Dusk Side Research Articles

Statistical properties of tail plasma sheet electrons above 40 keV

We use data from four tail seasons (2001–2004) of the Cluster spacecraft to study statistical features of energetic electrons (40–400 keV) in the magnetotail plasma sheet in combination with plasma parameters and indices of geomagnetic activity. We find that the horizontal magnetic field magnitude best orders the energetic electron observations. Using this method we observe a statistical flux gradient toward the neutral sheet. The fluxes also increase with an increase in the vertical magnetic field component (dipolarization). The intensity of supra‐thermal electrons is observed to increase strongly as function of plasma temperature. Although the energetic electron fluxes also appear to increase strongly with geomagnetic activity this is found to be mainly due to the increase in plasma temperature with increasing Kp. Investigation of neutral sheet fluxes and phase space densities at fixed first adiabatic invariant (μ) indicates that adiabatic heating can explain observed flux increases during field dipolarization. The spectral slope in the supra‐thermal range is observed to be independent of geomagnetic activity (Kp), while there is a significant local time dependence, with harder spectra observed at dawn compared to the dusk side.

Electron flux enhancement in the inner radiation belt during moderate magnetic storms

Abstract. During moderate magnetic storms, an electron channel (300–1100 keV) of the NOAA satellite has shown sudden electron flux enhancements in the inner radiation belt. After examinating the possibility of contamination by different energetic particles, we conclude that these electron flux enhancements are reliable enough to be considered as natural phenomena, at least for the cases of small to moderate magnetic storms. Here, we define small and moderate storms to be those in which the minimum Dst ranges between −30 and −100 nT. The electron flux enhancements appear with over one order of magnitude at L~2 during these storms. The enhancement is not accompanied by any transport of electron flux from the outer belt. Statistical analysis shows that these phenomena have a duration of approximately 1 day during the period, starting with the main phase to the early recovery phase of the storms. The flux enhancement shows a dawn-dusk asymmetry; the amount of increased flux is larger in the dusk side. We suggest that this phenomenon could not be caused by the radial diffusion but would be due to pitch-angle scattering at the magnetic equator. The inner belt is not in a stationary state, as was previously believed, but is variable in response to the magnetic activity.

Average thermospheric wind patterns over the polar regions, as observed by CHAMP

Abstract. Measurements of the CHAMP accelerometer are utilized to investigate the average thermospheric wind distribution in the polar regions at altitudes around 400 km. This study puts special emphasis on the seasonal differences in the wind patterns. For this purpose 131 days centered on the June solstice of 2003 are considered. Within that period CHAMP's orbit is precessing once through all local times. The cross-track wind estimates of all 2030 passes are used to construct mean wind vectors for 918 equal-area cells. These bin averages are presented in corrected geomagnetic coordinates. Both hemispheres are considered simultaneously providing summer and winter responses for the same prevailing geophysical conditions. The period under study is characterized by high magnetic activity (Kp=4−) but moderate solar flux level (F10.7=124). Our analysis reveals clear wind features in the summer (Northern) Hemisphere. Over the polar cap there is a fast day-to-night flow with mean speeds surpassing 600 m/s in the dawn sector. At auroral latitudes we find strong westward zonal winds on the dawn side. On the dusk side, however, an anti-cyclonic vortex is forming. The dawn/dusk asymmetry is attributed to the combined action of Coriolis and centrifugal forces. Along the auroral oval the sunward streaming plasma causes a stagnation of the day-to-night wind. This effect is particularly clear on the dusk side. On the dawn side it is evident only from midnight to 06:00 MLT. The winter (Southern) Hemisphere reveals similar wind features, but they are less well ordered. The mean day-to-night wind over the polar cap is weaker by about 35%. Otherwise, the seasonal differences are mainly confined to the dayside (06:00–18:00 MLT). In addition, the larger offset between geographic and geomagnetic pole in the south also causes hemispheric differences of the thermospheric wind distribution.

Two sources of magnetosheath ions observed by Cluster in the mid-altitude polar cusp

Double cusps have been observed on a few occasions by polar orbiting spacecraft and ground-based observatories. The four Cluster spacecraft observed two distinct regions, showing characteristics of a double cusp, during a mid-altitude cusp pass on 7 August 2004. The Wind spacecraft detected a southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF–Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion precipitation around 1 keV on the equatorward side of the cusp and a dense ion population in the cusp centre. Cluster 1, entering the cusp around 1 min later, observed only a partial ion dispersion with a low energy cutoff reaching 100 eV, together with the dense ion population in the cusp centre. About 9 min later, Cluster 3 entered the cusp and observed full ion dispersion from a few keV down to around 50 eV, together with the dense ion population in the centre of the cusp. The ion flow was directed poleward and eastward in the step/dispersion, whereas in the centre of the cusp the flow was directed poleward and westward. In addition the altitude of the source region of ion injection in the step/dispersion was found 50% larger than in the cusp centre. This event could be explained by the onset of dayside reconnection when the IMF turned southward. The step would be the first signature of component reconnection near the subsolar point, and the injection in the centre of the cusp a result of anti-parallel reconnection in the northern dusk side of the cusp. A three-dimensional magnetohydrodynamic (MHD) simulation is used to display the topology of the magnetic field and locate the sources of the ions during the event.

Observation of two distinct cold, dense ion populations at geosynchronous orbit: local time asymmetry, solar wind dependence and origin

Abstract. We report on the observation of two distinct cold (Ti<5 keV), dense (Ni>2 cm−3) ion populations at geosynchronous orbit. A statistical study was performed on measurements from the geosynchronous Los Alamos plasma instruments, for the period 1990–2004, and complemented by corresponding large-scale plasma sheet data obtained by mapping DMSP observations in the tail. The first population, which has previously been reported in several studies, is observed in the midnight region of geosynchronous orbit. The second population, which has drawn less attention, is detected on the dawn side of geosynchronous orbit. No such cold, dense population is observed on the dusk side of geosynchronous orbit on a frequent basis. The temporal evolution of various plasma parameters as a function of local time shows that the two populations appear at geosynchronous orbit as distinct populations, since the appearance of a midnight population is not usually associated with that of a dawn population, and vice versa. The midnight ion population is typically observed after the IMF has been northward for some time and is convected inward toward geosynchronous orbit after an observed mild southward turning of the average IMF. It is interpreted that the source of the midnight population is the cold, dense plasma sheet (CDPS). The dawn-side cold and dense ion population is associated with previously strong southward IMF and consequently occurs during substantial geomagnetic activity. These events are typically observed around the end of the main phase of the corresponding Dst decrease, down to −50 nT on average. It is unlikely that this dawn population is simply the low-latitude boundary layer (LLBL) moving closer to Earth because (1) no symmetric dusk population is observed and (2) on average a small sunward flow (~15 km/s) is observed for those events. The cold, dense population at dawn is thus observed during active times (based on Dst, Kp and AE indices) in comparison with the midnight case. However, since the dawn population is observed only around the end of the main Dst decrease, it is concluded that this population does not typically contribute to the Dst decrease during the main phase. This population may rather be transported to geosynchronous orbit by means of a compression and convection enhancement in the magnetosphere, with a preferential access from the dawn flank with no apparent counterpart at dusk. DMSP data suggest that a cold and dense plasma source is mainly present at dawn.

Magnus force in the Venus ionosphere

The retrograde superrotation motion of the Venus ionosphere is included in a study of the configuration of the ionospheric transterminator flow by using a fluid‐dynamic interpretation of the interaction that takes place between both flow motions. It is argued that the interaction leads to a fluid dynamic force (Magnus force) that produces an overall (∼10°) dawnward directed deflection of the transterminator flow which is comparable to that implied from the PVO observations. The Magnus force results from a pressure difference that is set across the planet by the difference in the speed of the plasma when both the rotation motion and the transterminator flow are in the same sense (dusk side terminator) to that where both motions are opposite to each other (dawn side terminator). The dawnward deflection of the ionospheric plasma in the transterminator flow also deviates the plasma channels (which serve to account for the ionospheric holes) that extend downstream from the magnetic polar regions and is responsible as well for that of the peak of the histogram of the electron density profiles with an upper ionospheric plateau that are seen near the midnight plane. The effects of the Magnus force are also related to the strong difference in the ionopause altitude observed between the dusk and the dawn side terminator (with higher altitudes seen in the dawn side) and to the larger ionospheric transterminator flow speeds measured at high altitudes by the dawn side terminator.

Simulations of the neutral structure within the dusk side aurora

Abstract. Observations of neutral winds from rocket release experiments within the premidnight and postmidnight substorm recovery phase aurora, show very large E-region neutral winds of several hundred m/s, where winds measured on the dusk side are even larger than those on the dawn side. These large winds are also associated with strong shears, and there is evidence that some of the regions below these shears may be unstable. The mechanisms which generate this strong vertical structure are not well understood. It is also not known whether the acceleration conditions in the pre and post midnight sectors of the aurora may produce significantly different neutral responses on the dawn and dusk sides. Simulations have been performed using a three-dimensional high resolution limited area thermosphere model to try to understand the neutral structure within the dawn and dusk side aurora. When simulations are performed using auroral forcing alone, for equivalent conditions within the dawn and dusk sectors, differences are found in the simulated response on each side. When measured values of auroral forcing parameters, and background winds and tides consistent with recent observations, are used as model inputs, some of the main features of the zonal and meridional wind observations are reproduced in the simulations, but the magnitude of the peak zonal wind around 140 km tends to be too small and the maximum meridional wind around 130 km is overestimated. The winds above 120 km altitude are found to be sensitive to changes in electric fields and ion densities, as was the case for the dawn side, but the effects of background winds and tides on the magnitudes of the winds above 120 km are found to be relatively small on the dusk side. The structure below 120 km appears to be related mainly to background winds and tides rather than auroral forcing, as was found in earlier studies on the dawn side, although the peak magnitudes of simulated wind variations in the 100 to 120 km altitude range are smaller than those observed. The source of the strong shears measured around 110 km altitude on the dusk side is uncertain, but may be related to different kinds of oscillations, such as gravity waves, non migrating semidiurnal tides, or secondary oscillations produced by non linear interactions between waves.

Equatorial distributions of the plasma sheet ions, their electric and magnetic drifts, and magnetic fields under different interplanetary magnetic field Bz conditions

To understand the nightside plasma sheet structure under different interplanetary magnetic field (IMF) Bz conditions, we have investigated statistically the equatorial distributions of ions and magnetic fields from Geotail when the IMF has been continuously northward or southward for shorter or longer than 1 hour. A dawn‐dusk density (temperature) asymmetry with higher density (temperature) on the dawn (dusk) side is seen in the near‐Earth plasma sheet during northward IMF, resulting in roughly dawn‐dusk symmetric pressure. As southward IMF proceeds, the density asymmetry weakens while the temperature asymmetry maintains, resulting in higher pressure on the dusk side. The plasma sheet is relatively colder and denser near the flanks than around midnight. The flux distributions show that the density asymmetry is due to ions <∼3 keV, and the temperature asymmetry is due to ions above thermal energy. The perpendicular flow shows that ions divert around the Earth mainly through the dusk side in the inner plasma sheet because of westward diamagnetic drift. The magnetic fields indicate that field lines are more stretched during southward IMF. Ions' electric and magnetic drift paths evaluated from the observations show that for thermal energy ions, magnetic drift is as important as electric drift. Comparison of the distributions of the observed phase space density with the evaluated drift paths at different energies indicates that the electric and magnetic drift transport is responsible for the observed dawn‐dusk asymmetries in the plasma sheet structure.

Mapping of the heavy ion outflows as seen by IMAGE and multifluid global modeling for the 17 April 2002 storm

Multifluid global modeling that includes both light and heavy ion dynamics is used to investigate changes in magnetospheric dynamics produced by enhanced ionospheric outflows for the 17 April 2002 storm. The predicted outflow rates and their effects on the magnetosphere are compared with AMIE and IMAGE/HENA data as a function of relative O+ concentration in the ionosphere. It is shown that O+ can be the dominant species on the dusk side in the inner magnetosphere during the periods of high activity even for ionospheric concentrations as low as 5% as a result of differential heavy ion convection patterns. However, for these low outflow conditions, the cross‐polar cap potential is much larger than that calculated by AMIE. As the O+ ionospheric concentration increases, the heavy outflow rate approaches canonical values, and the effective ionospheric resistivity falls in association with a decrease in cross‐polar cap potential and an increase in the auroral field aligned currents. Within the magnetosphere, the region where the O+ concentration exceeds 50% expands toward the duskside and down the tail. Pressures in excess of 1 nPa can be supported by the enhanced O+ outflows. The presence of these enhanced outflows appears at about the same time and over a similar region to enhancements seen in the HENA 51–180 keV oxygen data. These results provide a direct tie between heavy ionospheric outflows and their energization in the tail and suggest that during this period the heavy ions provide a substantial amount of the plasma required to support the magnetotail current sheet. The model results also show that the enhanced O+ outflow leads to the enhanced exclusion of light ions of both solar wind and ionospheric origin in the plasma sheet, albeit with the light ions experiencing enhanced heating in an O+ dominated current sheet. These results indicate that observed heavy ionospheric outflows and their energetization in the tail have global consequences for the magnetosphere including supporting the dynamics of the tail current sheet during active times and in global circulation including the cross‐polar cap potential and the region 1 and 2 currents.

Dawn-dusk asymmetries and sub-Alfvénic flow in the high and low latitude magnetosheath

Abstract. We present the results of a statistical survey of the magnetosheath using four years of Cluster orbital coverage. Moments of the plasma distribution obtained from the electron and ion instruments together with magnetic field data are used to characterise the flow and density in the magnetosheath. We note two important differences between our survey and the gasdynamic model predictions: a deceleration of the flow at higher latitudes close to the magnetopause, resulting in sub-Alfvénic flow near the cusp, and a dawn-dusk asymmetry with higher velocity magnitudes and lower densities measured on the dusk side of the magnetosheath in the Northern Hemisphere. The latter observation is in agreement with studies carried out by Paularena et al. (2001), Němeček et al. (2000), and Šafránková et al. (2004). In equations of hydrodynamics for a single-component additon to this we observe a reverse of this asymmetry for the Southern Hemisphere. High-latitude sub-Alfvénic flow is thought to be a necessary condition for steady state reconnection pole-ward of the cusp.

Localized fast flow disturbance observed in the plasma sheet and in the ionosphere

Abstract. An isolated plasma sheet flow burst took place at 22:02 UT, 1 September 2002, when the Cluster footpoint was located within the area covered by the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE). The event was associated with a clear but weak ionospheric disturbance and took place during a steady southward IMF interval, about 1h preceding a major substorm onset. Multipoint observations, both in space and from the ground, allow us to discuss the temporal and spatial scale of the disturbance both in the magnetosphere and ionosphere. Based on measurements from four Cluster spacecraft it is inferred that Cluster observed the dusk side part of a localized flow channel in the plasma sheet with a flow shear at the front, suggesting a field-aligned current out from the ionosphere. In the ionosphere the equivalent current pattern and possible field-aligned current location show a pattern similar to the auroral streamers previously obtained during an active period, except for its spatial scale and amplitude. It is inferred that the footpoint of Cluster was located in the region of an upward field-aligned current, consistent with the magnetospheric observations. The entire disturbance in the ionosphere lasted about 10min, consistent with the time scale of the current sheet disturbance in the magnetosphere. The plasma sheet bulk flow, on the other hand, had a time scale of about 2min, corresponding to the time scale of an equatorward excursion of the enhanced electrojet. These observations confirm that localized enhanced convection in the magnetosphere and associated changes in the current sheet structure produce a signature with consistent temporal and spatial scale at the conjugate ionosphere.

Simulation of the dawn-dusk magnetosheath asymmetry under quasi-steady states

The dawn-dusk asymmetry of the magnetosheath under quasi-steady states has been studied by using a newly developed 3D MHD magnetosphere simulation model. The results show that the dawn-dusk asymmetry is substantial because of the Parker spiral IMF. It is found that the dawn-dusk magnetosheath thickness asymmetry is the effect of different shock conditions. The plasma density and flux asymmetry are mainly caused by the different thickness of the dawn-dusk magnetosheath, and the magnetic reconnection on the magnetopause has no significant effects. It is also showed that the Plasma Depletion Layer in front of the dayside magnetopause can cause duskward plasma flow, and the total plasma flux on the dusk side will be higher.

Plasmoid in the high latitude boundary/cusp region observed by Cluster

A plasmoid‐like structure without a core magnetic field has been observed by Cluster spacecraft in the high latitude magnetopause region when the interplanetary magnetic field (IMF) is northward and has a dominant duskward component. Bursty electrons and ions are found to be well confined in this plasmoid‐like structure. The structure possesses a bipolar signature in the BN component. The structure is associated with a dramatic change of the plasma flow direction from tailward to dawnward. The plasma density in this structure is slightly enhanced whereas the ion temperature is twice as large as that of the ambient plasma. The plasma parallel temperature is higher than the perpendicular one. The observed structure is different from the traditional Flux Transfer Events (FTEs). The Walén test shows that change of plasma flow velocity are closely associated with change of Alfven velocity which indicates this structure is formed by reconnection process. The observed plasmoid is suggested to be formed by multiple X‐line reconnection in the dusk (dawn) side of the stagnant cusp region when the IMF is northward and possesses a dominant positive (negative) BY component.

Electrodynamics of transpolar aurorae

Polar UV image and Astrid-2 electric and magnetic field data are used to study the evolution of the electrodynamical configuration of the high-latitude ionosphere during a 5-h transpolar arc event. During the course of the event we observe Sunward convection in narrow regions associated with pairs of upward and downward transpolar arc-related field-aligned currents. We also observe Sunward convection deriving from more remotely located field-aligned currents as well as stagnant convection in the centre of the polar region associated with a decoupling of the dawn and dusk side current systems. Schematic pictures of the current closure and convective flows are presented for a variety of configurations. The different states evolve naturally into each other and do not represent fundamentally different magnetospheric configurations.

Latitudinal Variations of Diurnal Meteor Rates

The presence of a diurnal variation in meteor activity is well established. The sporadic meteor count rates are higher on the local dawn side and lower on the local dusk side. This phenomenon is caused by the Earth’s orbital motion and rotation. Meteor radar measurements have been compared from Esrange, Kiruna, Sweden, at 68° N, from Juliusruh, Germany, at 55° N, and from Ascension Island, at 8° S, to investigate how the diurnal variation depends on season at different latitudes. Data have been used from vernal and autumnal equinoxes and summer and winter solstices to locate the largest seasonal differences.

Evidence for pearl-like magnetic island structures at dawn and dusk side magnetopause

Two-dimensional magnetopause structure with a series of pearl-like magnetic islands at the dawn side of the equatorial plane was reported earlier by Hau and Sonnerup (1999) from the AMPTE/IRM data by solving the Grad-Shafranov equation as a spatial initial value problem. This paper presents another rare event that appears like the image of the dawn event at the dusk side magnetopause based on the analysis of two years satellite data and similar reconstruction method. The two crossings occur at (x, y, z) = (7.13,−6.64, 0.4) and (7.93, 9.06,−1.89)RE with magnetopause normal n̂ being (0.85,−0.52, 0.003) and (0.82, 0.55,−0.17) in the GSE coordinates, respectively. Both events are tangential discontinuity-like structures with shear angles being 124° and 97° and convect tailward with the deHoffmann-Teller frame velocity VHT of (−141.88,−215.47,72.48) and (−100.26, 144.52,20.90) (km/s), respectively, that form approximately 90° from n̂. The magnetic field configurations in both cases show great similarity with the same island width of ∼500 km and aspect ratio of ∼0.2.

Transpolar arc evolution and associated potential patterns

Abstract. We present two event studies encompassing detailed relationships between plasma convection, field-aligned current, auroral emission, and particle precipitation boundaries. We illustrate the influence of the Interplanetary Magnetic Field By component on theta aurora development by showing two events during which the theta originates on both the dawn and dusk sides of the auroral oval. Both theta then move across the entire polar region and become part of the opposite side of the auroral oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the Polar UVI instrument provides measurements of auroral emissions. Utilizing satellite data as inputs, the Royal Institute of Technology model provides the high-latitude ionospheric electrostatic potential pattern calculated at different times during the evolution of the theta aurora, resulting from a variety of field-aligned current configurations associated with the changing global aurora. Key words. Ionosphere (auroral ionosphere; electric fields and currents). Magnetospheric physics (magnetosphereionosphere interactions)

A study of Pc-5 ULF oscillations

Abstract. A study of Pc-5 magnetic pulsations using data from the Combined Release and Radiation Effects Satellite (CRRES) was carried out. Three-component dynamic magnetic field spectrograms have been used to survey ULF pulsation activity for the approximate fourteen month lifetime of CRRES. Two-hour panels of dynamic spectra were examined to find events which fall into two basic categories: 1) toroidal modes (fundamental and harmonic resonances) and 2) poloidal modes, which include compressional oscillations. The occurence rates were determined as a function of L value and local time. The main result is a comparable probability of occurence of toroidal mode oscillations on the dawn and dusk sides of the magnetosphere inside geosynchronous orbit, while poloidal mode oscillations occur predominantly along the dusk side, consistent with high azimuthal mode number excitation by ring current ions. Pc-5 pulsations following Storm Sudden Commencements (SSCs) were examined separately. The spatial distribution of modes for the SSC events was consistent with the statistical study for the lifetime of CRRES. The toroidal fundamental (and harmonic) resonances are the dominant mode seen on the dawn-side of the magnetosphere following SSCs. Power is mixed in all three components. In the 21 dusk side SSC events there were only a few examples of purely compressional (two) or radial (one) power in the CRRES study, a few more examples of purely toroidal modes (six), with all three components predominant in about half (ten) of the events. Key words. Magnetospheric physics (MHD waves and instabilities; magnetospheric configuration and dynamics) – Space plasma physics (waves and instabilities)

Evidence for energetic electron and iondispersive microinjections in theEarth's magnetotail as far as 27 <i>R<sub>E</sub></i>

Abstract. High energy and time resolution measurements of energetic electron and ion fluxes obtained by the DOK-2 experiment on board the Interball-tail satellite provide us the opportunity to study the short-lived (1–3min), localized, and often periodic and dispersive flux increases within the plasma sheet. We have deliberately selected and studied two intervals corresponding to the dawn and dusk magnetotail flanks. Dispersive electron (ion) bursts in the dawn side (dusk side) are observed from L=7 to L=27. These bursts, having an individual entity, are termed microinjections and are observed in radial distances greater than those predicted by the ``injection boundary model." In this paper we suggest that the dispersive fluxes at widely separated radial distances are produced by multiple pulsating isospectrum surfaces ordered in succession. At the inner edge of the plasma sheet, the isospectrum surface is considered by Sarafopoulos (2002) as a meandering injection boundary. Roughly, we estimate that the wavelength for an oscillating isospectrum surface is

Circulation of ionospheric and solar wind particle populations during extended southward interplanetary magnetic field

Single particles trajectories using time‐dependent electric and magnetic fields from multifluid simulations are used to examine the energy and mass transport due to entry of solar wind particles and from ionospheric outflows incorporating both light and heavy ions. It is shown the particles move with the convection cells derived from the multifluid simulations, and that strong particle acceleration occurs at both the dayside and nightside tips of the flow reversal regions. However, different particle populations have limited access to various parts of the convection pattern, and this leads to substantial dawn‐dusk asymmetries in the particle populations within the magnetosphere. For the strong southward interplanetary magnetic field (IMF) conditions for the event studied, fast solar wind particles are able to enter from the dawnside and contribute to the asymmetric ring current. The rest of the fast particles flow down the mantle and do not enter the near‐Earth magnetosphere. The remaining slow component is convected into the magnetosphere over a few tens of minutes to form cold populations in the mantle and LLBL. Solar wind He++ and O6+ are shown to penetrate deeper than the solar wind protons. Light ionospheric ions from the cusp and dawnside auroral oval have similar access problems and flow primarily down the tail and make little contribution to the plasma content to the near‐Earth plasma sheet. Nightside auroral protons are shown to form a cold component in the plasma sheet boundary layer. When they are eventually convected in to the near‐Earth plasma sheet they are accelerated preferentially to the duskside and experience further energy gains as they are convected sunward. These energetic protons contribute to the asymmetric ring current, with leakage from the magnetosphere occurring as they approach the dayside reconnection region. Heavy ionospheric ions enter the convection pattern differently from their light ions counterparts. O+ ions from the cusp can enter the near‐Earth plasma sheet to provide some mass loading, but the biggest contributor is the dawnside auroral oval. The dawn‐dusk electric field accelerates these ions toward the dusk side, but on sufficient low L shells that these ions can dominate the symmetric ring current composition. The O+ ions that originate from the midnight and dusk sectors go to forming hot populations in the dusk LLBL.