Velocity Dispersed Ion Structures Research Articles

Survey of reconnection signatures in auroral oval ion precipitation

The protons and electrons on newly reconnected field lines exhibit time-of-flight effects that have been observed and modelled on both the dayside and nightside, at both high and low altitudes. These reconnection signatures feature proton energy distributions that are cutoff toward low energy. In LEO the cutoff energy exhibits a dispersion with latitude, typically seen in the cusp on the dayside, and referred to as velocity dispersed ion structures on the nightside. Here, an automated algorithm for detecting such low-energy cutoffs in the energy spectra of precipitating ions was developed, without regard for any possible dispersion with latitude. The occurrences of LEC ion spectra were mapped over a year of DMSP observations. There are four distinct components to this map, two of which are produced by reconnection. On the dayside LEC ion spectra are seen in cusp, mantle, and open-LLBL precipitation, predominantly at sub-keV energies, as the result of dayside reconnection. On the nightside LEC ion spectra are seen at the poleward edge of the oval at supra-keV energies (usually dispersed with latitude), that indicate magnetotail reconnection. There is another supra-keV population seen on the dusk side at the equatorward edge of the oval, possibly indicating the onset of isotropy. Finally, there is a sub-keV population seen throughout the auroral oval that is thought to consist of ions accelerated out of the opposing hemisphere. The presence of the nightside reconnection signature is modulated by magnetic activity level. Superposed epoch analyses of the ionospheric flow velocity reveal flow through the open–closed boundary when reconnection signatures are present, and enhanced upflow on the dayside when reconnection signatures are present.

Instabilities driven by ion shell distributions observed by Cluster in the midaltitude plasma sheet boundary layer

Cluster observations in the near‐Earth plasma sheet boundary layer (PSBL) region have shown the presence of ion shell distributions related to velocity‐dispersed ion structures (VDIS) coincident with electrostatic emissions. We have examined ion shell instabilities in the presence of a cold ion and electron background using linear theory and particle in cell simulations. Linear theory shows that the shell instability is only excited when a cold ion background is present, generating a broad range of ion cyclotron harmonics. Numerical simulations confirm that ion Bernstein modes are preferentially excited transverse to the ambient magnetic field, along with a lower level of wave power at oblique angles. The background ions are heated primarily in the transverse direction because of a linear nonstochastic ion cyclotron heating mechanism, and overall saturation of the instability occurs because of thermalization of the shell combined with heating of the background ions and electrons. Comparison with Cluster observations shows that observed electrostatic waves with a spectrum from a few Hz to several hundred Hz is in good agreement with that expected from the shell instability. The cold background ions are observed to have a temperature anisotropy with T⊥ > T∥, while electrons are observed to have T∥ > T⊥, also in qualitative agreement with the shell instability. The wave‐particle effects due to the shell instability in the near‐Earth PSBL could have important consequences for auroral potential structure at lower altitudes and may cause the gaps in VDIS structure leading to beamlets.

A stochastic sea: The source of plasma sheet boundary layer ion structures observed by Cluster

On 14 February 2001 the Cluster Ion Spectrometry (CIS) experiment onboard three of the Cluster spacecraft observed velocity‐dispersed ion structures (VDIS) as the spacecraft passed from the tail lobes into the plasma sheet boundary layer. These are the first multiple spacecraft observations of the VDIS phenomenon. The Cluster 1 spacecraft (SC1) observed a dispersed ion signature with beamlets and a second structure like that expected to be produced by an echo, while Cluster 3 (SC3) observed much less pronounced structuring a few minutes later. During this same event and over an extended interval the ACE spacecraft observed an interplanetary magnetic field that was directed southward. We have inferred the sources and acceleration mechanisms of the ions in these VDIS observations by following millions of ion trajectories backward and forward in time through time‐dependent electric and magnetic fields obtained from a global MHD simulation. ACE data were used as input for the MHD model. We found that almost all of the particles comprising the first (A1) and second (A2) beamlets observed by SC1 had been nonadiabatic earlier in their history, while particles in the A3 beamlet exhibited a combination of adiabatic and nonadiabatic behavior. Beamlet A4 particles were always adiabatic. Moreover, for all of the beamlets the current sheet crossing that took place prior to their detection occurred between x = −13 RE and x = −16 RE in the tail, well earthward of the permanent stochastic “sea” from which all of the beamlets originated. Our model does not favor the multiple source scenario suggested by A. Keiling et al. Instead, it indicates that the source regions of the structures are spatially correlated. We have carried out a similar analysis of the SC3 observations. In general, SC3 beamlets have higher κ values, partly because of the depolarization of the field lines during these observations. In time forward calculations only a small fraction of ions from SC1 A structures returned to the spacecraft location. “Echoes” were more pronounced on SC3. In addition, in our calculations, some particles from SC1 A structures interacted with the current sheet and returned to the SC3 location, at the time when SC3 observed the A structures. When Cluster observations were organized by latitude instead of time, we found that all three Cluster spacecraft seemed to observe the same primary structure that persisted throughout the interval of observation.

Two types of energy‐dispersed ion structures at the plasma sheet boundary

We study two main types of ion energy dispersions observed in the energy range ∼1 to 14 keV on board the Interball‐Auroral (IA) satellite at altitudes 2–3 RE at the poleward boundary of the plasma sheet. The first type of structure is named velocity dispersed ion structures (VDIS). It is known that VDIS represent a global proton structure with a latitudinal width of ∼0.7–2.5°, where the ion overall energy increases with latitude. IA data allow to show that VDIS are made of substructures lasting for ∼1–3 min. Inside each substructure, high‐energy protons arrive first, regardless of the direction of the plasma sheet boundary crossing. A near‐continuous rise of the maximal and minimal energies of consecutive substructures with invariant latitude characterizes VDIS. The second type of dispersed structure is named time‐of‐flight dispersed ion structures (TDIS). TDIS are recurrent sporadic structures in H+ (and also O+) with a quasi‐period of ∼3 min and a duration of ∼1–3 min. The maximal energy of TDIS is rather constant and reaches ≥14 keV. During both poleward and equatorward crossings of the plasma sheet boundary, inside each TDIS, high‐energy ions arrive first. These structures are accompanied by large fluxes of upflowing H+ and O+ ions with maximal energies up to 5–10 keV. In association with TDIS, bouncing H+ clusters are observed in quasi‐dipolar magnetic field tubes, i.e., equatorward from TDIS. The electron populations generally have different properties during observations of VDIS and TDIS. The electron flux accompanying VDIS first increases smoothly and then decreases after Interball‐Auroral has passed through the proton structure. The average electron energy in the range ∼0.5–2 keV is typical for electrons from the plasma sheet boundary layer (PSBL). The electron fluxes associated with TDIS increases suddenly at the polar boundary of the auroral zone. Their average energy, reaching ∼5–8 keV, is typical for CPS. A statistical analysis shows that VDIS are observed mainly during magnetically quiet times and during the recovery phase of substorms, while sporadic and recurrent TDIS are observed during the onset and main phases of substorms and magnetic storms and, although less frequently, during substorm recovery phases. From the slope of the (velocity)−1 versus time dispersions of TDIS, we conclude that they have a sporadic source located at the outer boundary of the central plasma sheet, at distances from 8 to 40 RE in the equatorial plane. The disappearance of the PSBL associated with TDIS can be tentatively linked to a reconfiguration of the magnetotail, which disconnects from the Earth the field lines forming the “quiet” PSBL. We show that VDIS consist of ion beams ejected from an extended current sheet at different distances. These ion beams could be formed in the neutral sheet at distance ranging from ∼30 RE to ∼100 RE from the Earth. Inside each substructure the time‐of‐flight dispersion of ions generally dominate over any latitudinal dispersion induced by a dawn‐dusk electric field. These two main types of energy‐dispersed ion structures reflect probably two main states of the magnetotail, quiet and active. Finally, it must be stressed that only ∼49% (246 over 501) of the Interball‐Auroral auroral zone‐polar cap boundary crossings can be described as VDIS or TDIS. On the other 51% of the crossings of the plasma sheet boundary, no well‐defined ion dispersed structures were observed.

Oxygen ion beams at the boundary of velocity dispersed ion structures in the high-latitude magnetosphere under northward interplanetary magnetic field with a large by-component: Interball-tail observations

Here we discuss two cases of ionospheric ion beam observations by Interball-Tail at altitudes 6–9 R E in the high-latitude magnetosphere under conditions of a strong northward/ duskward directed interplanetary magnetic field (IMF). On November 22, 1997, the magnetosphere was immersed in the magnetic cloud characterized by a large, steady IMF and a tenuous, cold solar wind plasma. Interball-Tail (I-T) was in the prenoon high-latitude region of the south hemisphere, at R∼8 to 5 R E . Plasma data from PROMICS-3 and Corall instruments show that cold ionospheric ions were attached to the boundary of velocity and pitch-angle dispersed (VDIS) hot ion structures. Observed VDIS are similar to that seen typically at mid-altitudes and are known as coming either from the cusp injections or from the plasma sheet boundary layer and the plasma sheet. The other case of observations of ionospheric ion beams at the boundary of VDIS occurred under a somewhat weaker IMF and a more dense solar wind plasma (i.e., not a magnetic cloud event). This time O + beams were observed at the nightside high-latitude magnetosphere, just at the boundary of the plasma sheet. Later along the orbit, when IMF changed direction to a strongly dominating large northward BZ, oxygen ions were seen intermixed with a warm magnetosheath plasma apparently circulating within the dawn cell of the high-latitude NBZ convection system. This kind of data from PROMICS-3 are of a potential value for further coordinated analysis of ISTP events using multipoint observations.

Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations

Multiple and sporadic time‐of‐flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy‐dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection‐induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy‐dispersed ion beams.

The second pair in the INTERBALL quartet: Some main results

The INTERBALL-2 satellite (AURORAL PROBE) was launched on August 29, 1996, about a year after the INTERBALL-1 and MAGION-4 pair already worked on a higher orbit. It performed comprehensive measurements of magnetospheric plasmas and fields mainly at altitudes 2 – 3 R E during more than two years. Some of the main findings from these measurements are briefly reviewed, in particular: 1. Discovery of long-duration high-energy electron dispersion events above the polar edge of the post-midnight auroral oval. 2. Discovery of high-energy (> 20 keV, i.e. super-auroral energy) upward moving field-aligned electron beams accompanied by bi-directional electrons ∼ 1 keV, also in these auroral regions. 3. Mass-resolved velocity-dispersed ion structures (VDIS) extended down to superthermal energies injected in the plasma flux tubes at the auroral oval polar border in a wide range of local times. 4. Mid-altitude ion distribution functions in the cusp showing a combination of downward moving, and mirror reflected, magnetosheath ions, with upward moving thermal and superthermal H +, He +, and O + ions of the “cleft ion fountain” starting from eV energy range. 5. Thermal and superthermal plasma outflows of the classic polar wind type (only H + and He +, no O +) as contrasted to conditions when additional acceleration mechanisms are operating which accelerate thermal heavy ions also (O +, etc.).

Low‐altitude signatures of magnetotail reconnection

Precipitating ions on the poleward edge of the nightside auroral oval sometimes exhibit sharp low‐energy cutoffs in their energy spectra. These truncated spectra are interpreted as signatures of magnetic reconnection in the magnetotail. The energy cutoff is frequently smoothly dispersed in latitude, allowing an interpretation in terms of quasi‐steady reconnection. These events are designated velocity‐dispersed ion structures (VDIS) type 2. Roughly one third of type 2 VDIS are accompanied by a sharp transition in the polar rain near the open‐closed boundary that aids in their analysis. From 886 nightside open‐closed boundary crossings by DMSP spacecraft, 148 type 2 VDIS were identified. They were found most frequently within 2–3 hours of midnight and for 40% of the open‐closed boundary crossings between 2200 and 0100 magnetic local time. Minimum variance fits to the cutoff energies and polar rain transition are performed on 49 of these events. For four of them the information from the minimum variance fit and observed convection velocities are used to infer distances to the reconnection site that varied from 30 to 80 RE. In three of these four cases a sharp transition in the convection velocity is observed, coincident with the arrival of ions from the reconnection site. If the reconnection region is viewed as a voltage source, lobe field lines can be insufficiently populated to carry the current necessary to impose the required voltage on the ionosphere. This explains the coincidence between the arrival of ions and a discontinuity in convection, that is, that an electric field from the reconnection site is imposed on the ionosphere but only after sufficient density populates the field lines that connect the regions.

Toward an observational synthesis of substorm models: Precipitation regions and high‐latitude convection reversals observed in the nightside auroral oval by DMSP satellites and HF radars

A combination of simultaneous measurements from high, low and ground altitude instruments has been used to infer the rapid evolution of the coupled nightside magnetosphere‐ionosphere during substorms. Reversals from an eastward zonal convection to westward zonal convection become apparent inside the volume of the substorm bulge a few minutes after the intensification of the westward electrojet. These reversals persist for periods of 10–20 min and appear to have a one‐to‐one correspondence with the occurrence of dipolarizations. The changes in convection are accompanied by changes in precipitation. Flux depletion regions (FDR) are measured by the DMSP satellites inside the surge, near the equatorward portion of the westward electrojet intensification. The poleward boundary of every FDR is collocated with a convection reversal and an arc intensification that marks a poleward transition into a region initially dominated by intense discrete electron precipitation and velocity dispersed ion structures (VDIS). Convection in the FDR constitutes an eastward electrojet channel that may produce a transient recovery signature as observed by ground magnetometers. The observations of FDR's and the fast westward flows that accompany them are consistent with the scenario of a rarefaction and/or a neutral line in the near‐earth tail that produces fast earthward flows after the breakup. The arc intensification at the poleward boundary of the depletion region and the collocated transient convection reversal favor an enhancement of the magnetosphere‐ionosphere coupling and thus the continuation of the substorm expansion in a multiple cell convection system. Arguments are presented to explain how a series of pseudobreakups modify the near Earth magnetic field into an increasingly dipolar geometry until a breakup is possible.

Mapping of the Precipitation Regions to the Plasma Sheet

The progress in the mapping of the auroral regions in the Earth's polar ionosphere to outer magnetosphere reflects our growing understanding of the gross magnetospheric structure. Several natural tracers were identified and used by us for the mapping scheme advocated for more than a decade. A natural tracer is a plasma boundary identifiable at different altitudes which, from physical reasons, is aligned along the magnetic flux tube (accounting for cross-field convection). The boundaries' locations describe the current state of the magnetosphere. The following tracers to the tail were used in our studies : the low-latitude Soft Electron precipitation Boundary ; the large-scale Convection Boundary, or an Alfven Layer ; the Plasmapause ; the Stable Trapping Boundary for high energy electrons ; the precipitating hot ion Isotropy Boundary ; the two types of Velocity-Dispersed Ion Structures : VDIS-1 (adjacent to an electron inverted-V structure within the oval), and VDIS-2 (just poleward from the oval). A new Wall Region concept related to non-adiabatic (non-MHD) ion dynamics allows to add its effects in the list of natural tracers. Another newly discovered structure in the tail is the Low Energy Layer of counter-streaming low-energy (<100 eV) ions and electrons at the outer edge of the Boundary Plasma Sheet. Its physical origin in the far tail, and respective source location, are debatable. Physical limits to the MHD-mapping approach are placed by plasma and field fluctuations and turbulence, by finite Larmor radius effects including non-adiabatic particle dynamics, by finite Alfven propagation times in the magnetosphere, and by various medium- and large-scale disturbances- auroral activations.

Quiet‐time intensifications along the poleward auroral boundary near midnight

Radar and optical measurements from Sondrestrom are combined with satellite and Goose Bay data in a study of the poleward edge of the nightside auroral oval during a quiet period. The By and Bz components of the interplanetary magnetic field were close to zero, and the Bx component was ∼8 nT for more than 24 hours. On a large scale, the convection and precipitation patterns remained almost constant during this period; on a small scale, however, the conditions were quite dynamic. At 10‐ to 20‐min intervals the arc that marked the poleward auroral boundary intensified, and a new arc appeared poleward of it. About once per hour, stronger intensifications were observed. One such event is examined in detail. The auroral arcs first appeared to dim, and then they brightened, with a factor of 10 increase in E region electron density. At the time of the brightening a new arc formed poleward of all the arcs. The arcs then drifted southward at velocities of ∼270 m/s. A plasma drift disturbance, characterized by a doubling of the southward velocity and a reversal in the east‐west component, propagated westward at 900 m/s through the fields of view of the Sondrestrom and Goose Bay radars. A simultaneous satellite overpass close to the radars revealed the presence of an energetic ion event similar to the “velocity dispersed ion structures” observed on the Aureol satellite and presumed to be the signature of fast ion beams within the plasma sheet boundary layer. The stronger arc intensification events observed by the Sondrestrom radar are associated with an increase in plasma flow across the boundary between open and closed magnetic field lines. We interpret this increased flow as the ionospheric signature of abrupt, localized increases in the reconnection rate in the midnight sector.

Aureol‐3 observations of new boundaries in the auroral ion precipitation

Interesting and well‐separated structures in the 1–20 keV ion precipitation pattern have been revealed by an analysis of more than 50 crossings of the nightside (21–03 MLT) auroral zone by the AUREOL‐3 satellite. First, velocity‐dispersed ion structures (VDIS) are crossed near the poleward edge of the oval, and are the best ionospheric signature of ion beams flowing along the plasma sheet boundary layer. Proceeding equatorward, a large majority of VDIS events are bounded by a new and interesting narrow band of strongly reduced precipitation, or a gap, which delineates VDIS from the diffuse precipitation region connected to the CPS. A statistical analysis shows that the gap has an extent of about 1–2 degrees, which is almost independent of magnetic activity; its location, ∼70° ILAT, shifts significantly equatorward with higher magnetic activity levels. Intense electron arcs are observed near the equatorward edge of the gap. An important result is that the overall sequence of VDIS‐gap‐CPS can be explained in terms of orbital dynamics in the tail. The gap in precipitation appears as the counterpart of the “wall” regime in the equatorial plane, in which a cross‐tail current carried by energetic ions is strongly enhanced between 8 and 12 RE. This region has important consequences for the development of substorms.