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

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010333, 2004 Two types of energy-dispersed ion structures at the plasma sheet boundary J.-A. Sauvaud Centre d Etude Spatiale des Rayonnements, CNRS, Toulouse, France R. A. Kovrazhkin Space Research Institute of Russian Academy of Sciences, Moscow, Russia Received 22 November 2003; revised 16 June 2004; accepted 18 June 2004; published 16 December [1] 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 R E 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 , 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 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 R E 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 R E to 100 R E from the Earth. Inside each substructure the time-offlight 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. INDEX TERMS: 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; Copyright 2004 by the American Geophysical Union /04/2003JA of17

2 2764 Magnetospheric Physics: Plasma sheet; 2704 Magnetospheric Physics: Auroral phenomena (2407); 2744 Magnetospheric Physics: Magnetotail; 2716 Magnetospheric Physics: Energetic particles, precipitating; KEYWORDS: magnetotail, plasma sheet, boundary layer, ion beams, acceleration, ionosphere Citation: Sauvaud, J.-A., and R. A. Kovrazhkin (2004), Two types of energy-dispersed ion structures at the plasma sheet boundary, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] The processes of plasma heating, acceleration, and transport in the plasma sheet boundary layer (PSBL) and in the central plasma sheet (CPS) play a crucial role in the structure and dynamics of the Earth s magnetotail. Both regions are dynamic. Field-aligned plasma flows with velocities in the range km/s have been observed in the PSBL on board a number of satellites [e.g., DeCoster and Frank, 1979; Parks et al., 1979; Forbes et al., 1981; Lui et al., 1983; Eastman et al., 1984, 1985; Takahashi and Hones, 1988]. High-speed plasma flows in the central plasma sheet have been reported, in particular in data from the ISEE satellites [e.g., Hones, 1979; Nishida et al., 1981; Huang et al., 1987]. However, such flows have first received little attention because they were thought to be statistically insignificant based on the low average flow velocity in the CPS regardless of geomagnetic activity [see Angelopoulos et al., 1994]. However, the average velocity is small not just in the CPS but in all plasma sheet regions [Baumjohann et al., 1989]. Baumjohann et al. [1989, 1990] discover a class of short burst (1 min) of fast plasma flows in the CPS called bursty bulk flows (BBF). Angelopoulos et al. [1992, 1996] showed that BBF are imbedded in intervals of enhanced plasma flows, which last for 10 min. BBF velocities are one order of magnitude higher than the average convection velocity. Bursty bulk flows, which are mostly directed along the ±X axis of the tail, i.e., mainly perpendicularly to the neutral sheet magnetic field, constitute a key factor for the transport of magnetic flux from the distant tail to the inner magnetosphere. CPS flows are observed during substorms and moderately disturbed periods. Inside the PSBL, earthward flowing ions are more frequently measured, although reflected tailward beams are also observed. Inside the CPS and during substorms, Geotail data have shown that earthward (tailward) flows are more frequent earthward (tailward) of about 30 R E [Nagai et al., 1998]. It is generally accepted since the pioneering work of Cowley [1980] that the PSBL results from the acceleration of plasma mantle particle in the tail neutral sheet. CPS bursty bulk flows are frequently interpreted in the framework of a magnetic reconnection initiated in the midtail. However, recently, Petrukovich et al. [2001] experimentally showed that PSBL and BBF plasma flows could have a common origin, as CPS flows initiated in high beta region could result in field-aligned flows in the high Z PSBL where the plasma ion beta is lower. [3] Owing to the slow motion of satellites with high apogees, rapid cuts of the CPS-PSBL regions are nonavailable, except during disturbed times when the plasma sheet becomes unstable. This is the reason why the nearly instantaneous distribution of ion beams across the PSBL as a function of the distance to the neutral sheet is not known for various magnetic conditions. One way to solve such a difficulty is to take advantage of low and midaltitude orbiting satellites, which provide a snapshot of the ion distribution across the CPS/PSBL footprints in several minutes or tens of minutes, respectively. [4] Ion beams detected in the magnetotail obviously contribute to ion precipitation near the polar boundary of the nighttime auroral oval, as it is that zone into which the PSBL and the outer plasma sheet regions map along field lines. The search for evidences of ion beam precipitation using low-apogee satellite data led to the detection of energy-dispersed ion structures in the energy range 1 20 kev [Kovrazhkin et al., 1987; Bosqued, 1987; Zelenyi et al., 1990; Saito et al., 1992; Onsager and Mukai, 1995; Elphinstone et al., 1995]. Such structures were called VDIS (velocity dispersed ion structures). VDIS were described as having the maximum ion energy at the poleward edge of the precipitation zone. No clear dependence between VDIS occurrence rate and the magnetic activity has been observed. However, these structures have a tendency to appear during quiet or moderately disturbed magnetospheric conditions, in particular, during the recovery phase of a substorm [Elphinstone et al., 1995]. [5] A different type of ion energy-dispersed structure, clearly associated with substorms, has been identified from data delivered by the Interball-Auroral satellite. Data were taken at distances of 2 3 R E above the auroral region [Sauvaud et al., 1999]. Such structures, named TDIS (timeof-flight dispersed ion structures), represent sporadic and recurrent ion injections with timescales of 1 3 min. TDIS show some correlation with fast flows in the plasma sheet and with transient plasma jets [Sauvaud et al., 1999; Sergeev et al., 2000a]. They have been observed inside the poleward part of the auroral bulge. TDIS are systematically associated with prominent outflows of ionospheric H + and O + in 100 ev to 10 kev range [Popescu et al., 2002]. Their energy dispersion displays a specific temporal character: the first ions to be detected are high-energy protons followed by the low-energy ones. [6] It must be stressed that kev proton dispersed structures at the polar edge of auroral precipitation zone have also been reported from the data of the TIMAS mass-spectrometer on board the Polar satellite, at altitudes ranging from 4 to 7 R E,[Lennartsson et al., 2001]. These authors named the observed energy-dispersed structures blast-like structures and conclude that the proton velocity dispersions in the TIMAS data are by most indications temporal in nature and that there is, in any case, no consistent latitudinal dispersion per se. Note that Lennartsson et al. [2001] analyze the appearance of dispersed ion structures with respect to the geomagnetic activity index Kp and that this does not allow a study of the links between these observations and the substorm phases. Thus direct measurements from various satellites and altitudes indicate rather different results concerning the ion structures at the poleward edge of the auroral oval. 2of17

3 [7] This paper presents a detailed analysis of two kinds of energy-dispersed ion structures observed at the polar edge of auroral zone by the ION experiment on board the Interball-Auroral satellite. For a precise identification of the magnetotail source region, simultaneous measurements of electrons are also used. Our study has two main objectives. [8] The first objective is to determine the exact nature of the dispersed ion structures, i.e., to settle if the energy dispersions are the result of a temporal or of a latitudinal dispersion. In the first case it may mean that the ion structure formation signals rapid physical processes affecting the magnetic field configuration of the plasma sheet. In the second case one may expect the existence of a quasistable large-scale convection electric field in the magnetotail with timescales exceeding the time needed for the ion beam to propagate from its source to the polar ionosphere. [9] The second objective is to identify the magnetotail sources of ion beams, i.e., to determine whether the fieldaligned ion flows are generated inside the plasma sheet boundary layer (PSBL) and/or in the outer region of the central plasma sheet (CPS). [10] One key point of this study is the demonstration that velocity dispersed ion structures (VDIS) are made of substructures where the ions are dispersed mostly by time-offlight effects and that their overall energy increase with latitude probably comes from the acceleration mechanism in the extended tail during quiet times or during substorm recovery phases. Concerning TDIS, the estimation of the source location, together with the study of the characteristics of the electrons associated with them, confirms that their source is probably located inside the CPS, close to the Earth during substorm/storm periods. 2. Instrumentation and Data Sources [11] We investigate the poleward edge of the nighttime auroral zone using data obtained from the ION experiment flown aboard the Russian Interball-Auroral satellite (IA). This satellite was launched on 29 August 1996 into a 62.8 inclination orbit with an apogee of 20,000 km (3 R E ) and an orbital period of 6 hours. The satellite was spin stabilized with a rotation rate of 0.5 rpm. The ION experiment comprised two sets of experiments. Each set was made of an ion mass spectrometer (Wien filter) which measured fluxes of H +,He ++,He +, and O + in the energy range 5 14,000 ev/q and of an electron spectrometer measuring electron fluxes in the energy range 10 22,000 ev. The two experiment sets were looking in opposite directions, both normal to the satellite spin axis, which pointed toward the Sun [Sauvaud et al., 1998]. The time necessary to obtain a complete ion/electron energy spectrum varied from 1.5 to 7.5 s, depending on the operation mode. Generally, for the data selected for this study, the ION experiment gave energy spectra of H + and O + every 7.5 s, and electron energy spectra were obtained every 3.75 s. The measurements made with IA are correlated with far UV images from the Polar satellite. The UVI instrument is designed to operate over a wavelength range from 1300 to 1900 Å, i.e., to measure the Lyman-Birge-Hopfield system of bands, which results from electron impact ionization of N2. LBH bands are detected in two segments, one at short wavelength (LBHS), where strong absorption in the Shuman Runge continuum occurs, and one at longer wavelengths (LBHL), where the O 2 absorption is not significant [Torr et al., 1995]. The images from UVI have a spatial resolution of 0.5. We have also used the preliminary values of the AE index for , obtained from World Data Center C2 (Kyoto), as well as ground magnetic field data from the 210 MM magnetometer chain [Yumoto et al., 1996]. These data allow to characterize the geomagnetic activity during IA measurements. For the particular case of 18 January 1997 (IA orbit 590), we analyzed the IA measurements together with simultaneous data published by Lyons et al. [1999], concerning meridianscanning photometers and magnetometers (CANOPUS) as well as magnetic and plasma data obtained in the magnetotail on board the Geotail satellite. [12] In this study we have also used an example of VDIS from the CIS experiment flying on board the polarorbiting Cluster spacecraft. The Cluster apogee and perigee are equal to 19.6 R E and 4 R E, respectively, and the orbital period reaches 57 hours. The Cluster Ion Spectrometer (CIS) experiment comprises (1) a Hot Ion Analyzer, CIS-2, measuring the ion distributions from 5 ev/q to 31 kev/q without mass selection and (2) a time-of-flight mass spectrometer to measure the threedimensional (3-D) distribution functions of the major ion species (H +, He ++, He +, and O + ). The sensor primarily covers the energy range between 15 ev/q and 38 kev/q [Rème et al., 1997]. The three-dimensional ion distributions were measured with a time resolution of 4 s, the spin period of the Cluster satellites. 3. Observations 3.1. Ion Dispersed Structure Database [13] The ION database includes measurements from 27 September 1996 to 30 April For a preliminary analysis we selected IA passes through the polar cap in the Northern Hemisphere near the apogee part. The satellite crossed the auroral zone-polar cap boundary twice along such an orbit. The first crossing was poleward and the second was equatorward. For further analysis we selected passes in the night sector between 2100 and 0300 MLT. The criterion for the satellite entrance into the polar cap was either a drop of the electron fluxes below the detection threshold or the encounter of electron fluxes in the energy range kev with characteristics of the polar rain. Five hundred and one crossings including 278 poleward and 223 equatorward crossings were thus selected. Finally, we selected orbits where clear dispersed energy structures were measured near the polar edge of the auroral zone. As a result, 246 auroral zone crossings have been selected including 114 poleward and 132 equatorward crossings. These data, which constituted the database for this study, thus represent 49% of the polar cap boundary crossings in the 2100 to 0300 MLT sector. In 51% of the cases, although sometimes discrete ion precipitations with no clear energy dispersion are registered, diffuse ion populations are mainly observed. [14] These dispersed ion structures observed on board IA can be sorted out into two main classes: broad structures with a latitudinal width of (similar to VDIS observed on low-apogee satellites) and sporadic recurrent structures of TDIS type with 1 3 min duration and a 3of17

4 Figure 1. Velocity dispersed ion structures (VDIS) event on 3 November From top to bottom: The variations of the H component of the ground magnetic field at three stations of the MM210 magnetometer chain, Kotel nyi, Tixie, and Chokurdakh. The electron energy-time spectrogram between 0.5 and 22 kev and the proton energy time spectrogram between 0.7 and 14 kev taken on board Interball- Auroral from 1552:00 to 1607:40 UT. The (velocity) 1 -time spectrogram of protons between 1558:00 and 1606:20 UT. Differential energy fluxes expressed in kev/(cm 2 s sr kev) are color-coded. repetition period of 3 min (as reported for the first time by Sauvaud et al. [1999]). [15] In the following, we shall analyze specific examples of dispersed structures on poleward and equatorward crossings of the auroral zone-polar cap boundary and perform a statistical analysis of the distribution of these structures as a function of invariant latitude (ILAT), magnetic local time (MLT), and geomagnetic activity (AE index) Examples of VDIS Events: Poleward and Equatorward Crossings Poleward Crossings [16] A typical observation of ion structures during a poleward crossing of the auroral zone-polar cap boundary is illustrated in Figure 1 for a magnetically very quiet day, 3 November 1996 (orbit 277). The sum of Kp indices for this day was equal to 4. During the auroral zone crossing presented here, the Kp index was 0. Ground-based measurements at the Kotel nyi, Tixie, and Chokurdakh stations, located in the same local time sector as the IA satellite, show that the H-component of geomagnetic field did not exhibit noticeable variations and that its residual value was close to zero (Figure 1, upper panel). [17] The electron and ion energy time spectrograms are given in the second and third panels of Figure 1. Intense electron fluxes were recorded in the auroral oval. Inside the polar cap only an extremely weak electron precipitation, polar rain like, was recorded. From the electron character- 4of17

5 Figure 2. Detail of the energy-time spectrogram of protons between 1600:42 and 1605:00 UT on 3 November istics, the auroral zone can be separated into two regions with a boundary encountered at 1556 UT. The region located equatorward of this boundary displays energetic electron precipitation (up to 5 kev), which may be associated with the CPS in the magnetotail. Inside the region located poleward of the boundary, electrons have an average energy of the order of kev, typical for PSBL precipitation [e.g., Takahashi and Hones, 1988]. [18] A proton energy-dispersed structure was observed during 8 min, between 1558 and 1606 UT, which correspond to a latitude extend of 1 (third panel of Figure 1). The proton energy varies from 0.8 to 11.0 kev and differential energy fluxes exceed kev/cm 2 s sr kev. The structure displays a broad kind of dispersion, i.e., the energy rises with the latitude. It can be classified as being of the VDIS type, observed earlier at lower altitudes near the polar edge of the auroral region [e.g., Zelenyi et al., 1990; Saito et al., 1992]. However, this structure contains smaller scale substructures as illustrated in more details in the bottom panel of Figure 1, which represents a (velocity) 1 -time spectrogram between 1558:00 and 1606:20 UT, and in Figure 2, which gives an extended energy-time spectrogram. The repetition period of the substructures is min; the duration of each substructure is 1 2 min. Each one displays an energy-time dispersion opposite of that of the broader structure. Note that substructures ( beamlets ) inside VDIS have already been reported from observations made on board the low-apogee satellite AUREOL-3 [Ashour-Abdalla et al., 1992]. These beamlets have been tentatively explained by the spatial distribution of particles in the PSBL. According to Ashour-Abdalla et al. [1991], they should be narrow ion beams at certain Z location in the PSBL with gaps between them. The generation process of beams is acting in the current sheet. At some X location, particles escape into the PSBL after being accelerated and then follow Speiser orbit [Speiser, 1965, 1967], but at adjacent neighboring positions, where nonlinear conditions of enhanced trapping are satisfied, they remain in the CPS after being accelerated [see also Bosqued et al., 1993]. However, the velocity of AUREOL-3 at the apogee (H = 2000 km) being rather large and the structures having a small latitudinal extend due to the converging magnetic field, no detailed information on the energy shape of the beamlets were obtained. The IA satellite crossed these structures at altitudes 10,000 20,000 km and its velocity was small; therefore in the IA case the substructures were measured in more details. The key point revealed by IA is that the substructures show an energy dispersion with the most energetic ions arriving first. This means that the substructures are most likely temporal rather than spatial. [19] A second example of a broad energy dispersion with substructures is given in Figure 3, which pertains to a poleward crossing in the premidnight local time sector on 18 July 1997 between 0634:00 and 0641:30 UT. These measurements correspond to a quiet magnetic period following a substorm recovery phase. The average AE index was 46 nt and the Kp index was 2+. The top panel of the figure displays the ion energy-time spectrogram; the bottom panel gives the variations of the ion average energy. The broad energy-dispersed structure occupies 2.5 in invariant latitude and corresponds to an increase of the ion average energy from 6 to 13 kev. The substructures seen in the ion spectrogram last for about 1 min each. Inside substructures, higher-energy protons arrive before the low-energy ones. This leads to a decrease of the average energy of the ions inside a given substructure. Note that close to the polar cap boundary, the last substructure clearly comprises protons with energy higher than the upper detection limit of the ION IA spectrometer. [20] For the poleward crossings presented in Figures 1, 2, and 3 the proton energy increases inside the broad structures and decrease inside every substructure, which makes very clear the distinction between them. One has to realize that this is a characteristic encountered during poleward crossings. During equatorward crossings, if latitude still organize in the same way the energy changes inside broad structures and if substructures are most likely temporal rather than spatial, the ion energy will decrease inside both the broad structures and the substructures, which will somewhat tend to smooth the signatures Equatorward Crossings [21] An example of an equatorward crossing of the auroral zone is given by the IA pass 590 on 18 January 1997 (Figure 4). As in the previous cases, the magnetic activity on this day was rather weak. The daily Kp sum was 10, and the Kp index during the pass was 1+. The period of observation of a broad dispersed proton structure, from 0628:30 to 0635:00 UT, follows a substorm recovery phase (see the upper panel of Figure 4, where preliminary AE and AO indices are given). As in the preceding cases, in average, the ion energy increases as a function of the invariant latitude. Thus considering the overall proton structures displayed in Figures 1 and 3 for poleward crossings (orbits 277 and 1341) and in Figure 4 for an equatorward crossing (orbit 590), we can state that the broad 5of17

6 Figure 3. Proton energy-time spectrogram on 18 July 1997 between 0634:00 and 0641:30 UT (top panel) and corresponding proton average energy (bottom panel). latitudinal dispersion is preserved independently of the direction of the crossing of the auroral zone by the satellite. [22] The region of intense electron population (electron spectrogram in Figure 4) coincides with the occurrence of the proton structure. The electron average energy first increases, reaching 2 kev at maximum, and then falls down to 1 kev. During the subsequent motion of the satellite toward the equator, the electron energy increases almost monotonically with decreasing latitude. This is a typical signature of the plasma acceleration by betatron and Fermi mechanisms in the central plasma sheet [Galperin et al., 1978]. As in the previous case, the auroral zone shows a polar region (prior to 0638 UT) associated with electrons originating from the PSBL and an equatorial region connected to the CPS. [23] The proton broad energy-dispersed structure consists of several substructures which can be appreciated in both the 1/V spectrogram displayed at the bottom of Figure 4 and in the expanded energy-time spectrogram given in Figure 5. There is a tendency inside each substructure for the most energetic protons to arrive first; this can be better seen in the low-energy cut-off of the substructures in Figure 5. Although not as clear as for poleward crossings, the substructures are still consistent with a temporal nature. [24] We shall now compare the measurements made aboard the IA satellite on 18 January 1997 with simultaneous data from the CANOPUS meridian-scanning photometers and magnetometers and with measurements of magnetic and plasma parameters made on board the Geotail satellite, as described by Lyons et al. [1999, hereinafter referred as Ly-99]. The CANOPUS and Geotail data are well conjugated, with DMLT 0.6 hours. The longitudinal difference between IA and CANOPUS corresponds to DMLT 1.3 hours. [25] Magnetic and auroral activity during the period studied in Ly-99 was quiet, except for intensifications of the poleward boundary of the auroral zone. During the IA encounter with the polar cap boundary, weak (4 nt) Pi2 pulsations were registered at four CANOPUS stations located in the latitudinal range Between 0623 and 0640 UT, a weak poleward boundary intensification of 5577 Å emissions, characteristic of 1 kev electron precipitation, was noted. During the same period the energy of the maximal electron flux observed on board IA is 2 kev. This is a signature of the rather hot plasma sheet boundary, where active processes at the root of the generation of ion beam occur. Thus we may conclude that the poleward boundary intensification of the 5577 Å emissions corresponds to the appearance of intensive fluxes of hot electrons and ions near the polar edge of the auroral zone observed on board IA. According to Blanchard et al. [1997], the poleward boundary of 6300 Å emissions is within 1 in latitude of the magnetic separatrix, i.e., of the boundary between open and closed field lines. Observations of 6300 Å emissions by Ly-99 thus suggest that at 0630 UT, the poleward boundary of the particle precipitation is located at 73.5 invariant latitude for a MLT of 24.0 hours. The poleward boundary of the proton and electron precipitation 6of17

7 Figure 4. VDIS event on 18 January 1997; same presentation as Figure 1, except for the upper panel which displays the variations of the AE and AO magnetic indices. The number of stations used to produce the indices is given on the right side of the upper panel. as detected by IA at MLT 22.7 hours was at a somewhat lower latitude This difference can be tentatively explained by the longitudinal difference of the two measurements. To test this assumption we used the Polar Ultraviolet Imager (UVI) data displayed in Figure 6. At 0630:37 and 0633:41 UT the polar boundary of the UV emissions was located at 72.3, for MLT = 22.7 hours. This boundary clearly coincides, within the accuracy of UVI measurements, with the polar boundary of electron and proton precipitation determined by IA. Thus the direct comparison of the polar boundary of the particle precipitation seen by IA of the airglow emission given by CANOPUS and of UVI data from Polar allows to conclude that the dispersed proton structure is associated with ion beams generated in the PSBL and that this structure is located into a closed field line region. [26] Between 0633 and 0640 UT, coinciding with the observations of dispersed ion structure on IA, Geotail was entering into the plasma sheet boundary layer (Figure 2 of Ly-99). Ly-99 note that during intensifications of the poleward boundary of the auroral zone, bursty flows of plasma with significantly enhanced earthward particle fluxes were observed and that the ion distribution is typical of the field-aligned beams of PSBL ions that are ejected from the tail current sheet after undergoing Speiser motion and energization within the current sheet [Lyons and Speiser, 1982]. During the observations of the dispersed structures on board IA presented here, Geotail was located at X 30.4 R E. From the estimate made in Ly-99, the length of the current sheet tailward of Geotail in the X direction could be R E. IA measurements of the most poleward ion substructure registered around 0631 UT (see Figure 4) allow a very rough estimate of the source distance from the slope of the dispersion, assuming a time-of-flight dispersion 75 R E. Thus the comparative analysis of simultaneous Geotail and IA data shows that the ion beams 7of17

8 Figure 5. Detail of the energy-time spectrogram of protons between 0629:30 and 0635:30 UT on 18 January could be formed in the PSBL at distances ranging from 30 R E to R E from the Earth. [27] A second example of a broad proton energy-dispersed structure encountered close to the local midnight during an equatorward crossing is given in Figure 7 which pertains to 30 December 1996 between 1551:00 and 1606:00 UT. These measurements correspond to a quiet magnetic period just following a substorm recovery phase, clearly seen in the Alaska meridional chain of magnetometers (not shown). The corresponding Kp was 2. The top panel of Figure 7 displays the ion energy-time spectrogram; the bottom panel gives the variations of the ion average energy. The broad structure detected between 1553 and 1603 UT occupies 1 in invariant latitude and corresponds to a decrease of the ion average energy from 12 to 6 kev. The substructures seen in the ion spectrogram last between 1 and 3 min. Inside substructures, higher-energy protons arrive before the low-energy ones as expected from a timeof-flight dispersion. This leads to a decrease of the average energy of the protons inside a given substructure Examples of TDIS Events: Poleward (19 June 1997) and Equatorward (3 October 1997) Crossings [28] Electron and proton measurements during a poleward crossing on 19 June 1997 (orbit 1222) are presented in Figure 8. On that day the magnetic activity was moderate, with SKp =17, whereas during the pass Kp =2. The IA measurements in the auroral zone, however, were made during a substorm, just before its recovery phase, as evidenced by the AE and AO indices presented in the upper panel of Figure 8. Note that during the IA measurements the AE index was reaching 400 nt. [29] The electron fluxes near the polar edge of the auroral oval displayed large fluctuations. Very intense discrete precipitation with energies up to kev is clearly seen in the electron energy-time spectrogram. Electron fluxes show maxima in directions parallel and antiparallel to the magnetic field. The polar cap boundary at 1407 UT coincides with a sudden dropout of the electron fluxes. Between 1355 and 1406 UT, the proton energy time spectrogram displays successive intense injections appearing recurrently as energy-dispersed structures. The structure duration is 2 4 min and the repetition period is 3 min. For each structure, high-energy protons arrive first. [30] The properties of such TDIS structures have been considered in detail by Sauvaud et al. [1999] and Popescu et al. [2002]. These authors conclude that these structures have most likely a temporal rather than the spatial nature. Indeed, it is difficult to find a mechanism under which the low energy protons would appear more poleward than the high-energy ones. For example, the velocity filtering of protons participating in the radial E B drift in the X direction into the magnetotail would lead to the precipitation of low-energy protons at lower latitude than the highenergy ones. [31] The bottom panel of Figure 8 shows the 1/V-time spectrogram of protons of the TDIS structure. Under the assumption that the proton beams travel to the satellite from the equatorial plasma sheet, one can estimate the distance of the source using the slope of the structures (gray lines in the 1/V-time spectrogram). This estimation gives here a source distance in the range 18 to 35 R E. [32] Finally, let us note that the H + and O + spectrograms also display upflowing ions with large pitch angles and a maximum energy of 8 kev. These ion fluxes appearing Figure 6. For 18 January 1997, selected global UV auroral images from the UVI experiment on board the Polar satellite (see Figure 4). The crosses indicate the projection of the Interball-Auroral satellite along the field line to an altitude of 100 km. 8of17

9 Figure 7. Proton energy-time spectrogram on 30 December 1996 between 1551 and 1606 UT (top panel) and corresponding proton average energy (bottom panel). near the polar boundary of the auroral zone are conics associated with TDIS [e.g., Delcourt et al., 1999]. These simultaneous injections of plasma sheet protons and prominent outflows of ionospheric H + and O + indicate that the external plasma sheet is the site of a strong turbulence in such magnetic conditions. [33] It must be stressed that for all IA passes, in which sporadic proton injections in the auroral zone were observed, the electron fluxes have a sharp poleward boundary and that the mean electron energy reaches 4 6 kev. This energy range is characteristic of CPS rather than PSBL. This observation could be explained by the complete or partial disappearance of the PSBL during periods of reconfiguration of the magnetic field topology (dipolarization) in the magnetotail. During such periods the tail magnetic field takes a more dipolar configuration and the polar cap boundary will map between the external edge of the plasma sheet and the tail lobe. [34] We will now examine dispersed TDIS structures along an equatorward auroral zone crossing on 3 October 1997 during orbit 1660 (Figure 9). The magnetic activity on this day was moderate with SKp =16. During the pass, Kp = 2+. The upper panel of Figure 9, displaying the AE and AO indices, indicate the expansion phase of a substorm. [35] The electron fluxes increase sharply at 0801:30 UT when the IA was passing the polar cap-plasma sheet boundary. As for the poleward crossing (orbit 1222), discrete electron precipitation with mean energy of 2 4 kev are associated with proton energy-dispersed structures (see the energy-time spectrograms and the more detailed 1/V-time spectrogram). The slope of the two polar TDIS structures is about the same (gray lines in 1/V-time spectrogram). It must be stressed that proton structures with such a short duration are only observed in a narrow range of invariant latitudes. For example, in the case of the polar TDIS presented here, between 0803:25 and 0804:30 UT the IA satellite passed from ILAT = to ILAT = When the IA satellite moves more equatorward, other proton structures with different slopes are visible. They correspond to bouncing ion clusters first reported at geostationary orbit by Quinn and McIlwain [1979]. Such clusters were also detected at auroral altitudes from the Akebono [Hirahara et al., 1996] and the IA satellites [Sauvaud et al., 1999]. As evidenced by the 1/V-time spectrogram the bouncing ion cluster is initiated at 0806 UT, i.e., later than TDIS. The ions at the origin of these equatorward structures have their source in the inner magnetosphere, within quasi-dipolar geomagnetic field [Sauvaud et al., 1999]. [36] Oxygen TDIS structures are also observed as evidenced by the O + energy-time spectrogram in Figure 9. The first O + structure is starting at 0810:50 UT and the second at 0818:25 UT. Note that the intensity of the equatorial O + ion structure is larger than that of the polar one (this is observed also for H + TDIS). Estimations of the source distance from the structure slopes give value of the order of 11 to 16 R E both for H + and O +. Therefore the ion beams are generated rather close to the Earth. 9of17

10 Figure 8. Time-of-flight dispersed ion structures (TDIS) event on 19 June Same presentation as Figure 4, except that the middle panel include the O + energy-time spectrogram. [37] During this pass the Polar UVI imager simultaneously recorded the auroral oval emissions. Figure 10 illustrates the distribution of the airglow in an invariant latitude-magnetic local time plot. The cross indicates the footprint of the IA satellite at an altitude of 100 km. The auroral oval propagates successively to higher latitudes and a concurrent eastward motion of active auroral forms is visible. At 0802:40 UT the IA satellite enters inside a narrow polar auroral arc. This arc lasts for 6 min. That TDIS are observed by IA during the development of a bright form at the poleward edge of the oval confirms their temporal nature. However, even for such a good correlation between particle measurements and UV imaging, it cannot be completely ruled out that the IA satellite encountered longitudinally confined regions of briefly deformed (dipolarized) magnetic field lines that guided plasma sheet ions and electrons to the IA position. Previous literature has evidence of iongyroradii-sized high-latitude filaments [e.g., Huang et al., 1987]. [38] Finally, let us note that during the observations of TDIS, H + and O + conics were recorded with an energy extending up to 3 kev. This actually suggests, as for the poleward crossing along orbit 1222, that during TDIS 10 of 17

11 Figure 9. TDIS event on 3 October Same presentation as Figure 8. Figure 10. For 3 October 1997, selected global auroral images from the UVI experiment on board the Polar satellite, with same presentation as Figure of 17

12 Figure 11. Distribution of VDIS (left panel) and TDIS (right panel) structures as a function of invariant latitude (ILAT) and magnetic local time (MLT) (see text). formation the ionosphere could be a powerful source of plasma for the magnetosphere A Statistical Study of the Ion Structures [39] In the following statistical analysis, we use several criteria for classifying the dispersed structures as either of VDIS or TDIS types. For VDIS, (1) the low-energy cut-off of each ion substructure increases with the latitude and (2) the electron population is unstructured and the maximum electron energy fluxes are measured at energies lower than or equal to 2 kev near the polar boundary of the structure. On the contrary, for TDIS the low-energy cut-off of each recurrent ion injection is nearly constant (i.e., independent of the latitude) and the electron precipitation is structured with the maximum energy fluxes, corresponding generally to an energy higher than 2 kev. Figure 11 displays the occurrence frequency of VDIS and TDIS as the function of ILAT and MLT. Each cell corresponds to DILAT = 1 and DMLT = 0.5 hours. [40] There is no predominant narrow latitude range into which the events fall. The average latitudes of the polar border of VDIS and TDIS are given for each MLT bin by a heavy line. These latitudes vary in the range for VDIS and for TDIS. The overall average latitude of TDIS (71.9 ) is lower than that of VDIS (72.6 ). The distribution of VDIS and TDIS as a function of MLT shows that VDIS appear more frequently near midnight and in early morning hours, while TDIS are more frequently encountered in evening and midnight hours. We have also performed an analysis of the distribution of the maximum energy of VDIS as a function of various parameters (AE, MLT, invariant latitude) and we did not find a clear ordering. This could be linked to the quite low value of the ION/IA upper energy (14 kev). [41] Evidence for TDIS intensive ion injections into the high-altitude auroral bulge has been already reported [Sauvaud et al., 1999; Popescu et al., 2002]. Here we analyzed how VDIS and TDIS are distributed with respect to the global magnetic activity. From January 1997 to April 1998, 59 cases of VDIS and 123 cases of TDIS have been identified from the IA database and compared with the AE records. [42] As evidenced in Table 1 the maximal number of VDIS is observed during magnetically quiet time or during substorm recovery phases (93%), while TDIS are recorded during substorms (and magnetic storms), including onset, main, and recovery phases (90%). It must be stressed that for a small number of structures, the AE index was determined from only a small number of ground stations. Nevertheless, Table 1 provides the general dependence between the structure appearances and the magnetic activity. 4. Discussion and Conclusions [43] In section 3 we have studied the energy-dispersed structures near the polar edge of the plasma sheet boundary. The main experimental results can be formulated as follows. [44] 1. VDIS represent a broad ion structure with a extend in invariant latitude, which is observed during 5 20 min at altitudes of the order of 10,000 20,000 km on board the IA satellite. The key feature of VDIS emphasized in this paper is that they are made of a series of substructures. Each substructure lasts 1 3 min and inside each substructure, high-energy protons arrive first, regardless of the direction of the plasma sheet boundary crossing (poleward and equatorward crossings). A near-continuous rise in maximal and minimal energies of each consecutive substructure with invariant latitude is observed. Table 1. Relation of Dispersed Ion Structures With Geomagnetic Activity Poleward Crossing VDIS Equatorward Crossing Sum Poleward Crossing TDIS Equatorward Crossing Sum Quiet Onset Main Recovery Sum of 17

13 Figure 12. (Velocity) 1 -time spectrogram of an O + TDIS measured on 3 October 1997 from 0817 UT to 0824 UT (see Figure 6). The gray line approximates the experimental dispersion of O +. [45] 2. TDIS are recurrent structures in H + (and sometimes also in O + ) with a repetition period of 3 min and a duration of 1 3 min. The maximal energy of recurrent 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 energies up to 3 10 kev. In association with TDIS, bouncing H + clusters are observed. These clusters bounce in the inner magnetosphere, in quasidipolar magnetic field tubes, i.e., equatorward from TDIS. [46] 3. The electron populations at the poleward boundary of the auroral zone have different properties during observations of VDIS and TDIS. The electron flux accompanying VDIS increases smoothly starting from the polar boundary and smoothly decreases after the satellite has passed through the VDIS structure. The mean electron energy in such passes is in the range kev. This energy is typical for PSBL electrons in the magnetotail. The electron fluxes associated with TDIS observations increases suddenly at the auroral zone polar boundary. The electron flux displays sharp variations, i.e., discrete precipitations are visible. Their average energy reaches 4 8 kev and then does not drop down after the satellite has passed through TDIS. This electron energy, typical for CPS, suggests that during TDIS formation, either a sharp thinning of the PSBL takes place or that it disappears. [47] 4. A statistical analysis shows that VDIS are observed mainly in magnetically quiet time and during the recovery phase of substorms, including intensifications of the 5577 Å emissions at the auroral zone poleward boundary. In contrast to VDIS, sporadic recurrent proton injections (TDIS) are observed mainly during large disturbances of the magnetosphere. Most of TDIS were detected during the onset and main phases of substorms (and during magnetic storms). Nevertheless, TDIS are also observed during substorm recovery phases, although less frequently. [48] Let us consider now the dispersion properties of the VDIS and TDIS structures in more details. The question of the temporal or spatial nature of the structures is the most important in the context of this work. It is evident that the formation of dispersed structures is affected by two main effects: (1) time-of-flight dispersion affecting ions with different masses (H + and O + ) and velocities (energies) and (2) E B velocity filter dispersion in the large-scale dawndusk electric field. Both mechanisms operate during the motion of the ions from their magnetospheric source to the observation point (satellite). The predominance of one of these two mechanisms will lead to different signatures at the observation point. For example, assuming that the ion source remains stationary, when mechanism 1 dominates, energy-time dispersion will be much more pronounced that energy-latitude dispersion. [49] The 1/V versus time correlation for all TDIS structures means that it is the time-of-flight effect that is the dominating mechanism for the formation of such structure. The data from the IA pass 1660 show that this effect takes place for both H + and O + ions, and the dispersion slope 1/V as a function of time led to about the same distance of the source (11 16 R E ). However, along the ion trajectories, the E B drift also contributes to the motion of the beam toward the Earth, leading to a displacement of low-energy ions toward the equator. This can be appreciated in Figure 12, which displays the (1/V)-time spectrograms of the O + TDIS measured between 0817 and 0825 UT on board IA (see also Figure 9). The oxygen ions with the lower velocities are encountered later than expected from a simple time-of-flight dispersion, as indicated by the curved gray line in Figure 12. Here we attribute this delay to the convection velocity, which brings the low-velocity ions toward lower latitudes. However, the time that the ions spend within the convection electric field is rather short, as they are ejected at small distances from the Earth. Thus for TDIS the velocity filter effect seems to be of second order. [50] For VDIS structures the situation is more complex. As demonstrated in this paper, VDIS consists of substructures where high-energy protons arrive first (Figures 1 4, 6, 7). This result is reinforced by recent Cluster CIS data (see Rème et al. [1997] for a description of the CIS experiment). As evidenced in Figure 13, which displays a VDIS encountered during a Cluster-3 poleward crossing of the auroral zonepolar cap boundary, at altitudes (22,000 km) higher than those of IA, the same general trend is easily discernable. From VDIS data we can state that inside each substructure, the time-of-flight effect of protons generally dominates over the velocity filter effect. Therefore the substructures are likely to have a temporal rather than a spatial nature. Note that the energy range of each substructure increases toward the pole. A very rough estimation of the distance of the source using time-of-flight dispersion inside different substructures gives apparent distances between 30 and 100 R E. This indicates that the time spend by protons in the convection electric field during their propagation from the magnetotail can be 2 3 times larger than for protons 13 of 17

14 Figure 13. Example of a high energy VDIS structure measured on board Cluster on 14 February 2001 during a poleward crossing of the auroral zone-polar cap boundary at an altitude of 22,000 km. The upper panel displays the energy time spectrogram of ions. The middle panel gives the variations of the ion average energy and the lower panel displays their (velocity) 1 -time spectrogram. Differential fluxes, expressed in (cm 2 s sr kev) 1, are color-coded. The satellite radial distance, invariant latitude, and the magnetic local time of the conjugate point are given at the bottom of the figure. observed in TDIS. Therefore we expect that the E B dispersion, although not generally predominant, is not negligible here. The VDIS substructures represent the beams ejected from an extended current sheet at different distances. Note that the velocity filter dispersion will be more effective for more poleward structures, which is indeed generally seen. The VDIS structure as a whole represents the envelope containing substructures and display an energy-latitude dependence, which seems compatible with an E B drift of the ions when observed from low orbiting spacecraft. In this connection measurements made on past low-apogee satellites did not allow the determination of the nature of the energy dispersion of substructures embedded into VDIS due to insufficient spatiotemporal resolution. The analysis of such measurements has led most investigators to the conclusion that VDIS are strictly latitudinal structures [Zelenyi et al., 1990; Saito et al., 1992; Bosqued et al., 1993]. More detailed measurements presented here show that VDIS have a spatiotemporal nature. It must be stressed however that measurements performed on board the Polar satellite also show the presence of substructures (for example, middle panel of Figure 2 in the work of Lennartsson et al. [2001]). However, these authors did not recognized VDIS structures in their data set and proposed another classification differing from the separation between VDIS and TDIS. They classify these substructures to stepping up category. Similarly to the substructures within VDIS, the mean energy of stepping up substructures increases with the latitude. 14 of 17