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We investigate in detail a reversal of plasma flow from tailward to earthward detected by Cluster at the downstream distance of ~19 RE in the midnight sector of the magnetotail on 22August 2001. This flow reversal was accompanied by a sign reversal of the Bz component and occurred during the late substorm expansion phase as revealed by simultaneous global view of auroral activity from IMAGE. We examine the associated Hall current system signature, current density, electric field, Lorentz force, and current dissipation/dynamo term, the last two parameters being new features that have not been studied previously for plasma flow reversals. It is found that (1) there was no clear quadrupole Hall current system signature organized by the flow reversal time, (2) the x-component of the Lorentz force did not change sign while the other two did, (3) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source, (4) the electric field was occasionally dawnward, producing a dynamo effect, and (5) the electric field was occasionally larger at the high-latitude plasma sheet than near the neutral sheet. These observations are consistent with the current disruption model for substorms in which these disturbances are due to shifting dominance of multiple current disruption sites and turbulence at the observing location.

We present new results concerning electron temperature ( T e ) mapping in the inner magnetosphere of Saturn ( S ). The data are obtained by the Langmuir Probe (LP) instrument of the Radio and Plasma Wave Science (RPWS) investigation on board Cassini. The study is based on the fourteen first orbits around Saturn, and focus is on cold ( 5 cm −3 ) electron populations in the inner plasma torus (plasma disc). Apart from a general increase with distance from Saturn and away from the equatorial plane (latitude), we identify a possibly anisotropic electron temperature in the plasma disc, as well as the presence of photoelectrons from the spacecraft or E-ring dust particles. A polytropic index of γ ≈+0.4 is determined from the observations, which can be used in theoretical models of the plasma disc. We also infer local variations in T e near the equatorial plane, where the E-ring of dusty plasma and enhancements of water rich neutral gas exist, and determine constraints on the energy source required to heat the electrons in the plasma disc. We suggest that Joule heating or heating by low-frequency plasma waves as well as cooling by neutral gas and E-ring dust are involved in the energy balance, together with Coulomb collision heating by co-rotating ions.

We evaluate two prevailing substorm models with an event of plasma flow reversal from tailward to Earthward detected by Cluster at the downstream distance of ∼19 R E in the magnetotail during a substorm on August 22, 2001. We use the unique capability of Cluster measurements in determining gradients to examine the associated current density, Lorentz force, and current dissipation/dynamo term. In association with plasma flow reversal, it is found that (1) there was no clear quadrupole magnetic perturbation signature, (2) the x -component of the Lorentz force did not change sign, (3) the y -component of the product of the current density and the electric field was occasionally negative indicative of a dynamo effect, and (4) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source. These observations are consistent with the near-Earth initiation model for substorms with multiple current disruption sites moving progressively tailward near the late stage of substorm expansion.

In 1996 and 1997 during months when the orbit apogee was at XGSM⩾0 (May–October 1996 and April–August 1997), NASA's Polar spacecraft was skimming the high-latitude dayside magnetopause whenever the solar wind dynamic pressure (pdyn) was larger than ∼ 4 nPa. Magnetopause crossings occurred mostly under a northward and/or strongly dominating duskward-dawnward interplanetary magnetic field and took place at the poleward or dawn/dusk edge of the distant cusp. Polar data reveal that the indentation of cusp magnetopause may be deep, ∼2.5 RE below the axisymmetric surface of empirical magnetopause models. A significant IMF BY-dependence for probability of magnetopause encounter by Polar in a given local time sector suggests a formation of localized valleys extending away from the cusp proper along the merging lines on the magnetopause. Multipoint data in the distant cusp with the spacecraft separation on a ∼1–2 RE spatial scale could help to confirm this finding.

It is of considerable interest to compile a model of the low frequency electromagnetic wave intensity across the polar caps, in and around the auroral zones, as well as at lower latitudes. Waves are playing a dynamic role in the auroral region and can be used to characterize the level of activity in the magnetosphere. At middle latitudes, Very Low Frequency (VLF) and Extremely Low Frequency (ELF) waves are known to scatter trapped energetic electrons, alter their angular and energy distributions, and eventually cause their precipitation into the Earth's atmosphere where it affects the propagation of radio signals. We report here on the development of a data-based model of the electromagnetic power spectral density in the VLF band (10–46 kHz) as surveyed onboard the Swedish Viking spacecraft in the high-latitude region in the northern hemisphere. The data have been sorted into bins according to spatial location and wave frequency. A preliminary statistical model is presented showing the mean electric power spectral density versus magnetic local time and versus invariant latitude at fixed height intervals and for fixed frequency bands within the VLF range. Our data base includes 1162 data intervals, each of 15 min length, during March–December 1986. As a future goal, an empirical model fitting these averages with simple analytical functions may also be developed. This work also aims to associate observed plasma wave characteristics in a particular region of the magnetosphere with geomagnetic conditions. Progress in these directions is reported.

Multi-spacecraft tracing of the high latitude magnetopause (MP) and boundary layers and Interball-1 statistics indicate that: 1. (a) The turbulent boundary layer (TBL) is a persistent feature in the region of the cusp and ‘sash’, a noticeable part of the disturbances weakly depends on the interplanetary magnetic field By component; TBL is a major site for magnetosheath (MSH) plasma penetration inside the magnetosphere through percolation and local reconnection. 2. (b) The TBL disturbances are mainly inherent with the characteristic kinked double-slope spectra and, most probably, 3-wave cascading. The bi-spectral phase coupling indicates self-organization of the TBL as the entire region with features of the non-equilibrium multi-scale and multi-phase system in the near-critical state. 3. (c) We've found the different outer cusp topologies in summer/winter periods: the summer cusp throat is open for the decelerated MSH flows, the winter one is closed by the distant MP with a large-scale (∼several Re) diamagnetic ‘plasma ball’ inside the MP; the ‘ball’ is filled from MSH through patchy merging rather than large-scale reconnection. 4. (d) A mechanism for the energy release and mass inflow is the local TBL reconnection, which operates at the larger scales for the average anti-parallel fields and at the smaller scales for the nonlinear fluctuating fields; the latter is operative throughout the TBL. The remote from TBL anti-parallel reconnection seems to happen independently.

We study the high latitude turbulent boundary layer (TBL), where the essential magnetosheath (MSH) ion heating (in 1.5–3 times) is seen in more than 80 % cases within ‘diamagnetic bubbles’ -structures with highly reduced magnetic field. The high- energy tails of the core particle distributions in TBL exceed 20keV for ions and 10keV for electrons on April 21, 1996. We find indications also for further acceleration of a small fraction of protons there up to 300 keV. More energetic particles could have maximum intensity just inside magnetopause that is consistent with their magnetospheric origin. Simultaneous Polar and Interball-1 data in TBL and MSH on May 29, 1996 demonstrate heating of the MSH core ions in cusp, TBL and stagnant MSH. In the TBL the O + ions might be heated as well (up to few keV) and even be backscattered towards the ionosphere together with the protons > 18 keV . The dominant incompressible Alfvenic waves in TBL constitute an intermediate chain for the transformation of the solar wind kinetic energy into the auroral particle acceleration/heating. The most intensive energy transformation occurs over cusps and along the boundary between the mantle and low latitude boundary layer down to X = − 6 R E . It provides primary heating for MSH ions in both cusp proper and boundary cusp.

The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s-1. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 RE downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 RE downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction.

Multi-spacecraft data from Cluster allow for a more detailed magnetopause and boundary layer structure determination than ever before. Reconstruction methods, in which the time variability observed during a pass is interpreted as being due to boundary motion and/or spatial structure, are particularly well suited for this task. Such methods rely on the availability of plasma and field data and adopt the tangential discontinuity hypothesis to determine the motion, acceleration, boundary structure, boundary curvature and surface wave speed over an extended time interval. In this paper one- and two-dimensional reconstruction methods are applied to multi-spacecraft data for the first time.

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (waves and in-stabilities)

Photometer recordings of auroral emissions from N + 2 and OI have demonstrated that the ratios between the emissions show a characteristic plateau just prior to the passage of a westward travelling surge. The plateau may be interpreted as a temporary (approx. 1 min) cessation in the spectral hardening of the precipitating particles associated with the surge. The characteristic energy of the plateau is about one keV above the level prior to the beginning of the surge. The results have been interpreted in terms of an instability that is switched on during the time of the plateau and then turned off when the electron density is increased above a certain limit.

Information on the direction of auroral arcs, mostly all sky camera data during IGY, has been collected and organized in a geomagnetic coordinate system, corrected by the higher order spherical harmonic terms. By means of interpolation and integration between the measured values it was possible to obtain curves in a polar plot representing auroral arcs over the polar region in the latitude range 64–90 degrees. The ovals of auroral arcs do not follow the corrected geomagnetic latitudes. They are furthest from the pole on the night-side e.g. 70.5° at 24, 79.9° at 06, 79.2° at 12 and 75.5° of latitude at 18 hours. A small dawn-dusk asymmetry has also been observed, the curves being closer to the pole at 06 hours than at 18 hours. In the polar region between latitudes 80–85° for a time interval around 1600 hours it has been found that the auroral arcs have an angle relative to the solar direction of about 20 degrees. The shape of the auroral orientation curves corresponds (in the auroral zone region) with the auroral oval based on the occurrence of aurora.

An investigation has been made of the relation between auroral emission at λ5577 A and the cosmic noise absorption using a new technique. A photometer and the antenna of a riometer were mounted on a 60 m long rotating antenna boom which had a speed of 1 rev per 3 min. The instruments were directed at an elevation angle of 45°. From the analysis of several break-ups of the aurora it has been found that during a period of 15–20 min in the middle of a break-up there may be an increase of the absorption by a factor 2 to 4 which does not correspond to a similar increase of the auroral emission. These changes in the emission-absorption ratio has been interpreted as peaks in the energy spectrum of the incoming particles. The structures of auroral emission and auroral absorption are sometimes very similar over periods of hours and the appearance of the structure is usually in the form of an east-west oriented arc. The cross correlation coefficient may be as high as 0·9 during these events over long periods of time. However, a number of exceptional cases have appeared where little structure was found in the riometer record while the photometer showed structure and vice versa.

Highlights are presented from studies of the electric field data from various regions along the Cluster orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatial-temporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.Key words. Magnetospheric physics (electric fields) – Space plasma physics (electrostatic structures; turbulence)