Composition signatures in ion injections and its dependence on geomagnetic conditions

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A10, 1299, doi: /2001ja002006, 2002 Composition signatures in ion injections and its dependence on geomagnetic conditions S. Y. Fu, 1 Q. G. Zong, 2 T. A. Fritz, 2 Z. Y. Pu, 1 and B. Wilken 3 Received 29 June 2001; revised 18 December 2001; accepted 19 December 2001; published 16 October [1] Ion composition in substorm injections are investigated in detail by using data obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES). A total of 398 injection events are identified from 4 February to 10 October, It is found that the flux enhancements of O + ions in the with-oxygen injections (O + ions occupy more than 20% of the total energy density) are attributed to the flux intensifications of high-energy (E/q 100 kev/e) ions. In contrast, the flux of H + and He ++ ions in the without-oxygen injections (O + ions are less than 20% total energy density) are enhanced by the intensification of lower energy H + and He ++ (around 50 kev/e) ions. Statistical results also demonstrate that the abundance of O + ions in an injection strongly depends on the geomagnetic activity. Without-oxygen events take place most probably in relatively weak geomagnetic activity conditions (Dst index is around 50 nt and Kp is around 4). The averaged fractional energy density of O + ions increases roughly linearly with Kp index. All injections are found to be rich of oxygen ions during the strong magnetic storms (Dst < 100 nt). The present study indicates that the ionospheric oxygen ions are energized more readily during the storm conditions. When the near-earth reconnection occurs and expands from the central plasma sheet to the plasma sheet boundary layer or even to the lobe region, much more ionosphere origin O + ions could participate and be energized in the magnetospheric dynamic process. This leads to a with-oxygen injection in the inner magnetosphere. As to the without-oxygen injection, it seems that only preexisting oxygen ions in the near-earth plasma sheet are accelerated and become a part of the injected ions, thus a rather low abundance of O + ions can be observed. INDEX TERMS: 2730 Magnetospheric Physics: Magnetosphere inner; 2778 Magnetospheric Physics: Ring current; 2788 Magnetospheric Physics: Storms and substorms; 2720 Magnetospheric Physics: Energetic particles, trapped; KEYWORDS: oxygen ions, substorm injection, ion composition, storm time substorm Citation: Fu, S. Y., Q. G. Zong, T. A. Fritz, Z. Y. Pu, and B. Wilken, Composition signatures in ion injections and its dependence on geomagnetic conditions, J. Geophys. Res, 107(A10), 1299, doi: /2001ja002006, Introduction [2] Energetic particle injection or enhancement is an important feature of a substorm onset which is usually observed in the geosynchronous orbit. In the past, injections have been extensively studied [e.g., Baker et al., 1982; Reeves et al., 1990]. Since the different ion species H +, He ++,O +, and He + represent different origins (ionosphere or solar wind), the information of ion composition during injection events may provide useful clues about the physical processes which are responsible for the energization and transportation of these particles during geomagnetic activity. [3] The investigation of long-term average equatorial distributions of energetic He + and He ++ ions by Kremser 1 Department of Geophysics, Peking University, Beijing, China. 2 Center for Space Physics, Boston University, Massachusetts, USA. 3 Max-Planck-Institut fuer Aeronomie, Katlenburg-Lindau, Germany. Copyright 2002 by the American Geophysical Union /02/2001JA et al. [1994] showed that the distribution of He + ions is independent of the magnetic local time, whereas the distribution of He ++ ions depends strongly on the magnetic local time. He further concluded that the energetic He ++ ions originate in the solar wind and most of the He + ion are produced by charge exchange process from He ++ ions. [4] The ion composition in the near-earth magnetosphere has been investigated by many authors [Balsiger et al., 1980; Young et al., 1982; Lennartsson, 1989], Balsiger et al. [1980] surveyed the plasma composition in the energy per charge range below 16 kev/e at all local time from L =3to 8. He found that the heavier ions could be increased during both substorm and storm, and occasionally, they become the dominant species in the outer magnetosphere. Lennartsson et al. [1985] studied substorm effects on the plasma sheet composition on 22 March Prior to this substorm activity about 90 95% of the ion density was due to H + and He ++ ions. During this substorm activity, the density of O + ions increased by almost an order of magnitude through- SMP 14-1

2 SMP 14-2 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS out the measured energy ranged from 0.7 to 12 kev, resulting in comparable amounts of O + and H +. [5] It should be noted, however, that most of the earlier conclusions were based on observations made at lower energy plasma (below 16 kev) which is much lower than the ring current energy range (around 100 kev) because of the limitation of the instrument. Observations in the important intermediate energy range are only available since the application of the so called time-of-flight technique [Wilken, 1984; Gloeckler et al., 1985]. So far, there are only a few of studies concerning the composition signatures in the ring current energy range, especially for oxygen ions during substorm activity. [6] Sibeck et al. [1988] investigated the charge state of substorm injected particles. He concluded that the ionosphere is the source of He + and O + ions at low energy and that solar wind is the source for O + above 365 kev. Grande et al. [1982] examined a substorm event with double injections on 14 February The main observed features are that the first injection was rich of He ++ ions and the second one contained more O + ions. The alpha particles exhibited an intense increase while the oxygen ions demonstrated a pre-injection drop. Hall et al. [1998] argued that oxygen-rich injections are the result of the existing composition in the tail plasma sheet. [7] Recently the behavior of O + ions during geomagnetic activity has attracted more interest. Since O + ions are thought to be of ionospheric origin, its abundance is an indicator of the intensity of the coupling between the magnetosphere and the ionosphere. It has been pointed out that the abundance of O + ions often shows an obvious increase during substorms [e.g., Young et al., 1982; Daglis et al., 1983]. The importance of O + in the storm time ring current has also been emphasized by Daglis and Axford [1996]. Furthermore, the abundance of O + in terms of number density and energy density has been shown to have a good correlation with the storm activity [Fu et al., 2000]. However, it is still not clear whether the oxygen-rich substorms provide more oxygen to the ring current, or the storm time substorms have different ion composition from isolated substorms. The relationship between storm and substorm remains an unsolved question even from the view of ion composition. [8] Another weak but clear magnetospheric dynamic process is the so-called pseudo breakup [e.g., Akasofu, 1964; McPherron, 1991]. Pseudo breakups are similar to substorm processes and are often seen during the growth phase of substorms. The major difference between substorms and pseudo breakups is that the latter are more localized and much weaker. Davis and Hallinan [1976] suggested that pseudo breakups are in fact small localized substorms. However, McPherron [1991] pointed out that a pseudo breakup is usually followed by a real onset of expansion phase which is different from an isolated substorm. The reason for the limitation in intensity and spatial scale of pseudo breakups is directed to the lack of magnetotail energy storage or/and an ionosphere with unfavorable conditions Kan et al. [1988]. However, as to pseudo breakups, unfortunately, almost no composition information can be found in the past literature. [9] In addition, the global monitoring of energetic magnetospheric neutral atoms (ENAs) which are produced by charge-exchange collisions between the energetic ions and cold neutral hydrogen atoms of the geocorona can provide us the extra information of the evolution of energetic ions in the inner magnetosphere during substorms and storms [Jorgensen et al., 2000; Lui et al., 2001a, 2001b]. Also, the knowledge of ENA composition allows us to assess the total contribution of each ion species to the storm time ring current indirectly. Furthermore, Lui et al. [2001b] pointed out that a continual supply of energetic O + into the inner magnetosphere may be related to a more continual enhancement of AE index. [10] From 4 February to 10 October 1991, the apogee of CRRES orbits moved through local time from 2400 to Therefore the satellite spent a relatively long time in the midnight and premidnight region where the ion signatures of injection events are often observed. In this paper, ion composition in substorm injections has been examined in detail by using the MICS/CRRES data. A total of 398 substorm injection events are identified and categorized into three types. A schematic model is used to explain the observed composition signatures obtained from both the case and the statistical studies. 2. Mission and Instrument [11] The data presented in this paper are obtained from the spectrometer MICS onboard the CRRES satellite. The CRRES spacecraft was launched on 25 July 1990 into an orbit with apogee at km (5 R E ) and perigee at 323 km (both altitude above the Earth) and with an inclination of 18. It crossed L shells from L 1.05 to L 7, slightly off the equator, in less than 5 hours (the orbital period was 9 hours 52 min). The spin period was 30 s with the spin axis pointing to the Sun. This orbit provided an excellent opportunity for examining the ring current build-up and decay processes during geomagnetic active times. [12] The MICS instrument belonged to a family of advanced particle spectrometers which was able to identify different ion species (including the atomic charge state). An ESA (electrostatic analyzer) selected ions with a particular energy-to-charge ratio (E/Q), which was followed by a velocity/energy detection system (TOF (time-of-flight) / SSD (solid state detector)). A post-accelerating voltage was applied between the ESA and the TOF/SSD system to improve the detection efficiency for low-energy particles. [13] Ions entering the detector have a fixed energy per charge value which is determined by the electric field added in the curved trajectory. The energy range of MICS is from 1.2 to 426 kev/e which have 32 energy channels. The geometric factor of the MICS intrument is cm 2 sr. The low-energy cutoff was due to the SSD detection efficiency. More detailed information for the MICS instrument is given by Wilken et al. [1992]. 3. Selected Events [14] In the injection events observed by MICS/CRRES, the fluxes for H +, He +, He ++, and O + ions are often increased simultaneously, whereas their abundances vary from case to case. The abundances of O + ions increase in a continuous fashion as magnetic activity increases. The average fractional energy density ((O + )/ total )ofo + ions

3 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP 14-3 Figure 1. A color-coded energy per charge (E/Q) versus time spectrograms summarizing the energetic ions enhancements on 7 March The panels from top to bottom are counting rates of H +,He +,He ++, and O + ions, respectively. The magnetic local time (MLT), magnetic latitude (MagLat), L value and the distance are presented corresponding to the universal time. The arrow shows the beginning of the substorm injection. See color version of this figure at back of this issue. is 20% for the total of 398 substorm injection events. These events are easily divided into two groups: 290 cases with-oxygen injection ((O + )/ total > 20%) and 108 cases without-oxygen injection ((O + )/ total < 20%) Case 1: With-Oxygen Injection [15] Figure 1 displays a substorm injection event which occurred during the recovery phase of a moderate storm. The storm reached its maximum on with the Dst peak value of approximately 50 nt and experienced a long recovery phase in which many substorm injections have been detected by the MICS instrument. The injection event displayed in Figure 1 occurred in a geomagnetic active time with Kp =5 and AE around 600 nt. From Figure 1 we can see that a sudden flux enhancement began at 2145 UT and lasted for about 24 minutes. It was a relatively weak

4 SMP 14-4 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 2. The magnetic field (solid line) measured by CRRES in orbit 548. The values of the magnetic field from the Olson-Pfitzer model [Pfitzer et al., 1988] (dot-dashed lines) are also given as references. The dashed line marks the beginning of the magnetic field reconfiguration. The bottom panel shows a long time period for the magnetic magnitude and the upper four panels amplified the time period which is marked by a bold line in the bottom panel.

5 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP 14-5 Figure 3. The proton and electron flux intensities from 2100 UT to 2300 UT, 7 March, 1991 obtained by LANL Geosynchronous (upper two panels) and (bottom two panels). The dashed line marks the onset time of the injection which corresponds to the energetic ion enhancements observed by CRRES/MICS in Figure 2. See color version of this figure at back of this issue. but clear injection rich in O + ions. The flux of low-energy (E/q < 50 kev/e) He ++ ions showed a decrease while the fluxes of O + and H + and high-energy He ++ suddenly increased. [16] The drift echo beginning at about 2215 UT is clearly seen for He ++ and O + and discernable for H +. For 200 kev O + ions, it can be read that the time difference between the two echo is about 39 mins, which is consistent with the estimated gradient drift period at L = 6.4. [17] During the injection, the energy density of O + ions increased substantially. The fractional abundance n(o + )/ n total reached as high as 43%; in contrast it was only 18% before the injection. However, the ratio of He ++ energy density to the total density remained at the preevent level of 7.4%. It is worth noting that prior to the onset of the injection at 2145 UT, the background energy density levels for all ion species were relative high which are 8, 2, and 0.8 kev/cm 3 for H +,O +, and He ++, respectively. [18] Figure 2 shows the magnetic field data obtained by the CRRES satellite. Starting from 2015 UT, the magnetic field magnitude decreased gradually from 600 nt to around 110 nt at about 2145 UT. This is a spatial effect caused by the satellite moving from lower L shell (L =3.6)tolargeL shell (L = 5.8). The magnetic field also displayed a clear deviation from the model field, indicating a stretched magnetic field.

6 SMP 14-6 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 4. A color-coded energy per charge (E/Q) versus time spectrograms summarizing the energetic ions enhancements. The format of the figure is the same with Figure 2. The short arrows indicate the beginning of the ion injection. See color version of this figure at back of this issue. [19] A substantial change of the magnetic field took place at about 2145 UT. It was coincident with the ion flux enhancement observed by the MICS instrument as displayed in Figure 1. The main change occurred in the Bz component with a relatively slow increase over 20 min from 80 nt to about 135 nt, whereas the magnitude of Bx component fluctuates between 50 to 75 nt. This can be explained as a dipolarization process of the magnetic field in the vicinity of the geosynchronous orbit. The reconfiguration of the magnetic field and its associated energetic ion enhancement are typical signatures of substorm injections. [20] Figure 3 shows energetic particle fluxes obtained by LANL geosynchronous satellite and Both the injection fluxes of ions and electrons are dispersionless. The dashed line marks the onset time of injection at 2140 UT. It is noted that the injection (refer to Figure 1) and the dipolarization process observed at 2145 UT by CRRES (refer to Figure 2) are 5 min later than that observed by the geosynchronous satellites. This could be interpreted

7 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP 14-7 Figure 5. Color-coded energy per charge (E/Q) vs time spectrograms summarizing the energetic ions enhancements in the 19 March event (same format as Figure 2). The dashed arrows mark the time of pseudo breakup related injection, whereas the solid arrows indicate the beginning of the substorm injection. See color version of this figure at back of this issue. as that the substorm injection front moved inwards with a velocity of 16 km/s and passed through the geosynchronous orbit at 2140 UT and reached the orbit of CRRES (L = 5.8) at about 2145 UT. It pointed to an azimuthal electric field with an intensity of 2.27 mv/m which drove ions of all species to drift inwards at the same time Case 2: Without-Oxygen Injection [21] Time variations of spectrograms for the four ion species on 8 May 1991 are displayed in Figure 4 (same format as Figure 1). This event occurred in relatively quiet geomagnetic activity conditions with Dst = 2 nt,kp = 2.3 and AE = 300 nt. It was an isolated substorm injection. [22] The main observed facts for this event are listed as follows: 1. The O + flux is rather weak with a relative energy density ratio of only 11%. This is even less than the background level earlier on 7 March 1991 (case 1). 2. The enhancements of H + and He ++ ions are confined to the low-energy range (< 100 kev/e).

8 SMP 14-8 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 6. Scatter plot of the energy density of O + ions for all the 398 injection events (from February to September 1991) versus geomagnetic activity, indicated by Kp (top panel) and Dst (bottom panel). 3. The pre-injection intensities of all ions are lower than those in the 7 March 1991 event (case 1). 4. No obvious drift echo can be observed Case 3: Pseudo Breakup Related Injection [23] As mentioned previously, with-oxygen injections are often related to storm time substorms and without-oxygen injections are usually associated with isolated substorms. It is also interesting to know the ion composition variation in a substorm with multiple injections or a pseudo breakup. Is there any difference in such a complex substorm phenomena? [24] Figure 5 displays the composition variations for the two sequential injection events which occurred during the CRRES outbound pass on 19 March [25] In the time interval from 1800 and 2000 UT on 19 March, two injections were detected by MICS in the

9 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP 14-9 Figure 7. Events dependence on geomagnetic activity. The two panels in the left and two panels in the right side represent the event dependence on Dst and Kp index. The bottom panels are the total number of events which occurred in a certain index interval; The top panels are the relative probability of events with O + energy density ratio greater than 20% (solid line) and smaller than 20% (dot-dashed line). duskside of the magnetosphere. The first injection took place at 1845 UT with a population composed mostly of H + and He ++ ions from 20 to 100 kev/e. The O + flux was near its background level. On the contrary, during the second injection at 1915 UT, the O + flux increased significantly. Its intensity is even higher than that of He ++ ions, which had almost the same value as in the preceding event. The striking difference between these two injections is that in the first injection, there was almost no flux enhancement for high-energy H + and He ++ (E/Q > 100 kev/e), and an O + intensification was absent either. 4. Statistical Results [26] From 4 February to 10 October 1991, a total of 398 injection events are identified. Scatter plots of energy densities of O + ions versus Dst and Kp index for all these 398 events are displayed in Figure 6.

10 SMP FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 8. The averaged fractional energy density of O + ions and He ++ ions as a function of Dst and Kp index. The bottom panels are is the averaged oxygen energy density ratio and the top panels are that of He ++. [27] From the bottom panel of Figure 6 we can see that a majority of the events occurred during small or moderate storm time intervals (in terms of Dst index). The linear coefficient between the energy density and Kp and Dst index is 0.53 and 0.3, respectively. [28] In the energy versus Kp index plot, the top panel in Figure 6, injection events are nearly equally distributed over all Kp from 2 to 9. The injected O + energy density is roughly proportional to the Kp index as indicated by the linear fit: ED oxygen ¼ 5:6 þ 2:8 Kp: The linear correlation coefficient between the energy density and Kp is [29] The bottom panels in Figure 7 show the distribution of the raw numbers of total events which occurred in individual Kp or Dst index time intervals. The plots reveal that most of the injection events occur for Dst < 50 nt and Kp >4. [30] The top panels of Figure 7 show the appearance probability of with-oxygen events (with relative energy densities of O + ions greater than 20%, solid lines) and that of without-oxygen injections (O + energy density ratio less than 20%, dashed lines). The probability of each kind of

11 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP Figure 9. The injection events and composition abundance dependence on injection L values. Bottomleft: the raw number occurrence of events in L bins of L = 0.5. Bottom right: the occurrence probability of oxygen-rich events (solid line) and oxygen-void events (dashed line). Upper left: averaged fractional energy density of O + ions. Upper right: averaged fractional energy density of He ++ ions. injection has been normalized by the Dst and Kp occurrence probability. The appearance probability of a with-oxygen event increases strongly with the intensity of geomagnetic activity. In contrast, as the dashed lines indicate, a withoutoxygen event occurs most probably in the condition of relative low geomagnetic activity with Dst < 100 nt and/ or Kp < 7. It should be pointed out that all the injection events are O + rich if the geomagnetic activity index Kp >7 or Dst < 100 nt. [31] The averaged O + ion energy density ratio (ED) for different Dst and Kp indices are shown in the left (Dst index) and right (Kp index) in the bottom panels of Figure 8. [32] Obviously, the ratio increases substantially with enhanced geomagnetic activity from less than 20% to more than 65%. However, the averaged He ++ energy density ratio shown in the top panel varies much less compared with oxygen ions, which is only from about 7 to 11%. [33] Figure 9 gives the L shell distribution (the numbers of events detected in each 0.5 L value bin) of the observed injection events. Most of the injection events are observed around L = 6.5 (the bottom panel in the left column). The occurrence possibility of the injection events are also concentrated around L = 6.5 (the bottom panel in the

12 SMP FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Table 1. Summary of the Injection Events Sorted by the Fractional Energy Density of O + Ions ED-Ratio < 10% 10% 20% 20% 50% > 50% Total events Storm time events Nonstorm events Average Kp Average Dst Average He ++ ratio right column). Both the averaged fractional energy density of O + and He ++, which are around 30% and 7% 10%, show little change with the different L shells (see the top panels of Figure 9). [34] A summary of the 398 injection events observed by CRRES in 1991 are given in Table 1. There are 290 withoxygen events and 35 events among them have extremely high abundance of O + ions with fractional energy density greater than 50%). In the left 108 injection events, the oxygen ratio is less than 20%. The with-oxygen events occur in large Kp averaged Kp value is 5, and more than 80% of the events are observed during storm times. On the other hand, the without-oxygen events are detected predominately for small Kp values (averaged Kp value is 3.8). It is interesting to note that the abundance of He ++ changes only a little for without-oxygen and with-oxygen events. In fact, this can be interpreted as an evidence that the oxygen ions (O + ) and helium ions (He ++ ) have different origins and may have different favor conditions. Figure 10. phases. A comparison of O + injection events observed during storm main phases and recovery

13 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP [35] However, the oxygen ions (O + ), with the ionosphere as their origin, are strongly related to the geomagnetic conditions. The stronger geomagnetic activity, the more oxygen ions come out of the ionosphere into substorm injection regions. [36] Finally, in order to investigate the relation between substorm injections (with-oxygen injection) and magnetic storms, all the with-oxygen injection events are divided into two groups: events occurred in a storm main phase (Dst index decreases from 50 nt to its peak value Dst min ) and in a storm recovery phase (Dst index increases from Dst min to 50 nt). The statistic results are given in Figure 10. [37] It can be seen from Figure 10 that the distributions of injection events versus the absolute Dst values shows no major difference between a event appeared in a storm main phase and that in a storm recovery phase. Also, the variations of the fractional energy density of O + ions with Dst index are in the same fashion for events detected in the two phases. [38] However, despite the fact that the numbers of these two groups of injection events are found to be almost the same, the sampling time for storm main phases and recovery phases is 347 and 860 hours, respectively. This means that in a certain time interval the occurrence probability for a with-oxygen injection is much higher (more than two times higher) in main phases than in recovery phases. Such injections occurring frequently during storm main phases can lead to an oxygen-rich ring current which is often observed in large storms. It should be pointed out that for injections occurred at the same Dst interval, the averaged fractional energy density of O + ions shows no obvious difference for injections in main phases and in recovery phases (the top panels of Figure This implies that the abundance of substorm injection O + ions is independent of a storm process (whether it is in the main phase or recovery phase) but on its intensity (Dst value). 5. Discussion [39] It is well known that O + ions are of terrestrial origin and He ++ ions are coming from the solar wind. Our observations show that with-oxygen injections are usually associated with higher energy (larger than 100 kev) H + and He ++ ions. The injections without-oxygen ions often contain only lower energy H + and He ++ ions (< 100 kev/e). These observations suggest that these high-energy H + and He ++ ions should have been accelerated together with the ions of ionospheric origin (e.g., O + ions) in the magnetotail. This scenario is also consistent with the modified substorm model proposed by Baker and McPherron [1990] and is supported by Geotail observations in the distant magnetotail [Zong et al., 1997; Zong, 1999]. According to their results, a slow reconnection in the central plasma sheet may initially occur on closed field lines in the late growth phase of a substorm process, and further if the reconnection reaches to open field line or plasma sheet boundary layer where the Alfven speed is much higher, ions which origin from polar ionosphere are proprogating tailward into the lobe or into the plasma sheet boundary layer [Baker et al., 1996] and will be accelerated to a higher energy. A simple procedure representing this process has been illustrated in Figure 11. [40] At the beginning of the growth phase, the tail magnetic field is compressed and forms a more tail-like topology. Ions, mainly H + and He ++ from the solar wind are convected to the near-earth plasma sheet. In the late growth phase, a slow reconnection in the central plasma sheet occurs on closed field lines and the preexisting ions, mainly H + and He ++ are accelerated by this slow reconnection process and further accelerated by the dipolarization process. O + ions in the ionosphere at the beginning of geomagnetic activity are being dragged out by parallel electric field (formed beam distribution) or waves (formed conics distribution) and transported tailward along open magnetic field lines into the lobe and /or plasma sheet boundary layer. [41] If this substorm activity or the reconnection process stops to develop at this time, a weak injection without oxygen ions (case 2 type injection) should be observed because at this time the oxygen ions are still not participated in the reconnection process (Figure 11a). [42] However, if the activity is strong enough and even the lobe magnetic field lines, where the Alfven speed is much higher, begin to merge, the preexisting H +, He ++ together with the newly coming ionospheric O +,He + ions could be energized to relative high energies by the reconfiguration precess of magnetic field lines. In this case, O + ions, together with high-energy H + and He ++ ions (withoxygen event, case 1) could be observed (Figure 11b). [43] In addition, the ionospheric outflow fluxes may increase with an enhanced Kp index (exponential function) as reported by Yau et al. [1988] and Yau and André [1997]. Assuming the ionospheric outflow ions (H +,He +,O + )are with the same energy, e.g., 1 kev (but with different velocities), the drift velocities (driven by mantle plasma) of these ions towards the center plasma sheet [Baker et al., 1996] are independent on the ion species and have the same value of E B. This implies that a mass filter mechanism may exist in the magnetotail as indicated in the Figure 11c. Oxygen ions from the polar ionosphere with a small parallel velocity component (e.g., 110 km/s) will reach the near- Earth plasma sheet whereas lighter ions with a larger parallel velocity component (e.g., 440 km/s) will be transferred into the distant magnetotail. In this case, during strong magnetic activities, an extreme oxygen-rich injection could be expected. In fact, as an additional piece of evidence, such energetic oxygen ions will also be injected into the tailward moving plasmoid. This has been observed by the GEOTAIL mission [Zong et al., 1997, 1998; Wilken et al., 1998]. [44] Furthermore, if two sequential injections in which the time interval between the two injections is relative short, when the activity is not well developed, the first one must be very weak compared to the late coming main injection. In this case, an injection like pseudo breakup can be expected. [45] Although some acceleration mechanisams in the polar ionosphere have worked on these O + ions during their outflow, the energy that the ions can reach is about hundreds of ev or several kev, which is still far below the energy observed during injections. This implies that an additional energization process is needed in order to reach the observed energy range. As pointed out by Baumjohann

14 SMP FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 11. Schematic illustration how oxygen ions get into the inner magnetosphere by substorm injection process: (a) a weak injection and oxygen ions from the ionosphere are travelling in the lobe or plasma sheet boundary region and still not be accelerated by the near-earth reconnection process; (b) when the reconnection has been developed well and expanded to the lobe region or plasma sheet boundary region, the O + can be energized and be observed in the injections; (c) a mass filter mechanism (the ionosphere origin ions will be dispersed with their different mass) may exist in the magnetotail during strong magnetic activities. See color version of this figure at back of this issue.

15 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS SMP [1996], the lobe magnetic pressure is decreased only during the expansion phase of storm time substorms. This implies that the lobe magnetic field lines have indeed participated in the reconnection process during the storm time substorms. Further, he suggested that only closed magnetic field lines are involved in the process of nonstorm substorms. In this case, H + and He ++, can only be accelerated and be observed at low energies (<100 kev). This is also consistent with the present CRRES observations. 6. Conclusions [46] Although the ion composition in the near-earth magnetosphere has been investigated for many years, most of the earlier conclusions were based on observations made at lower energy plasma (below 16 kev). The observations presented in this paper are made for all ion species in the ring current energy range. A total of 398 substorm injection events are identified and examined in detail. The main results can be summarized as follows: 1. Not all of the injection events have the same abundance of each ion species. About 73% of the total events are rich in O + ions and 27% of them have very low admixtures of O + ions. 2. The O + extreme rich injections occur in high geomagnetic activity with averaged Kp > 6.8, Dst < 119 nt, whereas without-oxygen injections are the result of relatively weak activity (Kp < 3.7, Dst > 30 nt). 3. All injections are with-oxygen events when Dst < 100 nt. 4. The averaged abundance of O + ions in terms of energy density shows strong positive correlations with geomagnetic activity (indicated by Dst and Kp index). 5. The abundance of O + ions in terms of energy density in injections associated with pseudo breakups is lower than the averaged value in each individual Dst and Kp index interval. 6. No obvious difference has been found between injection events occurring during storm main phase and recovery phase except that the oxygen-rich injections are more often to be found during the main phase than during the recovery phase. 7. The composition variation in substorm injections can be explained in the framework of the modified reconnection model. A plasma sheet which is populated with ionospheric ions is a direct reason for with-oxygen injections. [47] Acknowledgments. This work was supported a grant of the Max- Planck Society and Chinese NSFC It was also supported in part by Chinese NSFC (grant and ) and the research Project G [48] Janet G. Luhmann thanks Finn Sørass and another referee for their assistance in evaluating this paper. References Akasofu, S.-I., The development of the auroral substorm, Planet. Space Sci., 12, , Baker, D. N., and R. L. 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16 SMP FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS composition with geomagnetic and solar activity, J. Geophys. Res., 87, , Zong, Q.-G., Energetic oxygen ions in geospace observed by the GEOTAIL spacecraft, Ph.D Thesis, p. T.U. Braunschweig, Zong, Q.-G., B. Wilken, G. Reeves, I. Daglis, T. Doke, S. Livi, K. Maezawa, J. Woch, T. Iyemori, T. Mukai, S. Kokubun, Z. Y. Pu, S. Ullaland, and T. Yamamoto, Geotail observation of energetic ion specics and magnetic field in plasmoid-like structures in the course of an isolated substorm event, J. Geophys. Res., 102, 11,409 11,428, Zong, Q.-G., B. Wilken, J. Woch, T. Mukai, G. Reeves, T. Doke, S. Livi, K. Maezawa, D. J. Williams, S. Kokubun, S. Ullaland, and T. Yamamoto, Energetic oxygen ion bursts in distant magnetotail as a product of intense substorms: Three case studies, J. Geophys. Res., 103, 20,339 20,363, S. Y. Fu and Z. Y. Pu, Department of Geophysics, Peking University, Beijing , China. T. A. Fritz and Q. G. Zong, Center for Space Physics, Boston University, MA 02215, USA. (zong@bu.edu) B. Wilken, Max-Planck-Institut fuer Aeronomie, D Katlenburg- Lindau, Germany.

17 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 1. A color-coded energy per charge (E/Q) versus time spectrograms summarizing the energetic ions enhancements on 7 March The panels from top to bottom are counting rates of H +,He +,He ++, and O + ions, respectively. The magnetic local time (MLT), magnetic latitude (MagLat), L value and the distance are presented corresponding to the universal time. The arrow shows the beginning of the substorm injection. SMP 14-3

18 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 3. The proton and electron flux intensities from 2100 UT to 2300 UT, 7 March, 1991 obtained by LANL Geosynchronous (upper two panels) and (bottom two panels). The dashed line marks the onset time of the injection which corresponds to the energetic ion enhancements observed by CRRES/MICS in Figure 2. SMP 14-5

19 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 4. A color-coded energy per charge (E/Q) versus time spectrograms summarizing the energetic ions enhancements. The format of the figure is the same with Figure 2. The short arrows indicate the beginning of the ion injection. SMP 14-6

20 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 5. Color-coded energy per charge (E/Q) vs time spectrograms summarizing the energetic ions enhancements in the 19 March event (same format as Figure 2). The dashed arrows mark the time of pseudo breakup related injection, whereas the solid arrows indicate the beginning of the substorm injection. SMP 14-7

21 FU ET AL.: COMPOSITION SIGNATURE IN ION INJECTIONS Figure 11. Schematic illustration how oxygen ions get into the inner magnetosphere by substorm injection process: (a) a weak injection and oxygen ions from the ionosphere are travelling in the lobe or plasma sheet boundary region and still not be accelerated by the near-earth reconnection process; (b) when the reconnection has been developed well and expanded to the lobe region or plasma sheet boundary region, the O + can be energized and be observed in the injections; (c) a mass filter mechanism (the ionosphere origin ions will be dispersed with their different mass) may exist in the magnetotail during strong magnetic activities. SMP 14-14