Tailward Streaming Research Articles

Assessing the Sources of the O+ in the Plasma Sheet

AbstractTo study the average contributions of the cusp outflow through the lobes and of the nightside auroral outflow to the O+ in the plasma sheet (PS), we performed a statistical study of tailward streaming O+ in the lobes, plasma sheet boundary layer|the plasma sheet boundary layer (PSBL) and the PS, using MMS/Hot Plasma Composition Analyzer (HPCA) data from 2017 to 2020. Similar spatial patterns illustrate the entry of cusp‐origin O+ from the lobes to the PS through the PSBL. There is an YGSM‐dependent energy pattern for the lobe O+, with low‐energy O+ streaming closer to the tail center and high energy (1–3 keV) O+ streaming near the flanks. Low energy (1–100 eV) O+ from the nightside auroral oval is identified in the near‐Earth PSBL/PS with high‐density (>0.02 cm−3), and energetic (>3 keV) streaming O+ with similar density (∼0.013 cm−3) is observed further out on the duskside of the PSBL/PS. The rest of the nightside auroral O+ in the PSBL is mixed with O+ coming in from the lobe, making it difficult to distinguish the source. We estimated the contributions of the different sources of H+ and O+ ions through the PS between 7 and 17 RE, using estimates from this work and data extracted from previous studies. We conclude that, during quiet times, the majority of the near‐Earth PS H+ are from the cusps, the polar wind and Earthward convection from the distant tail. Similarly, while the O+ in the same region has a mixed source, cusp origin outflow provides the highest contribution.

Ion acceleration and reflection on magnetotail antidipolarization fronts

AbstractAntidipolarization fronts (ADFs), tailward moving structures in the Earth's magnetotail with sharp decreases in their magnetic Bz component, are thought to be mirror images of earthward propagating dipolarization fronts (DFs) generated on the opposite side of the reconnection site. We use ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) observations and numerical simulations to study the role of ADFs in accelerating and reflecting ambient plasma sheet ions. In both case and statistical observations before ADF arrival, tailward streaming, energy‐dispersed ions are seen first. After about 1 min, the ion fluxes are enhanced significantly with the peak shifted duskward, and then the peak gradually shifts back to the tailward direction until the ADF arrives. All these signatures are reproduced by our simulation model of ion acceleration and reflection on ADFs. We further examine typical ion trajectories before and after ADF reflection, to understand these seemingly complicated ion signatures as well as their similarities with and differences from the DF preceding signatures.

Cusp as a source for oxygen in the plasma sheet during geomagnetic storms

We have used the ion composition data from the CIS/CODIF instrument on Cluster to determine how the O+ population in the plasma sheet and the adjacent lobes changes during geomagnetic storms. The Cluster trajectory, which moves over the polar cap, into the lobe, and then into the plasma sheet on each orbit, allows us to track the changes in O+ in these regions for a prestorm orbit, main‐phase orbit, and recovery phase orbit. We find that changes in the O+ density and pressure in the plasma sheet are similar to those commonly observed in the ring current during a storm. The O+ is low prestorm. It increases by about a factor of 10 just prior to or during the early main phase of the storm, and is reduced, but usually not down to prestorm levels, in the recovery phase. The lobes contain tailward streaming O+ which originates in the “cleft ion fountain”. During the storms main phase, this population also increases. A detailed look at the main‐phase passes shows that a significant increase in the O+/H+ ratio is observed when this lobe population reaches the plasma sheet, and the tailward streaming O+ is observed continuously as the spacecraft moves from the lobe into the plasma sheet. The enhanced O+ in the lobe and the plasma sheet is observed for many hours during the storm. The inward convection of this population is likely a significant contributor to the storm time ring current.

Morphology of the dayside ionosphere of Mars: Implications for ion outflows

Significant fluxes of tailward streaming ions have been detected in the Martian wake by instruments on spacecraft. Imposing outward fluxes at the top of a model will produce dayside ion density profiles that are characterized by smaller scale heights than those of diffusive equilibrium. We determine the maximum outward fluxes of ions, and those implied by radio occultation data, by constructing ∼180 models, with upward velocity boundary conditions in the range from 0 to (7–8) × 105 cm s−1 in small increments. As the upward velocity is increased, the topside ion or electron densities decrease until eventually the computed ion fluxes cease to increase, implying that this is the maximum outward flux that the ionosphere can sustain. By comparison to data, we derive a low solar activity upward flux of O2+ of ∼5 × 107 cm−2 s−1, and a maximum of ∼8 × 107 cm−2 s−1. For O+, the analogous fluxes are ∼4 × 106 cm−2 s−1 and ∼1.1 × 107 cm−2 s−1. We derive high solar activity upward fluxes of O2+ in the range ∼(1.2–1.6) × 108 cm−2 s−1, and a maximum of 2.4 × 108 cm−2 s−1. The O+ derived and maximum fluxes at high solar activity are ∼(1.5–2) × 107 and 5 × 107 cm−2 s−1, respectively. If these fluxes are averages over the dayside, we estimate total loss rates of O+ and O2+ of (2.8–11) × 1024 and (3.6–8.7) × 1025 s−1, respectively. Our computed escape rates of O+ are in substantial agreement with the models and data, but our O2+ escape rates are an order of magnitude larger. We discuss various mechanisms that would bring our O2+ escape rates or the O+/O2+ ratio into agreement with the measurements and models.

Timing and localization of near‐Earth tail and ionospheric signatures during a substorm onset

On 16 February 2008, the THEMIS spacecraft (probes) bracketed the near‐Earth signatures of a substorm onset as identified in the THEMIS ground‐based observatories measuring an AETH index up to 180 nT. The main onset was associated with the formation and tailward release of a plasmoid (a proto‐plasmoid) at XGSM = −18.3 RE and a dipolarization in the inner part of the plasma sheet at XGSM = −11.0 RE. The time history and geometry of the event in the tail are consistent with magnetic reconnection as the cause of the substorm expansion onset process. Two activations of the plasma sheet, evidenced by tailward streaming of energetic ions and southward or bipolar signatures of the magnetic field, preceded the main substorm. The first activation was associated with an intensification of a high‐latitude arc, while the second was associated with the onset of ULF pulsations at midlatitude and low‐latitude stations. We conclude that near‐Earth plasma sheet activity that may also be due to reconnection and may be related to nonsubstorm arc intensifications can precede substorm onset by several minutes. In particular, high‐latitude arcs do not appear to result in substorm initiation even though they may occur in close temporal and spatial proximity to the substorm arc.

Ultra‐low‐frequency waves and associated wave vectors observed in the plasma sheet boundary layer by Cluster

Waves in the Pc1–Pc2 frequency range (0.1–5 Hz) are studied using Cluster magnetic field data. In the plasma sheet boundary layer, Cluster observed harmonically related waves with the fundamental near the local proton cyclotron frequency (Ωp). These waves had components both parallel and perpendicular to the local magnetic field (B0). Application of the wave telescope yielded the full wave vector (k) of the waves and showed that they propagated nearly perpendicular to B0. Associated with the waves were two counterstreaming ion beams with a relative velocity of approximately 4000 km/s. During wave activity, the tailward streaming beam increased in energy flux, and subsequently, energy flux increased in directions perpendicular to B0. We suggest two possible sources for the waves: inverse Landau resonance with an accelerated electron beam and an electromagnetic ion/ion cyclotron instability. The change in the ion distribution suggests these waves could contribute to the formation of the hot protons of the central plasma sheet.

Ionospheric signature of a magnetic flux rope in the magnetotail

A flux rope was detected in the near‐Earth magnetotail at X ≈ −20 RE by Geotail with simultaneous viewing of the northern polar region by Polar on January 11, 1997. This offers the opportunity to ascertain the optical signature in the ionosphere of a flux rope for the first time. The magnetotail flux rope was associated with tailward streaming of energetic particles and had an estimated core radius of ∼ 1.5 RE. The composition of energetic ions within the flux rope showed a temporal increase in the abundance of ionospheric N+ and O+ ions. Simultaneous with the flux rope encounter were auroral activations at ∼23 MLT (substorm onset) and ∼ 18 MLT. An extensive consideration of magnetic field projection of the Geotail location to the ionosphere and the associated auroral and particle signatures of this event lead to the inference that the optical signature of the flux rope was an auroral arc system at ∼ 18 MLT, which had an ∼2 h extent in MLT and was moving progressively poleward and westward.

Dawn‐dusk magnetic field effects on ions accelerated in the current sheet

The effects of a By field on ions in unbounded transient orbits in the current sheet of the geomagnetic tail are studied. The trajectories and the energization of ions injected at the north edge of the current sheet (z = +L) are investigated varying the dawn to dusk magnetic field strength By. The current sheet with +By produces less energetic ions than that with −By. At By = 0 the exit positions of ions after acceleration from the current sheet are scattered almost equally above and below the current sheet. Finite By introduces a helicity to the magnetic tail structure that breaks this symmetry. When the By field reaches 3 nT, almost all of the tailward streaming ions injected with initial velocities (0 < V0 < 2000 km/s and a given initial kinetic energy ½mV02) are ejected above the current sheet after acceleration. However, a current sheet with a negative dawn‐dusk magnetic field ejects all of them below itself even for small negative By(−1 nT) values. The nonzero EyBy produces E‖ = EyBy/B with E · B being a Lorentz scalar invariant. The finite E‖ makes the electrons move so as to create a cancelling electrostatic potential difference along the magnetic field line. On the basis of the ion orbits found here for the standard model E and B fields we estimate the self‐consistent electrostatic field expected from the difference in the large ion orbits and the well‐magnetized electrons. We suggest that the onset of geomagnetic activity may be correlated with the By field and the associated electrostatic field in the central plasma sheet.

Particle diffusion and adiabatic expansion of plasmoids

In this paper neutral point (coalescence) instability (NPI) of plasmoids (PMD) associated with tailward streaming O+, H+ and e− particles and diffusion region (spatial extent and particle life time) are studied. Also, radial and pitch angle diffusion studies have been carried out in the near-earth region (10 and 11 RE during substorm onset. Our study revealed that coalescence instability is favoured by a heavier ion like O+ and the time required for a heavier ion to reach from one X point to an adjacent X point is minimum. Near an X type neutral line (diffusion region) ions spend a fairly long time and gain a significant amount of energy. Radial and pitch angle diffusion studies showed that higher energy particles are largely confined near the reconnection region (near-earth region) and lower energy particles are rapidly depleted from the near-earth region during substorm onset.

Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1 ∼30‐keV ion observations

We present a statistical survey of the near‐Earth plasma sheet (r = 7‐23 RE, distance from the neutral sheet center Zns = 0‐10 RE, LT = 2100‐0200) during (late) substorm recovery phase and quiet time based on ISEE 1 medium energy particle instrument observations of first‐order ∼30‐keV ion anisotropies, comprising ∼112 hours of the 1978 tail season. The data reveal predominantly isotropic conditions, with intervals of generally weakly anisotropic (<10 highest‐to‐lowest flux ratio) field‐aligned ion flows (FAIFs) and ion flux gradients. Both earthward and tailward streaming FAIFs are observed for Zns between 0 and 10 RE and are probably unassociated with the plasma sheet boundary layer. The occurrence rate of tailward FAIF (∼4.5%) is relatively insensitive to AE conditions. The occurrence rate of earthward FAIF increases during “active” conditions to as much as 12.4% and decreases to as low as 1.3% during “quieter” conditions. An increase in the occurrence rate of tailward FAIFs is observed with increasing Z (mapping along B to the more distant current sheet). We suggest that the tailward streaming flows may arise during stretching of especially distant closed field lines during extended recovery and quiet time. Ions will lose energy as they cross the tailward moving current sheet, just as they gain energy in crossing an earthward moving field reversal region. No evidence for a quiet time distant tail source is seen in the energetic (24 keV to 2 MeV) ion data during these late recovery phase and quiet time periods. A weak local time asymmetry in the ratio of occurrence rates for tailward and earthward FAIFs (T∶E) is observed, with earthward streaming FAIFs outnumbering tailward streaming FAIFs in the dusk sector (T∶E = 0.9‐1.0) and vice versa in the dawn sector (T∶E = 1.4‐ 2.1). The occurrence rate of dawn‐to‐dusk flux anisotropies, interpreted here as gradients indicating higher ion flux toward the plasma sheet equatorial plane, increases during active conditions and decreases during quieter conditions. North‐to‐south and south‐to‐north flux anisotropies, interpreted here as dawnward and duskward flux gradients (3‐5%), are independent of AE conditions.

Plasma distribution functions in the Earth's magnetotail (XGSM∼ −42RE) at the time of a magnetospheric substorm: GEOTAIL/LEP observation

Three plasma burst events were observed with the Low Energy Particle (LEP) instrument onboard GEOTAIL at XGSM=−42 RE. Two of these events were detected when the spacecraft was in the plasma sheet. In these events, the first signature of the burst was the tailward flow of the accelerated electrons with energies of about 5 keV. This was followed by the appearance of the tailward streaming ions with energies > 20 keV. Importantly, these tailward streaming ions were observed together with the non‐streaming ions of the plasma sheet which had not experienced acceleration or heating. Then the electron distribution function started to develop an elongated core component at < 1 keV and isotropic or perpendicularly heated structure at > 1keV. These two events lasted for about two minutes. The spacecraft was in the tail lobe before the third event took place. In this event the first signature was the bistreaming electrons with peak energy at 2 keV. After a short interval of the flux decrease, hot electrons with energy ∼3 keV appeared. The velocity distribution function of the ions had an incomplete shell structure with an average tailward velocity of ∼900 km/sec. The non‐streaming component was absent in this case. It is suggested that in the first two events the spacecraft was in the plasma jet emanating from the near Earth neutral line, while it traversed the edge of the plasmoid in the third event. The associated substorm onset can be identified for the first event after which the disturbance continued.

Energetic ion anisotropies in the geomagnetic tail: 2. Magnetic field and substorm characteristics

In a companion paper we have reported results from a survey of strong energetic ion (E > 25 keV) anisotropies in the central geomagnetic tail based on 2 years of data from the ISEE 2 spacecraft. Here we extend this study with particular emphasis on the relation of high ion anisotropies to recent models of geomagnetic substorms and include magnetic field and ground‐based data. Beyond 16 RE downtail distance, we find a correlation between the magnetic field north‐south component (Bz) and the streaming direction of the energetic ions. Earthward ion streaming is observed almost exclusively when Bz is positive while tailward streaming tends to occur with negative Bz. Strong ion anisotropies are magnetic field aligned rather than perpendicular to it suggesting that most of them are observed at the plasma sheet boundary. However, they are also found in the central plasma sheet. A superposed epoch analysis using the AL index yields a relation between substorm phases and the observation of high ion anisotropies. Earthward streaming at larger distances from the Earth and/or the neutral sheet usually occurs during substorm recovery while earthward streaming closer to the Earth and tailward streaming events are related to the substorm expansion phase. Three events characterized by strong tailward ion and plasma streaming detected first changing to earthward streaming later on are presented in some detail. The overall features of two of these events strongly suggest the formation of a magnetic neutral line earthward of the satellite at substorm onset moving tailward around recovery. While most observations during the third event also indicate such a sequence, there are some puzzling details which do not match this interpretation. Although a certain By signature proposed by numerical simulations of near‐Earth reconnection could not be found, we conclude that the statistical properties of strong ion anisotropies can consistently be explained in terms of the near‐Earth neutral line model of substorms if the substorm neutral line sometimes forms well inside the ISEE orbit. Other recently published models of substorm onset such as the thermal catastrophe model and the current disruption model can also account for part of the observations but still require further refinement.

A two-satellite study of nightside flux transfer events in the plasma sheet

High time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X ∼ −21 R e in the central magnetotail were used in the study of short-duration (time scale 1 min) magnetic field events during two consecutive substorms on 23 March 1979. These events were observed at ISEE-1 and 2 (having 0.3 R e separation along the Z-coordinate), both inside and outside of the thinned plasma sheet, close (in time) to the polarity reversal of the plasma flow from tailward to earthward direction in association with the transient plasma sheet expansions. The enhanced magnetic flux closure (southward B z during tailward streaming events, northward during earthward streaming events), plus enhanced plasma flow during these events, suggest that they are nightside magnetic flux transfer events (NFTEs). The typical behaviour of the magnetic field during a NFTE (compression, rotation then field decrease), the signatures of overpressure in such structures and their close correlation with ground-based observed auroral activations and high energy particle bursts signify that NFTE structures may result from the impulsive reconnection in the plasmasheet. Systematic differences in the magnetic variations found at ISEE-1 and 2 indicate current concentration at the outer plasmasheet boundary during the passage of a NFTE. Two NFTEs appeared to have structures similar to magnetic flux ropes. The NFTE structure and associated current system, as well as the interpretation of NFTE signatures in terms of the impulsive reconnection, are discussed. During the “poleward leap” stage of one substorm, the spacecraft within the central part of the plasma sheet observed the intense earthward magnetic flux transport associated with superimposed NFTE structures. The estimated amount of this flux transfer explained well the observed amplitude of the poleward expansion of the westward electrojet in the conjugate auroral zone.

Quasi‐steady current sheet structures with field‐aligned flow

General properties of field‐aligned plasma flow in quasi‐steady ideal MHD configurations are discussed, and explicit solutions, modeling compressible flow around a plasmoid in the distant magnetotail, are presented. The explicit solutions are based on the assumption that plasma and field gradients along the x and y directions parallel to the tail current sheet are typically much smaller than those in the z direction perpendicular to the current sheet. In contrast to the incompressible case, two different flow solutions exist in the compressible case in the subalfvénic regime. The different regimes are distinguished by whether the flow speed is below or above a critical speed υo, which represents the group velocity of a slow magnetosonic wave (aligned with the magnetic field) in the limit of phase propagation perpendicular to the magnetic field, where the phase speed vanishes. In the incompressible case, only the subcritical flow solution is realized in the subalfvénic regime. In the compressible case, both regimes may exist simultaneously in the flow around the plasmoid. They are separated by a tangential discontinuity which may be considered as the limit of a slow shock for vanishing normal flow and normal magnetic field. The subcritical flow region surrounding the plasmoid inside of a supercritical region possibly corresponds to the layers of tailward streaming ions observed adjacent to the plasmoid proper. The signatures of a passage from the supercritical through the subcritical flow region in the vicinity of the plasmoid can be similar to those of a passage through the plasmoid itself: a reduction of Bx followed by a local enhancement in the middle of the encounter, as observed, for instance, by Hones et al. (1984) and Slavin et al. (1989). This enhancement is found for a simple loop structure within the plasmoid and hence need not be associated with multiple loops or a tail flapping as suggested by Hones et al. Field signatures outside the plasmoid are typically those of encounters of a traveling compression region: an enhancement of Bx accompanying a north‐south signature of Bz. In certain parameter regimes, however, unusual signatures are also possible: a reduction of Bx with a north‐south Bz signature for a close plasmoid encounter (without penetration) and a south‐north signature of Bz accompanying a Bx enhancement for a distant plasmoid encounter at high latitudes.

The magnetosphere of Neptune: Hot plasmas and energetic particles

A comprehensive overview is provided of the hot plasmas and energetic particles (≳keV) observed in the vicinity of Neptune by the low energy charged particle (LECP) experiment on the Voyager 2 spacecraft. The LECP data are ordered with respect to magnetic field data and models derived from the Voyager magnetometer experiment. The findings include the following: (1) Weakly enhanced ion and electron fluxes were observed at the position of the subsolar bow shock. (2) Magnetic‐field‐aligned, antiplanetward streaming ions and electrons were sporadically observed within the inbound (subsolar) and outbound (tail flank) magnetosheaths, and within the unique “pole‐on” cusp region encountered during the inbound trajectory. (3) Tangential ion streaming was observed at the positions of both the inbound (dawnward streaming) and outbound (tailward streaming) magnctopauscs. (4) A distinct “trans‐Triton” ion population outside the minimumL shell of Triton is characterized by large angular anisotropics that show that heavy ions (presumably N+) are a likely constituent This population is at least partially corotating with Neptune out to at least L = 27 RNand is also characterized at times by cigar‐shaped (field‐aligned) pitch angle distributions, possibly indicative of an interaction with a neutral torus. (5) Within the middle magnetospheric regions (inside Triton), pitch angle distributions have well‐developed trapped or “pancake” shapes. Also, in contrast to Uranus, flux profiles show no evidence of substorm‐generated azimuthal asymmetries. (6) Triton (and/or Triton‐generated neutral gas) controls the outer bounds of the hot plasmas and energetic particles, although the mechanism of that control is unclear. Also, there are clear charged particle signatures of satellite 1989NI and of ring 1989N3R. However, the large number of calculated criticalLshell positions associated with all of the rings and satellites renders impractical at this time the unique determination of causal relationships between the many observed particle signatures and known material bodies. (7) Concerning the bulk (integral) and spectral parameters of the not plasmas, if it is assumed that the trans‐Triton population is dominated by N+, the plasma β parameter reaches ∼1 within the near‐planet magnetotail (L ∼28 RN; in conjunction with a magnetic field depression “tail event”), having only reached ∼0.2 in the more planetward regions. Integral electron energy intensities are such that the more localized Neptune UV aurora can be explained if loss cone intensities are ≲1% of trapped intensities. In contrast to the Uranian magnetosphere, the lower‐energy electron distributions appear generally to be at least as well characterized by hot Maxwellian distributions (kT = 10 to 30 keV) as by power law distributions inside L ∼20 RN, a characteristic generally exhibited at the other planets by the ions. At Neptune the ions have kT = 12 to 100 keV, and kTis strongly correlated with position relative to Triton's L shell. (8) Within the Neptunian magnetotail, planetward, magnetic‐field‐aligned streaming of ions and electrons is observed within the distant (∼67RN) plasma sheet and within a closer region thought to be a detached or striated portion of the plasma sheet population. Within the near‐planet magnetotail (L ∼28 RN), where the spacecraft crossed from the plasma sheet to the tail lobes, cigarlike electron distributions are observed, suggestive of shell‐splitting/magneto pause‐sweeping effects. Consistent with the middle magnetospheric observations, and in sharp contrast to the Uranian magnetotail, the Neptunian magnetotail shows no evidence of substorm processes.

Substorm-associated energetic ion (E ≅ 45 keV) flows at the plasma sheet boundary layer: A dawn-dusk flow reversal

This work presents statistical results of ISEE-1 observations of energetic ions (E ≅ 45 keV) streaming at the plasma sheet boundary layer (PSBL), during thinnings and expansions of the tail plasma sheet associated with substorms. It is shown that the recovery PSBL events are more anisotropic, with higher peak intensities and shorter time durations than the thinning ones. Earthward streaming is found to be prevalent in the pre-midnight PSBL region, while tailward streaming is more frequent in the post-midnight flank. This flow reversal in the dominant streaming direction is more distinct for the entry events and is most probably the result of an underlying circulation pattern of energetic ions in the magnetotail. It is suggested that dawnward drifts of earthward-injected energetic ions, which take place in the presence of strong magnetic field gradients in the vicinity of the PSBL, play an important role in the generation of the circulation pattern and in particular the distinct tailward streaming at the dawn PSBL.

ISEE 1 and 2 observations of ion distributions at the plasma sheet‐tail lobe boundary

Observations of plasma ions (0.075–29 keV) at or near the plasma sheet‐tail lobe boundary made with the ISEE 1 and 2 spacecraft are the principal topic of this paper. We describe crossings of this boundary that occurred during various levels of geomagnetic activity at distances XSM = −7 to −13 RE, near local midnight, and 2 RE or more above the estimated location of the tail current sheet. Fluxes of ions directed along the magnetic field lines (referred to as ion beams) with kinetic energies of 1‐ > 29 keV are commonly observed during crossings of this boundary. The ion beams nearest the edge of the plasma sheet are directed earthward, while deeper within the plasma sheet, additional ion beams, directed tailward, are encountered, usually resulting in bidirectional or counterstreaming ion distributions. The ion distribution functions for these clearly indicate that the tailward directed beams are reflected versions of the earthward directed beams, returning after mirroring in the stronger magnetic field near the Earth. From an intersatellite timing analysis the thickness dB of the region of solely earthward streaming ions is estimated to be a few tenths of an Earth radius. The fastest particles (plasma electrons and the highest‐energy ions) are the first to be seen as a satellite enters the plasma sheet from the lobe and last to be seen as it leaves the plasma sheet to enter the lobe. This suggests that the ion beams are components of a layer of plasma that is projected earthward along field lines from a more or less steady source tailward of the satellite and that is E × B drifting equatorward. The observations are consistent with theoretical predictions for an open model of the magnetosphere having a neutral line tailward of the satellite and having the plasma sheet‐lobe interface coincident with the separatrix between closed and open field lines. We therefore identify the beam‐containing plasma layer contiguous to, and extending equatorward from, this interface as the separatrix layer. The velocity, temperature, and kinetic energy flux of the earthward beams in the separatrix layer are found to correlate positively with the AE index. We rarely observe isolated tailward streaming ion beams near the boundary, although slow (<300 km/s) tailward flows there sometimes result from an imbalance of particle fluxes carried by the oppositely streaming beams. We show that such net tailward ion anisotropies, occasionally observed in our data set, differ distinctly from the tailward bulk flow event reported by Hones et al. (1986). That event had all ion components moving tailward with a net velocity of ∼500 km/s; it was interpreted to be the result of magnetic reconnection at a near‐Earth neutral line that formed at the onset of a substorm and tailward departure of the resulting plasmoid.

Quasi‐stagnant plasmoid in the middle tail: A new preexpansion phase phenomenon

From the analysis of ISEE 3 data we have found that a plasmoid is sometimes formed in the middle tail outside the intervals of the substorm expansion phase. This plasmoid is produced by reconnection at the X‐type neutral line, which is located earthward of the distant neutral line but beyond the substorm‐associated near‐tail neutral line, and it is almost stagnant in that the associated flow speed is less than 300 km/s. The blocking effect of the distant neutral line is the most probable reason for the slow movement. The quasi‐stagnant plasmoid is observed at x = −60 to −100 RE for a duration of a few tens of minutes, and in about one half of the cases it is followed by the fast tailward streaming. The onset of this streaming tends to coincide with the onset of the substorm expansion phase, and this probably occurs when the reconnection at the middle‐tail neutral line comes close to processing the last closed field line. Intensification of the dawn‐to‐dusk electric field that causes the mantle plasma to reach the plasma sheet boundary closer to the earth is suggested as the reason for the formation of the middle‐tail neutral line earthward of the distant neutral line. The effects on the energetic particle flux and relation to the near‐tail reconnection are also discussed.

Observations of the right‐hand resonant ion beam instability in the distant plasma sheet boundary layer

From the ISEE 3 distant tail prime intervals of study (days 24–43 and 169–193, 1983), 66 large amplitude wave events at the plasma sheet boundary layer were identified, analyzed and tabulated. It is found that (1) 83% of the wave events are clearly right‐hand polarized in the spacecraft frame (arguments are given as to why they must also be right‐hand polarized in the plasma frame), (2) the waves are propagating nearly parallel to the ambient magnetic field (θkB < 15°), (3) they have a significant compressional‐to‐transverse amplitude ratio (typically 0.2–0.5), of which only part can be ascribed as being due to cross‐field propagation, (4) the waves have a maximum amplitude of 6–7 nT peak to peak (presumably caused by a presently unexplained saturation effect), (5) the waves have rest frame frequencies within the range 0.3 to 2.0 Ω+, and (6) the waves are strongly substorm dependent. Several case studies involving simultaneous energetic (>35 keV) ion measurements have been analyzed and are presented. Highly anistropic, tailward streaming, relatively cold ion beams (1000–100∶1 anisotropy ratio) are found to occur in near time coincidence with the waves. The ion beam properties are calculated and are compared with theoretical predictions of right‐hand resonant and nonresonant ion beam instabilities. It is demonstrated that for the cases examined, the measured beam and wave parameters are in agreement with the presence of the resonant instability. It is believed that this is the first simultaneous observation of the waves and the unscattered cold ion beam of the right‐hand resonant ion beam instability in either space plasma or laboratory measurements. Because of the absence of strong convective effects separating the waves and the generating ion beam, the plasma sheet boundary layer is an excellent region of space to study the details and development of this very important plasma instability.

Plasmoid‐associated energetic ion bursts in the deep geomagnetic tail: Properties of the boundary layer

Energetic (>35 keV) ion bursts in the deep geomagnetic tail associated with the passage of 37 plasmoids are examined using data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3. These bursts can usually be divided into four distinct phases: (1) strongly tailward streaming ions observed in the lobe for a few minutes prior to plasmoid entry, commencing ∼25 min after geomagnetic substorm onset; (2) the plasmoid interval, when the energetic ions have a broader tailward angular distribution arising from convection with the plasmoid; (3) the “post‐plasmoid” plasma sheet, where more strongly tailward streaming ions are observed in the plasma sheet on field lines disconnected from the earth at the substorm neutral line; and (4) a strongly tailward streaming ion population extending into the lobe for a few minutes after exit from the plasma sheet. We concentrate here on the streaming ion “boundary layers” observed in the lobe at the leading and trailing edges of these bursts. In a majority of these layers, a clear dawn‐dusk gradient anisotropy and energy dispersion are evident at the leading edge, and a similar gradient anisotropy with “reverse” dispersion is evident at the trailing edge. It is shown however that the dispersion at onset is not consistent with simple time of flight from a near‐earth neutral line or from a neutral line retreating tailward during substorm recovery. Instead, observations of 90° pitch angle ions with a time resolution of 16 s are used to infer that the ion onset is due to a layer of energetic ions expanding outward from the tail center plane and engulfing the spacecraft. At the trailing edge of the burst, this layer contracts back across the spacecraft toward the center plane. Mean expansion and contraction speeds are 94±74 km s−1 and 99±100 km s−1 respectively, with boundary layer thicknesses of ∼3 RE. From these observations, it is concluded that the expansion of the ion layer is caused predominantly by the ion layer being swept across the spacecraft by the arrival of the plasmoid in the deep tail, contributing ∼60 km s−1 to the expansion speed, rather than by a thickening of the region of lobe field lines disconnected at the substorm neutral line which expands at ∼35 km s−1. The energy dispersion at the leading edge can be reconciled with a near‐earth neutral line in this case. Using this dispersion and the measured expansion speed of the layer, the electric field along the near‐earth substorm neutral line is deduced. Values derived from layer expansions and contractions are both ∼0.4 mV m−1, equivalent to ∼110 kV across a ∼40‐RE tail width.