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In the context of the onset of substorms, observations had revealed the frequent occurrence of an unidentified trigger process. Inspections of such onsets had led to the discovery that high-beta plasma at the magnetosphere-tail boundary became suddenly unstable when so-called auroral streamers lined up closely with that boundary. The manifestation was the appearance of bead-like auroral structures preceding the auroral breakup and subject to nonlinear growth. Highly resolved video coverage of a few events showed that the fast moving beads moved opposite to the convective motions on either side. This led to the proposal [Haerendel & Frey 2021] that the trigger of the instability was the formation of a new current circuit, by a non-MHD process, in the gap between the two adjacent circuits of the high-beta plasma boundary and the auroral streamer. Observational evidence and a model of the current structure will be presented.

The paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 RE. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high‐beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high‐resolution videos of two substorm onsets. We conclude that they may be low‐entropy content bubbles with a balanced field‐aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non‐MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high‐beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos, Plain Language Summary: The substorm begins with two independent events of common origin. The first one is normally the brightening of the growth phase arc, which is the trace of a hot plasma layer forming at the inner border of the tail. The second one is due to flow bursts, emerging from reconnection in the tail, arriving at the inner edge of the tail, the very reason for the substorm. The common origin is the thinning of the tail current sheet due to the stretching by the solar wind. The paper is devoted to the understanding of the first event. As observed about 10 years ago, the brightening of the growth phase arc with beadlike structures and growth of instability is often triggered by a weak arc, an auroral streamer, arriving from high latitudes and lining up with the growth phase arc. The paper proposes that the trigger process involves the formation of a new current circuit between the two arcs by the mating of the neighboring current sheets, which involves a non‐MHD process. It generates the sudden appearance and motion of the beads and constitutes a channel for the outflow of internal energy of the high‐energy plasma., Key Points: Growth phase arc brightening and instability and reconnection in the near‐Earth tail are completely separate processes. Auroral streamers may be low‐entropy content bubbles with a flow channel attached and manifested by an Alfvénic arc. Mating of two unconnected current sheets by a non‐MHD process creates a channel for outflow of energy from the high‐beta plasma layer.

The electron drift instrument (EDI) on Cluster pioneered a new method of measuring electric fields, using a beam of electrons to sample the drift velocity over a km���scale gyro orbit. The technique is especially well suited to measuring weak, sub���mV/m, convection fields due to its sensitivity (to both components in the plane perpendicular to B) and because it is unaffected by the anomalous local electric fields that are generated by spacecraft���plasma interactions. Because EDI requires exquisite beam pointing with active tracking of the firing directions, measurements are less regular, or even impossible, in rapidly varying electric and magnetic fields; however, in the many regimes where tracking is successful the resulting measurements are reliably accurate. We review the EDI technique and instrumentation, and present six areas of investigation using Cluster data: (1) Detailed comparisons of EDI data with the electric field and waves double probe measurements show excellent agreement in many cases but identify large discrepancies where strong ion outflow in the polar regions creates local spacecraft wake effects. (2) The wake effect is exploited to infer quantitative ion outflow rates. Detailed convection patterns in the (3) polar cap, (4) lobe, and (5) inner magnetosphere are derived under various driver conditions using statistical analyses of long term measurements during the Cluster mission. (6) EDI's large geometric���factor detector is used for extremely high time resolution measurements of electrons at a specified energy and pitch angle., Key Points: Complementarity of electron drift and double���probe techniques established. Inner magnetosphere, polar cap, and tail���lobe convection patterns determined. Key contribution to ion outflow velocities made.

The fracture theory for auroral arcs, developed by the author since 1980, compares the decoupling of the magnetic field from the ionosphere by the auroral acceleration region (AAR) with the breaking of a solid rod. In the latter elastic energy stored by the bending is converted into kinetic energy of the stress release motion. Similarly, magnetic energy stored in sheared magnetic fields is temporarily converted into stress release motions and finally transported as Poynting flux into the AAR. The fracture theory has been especially applied to arcs embedded in the convection of the evening auroral oval. The present study subjects the different steps in the fracture process to a critical analysis in the light of new physical insights. This boils down to a revision of the illustrating cartoon used in the earlier publications, without having affecting the quantitative evaluations. The first revision concerns the height extent of the AAR. It must be largely increased. The second revision introduces a nearly 2‐D magnetohydrodynamics (MHD) turbulence into the state of the AAR. This is supported by high‐altitude electric field data and leads to new view of auroral rays. The third revision describes the transition from the AAR to the ionosphere as structured by so‐called potential fingers, which contain substantial fractions of the total field‐parallel potential drop. The most important modification pertains to the average U‐shaped potential of a spontaneously propagating AAR. While the leading edge of the auroral current sheet is structured by stress release motions, the reverse flow in the rear section escapes simple interpretation. It is proposed that this flow is driven by a turbulent transport of reversed momentum from front to rear in response to the incompressibility of the magnetic field in the acceleration region. This leads to a revision of the field‐aligned currents and wavefield in the rear of the arc.

The decision of the Max Planck Society (MPG) to get involved in the establishment of an incoherent scatter radar in northern Europe was intimately linked to the future of the Max Planck Institute for Aeronomy (MPAe) in Katlenburg-Lindau. Delegates of the MPG played an important role in defining the rules for participation in EISCAT during the period from 1973 to 1975. The "technical" period from 1976 to 1981 was mainly devoted to the development of the UHF transmitter and the klystrons. The latter encountered great difficulties, causing substantial delays. During the same period the ionospheric heating facility was established by MPAe at Ramfjordmoen, Norway. The period following the inauguration in August 1981 saw a great number of changes in the leading personnel. In this context much attention had to be given to taxation rules. Besides continuing hardware problems with the UHF radar, severe problems arose with design and manufacturing of the VHF klystrons, requiring changes of the contractor. However, by fall of 1983 the UHF radar was able to reach the intended operational level. In 1984 important steps were made for archiving and for proper exploitation of the EISCAT data.

A new scenario is presented for the onset of a substorm and the nature of the breakup arc. There are two main components, current sheet avalanche and stop layer. The first refers to an earthward flow of plasma and magnetic flux from the central current sheet of the tail, triggered spontaneously or by some unknown interaction with an auroral streamer or a suddenly appearing eastward flow at the end of the growth phase. The second offers a mechanism to stop the flow abruptly at the interface between magnetosphere and tail and extract momentum and energy to be partially processed locally and partially transmitted as Poynting flux toward the ionosphere. The stop layer has a width of the order of the ion inertial length. The different dynamics of the ions entering freely and the magnetized electrons create an electric polarization field which stops the ion flow and drives a Hall current by which flow momentum is transferred to the magnetic field. A simple formalism is used to describe the operation of the process and to enable quantitative conclusions. An important conclusion is that by necessity the stop layer is also highly structured in longitude. This offers a natural explanation for the coarse ray structure of the breakup arc as manifestation of elementary paths of energy and momentum transport. The currents aligned with the rays are balanced between upward and downward directions. While the avalanche is invoked for explaining the spontaneous substorm onset at the inner edge of the tail, the expansion of the breakup arc for many minutes is taken as evidence for a continued formation of new stop layers by arrival of flow bursts from the near-Earth neutral line. This is in line with earlier conclusions about the nature of the breakup arc. Small-scale structure, propagation speed, and energy flux are quantitatively consistent with observations. However, the balanced small-scale currents cannot constitute the substorm current wedge. The source of the latter must be located just earthward of the stop layer in the near-dipolar magnetosphere and be powered by the internal energy of the flow bursts. The stop layer mechanism is in some way the inverse of reconnection, as it converts flow into electromagnetic energy, and may have wide applicability in astrophysical plasmas.

Alfvenic arcs are often found adjacent to the poleward border of the substorm bulge. They are created by precipitating narrow electron beams with a broad energy distribution, mainly below 1 keV, and dominated by Alfvenic structures of small scales and frequencies in the 1 Hz range. They are also associated with transverse ion heating. A balanced field-aligned current system accompanying the poleward Alfvenic arc reveals its underlying structure being a broad channel of primary energy inflow of several tens of kilometers width. The associated magnetic shear stresses drive a flow along the poleward side of the substorm current wedge. Polar cap convection passes through this channel before entering the substorm bulge. The small-scale electromagnetic structures are developing from the primary energy inflow by scale breaking and multiple reflections in the ionospheric Alfven resonator. By their parallel electric field components these structures decouple effectively from the ionosphere. Their energy content is converted into kinetic energy of auroral particles in the topside ionosphere. A selected data set from FAST, DE 2, and Cluster is presented for characterizing the energetic and electromagnetic properties of the Alfvenic arc. Its function in the substorm can be summarized as a preconditioning of the polar cap plasma and magnetic field for entry into the magnetosphere through the poleward arc. This occurs under substantial density depletion.

The paper reviews three magnetosphere-ionosphere coupling scales characterizing (1) inverted-V auroral arcs, (2) so-called Alfvenic arcs, and (3) dense plasma clouds artificially injected into the magnetosphere. The three scale-breaking processes are different but follow all from the principle of perfect matching of the wave impedance of the energy and momentum carrier with the effective resistance of the energy dump. As a consequence, wave reflections from the ionosphere are absent or present only in a short initial phase and lead to quasi-stationary wave fields. In inverted-V auroral arcs the energy conversion occurs in the low magnetosphere by postacceleration of the current-carrying hot electrons. For the Alfvenic arc the energy is transferred first to dispersive Alfven waves which are then damped in the topside ionosphere by accelerating electrons along B and ions transversely to B. Barium plasma clouds break into narrow striations mapping to ionospheric scales that experience a strongly reduced Pedersen conductivity and thus achieve the perfect matching. It is also argued that the scale ΣP/K, derived from electrostatic mapping, is not suited to describe M-I coupling.

[1] We performed a statistical study of low frequency (LF) auroral hiss recorded at South Pole Station in 2004, 2005, and 2007, and very low frequency (VLF) hiss recorded in 2000–2008. As expected, most auroral hiss occurs in the pre-midnight sector. However, there is a secondary peak in occurrence in the pre-noon sector (1000–1530 UT; ∼ 0630–1200 magnetic local time (MLT)) and somewhat more events occur in the post-noon sector (1530–2100 UT; ∼ 1200–1730 MLT), with a null in occurrence around noon MLT. Individual dayside events appear similar to nightside hiss, but statistically they do not extend to as high frequencies. Solar wind discontinuities or impulses on the magnetopause are not correlated with these events. All-sky camera, photometer, magnetometer, riometer, and VLF receiver data show that dayside LF hiss almost always extends to the VLF range and is often associated with active aurora. Examination of interplanetary magnetic field (IMF), substorm conditions, and Kp/AE/QI indices at times of dayside hiss suggests differences between the pre-noon and post-noon events: pre-noon events are associated with IMF By 0. The correlation between pre-noon events and By

The in situ cometary dust particle instrument COSIMA (COmetary Secondary Ion Mass Analyser) onboard ESA's Rosetta mission has collected about 31 000 dust particles in the inner coma of comet 67P/Churyumov–Gerasimenko since August 2014. The particles are identified by optical microscope imaging and analysed by time-of-flight secondary ion mass spectrometry. After dust particle collection by low speed impact on metal targets, the collected particle morphology points towards four families of cometary dust particles. COSIMA is an in situ laboratory that operates remotely controlled next to the comet nucleus. The particles can be further manipulated within the instrument by mechanical and electrostatic means after their collection by impact. The particles are stored above 0°C in the instrument and the experiments are carried out on the refractory, ice-free matter of the captured cometary dust particles. An interesting particle morphology class, the compact particles, is not fragmented on impact. One of these particles was mechanically pressed and thereby crushed into large fragments. The particles are good electrical insulators and transform into rubble pile agglomerates by the application of an energetic indium ion beam during the secondary ion mass spectrometry analysis. This article is part of the themed issue ‘Cometary science after Rosetta’.

The importance of reconnection in astrophysics has been widely recognized. It is instrumental in storing and releasing magnetic energy, the latter often in a dramatic fashion. A closely related process, playing in very low beta plasmas, is much less known. It is behind the acceleration of auroral particles in the low-density environment several 1000 km above the Earth. It involves the appearance of field-parallel voltages in presence of intense field-aligned currents. The underlying physical process is the release of magnetic shear stresses and conversion of the liberated magnetic energy into kinetic energy of the particles creating auroral arcs. In this process, field lines disconnect from the field anchored in the ionosphere and reconnect to other field lines. Because of the stiffness of the magnetic field, the process resembles mechanical fractures. It is typically active in the low-density magnetosphere of planets. However, it can also lead to significant energy conversion with high-energy particle production and subsequent gamma ray emissions in stellar magnetic fields, in particular of compact objects.

As a result of increasing public and political interest in ‘space’ (i.e. solar system) exploration at the global scale, the Space Advisory Group of the European Commission has evaluated the situation in Europe with regard to its potential to participate in this ambitious global enterprise. Aspects of science, technology, environment and safety, society, spin-offs and international cooperation were all considered. The group concluded that Europe possesses sufficient key technologies and scientific expertise to play a major role in international space exploration and has recommended that the EU take a central role to ensure the success of future European space exploration, not only to give a clear political signal for the way forward but also to ensure an appropriate financial framework. In this way Europe would embrace the spirit of the European Space Policy and contribute to the knowledge-based society by investing significantly in space-based science and technology, thereby playing a strong role in international space exploration.

Reconnection of sheared magnetic fields is commonly treated by regarding the component perpendicular to the antiparallel components as a largely inert guide field. In this paper an alternative is proposed in which the free energy residing in the shear field is being converted prior to reconnection. This happens in high-density, dissipative current sheets bordering the reconnection site. A global scenario is presented in which low-intensity currents out of the photosphere are converging into the narrow, high-intensity currents at high altitude. This is enabled by the obliqueness of the latter. The very short timescale of the energy conversion causes a lateral propagation of the current sheets. In a quasi-stationary situation, it balances the reconnection rate, which turns out to be much lower than in guide-field approaches. Another important consequence of the obliqueness is the field-parallel emission of runaway electrons. Accelerated up to tens of keV, they are possibly important contributors to the production of hard X-rays during the impulsive phase of a flare, but only in areas of upward-directed currents. Quantitative evaluation of the model predicts various potentially observable properties, such as width and propagation speed of the generated flare ribbons, spatial dependences of the electron spectrum, size of the area of energy deposition, and successive decrease of the shear angle between conjugate footpoints. The presented theoretical model can account for the observed brightness asymmetry of flare ribbons with respect to the direction of the vertical currents.

This paper presents a scenario for the chromospheric evaporation during solar flares, which is inspired by the chain of events leading to the formation of auroral arcs and ionospheric evacuation during magnetospheric substorms. The plasma, ejected from high coronal altitudes during a flare reconnection event, accumulates at the tops of coronal loops by braking of the reconnection flow, possibly by fast shock formation. A high-beta layer forms and distorts the magnetic field. Energy contained in magnetic shear stresses is transported as Alfven waves from the loop-top toward the chromosphere. It is shown that under these conditions the Alfven waves carry enough energy to feed the chromospheric evaporation process. The second subject of this investigation is identification of the most effective energy dumping or wave dissipation process. Several processes are being analyzed: ion-neutral collisions, classical and anomalous field-aligned current dissipation, and critical velocity ionization. All of them are being discarded, either because they turn out to be insufficient or imply very unlikely physical properties of the wave modes. It is finally concluded that turbulent fragmentation of the Alfven waves entering the chromosphere can generate the required damping. The basic process would be phase mixing caused by a strongly inhomogeneous distribution of Alfvenic phase speed and laminar flow breakup by Kelvin-Helmholtz (K-H) instability. The filamentary (fibril) structure of the chromosphere thus appears to be essential for the energy conversion, in which the K-H instability is the first step in a chain of processes leading to ion thermalization, electron heating, and neutral particle ionization. Quantitative estimates suggest that a transverse structure with scales not far below 100 km suffices to produce strong wave damping within a few seconds. Nonthermal broadening of some metallic ion lines observed during the pre-impulsive rise phase of a flare might be a residue of the turbulent breakup process.

International audience; In 2005 the then ESA Directorate for Human Spaceflight, Microgravity and Exploration (D-HME) commissioned a study from the European Science Foundation's (ESF) European Space Sciences Committee (ESSC) to examine the science aspects of the Aurora Programme in preparation for the December 2005 Ministerial Conference of ESA Member States, held in Berlin. A first interim report was presented to ESA at the second stakeholders meeting on 30 and 31 May 2005. A second draft report was made available at the time of the final science stakeholders meeting on 16 September 2005 in order for ESA to use its recommendations to prepare the Executive proposal to the Ministerial Conference. The final ESSC report on that activity came a few months after the Ministerial Conference (June 2006) and attempted to capture some elements of the new situation after Berlin, and in the context of the reduction in NASA's budget that was taking place at that time; e. g., the postponement sine die of the Mars Sample Return mission. At the time of this study, ESSC made it clear to ESA that the timeline imposed prior to the Berlin Conference had not allowed for a proper consultation of the relevant science community and that this should be corrected in the near future. In response to that recommendation, ESSC was asked again in the summer of 2006 to initiate a broad consultation to define a science-driven scenario for the Aurora Programme. This exercise ran between October 2006 and May 2007. ESA provided the funding for staff support, publication costs, and costs related to meetings of a Steering Group, two meetings of a larger ad hoc group ( 7 and 8 December 2006 and 8 February 2007), and a final scientific workshop on 15 and 16 May 2007 in Athens. As a result of these meetings a draft report was produced and examined by the Ad Hoc Group. Following their endorsement of the report and its approval by the plenary meeting of the ESSC, the draft report was externally refereed, as is now normal practice with all ESSC-ESF reports, and amended accordingly. The Ad Hoc Group defined overarching scientific goals for Europe's exploration programme, dubbed "Emergence and co-evolution of life with its planetary environments,"focusing on those targets that can ultimately be reached by humans, i.e., Mars, the Moon, and Near Earth Objects. Mars was further recognized as the focus of that programme, with Mars sample return as the recognized primary goal; furthermore the report clearly states that Europe should position itself as a major actor in defining and leading Mars sample return missions. The report is reproduced in this article. On 26 November 2008 the Ministers of ESA Member States decided to give a high strategic priority to the robotic exploration programme of Mars by funding the enhanced ExoMars mission component, in line therefore with the recommendations from this ESSC-ESF report.

International audience; The presence of solid carbonaceous matter in cometary dust was established by the detection of elements such as carbon, hydrogen, oxygen and nitrogen in particles from comet 1P/Halley1, 2. Such matter is generally thought to have originated in the interstellar medium3, but it might have formed in the solar nebula—the cloud of gas and dust that was left over after the Sun formed4. This solid carbonaceous material cannot be observed from Earth, so it has eluded unambiguous characterization5. Many gaseous organic molecules, however, have been observed6, 7, 8, 9; they come mostly from the sublimation of ices at the surface or in the subsurface of cometary nuclei8. These ices could have been formed from material inherited from the interstellar medium that suffered little processing in the solar nebula10. Here we report the in situ detection of solid organic matter in the dust particles emitted by comet 67P/Churyumov–Gerasimenko; the carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites11, 12. The organic matter in meteorites might have formed in the interstellar medium and/or the solar nebula, but was almost certainly modified in the meteorites’ parent bodies11. We conclude that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before and/or after being incorporated into the comet.

Three types of processes, occurring in the weakly ionized plasmas of the Earth’s ionosphere as well as in the solar chromosphere, are being compared with each other. The main objective is to elaborate on the differences introduced primarily by the grossly different magnitudes of the densities, both with respect to the neutral and, even more so, to the plasma constituents. This leads to great differences in the momentum coupling from the plasma to the neutral component and becomes clear when considering the direct electric current component transverse to the magnetic field, called “Pedersen current” in the ionosphere, which has no quasi-static counterpart in the chromosphere. The three classes of processes are related to the dynamical response of the two plasmas to energy influx from below and from above. In the first two cases, the energy is carried by waves. The third class concerns plasma erosion or ablation in the two respective regions in reaction to the injection of high Poynting and/or energetic particle fluxes.

Knowledge of the magnetization of planetary bodies constrains their origin and evolution, as well as the conditions in the solar nebular at that time. On the basis of magnetic field measurements during the descent and subsequent multiple touchdown of the Rosetta lander Philae on the comet 67P/Churyumov-Gerasimenko (67P), we show that no global magnetic field was detected within the limitations of analysis. The Rosetta Magnetometer and Plasma Monitor (ROMAP) suite of sensors measured an upper magnetic field magnitude of less than 2 nanotesla at the cometary surface at multiple locations, with the upper specific magnetic moment being −5 ampere–square meters per kilogram for meter-size homogeneous magnetized boulders. The maximum dipole moment of 67P is 1.6 × 10 8 ampere–square meters. We conclude that on the meter scale, magnetic alignment in the preplanetary nebula is of minor importance.

In the morning sector of its near equatorial orbit Equator-S observed large magnetic oscillations in the Pc 5 frequency range. The poloidal part of the observed oscillations is usually interpreted as being caused by surface waves of the magnetopause driven by a Kelvin-Helmholtz instability (KHI). We found, however, that these oscillations are sometimes additionally superposed with reconnection signatures. This occurs during periods of northward interplanetary magnetic field. In fact, the Walen relation as a test for reconnection is satisfied for many short periods of time. This indicates transient and thin layers of reconnection, embedded in the large scale motion. We compared our observations with appropriate magnetohydrodynamic simulations allowing reconnection in the case of shear flows. Comparing the simulation results with the magnetic field and plasma observations we found good agreement of simulations and observations. We conclude that Equator-S has observed reconnection embedded in Kelvin-Helmholtz instability generated vortices in its nonlinear phase.

Auroral particle acceleration is the result of the appearance of effective resistance in field-parallel electric currents and of the voltages needed to overcome them. The basic condition for that to occur is that the field-aligned current density somewhere in the system approaches the thermal flux of the background electrons. The generation of such high-current density is a result of the dynamic interaction of the magnetospheric and ionospheric plasmas. The free magnetic energy stored in the current system is converted into kinetic energy by the auroral acceleration process and, consequently, magnetic shear stresses are released. Thus auroral acceleration can be seen as the manifestation of a magnetic elasticity problem in low-beta plasmas, a problem that may arise in any cosmic system with sufficiently strong magnetic field. The lecture concentrates on mechanisms for current concentration and the dynamic consequences of energy conversion and stress release on the overall system.

It is proposed that the coincidence of higher brightness and upward electric current observed by Janvier et al. during a flare indicates electron acceleration by field-parallel potential drops sustained by extremely strong field-aligned currents of order 104 A m−2. A few consequences are discussed here.

It is proposed that the coincidence of higher brightness and upward electric current observed by Janvier et al. during a flare indicates electron acceleration by field-parallel potential drops sustained by extremely strong field-aligned currents of the order of 104 A m−2. A consequence of this is the concentration of the currents in sheets with widths of the order of 1 m. The high current density suggests that the field-parallel potential drops are maintained by current-driven anomalous resistivity. The origin of these currents remains a strong challenge for theorists.

Knowledge of the magnetization of planetary bodies constrains their origin and evolution, as well as the conditions in the solar nebular at that time. On the basis of magnetic field measurements during the descent and subsequent multiple touchdown of the Rosetta lander Philae on the comet 67P/Churyumov-Gerasimenko (67P), we show that no global magnetic field was detected within the limitations of analysis. The Rosetta Magnetometer and Plasma Monitor (ROMAP) suite of sensors measured an upper magnetic field magnitude of less than 2 nanotesla at the cometary surface at multiple locations, with the upper specific magnetic moment being3.1 × 10(-5) ampere-square meters per kilogram for meter-size homogeneous magnetized boulders. The maximum dipole moment of 67P is 1.6 × 10(8) ampere-square meters. We conclude that on the meter scale, magnetic alignment in the preplanetary nebula is of minor importance.

A unique multispectral imager and an associated multispectral analysis framework are described which together constitute a new diagnostic tool for auroral research. By acquiring spatial and spectral data simultaneously, multispectral imaging allows one to exploit physical connections between auroral morphology and the auroral optical spectrum in a way that sequential spectral imaging cannot. The initial research focus is on imaging the transition in the incident energy spectrum during the formation of discrete arcs—that is, when the precipitating population is characterized by keV electrons. A technique is presented which uses two spectral bands (centered at 4278 and 7325 A) to extend the effective dynamic range of passive imaging to much lower energies.

The analysis of 37 Equator-S magnetopause crossings with exceptionally smooth profiles in magnetic field magnitude is presented. The crossings are distributed through 10 orbits and occur under both northward and southward interplanetary magnetic field (IMF) conditions. Under northward IMF, when subsolar reconnection is not believed to occur, the ramp and the field rotation generally overlap. In all 17 cases with a southward magnetosheath field, the field rotation to a northward direction lies sunward of the start of the ramp in field magnitude. Of these cases, five show a field magnitude dip during the field rotation, with shorter field rotations tending to be associated with deeper minima. We suggest that this characteristic of some of the crossings under southward IMF might reflect the distance of the crossing from a reconnection site. Field maxima sometimes appear just inside the magnetopause, but the occurrence of these features is not confined to those crossings showing field minima in the magnetosheath. Examination of the field rotation shows that the rotation is rarely circularly polarized, even when a field magnitude minimum is absent. Comparison with recent two-dimensional hybrid models shows good qualitative agreement.

EDI measures the drift velocity of artificially injected electron beams. From this drift velocity, the perpendicular electric field and the local magnetic field gradients can be deduced when employing different electron energies. The technique requires the injection of two electron beams at right angles to the magnetic field and the search for those directions within the plane that return the beams to their associated detectors after one or more gyrations. The drift velocity is then derived from the directions of the two beams and/or from the difference in their times-of-flight, measured via amplitude-modulation and coding of the emitted electron beams and correlation with the signal from the returning electrons. After careful adjustment of the control parameters, the beam recognition algorithms, and the onboard magnetometer calibrations during the commissioning phase, EDI is providing excellent data over a wide range of conditions. In this paper, we present first results in a variety of regions ranging from the polar cap, across the magnetopause, and well into the magnetosheath.Key words. Electron drift velocity (electric fields; plasma convection; instruments and techniques)

An analysis of ground-based images and rocket-borne electron data from the Physics of Auroral Zone Electrons II rocket experiment suggests that suprathermal ambient electrons, precipitating in field-aligned bursts, may play an active role in regulating the decay of auroral potential structures. A set of discrete arcs were observed to form in quasiperiodic succession behind a passing westward traveling surge. The nascent arcs faded rapidly, losing 90% of their luminosity within 15 s. The fading then abruptly stopped at a brightness consistent with a peak electron energy of ∼1.5 keV. Rocket-borne measurements of energetic electrons over the residual arcs 2 min later revealed a superposition of hot isotropic and cold ionospheric electrons, the latter precipitated in dispersive periodic (3-5 Hz) bursts from a source altitude of 3000-6000 km. A combined analysis of the optical and particle data showed that the FAB component (1) was present for at least 1 min, (2) precipitated in spatial scales of ∼10 km and (3) carried a number flux comparable to that of the overlying isotropic population. Bulk plasma parameters calculated in the vicinity of the fading arcs revealed an inverse relationship between parallel potential drop and number flux carried by the field-aligned population.

The essentials of the auroral acceleration process are summarized, and a simple set of relations is derived which enables the analysis of parallel potential drops and related parameters in relevant astrophysical systems. The main condition for the occurrence of such potential drops is the existence of intense field-aligned currents driven by forces in the outer, weak magnetic fields of an astrophysical object. In the strong fields near the object the currents may reach a critical limit where the mirror force or current-driven anomalous resistivity require the setup of parallel potential drops for the maintenance of current continuity. Analysis of three candidate systems, solar flares, cataclysmic variables and accreting neutron stars, reveals the possibility of fast and efficient production of highly relativistic electrons manifesting themselves by gamma ray emission.

Equator-S made many magnetopause crossings between 06 00 and 10 40 LT, taking magnetic field data at 64 or 128 vectors/s. This allows structure within the shortest crossings to be examined, and examples having an exceptionally smooth ramp in |B| are presented. These crossings show no significant structure at sub-kilometre scales. Strong electromagnetic wave activity did not occur within the ramp, although some crossings have extremely low amplitude compressional waves in part of the field ramp close to the magnetosphere, with frequencies close to the lower hybrid frequency. These initial results suggest that physical processes on sub-km scales play no role in determining the structure of the current layer.

The paper deals with the conditions enabling auroral acceleration, the coupling of the various processes involved, and the feedback mechanisms on the source of energy and mass. Particular emphasis is given to the energy flow, for which four different paths are identified, whose efficiencies and time-scales are being compared. Furthermore, the necessary ingredients of auroral arc formation are discussed, among which current filamentation and dynamical feedback on the source plasma are still largely unknown elements. The paper closes with a conjecture about the origin of auroral folds and curls, which is attributed to the evacuation process of the topside ionosphere, the ensuing modification of the electric potential distribution, generation of small-scale shear flows and their instability.

We report evidence of a long-lasting reconnection event during which the accelerated plasma flow direction changes in response to an interplanetary magnetic field (IMF) By reversal, indicating a change in the reconnection site location. The observations were made by Equator-S on the dawn flank of the magnetopause and consist of a large number of plasma jets detected mostly within magnetospheric flux transfer events. The plasma jets were found in quantitative agreement with the theoretical predictions for reconnection. The reversal of the plasma flow direction in the jets following the reversal of the By component not only confirms that the dayside reconnection configuration is controlled by the IMF, as opposed to local control, but also stresses the importance of the IMF dawn-dusk component, in addition to the north–south component, in determining the global configuration of the reconnection.

In the morningside plasma sheet close to the equatorial plane, satellites often observe magnetic field amplitude variations on timescales of several minutes (Pc5 range). Here we present an event study which shows that in some cases these variations can be a result of magnetospheric regions with different magnetic field amplitudes sloshing back and forth past the satellite. The sloshing is caused by toroidal Alfven oscillations.

Electron density and temperature, adiabatic thermal current, and field-aligned conductivity have been estimated on the basis of observations by the Defense Meteorological Satellite Program (DMSP) satellites above the auroral oval and from measurement by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM) satellite in the near-Earth plasma sheet. We found that the estimated densities are comparable between these two satellites, while the temperatures obtained from an accelerated Maxwellian fitting procedure used on the DMSP spectra are far lower than those measured by IRM in the near-Earth plasma sheet. From this temperature discrepancy we conclude that the accelerated electrons do not come from the equatorial plane of the magnetosphere but from the region just above the field-aligned potential difference at an altitude of a few RE. The DMSP data show that a large field-aligned potential difference, which accelerates auroral electrons downward, is formed in the region with low field-aligned conductivity. The IRM data show that the field-aligned conductivity decreases with increasing XGSM distance, increasing AE index, and after earthward high-speed flow passage. The adiabatic thermal current estimated from the IRM data is found to be not enough to supply typical auroral current. Though the mechanism that produces field-aligned potential difference has not been identified yet, these results suggest that field-aligned potential difference is formed to keep the balance between the field-aligned current generated by magnetospheric processes and the current carried away from the generator region by accelerated electrons.

The paper deals with five selected issues of the dynamical coupling of the near-Earth plasma sheet and magnetosphere, (1) substorm initiation, (2) dipolarization, (3) pressure release of the outer magnetosphere via the auroral energy conversion process, (4) magnetization of the very high beta plasma assembling at the inner edge of the tail, and (5) penetration of energetic particles into the ring current below L ≅ 4. One outstanding and strongly debated subject is not discussed here, the origin of the substorm current wedge. The main conclusions (or personal preferences) are: (1) the substorm is initiated by formation of a near-Earth neutral line; (2) dipolarization occurs through magnetic flux transport by the earthward reconnection flow and not by current diffusion; (3) the auroral energy conversion process, the “auroral pressure valve”, contributes substantially to the pressure release during the substorms; (4) high beta (≫ 10) plasma breaks up into smaller scale blobs under slow magnetization; and (5) deep and prolonged penetration of hot plasma sheet plasma into the middle magnetosphere produces currents and electric fields which lead to the growth of the storm-time ring current.

An overview of the origin of the EQUATOR-S mission, the spacecraft development and the mission history is given. In spite of its short operational life of only five months, the major goals were fulfilled, except that its contribution to IASTP science is restricted to the morning sector of the outer magnetosphere. A few data sets illustrating the satellite's capability are presented.

In space plasmas the phenomenon of mass loading is common. Comets are one of the most evident objects where mass loading controls to a large extent the structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through our solar system. In this review we summarize both the current observations and the relevant theoretical approaches. First we survey the MHD methods, starting with a discussion how mass loading affects subsonic and supersonic gasdynamics flows, continuing this with single and multi-fluid MHD approaches to describe the flow when mass, momentum and energy is added, and we finish this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitch angle and energy scattering in linear and quasi-linear approximations. The different descriptions differ in assumptions and conclusions; we point out the differences, but it is beyond the scope of the paper to resolve all the conflicts. Applications of these techniques to comets, planets, artificial ion releases, and to the interplanetary neutrals are reviewed in the last section, where observations are also compared with models, including hybrid simulations as well. We conclude the paper with a summary of the most important open, yet unsolved questions.