Nightside Magnetotail Research Articles

Energization of Cold Ions in Magnetic Reconnection: Particle‐in‐Cell Simulation

AbstractCold ions in the ionosphere can reach the nightside magnetotail along the magnetic field lines, which may substantially affect the magnetotail reconnection and reconnection‐driven phenomena. Although reconnection has been intensively studied in the past decades, the dynamics of cold ions in reconnection are poorly known. We performed a 2.5‐D full kinetic simulation to study the energization of cold ions in a symmetric anti‐parallel reconnection. We find that the cold ions can be considerably energized during reconnection. The average energy gain per cold ion is smaller than that per hot ion but larger than that per electron. The cold ions with the highest energy are firstly accelerated around the X‐line, and further accelerated around the reconnection front in the outflow region. They perform the meandering motion at the X‐line, while simultaneously accelerated by the reconnection out‐of‐plane electric field. The velocity distribution of cold ions around the X‐line is characterized by the counter‐streaming inflow component and the accelerated component in the +y quadrant. The cold ions are picked up at the reconnection front and trapped by the secondary island, resulting in a bump in the energy spectrum. The energy of the bump is close to the energy of the propagating speed of the reconnection front and the secondary island. Furthermore, a crescent‐shaped velocity distribution is observed at the reconnection front, caused by the finite Larmor radius effect of the high‐energy cold ions in the flux pileup region. Our result presents an important building block toward a fully understanding of the magnetosphere‐ionosphere coupling.

Global Spatial Distribution of Dipolarization Fronts in the Saturn's Magnetosphere: Cassini Observations

AbstractDipolarization front (DF), characterized by a sharp increase of the south‐north component of the magnetic field, is suggested to play an important role in transferring plasmas, magnetic fluxes, and energy in the planetary magnetosphere. Using the measurements from the Cassini spacecraft between January 1, 2005 and June 15, 2011, we successfully selected 96 DF events, and obtained the global spatial distribution of DFs in the Saturn's magnetosphere. For the first time, we found that DFs are distributed not only in the nightside magnetotail but also in the dayside magnetosphere. The dayside DF events are mainly located from X = 10 RS to X = 30 RS (RS is the Saturn's radius) while the nightside events have a wide range, up to X∼−50 RS. Moreover, the DFs are observed to be asymmetric in the south‐northern hemisphere: ∼70% of events in the northern hemisphere and ∼30% of events in the southern hemisphere, which is likely to be due to the asymmetric orbit coverage of the Cassini in south‐northern hemisphere. The occurrence of dayside DFs provides a strong evidence that the magnetic reconnection could also occur in the dayside of Saturn's magnetosphere. Thus, our results concerning the location and distribution of DFs are helpful for the study of the site of magnetic reconnection and energy transport/dissipation in Saturn's magnetosphere.

Saturn's Nightside Ring Current During Cassini's Grand Finale

AbstractDuring Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

ANALYSIS OF THE DAILY HOURLY DEPARTURES OF THE GEOMAGNETIC H FIELD OVER LOW-LATITUDE STATIONS

The telecommunication technologies deployed in the last decades have shown that there are better days ahead. From electric telegraphs systems in the 19th century to today’s wireless communication using satellites and land links (Lanzerotti, 2001). The horizontal component (H-field) of the geomagnetic field has great impacts in the present-day communication technologies. The aim of this paper was to analyse the daily hourly departures of geomagnetic H-field along the low-latitude stations on the equatorial chain in relation to their correlation coefficients with their longitudes. The magnitude of H-component is a very significant factor in the determination of the total magnetic field of any location on the earth. The baseline values (H0) which is the average of H components at 23LT, 24LT, 1LT and 2LT were subtracted from the hourly values of H (Ht) to obtain the hourly departures of H (dH). The hourly departures of H (dH) from the baseline is non-cyclic since H1 ≠ H24 as observed over the three MAGDAS stations in Nigeria. The correlation coefficients between dHs of Lagos/Ilorin, Ilorin/Abuja and Abuja/Ilorin are 0.97, 0.75 and 0.66 respectively. This suggests that substorms are deflected eastwards along different longitudes due to the rotation of the earth such that locations in the same longitudes have the same hourly departures. There is no solar radiation during the nighttime, thus dH should be zero but for the energy stored in the magnetotail, the reduction of hourly departures towards nighttime in Nigeria is evident. This is further revealed as the midnight dH amplitudes decrease regularly. The eastward motion of the electrons during substorms was suggested to be responsible for the reduction of the H-field at the nightside magnetotail region for all the stations. These findings will assist the telecommunication companies to know which location is best for their transmitters to be sited in order to reduce loss of signals along the optical cables to the receivers. Furthermore, global telecomms companies should have business continuity plan which should include risk assessment and prompt response to geomagnetic disturbances being one of the causes for telecommunication outages.

Shear Flow‐Interchange Instability in Nightside Magnetotail as Proposed Cause of Auroral Beads as a Signature of Substorm Onset

AbstractA geometric wedge model of the near‐Earth nightside plasma sheet is used to derive a wave equation for low‐frequency shear flow‐interchange waves which transmit sheared zonal flows along magnetic flux tubes toward the ionosphere. Discrepancies with the wave equation result used in Kalmoni et al. (2015, https://doi.org/10.1002/2015JA021470) for shear flow‐ballooning instability are discussed. The shear flow‐interchange instability appears to be responsible for substorm onset. The wedge wave equation is used to compute rough expressions for dispersion relations and local growth rates in the midnight region of the nightside magnetotail where the instability develops, forming the auroral beads characteristic of geomagnetic substorm onset. Stability analysis for the shear flow‐interchange modes demonstrates that nonlinear analysis is necessary for quantitatively accurate results and determines the spatial scale on which the instability varies.

Locally Generated ULF Waves in the Martian Magnetosphere: MAVEN Observations

AbstractWe investigate Martian ultralow frequency (ULF) electromagnetic waves generated by local plasma instabilities below the Martian bow shock. Recent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown that ULF waves generated upstream of the Martian bow shock can propagate down to the upper ionosphere, possibly facilitating heavy ion escape from Mars by heating the ionospheric plasma. In contrast to the upstream waves oscillating near the upstream proton cyclotron frequency, we identify narrow band ULF magnetic field fluctuations with frequencies near the local proton cyclotron frequency (fcp(local)) from MAVEN data. In addition to expected proton cyclotron waves locally generated in the magnetosheath, we newly identify compressional narrow band emissions near fcp(local) (and its harmonics for some cases) in the dayside upper ionosphere and in the nightside magnetotail. The dayside waves are preferentially observed for high solar extreme ultraviolet (EUV) conditions and are often associated with ring/shell‐like, hot protons of magnetosheath origin in the presence of cold, dense ionospheric protons. The nightside waves exhibit distinct preference for high‐solar‐EUV, strong‐solar‐wind conditions, under which both warm and cold protons are enhanced. The observed properties of these compressional waves are generally consistent with a proton Bernstein mode instability driven by a positive perpendicular slope in proton velocity distribution functions. The excited waves can cause perpendicular heating of thermal protons, thereby transferring energy from precipitating hot protons to cold ionospheric protons.

Investigating the Development of Abnormal Subauroral Ion Drifts (ASAID) During the Magnetically Quiet Times of October 2003

AbstractAppearing as inverted subauroral ion drifts (SAID) during magnetically disturbed and quiet times, the abnormal SAID (ASAID) is a recently reported phenomenon. Disturbed‐time ASAID development can be explained with a magnetospheric cold‐plasma‐shoulder‐hot‐ring‐current interface layer that generates an earthward (or inward) magnetospheric electric (E) field, which maps down to the ionospheric cold‐plasma‐trough‐hot‐ring‐current interface layer as a southward (or equatorward) ionospheric E field. However, this plasmaspheric shoulder cannot develop under magnetically quiet conditions. Therefore, the quiet time ASAID development is still not clear and our main aim is to unravel its development. We analyze multisatellite and multiinstrument observations and show four scenarios demonstrating ASAID events during the magnetically quiet period of 9–11 October 2003. Results obtained for the longer 9–12 October 2003 time period provide observational evidence of the continuous presence of solar wind Alfven waves (i) driving dayside magnetopause and nightside magnetotail reconnections, (ii) bouncing back and forth in the coupled magnetosphere and ionosphere, and thus (iii) creating bouncing Alfven waves that ripple the plasma sheet's inner edge during the ASAID events investigated. These observational results suggest that the bouncing Alfven waves led to the development of a magnetospheric cold‐plasma‐ripple‐hot‐ring‐current interface layer during these quiet times leading to quiet time ASAID development. Confirming this suggestion, we demonstrate and analyze 10 February and 5 March 1991 scenarios, depicting ASAID events, and invoke published magnetospheric cross‐tail EY observations evidencing bouncing Alfven waves and plasma sheet rippling during the underlying energetic particle injection events.

Investigating the Development of Localized Neutral Density Increases During the 24 August 2005 Geomagnetic Storm

AbstractWe investigate localized thermospheric neutral density increases (or density spikes) developed during the 24 August 2005 geomagnetic storm and utilize a multi‐instrument database for monitoring the underlying thermospheric, ionospheric, magnetospheric, and solar wind conditions. We study five scenarios occurring under eastward/westward BY dominations and demonstrating the development of density spikes during various events. These scenarios show (1) a northern density spike associated with enhanced antisunward polar flows driven by dayside merging along old‐open field lines during the initial phase, (2) some southern density spikes associated with enhanced sunward auroral flows caused by auroral brightening events during the storm initial phase and main phase, (3) two pairs of southern dayside‐nightside density spike configuration associated with enhanced antisunward polar flows caused by dayside merging and nightside magnetotail reconnections along old‐open field lines during the underlying substorm activity, and (4) a series of enhanced polar sunward flows developed in the polar cap driven by different processes during the recovery phase. We provide a detailed analysis of these events including the specification of underlying field aligned current systems, flow channel types, and auroral forms. Observational results demonstrate the various density spikes and their associated auroral forms and/or flow channels and thus provide a better understanding of their development during this intense geomagnetic storm.

On the origins of magnetic flux ropes in near‐Mars magnetotail current sheets

AbstractWe analyze Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of magnetic flux ropes embedded in Martian magnetotail current sheets, in order to evaluate the role of magnetotail reconnection in their generations. We conduct a minimum variance analysis to infer the generation processes of magnetotail flux ropes from the geometrical configuration of the individual flux rope axial orientation with respect to the overall current sheet. Of 23 flux ropes detected in current sheets in the near‐Mars (∼1–3 Martian radii downstream) magnetotail, only 3 (possibly 4) can be explained by the magnetotail reconnection scenario, while the vast majority of the events (19 events) are more consistent with flux ropes that are originally generated in the dayside ionosphere and subsequently transported into the nightside magnetotail. The mixed origins of the detected flux ropes imply complex nature of generation and transport of Martian magnetotail flux ropes.

Investigating the development of double‐peak subauroral ion drift (DSAID)

AbstractThis study focuses on the newly described ionospheric feature, called double‐peak subauroral ion drift (DSAID), which is a subclass of the well‐known single‐peak SAID. Double‐layer Region 2 (R2) field aligned currents (FACs) could be the main driver of DSAID. Our aim is to gain new insights into the development of DSAID during its two‐stage progression. Observational results are provided by five scenarios, each demonstrating a certain progression sequence of DSAID. Results show that SAID/DSAID occurred during flux transfer events and was accompanied by flow channels (FCs) associated with dayside magnetopause (FC‐2) and nightside magnetotail (FC‐3) reconnections, with westward electrojet (eastward FC), and with auroral streamers (FC‐4). In the premidnight magnetic local time (MLT) sector of stage 2, DSAID development was due to the short‐circuiting of the reconnection‐injected plasma jets during substorms or pseudobreakups. Thus, the related ring current pressure buildup enhanced the downward R2 FACs leading to double/multiple circuits forming double‐layer R2 FACs. During the midnight MLT hours of stage 2, DSAID development was closely related to the westward traveling surge (WTS)/substorm current wedge (SCW). WTS/SCW‐related strong upward R1 FACs closed with meriodional currents producing eastward and downward (i.e., downward R2 FAC‐style) return currents enhancing the downward R2 FACs and thus leading to double/multiple circuits forming double‐layer R2 FACs. Auroral streamers/FC‐4 represent a substorm substructure and their occurrence with DSAID after stage 2 demonstrates that this substructure occasionally includes DSAID. Our results demonstrate also that the short‐circuited system underlying SAID/DSAID acted sometimes as a current generator and sometimes as a voltage generator.

The FIELDS Instrument Suite on MMS: Scientific Objectives, Measurements, and Data Products

The FIELDS instrumentation suite on the Magnetospheric Multiscale (MMS) mission provides comprehensive measurements of the full vector magnetic and electric fields in the reconnection regions investigated by MMS, including the dayside magnetopause and the night-side magnetotail acceleration regions out to 25 Re. Six sensors on each of the four MMS spacecraft provide overlapping measurements of these fields with sensitive cross-calibrations both before and after launch. The FIELDS magnetic sensors consist of redundant flux-gate magnetometers (AFG and DFG) over the frequency range from DC to 64 Hz, a search coil magnetometer (SCM) providing AC measurements over the full whistler mode spectrum expected to be seen on MMS, and an Electron Drift Instrument (EDI) that calibrates offsets for the magnetometers. The FIELDS three-axis electric field measurements are provided by two sets of biased double-probe sensors (SDP and ADP) operating in a highly symmetric spacecraft environment to reduce significantly electrostatic errors. These sensors are complemented with the EDI electric measurements that are free from all local spacecraft perturbations. Cross-calibrated vector electric field measurements are thus produced from DC to 100 kHz, well beyond the upper hybrid resonance whose frequency provides an accurate determination of the local electron density. Due to its very large geometric factor, EDI also provides very high time resolution (∼1 ms) ambient electron flux measurements at a few selected energies near 1 keV. This paper provides an overview of the FIELDS suite, its science objectives and measurement requirements, and its performance as verified in calibration and cross-calibration procedures that result in anticipated errors less than 0.1 nT in B and 0.5 mV/m in E. Summaries of data products that result from FIELDS are also described, as well as algorithms for cross-calibration. Details of the design and performance characteristics of AFG/DFG, SCM, ADP, SDP, and EDI are provided in five companion papers.

Magnetospheric multiscale (MMS) mission attitude ground system design

This paper describes the attitude ground system (AGS) design to be used for support of the Magnetospheric MultiScale (MMS) mission. The AGS exists as one component of the mission operations control center. It has responsibility for validating the onboard attitude and accelerometer bias estimates, calibrating the attitude sensors and the spacecraft inertia tensor, and generating a definitive attitude history for use by the science teams. NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland is responsible for developing the MMS spacecraft, for the overall management of the MMS mission, and for mission operations. MMS is scheduled for launch in 2014 for a planned two-year mission. The MMS mission consists of four identical spacecraft flying in a tetrahedral formation in an eccentric Earth orbit. The relatively tight formation, ranging from 10 to 400 km, will provide coordinated observations giving insight into small-scale magnetic field reconnection processes. By varying the size of the tetrahedron and the orbital semi-major axis and eccentricity, and making use of the changing solar phase, this geometry allows for the study of both bow shock and magnetotail plasma physics, including acceleration, reconnection, and turbulence. The mission divides into two phases for science; these phases will have orbit dimensions of 1.2 x 12 Earth radii in the first phase and 1.2x25 Earth radii in the second in order to study the dayside magnetopause and the nightside magnetotail, respectively. The orbital periods are roughly one day and three days for the two mission phases. Each of the four MMS spacecraft will be spin stabilized at 3 revolutions per minute (rpm), with the spin axis oriented near the ecliptic north pole but tipped approximately 2.5 deg towards the Sun line. The main body of each spacecraft will be an eight-sided platform with diameter of 3.4 m and height of 1.2 m. Several booms are attached to this central core: two axial booms of 14.9 m length, two radial magnetometer booms of 5 m length, and four radial wire booms of 60 m length. Attitude and orbit control will use a set of axial and radial thrusters. A four-head star tracker and a slit-type digital Sun sensor (DSS) provide input for attitude determination. In addition, an accelerometer will be used for closed-loop orbit maneuver control. The primary AGS product will be a daily definitive attitude history. Due to power limitations, the star tracker and accelerometer data will not be available at all times. However, tracker data from at least 10 percent of each orbit and continuous DSS data will be provided. An extended Kalman filter (EKF) will be used to estimate the three-axis attitude (i.e., spin axis orientation and spin phase) and rotation rate for all times when the tracker data is valid. For other times, the attitude is generated by assuming a constant angular momentum vector in the inertial frame. The DSS sun pulse will provide a timing signal to maintain an accurate spin phase. There will be times when the Sun is occulted and DSS data is not available. If this occurs at the start or end of a definitive attitude product, then the spin phase will be extrapolated using the mean rate determined by the EKF.

EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere

Abstract. We report observations of a sequence of quiet-time Earthward bursty bulk flows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet (XGSM ~ −12 to −14 RE) in the evening sector, and by simultaneous high-resolution measurements in the northern conjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera and magnetometers, as well as a meridian-scanning photometer (MSP) in the Scandinavian sector on 17 October 2005. The BBFs at Cluster show signatures that are consistent with the plasma "bubble" model (Chen and Wolf, 1993, 1999), e.g. deflection and compression of the ambient plasma in front of the Earthward moving bubble, magnetic signatures of a flow shear region, and the proper flows inside the bubble. In addition, clear signatures of tailward return flows around the edges of the bubble can be identified. The duskside return flows are associated with significant decrease in plasma density, giving support to the recent suggestion by Walsh et al. (2009) of formation of a depleted wake. However, the same feature is not seen for the dawnside return flows, but rather an increase in density. In the ionosphere, EISCAT and optical measurements show that each of the studied BBFs is associated with an auroral streamer that starts from the vicinity of the polar cap boundary, intrudes equatorward, brakes at 68–70° aacgm MLAT and drifts westward along the proton oval. Within the streamer itself and poleward of it, the ionospheric plasma flow has an equatorward component, which is the ionospheric manifestation of the Earthward BBF channel. A sharp velocity shear appears at the equatorward edge of a streamer. We suggest that each BBF creates a local velocity shear in the ionosphere, in which the plasma flow poleward of and inside the streamer is in the direction of the streamer and southeastward. A northwestward return flow is located on the equatorward side. The return flow is associated with decreased plasma densities both in the ionosphere and in the magnetosphere as measured by EISCAT and Cluster, respectively. In summary, we present the first simultaneous high-resolution observations of BBF return flows both in the plasma sheet and in the ionosphere, and those are in accordance with the bubble model. The results apply for the duskside return flows, but the manifestation of dawnside return flows in the ionosphere requires further studies. Finally, EISCAT measurements indicate increased nightside reconnection rate during the ~35-min period of BBFs. We suggest that the observed temporal event of IMF rotation to a more southward direction produces enhanced open flux transport to the nightside magnetotail, and consequently, the nightside reconnection rate is increased.

Critical Issues on Magnetic Reconnection in Space Plasmas

The idea of expedient energy transformation by magnetic reconnection (MR) has generated much enthusiasm in the space plasma community. The early concept of MR, which was envisioned for the solar flare phenomenon in a simple two-dimensional (2D) steady-state situation, is in dire need for extension to encompass three-dimensional (3D) non-steady-state phenomena prevalent in space plasmas in nature like in the magnetosphere. A workshop was organized to address this and related critical issues on MR. The essential outcome of this workshop is summarized in this review. After a brief evaluation on the pros and cons of existing definitions of MR, we propose essentially a working definition that can be used to identify MR in transient and spatially localized phenomena. The word “essentially” reflects a slight diversity in the opinion on how transient and localized 3D MR process might be defined. MR is defined here as a process with the following characteristics: (1) there is a plasma bulk flow across a boundary separating regions with topologically different magnetic field lines if projected on the plane of MR, thereby converting magnetic energy into kinetic particle energy, (2) there can be an out-of-the-plane magnetic field component (the so-called guide field) present such that the reconnected magnetic flux tubes are twisted to form flux ropes, and (3) the region exhibiting non-ideal MHD conditions should be localized to a scale comparable to the ion inertial length in the direction of the plasma inflow velocity. This definition captures the most important 3D aspects and preserves many essential characteristics of the 2D case. It may be considered as the first step in the generalization of the traditional 2D concept. As a demonstration on the utility of this definition, we apply it to identify MR associated with plasma phenomena in the dayside magnetopause and nightside magnetotail of the Earth’s magnetosphere. How MR may be distinguished from other competing mechanisms for these magnetospheric phenomena are then discussed.

A physics based nonlinear dynamical model for the solar wind driven magnetosphere-ionosphere system

The predictive performance of a six-dimensional nonlinear dynamical model of the coupled magnetosphere-ionosphere system is compared to magnetospheric activity data. The model couples the basic energy components of the night-side magnetotail to the ionosphere by the region 1 currents. It predicts both the state of the geotail and the ionospheric westward electrojet index from the value of the solar wind speed. Except for the ionospheric dissipation, the system is both Kirchhoffian and Hamiltonian. This ensures that the solar wind input power is divided into physically realizable sub power components, a property not shared by signal processing filters. All the model parameters can be calculated analytically for different substorm scenarios. Agreement between the predicted and observed magnetospheric indices improves significantly when the system is coupled to particle simulations to produce realistic parameters for the model. Including the nonlinear increase of the ionospheric conductance with power deposited in the ionosphere is also essential.

IMF induced changes to the nightside magnetotail: A comparison between WIND/Geotail/IMP 8 observations and modeling

A 3‐D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross‐section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.

Multiple‐Branch Model of the open magnetopause

When the magnetosphere is magnetically open, the magnetopause must cut across open field lines and is usually identified with a rotational discontinuity or finite‐amplitude Alfvén wave. The merging of magnetospheric and interplanetary magnetic fields in the nose region of the magnetosphere generates two pairs of such Alfvén waves; the two interior ones propagate down to the ionosphere, and the two exterior ones propagate into the incoming magnetosheath plasma and thus can be identified with the magnetopause in the dayside. However, continuing this magnetopause into the nightside magnetotail region would require reversing the direction of magnetic‐field rotation at some point on the propagating wave, which is not possible. To resolve this problem, I propose that the magnetopause on the nightside is not a continuation of that on the dayside but is a distinct surface formed by a separate Alfvén wave generated near the downstream edge of the dayside polar cusp. This multiple‐branch model also explains the boundary of the exterior cusp (identified with the continuation of the dayside magnetopause) and accounts for the non‐stagnant flow within the exterior cusp (the flow simply proceeds across the nightside magnetopause into the plasma mantle region). Where the two branches are adjacent to the magnetotail, their spatial separation can be estimated as typically no more than about 2 RE, not likely to have been noticed unless specifically looked for. A possible mechanism for generating the nightside Alfvén wave is the expansion of plasma within the polar cusp as the flux tubes are advected into the rapidly increasing volume of the magnetotail.

Nonlinear dynamics of charged particles in the magnetotail

An important region of the Earth's magnetosphere is the nightside magnetotail, which is believed to play a significant role in energy storage and release associated with substorms. The magnetotail contains a current sheet which separates regions of oppositely directed magnetic field. Particle motion in the collisionless magnetotail has been a long‐standing problem. Recent research from the dynamical point of view has yielded considerable new insights into the fundamental properties of orbits and of particle distribution functions. A new framework of understanding magnetospheric plasma properties is emerging. Some novel predictions based directly on nonlinear dynamics have proved to be robust and in apparent good agreement with observation. The Earth's magnetotail may serve as a paradigm, one accessible by in situ observation, of a broad class of boundary regions with embedded current sheets. This article reviews the nonlinear dynamics of charged particles in the magnetotail configuration. The emphasis is on the relationships between the dynamics and physical observables. At the end of the introduction, sections containing basic material are indicated.

Non-substorm transient injection events in the ionosphere and magnetosphere

High resolution ground-based observations in combination with observations made in the central plasma sheet by the ISEE-1 and -2 spacecrafts provided evidence for a new morphological class of transient events—plasma injections. They occurred in the nightside magnetotail during periods of prolonged steady convection without any activity of the type traditionally attributed to substorms. The main ionospheric signature of the plasma injections was an enhancement of equatorward plasma flow for a few minutes. The flow came into the STARE field of view at Λ ⩾ 70° and then proceeded equatorwards with a speed of 800–1000 km s −1. Its leading edge was often rather sharp (⩽20 km). In the central plasma sheet region, at r ∼ 20 R e, the main signatures were transient enhancements in the earthward convection speed and a rise of the B 2 component as well as the total magnetic field. They were sometimes accompanied by energetic particle bursts. Using the high time resolution measurements of the ISEE satellite pair it was possible to determine more about the structure of the fast spikes. The leading edge of the propagating disturbance was often quite sharp but could not be classified as any simple type of planar discontinuity. The observed features are discussed in terms of transient plasma injections produced by impulsive enhance- ments in the reconnection rate in the distant tail

Detached plasma in Saturn's front side magnetosphere

We discuss plasma observations in the outer front side Saturnian magnetosphere which indicate the existence of dense flux tubes outside the plasma sheet. We suggest that flux tubes are detached from the plasma sheet by a centifugally driven flute instability. The same instability leads to a dispersal of Titan‐injected plasma. It is shown that the detached flux tubes will probably break open as they convect into the nightside magnetotail and lose their content in form of a planetary wind.