reconnection region. In the present case study we find the cold plasma enters the plasma sheet in the near Earth tail where it is observed by Cluster.

The October 22-24, 2003 interplanetary magnetic cloud was characterized by an exceptionally long interval (~32 hours) of nearly purely northward interplanetary magnetic field (IMF). Following the northward IMF turning Cluster observed a gradual transition to a cold (<1 keV) and dense (~1-2 cm^-3) plasma sheet (CDPS). Cluster observed CDPS continuously for the following ~30 hours while passing through the neutral sheet from the northern to the southern hemisphere. DMSP observations mapped to the equatorial plasma sheet reveal that the CDPS extended to all nightside local times. The FAST satellite observed reversed ion dispersion signatures in the cusp indicative of poleward-of-cusp reconnection, and nearly no polar cap. The CDPS observations show good agreement with a global MHD simulation where the CDPS is formed by poleward-of-cusp reconnection capturing magnetosheath plasma and convecting it to the tail. The process shrinks the size of the lobes (and therefore the polar cap) significantly, as observed.

Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ~1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ~10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ~10 microA/m², and upward electron beams with characteristic energies of a few hundred eV to a few keV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. -- abstract truncated --

Space plasmas present intriguing and challenging puzzles to the space community. Energy accessible to excite instabilities exists in a variety of forms, particularly for the magnetospheric environment prior to substorm expansion onsets. A general consensus of the pre-expansion magnetosphere is the development of a thin current sheet in the near-Earth magnetosphere. This review starts with a short account of the two major substorm paradigms. Highlights of some observations pertaining to the consideration of potential plasma instabilities for substorm expansion are given. Since a common thread of these paradigms is the development of a thin current sheet, several efforts to model analytically a thin current sheet configuration are described. This leads to a review on the instability analyses of several prominent candidates for the physical process responsible for substorm expansion onset. The potential instabilities expounded in this review include the cross-field current, lower-hybrid-drift, drift kink/sausage, current driven Alfvénic, Kelvin-Helmholtz, tearing, and entropy anti-diffusion instabilities. Some recent results from plasma simulations relevant to the investigation of these plasma instabilities are shown. Although some of these instabilities are generally conceived to be excited in spatially localized regions in the magnetosphere, their potentials in yielding global consequences are also explored.

Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions - in the absence of active reconnection at low latitudes - there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin-Helmholtz instability. This can supply plasma sources for various space weather phenomena.

Isolated electrostatic structures are observed throughout much of the 4R_E by 19.6R_E Cluster orbit. These structures are observed in the Wideband plasma wave instrument's waveform data as bipolar pulses (one positive and one negative peak in the electric field amplitude) and tripolar pulses (two positive and one negative peak, or vice versa). These structures are observed at all of the boundary layers, in the solar wind and magnetosheath, and along auroral field lines at 4.5-6.5R_E. Using the Wideband waveform data from the various Cluster spacecraft we have carried out a survey of the amplitudes and time durations of these structures and how these quantities vary with the local magnetic field strength. Such a survey has not been carried out before, and it reveals certain characteristics of solitary structures in a finite magnetic field, a topic still inadequately addressed by theories. We find that there is a broad range of electric field amplitudes at any specific magnetic field strength, and there is a general trend for the electric field amplitudes to increase as the strength of the magnetic field increases over a range of 5 to 500nT. We provide a possible explanation for this trend that relates to the structures being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding dependence of the duration of the structures on the magnetic field strength, although a plot of these two quantities reveals the unexpected result that with the exception of the magnetosheath, all of the time durations for all of the other regions are comparable, whereas the magnetosheath time durations clearly are in a different category of much smaller time duration. We speculate that this implies that the structures are much smaller in size. The distinctly different pulse durations for the magnetosheath pulses indicate the possibility that the pulses are generated by a mechanism which is different from the mechanism operating in other regions.

The electron density profiles derived from the EFW and WHISPER instruments on board the four Cluster spacecraft reveal density structures inside the plasmasphere and at its outer boundary, the plasmapause. We have conducted a statistical study to characterize these density structures. We focus on the plasmasphere crossing on 11 April 2002, during which Cluster observed several density irregularities inside the plasmasphere, as well as a plasmaspheric plume. We derive the density gradient vectors from simultaneous density measurements by the four spacecraft. We also determine the normal velocity of the boundaries of the plume and of the irregularities from the time delays between those boundaries in the four individual density profiles, assuming they are planar. These new observations yield novel insights about the occurrence of density irregularities, their geometry and their dynamics. These in-situ measurements are compared with global images of the plasmasphere from the EUV imager on board the IMAGE satellite.

In this case study we investigate the source region of whistler-mode chorus located close to the geomagnetic equator at a radial distance of 4.4 Earth radii. We use measurements from the four Cluster spacecraft at separations of less than a few hundreds of km, recorded during the geomagnetic storm of 18 April 2002. The waveforms of the electric field fluctuations were obtained by the WBD instruments in the frequency range 50Hz-9.5kHz. Using these data, we calculate linear and rank correlation coefficients of the frequency averaged power-spectral density measured by the different spacecraft. Those coefficients have been recently shown to decrease with spacecraft separation distance perpendicular to the static magnetic field with a characteristic scale length of 100km. We find this characteristic scale varying between 60 and 200km for different data intervals inside the source region. We examine possible explanations for the observed large scatter of the correlation coefficients, and we suggest a simultaneously acting effect of random positions of locations at which the individual chorus wave packets are generated. The statistical properties of the observations are approximately reproduced by a simple 2-D model of the source region, assuming a perpendicular half-width of 35km (approximately one wavelength of the whistler-mode waves) for the distribution of power radiated from individual active areas.

On April 18, 2002, the Cluster spacecraft were outbound in the northern hemisphere over the pole and entered the cusp. A cusp-like region was observed consecutively three times from 1620 to 1830 UT by all four Cluster Spacecraft although the solar wind dynamic pressure was small and stable. All three cusp encounters were characterized by turbulent magnetic fields, high density plasma and stagnant plasma flow. The cusp region identifications were further confirmed by the clock angle criterion. All three cusps were found to be associated with clear signatures of energetic ions, high He/H and O/H ratios obtained by the RAPID instrument. The observed triple cusps may be either explained as a funnel-shaped cusp trifurcated or swiveled into a complicated geometry in space or as a cusp which shifted back and forth three times in a two hour interval. Observational evidence shows that the observed triple cusps were a temporal sequence rather than a spatial effect. We suggest further that the solar wind azimuthal flow was the controlling factor of the cusp position and was stronger factor than the IMF BY/BZ components. The importance of the solar wind azimuthal and north/south flow as a dynamic driver of the cusp, and even the whole magnetosphere has been more or less neglected or underestimated.

The first simultaneous measurements of discrete chorus emissions on multiple spacecraft, realized in the context of the Cluster mission, revealed a rather unexpected frequency difference of around 1 kHz between nearly identical discrete elements observed on different spacecraft [Gurnett et al., 2001 ]. This frequency difference is interpreted herein as a natural outcome of the dependence of the whistler-mode refractive index on the wave normal angle between the wave vector k and the static magnetic field B₀ and the rapid motion of highly localized source region(s) of chorus of 400 km to 1700 km in extent along the field line, but only less than 100 km transverse to the magnetic field, and moving at speeds of 20,000 km/s to 25,000 km/s. Wave packets emanating from the localized regions propagate to two spacecraft at different wave normal angles, and are observed at different frequencies due to the differential Doppler shift between the two spacecraft. These differences in frequency, as well as the different times of arrival of the similar emissions at the different spacecraft, provide a unique opportunity to estimate the source characteristics, using a model involving rapidly moving sources traveling at speeds comparable to the parallel resonant velocity of counterstreaming electrons moving along the Earth's magnetic field lines. We report the determination of chorus emission source region motion for two different cases observed during 20002001, where these differences in frequency were readily observable due to the relatively large separation of the Cluster spacecraft. We also report a case in 2002 where the spacecraft separations were smaller, so that these frequency differences were not as evident but nevertheless measurable. In general, our results provide the first experimental evidence that the sources that generate the discrete chorus emissions are in rapid motion.

Measurements of a spacecraft floating potential, on the four Cluster spacecraft, are used as a proxy for electron plasma density to study, for the first time, the macroscopic density transition scale at 98crossings of the quasiperpendicular terrestrial bow shock. A timing analysis gives shock speeds and normals; the shock speed is used to convert the temporal measurement to a spatial one. A hyperbolic tangent function is fitted to each density transition, which captures the main shock transition, but not overshoot or undershoot nor foot features. We find that, at a low Mach number M, the density transition is consistent with both ion inertial scales c/ω_pi and convected gyroradii v_sh,n/Ω_ci,2, while at M ≥ 4-5 only the convected gyroradius is the preferred scale for the shock density transition and takes the value L ≈ 0.4v_sh,n/Ω_ci,2.

We investigate intense whistler-mode chorus emissions which occurred during the geomagnetic storm on 31 March 2001. We use multipoint measurements obtained by the Cluster spacecraft in the premidnight equatorial region outside the plasmasphere at a radial distance of 4 Earth radii (L = 4.0 - 4.2). Observed spatio-temporal variations of the direction of the Poynting flux manifest a consistent pattern: the central position of the chorus source fluctuates at time scales of minutes within 1000-2000 km of the geomagnetic equator. We demonstrate that estimates of the electromagnetic planarity can be used to characterize the extent of the source, obtaining a range of 3000-5000 km. Discrete wave packets of chorus are observed to rise in frequency between 0.13 and 0.5 of the local electron cyclotron frequency, at a rate up to 20 kHz/s, having the maximum peak amplitudes of <20 mV/m. We observe a fine structure of subpackets with large amplitudes embedded in the interior of the wave packets. This fine structure has a typical delay of a few milliseconds between the two neighboring maxima of the wave amplitude. Longer delays occur with a decreasing probability density.

Electric and magnetic field observations on the Polar satellite at the subsolar magnetopause show that the magnetopause current is often striated. The largest of the resulting current channels are interpreted as electron diffusion regions because their widths are several electron skin depths and the electron flow U_e within them does not satisfy E + U_e x B = 0. The data suggest that the magnetopause contains many such electron diffusion regions and that they are required because E x B/B² drifting electrons cannot carry the large filamentary currents imposed on the local plasma. The most probable interpretation of E + U_e x B is not equal to 0 is that the pressure term on the right side of the generalized Ohm's law balances this inequality.

Measurements of the Lyman alpha column brightness by the Geocoronal Imager (GEO), part of the FUV imaging system on board the IMAGE satellite, have been used to derive an empirical model of the neutral hydrogen density distribution at high altitudes (>3.5 R_E geocentric distance) on the night-side of the Earth. The model presented is an effort to provide the density profiles needed to analyze the energetic neutral atom imaging data at ring current altitudes and above. The variable solar Lyman alpha flux is obtained from the UARS/SOLSTICE measurements and the scattered solar Lyman alpha emissions from interplanetary hydrogen are obtained from a model. Assuming that the exosphere at high altitudes (>3.5 R_E geocentric distance) can be considered as an optical thin medium and that the hydrogen density profile can be expressed as a double exponential we show that the Lyman alpha column brightness can be converted to hydrogen density profiles. The hydrogen density above 5 R_E is found to be slightly higher for large solar zenith angles than for 90° solar zenith angle. The hydrogen density shows temporal variations which are not controlled by any solar quantity or geomagnetic parameter alone. Our Lyman alpha profiles and derived hydrogen density profiles are close to what was observed by Dynamics Explorer 1 [Rairden et al., 1986]. Above 8 R_E we find higher densities than they did for all solar zenith angles >90°. We do not find any evidence of depletion due to charge exchange with solar wind protons outside the magnetopause. Our results are only valid above 3.5 R_E

We discuss chorus emissions measured by the four Cluster spacecraft at close separations during a geomagnetically disturbed period on 18 April 2002. We analyze the lower band of chorus below one half of the electron cyclotron frequency, measured at a radial distance of 4.4 Earth's radii, within a 2000 km long source region located close to the equator. The characteristic wave vector directions in this region are nearly parallel to the field lines and the multipoint measurement demonstrates the dynamic character of the chorus source region, changing the Poynting flux direction at time scales shorter than a few seconds. The electric field waveforms of the chorus wave packets (forming separate chorus elements on power spectrograms) show a fine structure consisting of subpackets with a maximum amplitude above 30 mV/m. To study this fine structure we have used a sine-wave parametric model with a variable amplitude.

The four Cluster spacecraft were successfully launched in pairs by two Russian Soyuz rockets on 16 July and 9 August 2000. On 14 August, the second pair joined the first pair in highly eccentric polar orbits, with an apogee of 19.6 Earth radii and a perigee of 4 Earth radii. The very accurate orbital injection and low fuel consumption mean that spacecraft operations could continue for at least two more years after the nominal two-year mission. This is the first time that the Earth's magnetic field and its environment have been explored by a small constellation of four identical spacecraft. Preliminary results show that, as predicted, with four spacecraft we can obtain a detailed three-dimensional view of the Sun-Earth connection processes taking place at the interface between the solar wind and the Earth's magnetic field.

Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001.

Presentation from the press event marking the beginning of Cluster's operational phase - held at ESA HQ, 16 February 2001.