Multisatellite observations of MeV ion injections during storms

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A9, 1231, doi: /2001ja000276, 2002 Multisatellite observations of MeV ion injections during storms K. R. Lorentzen, J. E. Mazur, M. D. Looper, J. F. Fennell, and J. B. Blake Space Science Applications Laboratory, The Aerospace Corporation, Los Angeles, California, USA Received 24 August 2001; revised 22 October 2001; accepted 14 November 2001; published 11 September [1] We report here on the formation of new ion radiation belts observed in connection with several solar energetic particle events and large geomagnetic storms in 1998 and We use observations from the Polar spacecraft, the highly elliptical orbit (HEO) , and the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), to study details of the inner zone radiation belt at high and low altitudes. We focus specifically on the four International Solar Terrestrial Physics events of August and September 1998 and April and July In several events we find new 2 15 MeV proton belts at various locations between L = 2.0 and L = 3.5. The low-altitude SAMPEX observations revealed features not visible at high altitudes, such as radiation belts with multiple peaks in L shell. During the July 2000 event, energetic helium and iron were observed at L 2, suggesting a solar energetic particle source for these injected ions. We compare observations of these new belts and remark on the significant differences from event to event. INDEX TERMS: 2720 Magnetospheric Physics: Energetic particles, trapped; 2788 Magnetospheric Physics: Storms and substorms; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; KEYWORDS: radiation belts, solar energetic particles, Polar, SAMPEX Citation: Lorentzen, K. R., J. E. Mazur, M. D. Looper, J. F. Fennell, and J. B. Blake, Multisatellite observations of MeV ion injections during storms, J. Geophys. Res., 107(A9), 1231, doi: /2001ja000276, Introduction Copyright 2002 by the American Geophysical Union /02/2001JA [2] Although MeV protons in the inner zone radiation belt are fairly stable, significant variations can occur at the edge of the inner zone beyond L 2 in association with solar energetic particle (SEP) events and geomagnetic storms. These phenomena appear to be somewhat rare and do not accompany every storm or SEP event. These longterm, non-adiabatic changes appear as both increases and decreases in the intensity of protons and other ions at fixed L shells. Shown in an L shell versus time format, the increases cause the radiation belt boundary to appear to move outward and the decreases cause the radiation belt boundary to appear to move inward. The increases sometimes also take the form of distinct new belts when data are plotted against L shell. Previous observations of these phenomena have been limited to only a few examples of each type. These observations are summarized in Table 1 and described in detail below. [3] The most dramatic variation in the proton population is the formation of a distinct new proton belt. New secondary proton belts have been previously observed in the slot region on only a few occasions. McIlwain [1963] first saw a secondary peak in MeV protons at L = 2.2 using the Explorer XV satellite in These observations were verified by the Relay I satellite [Fillius and McIlwain, 1964]. Although the creation of this second belt was not observed, it was later attributed to a geomagnetic storm in November 1960 [Bell et al., 1997]. More recently, Gussenhoven et al. [1989] reported DMSP dosimeter observations of a >35 MeV proton belt at L = 2.8 lasting for more than 100 days after the February 1986 storm. [4] It was not until launch of the CRRES satellite that these secondary proton belts were studied in detail, however. In March 1991 a very large storm sudden commencement (SSC) created new belts in both protons (>20 MeV) and electrons (>6 MeV) at L = 2.5 [Mullen et al., 1991; Blake et al., 1992; Gussenhoven et al., 1993]. The CRRES satellite also observed three other examples of lower-energy (10 MeV) proton belts at higher L shells in association with smaller SSCs [Gussenhoven et al., 1994; Hudson et al., 1997, 1998]. [5] Some storms display enhancements in proton belt flux, without the distinctive double-peaked structure described above. Moritz [1973] reported an early observation of this type. Spjeldvik and Fritz, [1981a, 1981b] described another storm time enhancement in 1 3 MeV helium, as well as heavy ions. Gussenhoven et al. [1994] and Hudson et al. [1997, 1998] reported a single CRRES observation of such an increase in 10 MeV protons. [6] Several mechanisms have been suggested as the source of these increases and distinct new protons belts. The March 1991 event was accompanied by a large amplitude electric field oscillation [Wygant et al., 1994]. Simulations showed that this electric field pulse could create a new electron belt on a timescale of one minute [Li et al., 1993]. The same mechanism was later shown to produce the new proton belt from a source population of primarily solar protons [Hudson et al., 1995]. This mechanism may also account for lower-energy proton belts at higher L shells, given a lower-energy solar particle pop- SMP 7-1

2 SMP 7-2 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS Table 1. Previous Observations Storm Date Reference Satellite L Shell Energy Type Duration Nov. 1960? McIlwain [1963] Explorer XV L = MeV new belt 2 years? Fillius and McIlwain [1964] Relay I Sept McIlwain [1965] Relay I L < 2.0 >35 MeV decrease 5 months April 1965 McIlwain [1966] Explorer 26 L < MeV decrease >20 days March 1970 Moritz [1973] Azur L = MeV increase 3 months June 1972 Spjeldvik and Fritz [1981a, 1981b] Explorer 45 L > MeV He increase months Aug Spjeldvik and Fritz [1981a, 1981b] Explorer 45 L = MeV He increase months Feb Gussenhoven et al. [1989] DMSP L = 2.8 >20 MeV new belt >100 days Aug Gussenhoven et al. [1994] CRRES L = <13 MeV new belt 12 days Hudson et al. [1997, 1998] Feb Hudson et al. [1997, 1998] CRRES L = MeV new belt 1 month March 1991 Mullen et al. [1991] CRRES L = MeV peak new belt years Blake et al. [1992] Gussenhoven et al. [1993] June 1991 Hudson et al. [1997, 1998] CRRES L < MeV decrease 1 week June 1991 Hudson et al. [1997, 1998] CRRES L < MeV increase weeks July 1991 Gussenhoven et al. [1994] CRRES L < MeV decrease days Hudson et al. [1997, 1998] July 1991 Gussenhoven et al. [1994] CRRES L < MeV decrease months Hudson et al. [1997, 1998] Aug Hudson et al. [1997, 1998] CRRES L MeV new belt 3 days ulation [Hudson et al., 1997]. On a timescale of days, Hovestadt et al. [1978] suggested that relative abundance of radiation belt heavy ions can be accounted for by solar wind particles moving inward by radial diffusion, and Spjeldvik [2000] modeled the radial diffusion of 1, 10, and 100 MeV ions and found that effects on the radiation belts depend strongly on the duration of the SEP event. Drift-resonant interaction with ultra-low frequency (ULF) waves has been suggested to enhance radial diffusion [Perry et al., 2000], for the formation of new proton belts on a timescale of one day. [7] In contrast to the increases in inner-zone protons described above, other storms were associated with decreases in the same population. McIlwain [1965, 1966] described two early observations of this type. Later, Gussenhoven et al. [1994] and Hudson et al. [1997, 1998] reported three CRRES observations of these decreases. It should be noted that some of these decreases were also accompanied by short-term, adiabatic decreases in protons inside L = 2.4, associated with large-scale changes in the Earth s magnetic field [McIlwain, 1966]. These effects are similar to the adiabatic decreases of kev protons at L = associated with the December 1971 storm [Lyons and Williams, 1976]. However, these short-term, adiabatic effects are not the focus of this paper. [8] Dragt [1961] first suggested that hydromagnetic waves of a few Hz could cause loss of inner zone protons. Based on later observations, McIlwain [1965] suggested that non-adiabatic decreases in inner zone protons may be associated with mhz magnetic fluctuations. Gussenhoven et al. [1994] proposed that depletions are the result of SSCs perturbing the boundary of the inner zone protons, in the absence of SEP events. Hudson et al. [1997, 1998], Anderson et al. [1997], and Young [2001] suggested an alternate explanation that protons are lost when the magnetic field is disturbed in such a way that adiabatic trapping of protons is no longer possible. Other possible explanations for loss may be pitch angle scattering due to interaction with electromagnetic ion cyclotron (EMIC) waves [Hudson et al., 1998]. Although generally found at higher L shells, EMIC waves have been observed to move below L = 2.4 during geomagnetic storms [Bräysy et al., 1998]. Loss of lower-energy ring current protons by interaction with EMIC waves was suggested by Cornwall et al. [1970] but this wave mode could potentially interact with higher-energy protons as well. [9] Variations in proton belts have been observed for almost 40 years, yet the number of examples of each type is still small. In addition, the examples described above generally used observations from only a single spacecraft. Questions remain about the processes causing these increases and decreases in the inner zone proton population. These inner zone phenomena may have an impact on satellite operations since ions with MeV energies can cause damage to spacecraft [Feynman and Gabriel, 2000]. [10] In this paper we present four new examples of variations in the proton radiation belts for which we have measurements from three different spacecraft. The four examples are from the ISTP Sun-Earth Connection events of August 24 26, 1998 (days ), September 23 26, 1998 (days ), April 4 7, 2000 (days 95 98), and July 14 16, 2000 (days , Bastille Day). We use data from the Polar spacecraft, the highly elliptical orbit (HEO) , and the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft to characterize the variations at both high and low altitudes. Following the SEP injections associated with each of these large geomagnetic storms, the three spacecraft observed changes in the inner zone protons at various L shells. The four events studied here show the wide range of phenomena associated with geomagnetic storms, including increases, decreases, and distinct new belts. 2. Instrumentation [11] The Polar satellite was launched on February 24, 1996 into an orbit of R E with an inclination of 86 [Harten and Clark, 1995]. The orbit period is 18 hours. In 1998 the orbit reached a minimum L shell of 2.5, but in 2000 the orbit reached below L = 2. The Polar Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment High Sensitivity Telescope (HIST)

3 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS SMP 7-3 Figure 1. (a e) Summary of Dst, Polar, HEO , and SAMPEX data from Polar data are summed over all pitch angles. The arrow in Figure 1e corresponds to the time shown in Figure 2a.

4 SMP 7-4 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS Figure 2. SAMPEX HILT (solid) and MAST (dotted) data from (a) August 31, 1998 and (b) April 7, Note that HILT is saturated during part of the August pass, and that the efficiency of MAST for detecting protons in 2000 was 1% of its value in [Blake et al., 1995] measures trapped electrons (300 kev 10 MeV) and protons (4 80 MeV) in the radiation belt with a geometry factor of cm 2 sr. The instrument measures data in 16 pitch angle sectors, with full coverage every 12 s. However, it cannot resolve the loss cone in any detail. In addition, saturation of the instrument by penetrating radiation prevents us from using data below L =2. [12] The HEO spacecraft is in a R E orbit with a 62 inclination. The satellite orbit period is 12 hours. The spacecraft dosimeters measure protons using omnidirectional sensors with geometry factors of 0.45 cm 2 sr. The sensors cover the energy ranges >8.5 MeV, >16 MeV, and >27 MeV. [13] The SAMPEX satellite was launched on July 3, 1992 into an orbit of km altitude with 82 inclination [Baker et al., 1993]. The orbit period is 96 min. The satellite carries four instruments that measure various types of charged particles. The Heavy Ion Large Telescope (HILT) measures both protons and electrons [Klecker et al., 1993]. For this study, we use a channel that measures both >5 MeV protons and >3 MeV electrons, but with very low efficiency for electrons. HILT has a geometry factor of 60 cm 2 sr and a view angle of The Proton/Electron Telescope (PET) measures 1 4 MeVelectrons and MeV protons in separate channels [Cook et al., 1993b]. It has a geometry factor of 1.8 cm 2 sr. The Mass Spectrometer Telescope (MAST) measures protons and ions with energies >10 MeV/nucleon [Cook et al., 1993a]. It has a geometry factor of 7 14 cm 2 sr. The efficiency of MAST for measuring protons changed several times during the interval covered by this study. The Low-Energy Ion Composition Analyzer (LICA) is a time-of-flight mass spectrometer that measures MeV/nucleon heavy ions [Mason et al., 1993]. It also measures 2 4 MeV protons with very low efficiency (<1%). 3. Observations [14] Using Polar, HEO, and SAMPEX, we have observed several different types of storm time variations in radiation belt ions. In this section, we give an overview of proton measurements from the different satellites and then describe details of energy spectra, timing, and ion composition for each event Overview of 1998 Events [15] Figure 1 shows Dst along with data from the three spacecraft in 1998, covering the first two events of this study. Large geomagnetic storms occurred on days August 27 (day 239) and September 25 (day 268) as indicated by the Dst index. Several other storms occurred in October, November, and December, but we have chosen to focus only on the August and September storms. Figure 1b shows Polar HIST 5 8 MeV spin-averaged protons. SEP injections can be seen extending down from high L shells on a number of occasions, generally preceding drops in Dst. Enhancements in proton flux are seen near L 3.5 in

5 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS SMP 7-5 Figure 3. (a e) Summary of Dst, Polar, HEO , and SAMPEX data from Polar data is summed over all pitch angles. The arrow in Figure 3e corresponds to the time shown in Figure 2b.

6 SMP 7-6 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS Table 2. Relative Timing of Geomagnetic Phenomena Event SEP Max GOES8 Protons SSC Dst min Dst Type Duration day UT #/cm 2 ssr day UT day UT nt Aug new belts 20 days Sept new belt 10 days April decrease 6 months 1 new belt July increase 1 month association with the largest storms. The August enhancement shows a gradual radial decay over about 20 days, while the other enhancements disappeared more abruptly when new geomagnetic disturbances occurred. Figure 1c shows >8.5 MeV proton data from the HEO spacecraft. These data show the same SEP events and new proton belts as the Polar data, and serve primarily to confirm the Polar observations. Figure 1d shows >2.9 MeV electron data from the HEO spacecraft, which is similar to Polar electron data for the same energy range (not shown). Figure 1e shows SAMPEX HILT >3 MeV electrons and >5 MeV protons. Again, SEP events are visible at high L shells. Although we cannot separate proton and electron contributions in this channel, comparisons with other measurements made by SAMPEX allow us to identify which features are dominated by electrons and which features are dominated by protons. Above L 3 there is a significant electron contribution to these measurements. However, at L < 3, the observations are predominantly protons and we find different features than Polar or HEO. The SAMPEX data show new belts starting at L 3 and several days later splitting into 2 distinct new belts with a slot-like region in the middle. [16] In order to illustrate these features more clearly and to differentiate proton and electron contributions, we have included a figure showing a single SAMPEX pass through these new belts. Figure 2a shows SAMPEX data from HILT and MAST on day 243, showing clearly this triple proton belt, although HILT is saturated at the highest count rates. The HILT data is the same combined electron and proton channel as shown in Figure 1e. The MAST 8 15 MeV proton data provide definitive identification of the proton component of the new belts. In September 1998, the MAST instrument mode was changed to one of very low efficiency for protons, so it was unable to observe protons for the other storms in Overview of 2000 Events [17] Figure 3 shows data from 2000 in the same format as Figure 1. The Dst index shows the two very large storms that occurred on April 7 (day 98) and July 15 (day 197). These storms were significantly larger than those in However, the Polar HIST 5 8 MeV protons in Figure 3b do not show the dramatic enhancements in proton flux rising from a steady inner zone, as in Instead, the boundary of the inner zone displays large shifts in position, inward in April, and outward again in June. These variations are similar to those seen by CRRES [Gussenhoven et al., 1994; Hudson et al., 1998]. The shift in June 2000 was associated with a moderate SEP event, but an insignificant storm. The extremely large SEP event in July, however, was only accompanied by a slight outward shift in the inner zone protons seen by Polar. Figure 3c shows again that the HEO proton data confirm the Polar proton observations. Figure 3d shows the HEO electron observations, which are similar to the Polar electron observations (not shown). Figure 3e shows SAMPEX HILT >3 MeV electrons and >5 MeV protons. The SAMPEX data in April and July show a bifurcating structure similar to that observed in the August 1998 event, but with the addition of a new population at L = 2 that persists for months following the April storm. Although this bifurcating structure is similar to that observed in the August 1998 event, we do not see the same evidence of proton content above L 2.5 in the other SAMPEX detectors, or in the Polar or HEO data. However, the new population at L = 2 definitely contains protons. This new L = 2 belt is observed only when the spacecraft is located near the South Atlantic anomaly and sampling a different pitch angle population. [18] Figure 2b shows a single SAMPEX pass through the radiation belts during the April 2000 storm. At this time, the MAST efficiency for 8 15 MeV protons was about 1% of its value in August However, it was still able to observe the belt at L = 2. There is no belt visible at L 2.5 in MAST, indicating one of three possibilities: 1) the efficiency is too low to detect the smaller peak, 2) the HILT peak at L 2.5 consists of protons with lower energy (<8 MeV), or most likely 3) the HILT peak is dominated by electrons. [19] Table 2 summarizes the important features of each event, along with solar and geophysical parameters from NOAA and the World Data Centers in Copenhagen and Kyoto Energy Spectra and Timing [20] Figure 4 shows proton count rates from various energy channels on the HEO satellite during the events of this study. Error bars indicate one standard deviation. We summed the count rates over the L regions where the changes in the new proton belts were most obvious in Figures 1 and 3 (L = 3.25 ± in 1998 and L = 2.5 ± in 2000). For the August and September 1998 storms, the new belts are visible in the >8.5 MeV and >16 MeV channels, but not in the >27 MeV channel. For the April and July 2000 storms, the variations are obvious in all three channels. However, for the July 2000 storm, the variations are more prominent in the higher energy channels.

7 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS SMP 7-7 Figure 4. (a d) HEO counts vs. time for >8.5 MeV, >16 MeV and >27 MeV protons during each of the four events. [21] Figure 4 also illustrates the relative timing of the events. For both of the storms in 1998, the >8.5 MeV proton count rate rose one orbit (half a day) earlier than the >16 MeV proton count rate. However, in April 2000, there was an unfortunate data dropout, so we can only say that the count rate in all three detectors decreased within one day of one another. The July 2000 storm displayed somewhat different behavior. The highest-energy >27 MeV channel rose first, followed half a day later by the >16 MeV channel. The lowest-energy >8.5 MeV channel actually showed a dropout on the same day. [22] SAMPEX can provide additional information about the timing of the April 2000 event. The belt at L = 2 was first observed on day However, because the particles are only observed near the South Atlantic anomaly, the new belt could have formed up to 6 h earlier when the spacecraft was not in position to see it. SAMPEX was near the South Atlantic anomaly at the time of the SSC on day 97.69,

8 SMP 7-8 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS Figure 5. Polar 5 8 MeV proton pitch angle anisotropy vs. time during (a) 1998 at L = 3.25 ± and during (b) 2000 at L = 2.5 ± however, and did not observe the new belt at this time. Therefore, the formation of the new belt was delayed at least a few hours from the time of the SSC Pitch Angle Distributions [23] Although the Polar HIST instrument cannot resolve the loss cone in any detail, the overall shape of the pitch angle distributions can provide information on the mechanisms that may cause changes in the proton population. For example, inward radial diffusion would lead to an increase in anisotropy when the first and second adiabatic invariants are conserved. In contrast, pitch angle diffusion would lead to a decrease in anisotropy. SEP injections would be expected to be fairly isotropic in the radiation belts. [24] We used fits to the pitch angle spectra from HIST of the form: jðaþ ¼ c 1 sin n ðaþþc 0 ð1þ where a is the pitch angle, c 0 and c 1 are constants, and n is a measure of the anisotropy. Figure 5 shows this anisotropy parameter, n, for the 5 8 MeV protons seen by Polar HIST. The data have been smoothed to de-emphasize orbital variations. As in Figure 4, the L ranges for the two periods are different, and were chosen to cover the region where the changes in the new proton belts were most obvious in Figures 1 and 3 (L = 3.25 ± in 1998, and L = 2.5 ± in 2000). [25] Before the August 1998 storm (day 239), the pitch angle distributions have an anisotropy of 4, indicating a distribution somewhat peaked around 90. Following the August 1998 storm, the anisotropy decreased to 2, indicating that the distribution flattened significantly. Later in the year slight increases lasting only a few days each are noted in connection with storms, and are probably due to adiabatic effects of magnetic field variation. In 2000, another large decrease in anisotropy is visible in association with the April storm (day 98) and a smaller increase in

9 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS SMP 7-9 Figure 6. SAMPEX LICA composition data for (a) April 2000 and (b) July Hydrogen is measured at 2 4 MeV/nucleon, helium-4 is measured at MeV/nucleon and iron is measured at MeV/nucleon. anisotropy is visible in association with the July storm (day 197) Ion Composition Data [26] Thus far, we have discussed the new radiation belts observed in energetic protons only. However, ion composition instruments on SAMPEX can also provide important information about these new radiation belts. Figures 6a and 6b show SAMPEX LICA data from the year 2000 storms. The efficiency of this instrument is very low, and each point plotted corresponds to a single ion. No data from 1998 is shown, because no significant features were visible above the background. Following the April SEP event, LICA data show a striking increase in hydrogen (2 4 MeV/nucleon) and helium-4 (0.7 2 MeV/nucleon) at L = 2. Hydrogen from the April event persists at L = 2 until the July SEP event, when iron (0.5 3 MeV/nucleon) also reaches these regions. Note that the first iron particle below L = 2 was detected at 197.6, approximately the same time as the third SSC. [27] MAST observed similar increases or these two storms in both hydrogen and helium at 8 15 MeV/nucleon (not shown). These increases are similar to the features seen by SAMPEX HILT in Figure 3e. For all the SAMPEX instruments, the ions at L = 2 are observed only near the South Atlantic Anomaly region, the same region where anomalous cosmic rays (ACRs) are observed. However, the ACR trapping mechanism is not expected to affect protons and helium, and iron is not considered an ACR element [Cummings et al., 1993]. 4. Summary and Discussion [28] Previous observations of storm time variations in energetic protons were easily classified as either increases, decreases, or distinct new belts. However, multisatellite observations further complicate the picture. Events displaying one feature at high altitudes can display totally different features at low altitudes. In the region above L = 3, observations of protons from low- and high-altitude space-

10 SMP 7-10 LORENTZEN ET AL.: STORM TIME MEV ION INJECTIONS craft generally resemble one another, but in the region below L = 3, SAMPEX is best situated to observe the distinct new belts. Closer to the equator, the stably trapped inner zone protons tend to obscure the more transient features. The four examples presented in this paper display a variety of characteristics, many of which lend support to the various suggested source and loss mechanisms. [29] The only event whose timing is consistent with acceleration by shock compression of the magnetosphere [Li et al., 1993; Hudson et al., 1995] is the July 2000 event. In all other events, the delay between the SSC and the formation of the new belt is too long. The presence of iron in the July 2000 event indicates a connection to the SEP event since this element is not normally seen in the radiation belts. However, L = 2 is well below the geomagnetic cosmic ray cutoff location. Using SAMPEX MAST 8 15 MeV/ nucleon helium data, Leske et al. [2001] measured geomagnetic cutoffs of L = 3.4 and L = 4.0 for the August and September 1998 storms, respectively. For the July 2000 storm, the helium cutoff location was L = 2.7 (R. Leske, personal communication, 2001). In all cases, the cutoff location is well above the location of the new proton belts. Since the cutoff location depends on the particle rigidity, the proton cutoffs will be at even higher L shells. Thus, some additional mechanism such as acceleration by shock compression is needed to transport solar particles to lower L shells. However, the delayed increase in proton fluxes as seen by HEO (see Figure 4d) remains a puzzle. Since the July 2000 storm was accompanied by 3 SSCs, it is possible that the close timing between the observation of iron and the third SSC is merely a coincidence. [30] In order to further examine the relationship between the iron and the shock injection mechanism, we calculated the drift velocity of marginally trapped iron. Labrador et al. [2001] found that MeV/nucleon iron had a charge state of 14 during the July 2000 storm. Since the charge state decreases with decreasing particle energy [Mazur et al., 1999] we take a charge state of 11 for the MeV/nucleon iron observed by SAMPEX LICA during this event. This charge state gives a drift velocity of 900 km/s for marginally trapped MeV/nucleon iron at L = 3, which is comparable to the magnetosonic pulse velocity of km/s used by Li et al. [1993] to model the March 1991 shock injection. If shock acceleration is in fact the source of the trapped iron, the dependence of the drift velocity on the particle charge state may explain the delayed increase in the proton flux relative to the iron flux. [31] Although classical radial diffusion may be too slow to account for the increases in proton population and new belts shown in these four events, ULF enhanced radial diffusion may be able to account for the outermost belt observed in the August 1998 event [Perry et al., 2000]. This mechanism is consistent with the observation that increases in lower-energy ions are observed first at a fixed L shell (see Figure 4a). The September 1998 event may also be explained by this mechanism. However, ULF enhanced radial diffusion cannot account for the bifurcation of the two belts during the August 1998 event. The decreases in anisotropy seen in Figure 5 may also support an SEP source for the particles, since SEP events would be expected to be isotropic in the radiation belts. [32] The gradual loss of the new belts formed in August and September 1998 and the sudden decrease in April 2000 may be the result of pitch angle scattering due to the breakdown of adiabatic motion in regions of field line stretching [Anderson et al., 1997; Young, 2001]. During the April 2000 storm, Dst reached as low as 321 nt, resulting in significant distortion of the Earth s magnetic field. This pitch angle scattering may also explain the appearance of the new belt at L = 2 in the SAMPEX data and the decrease in anisotropy seen by Polar. As particles are scattered toward smaller pitch angles in the equatorial region, they become visible to low-altitude spacecraft such as SAMPEX. The high stability for particles at L = 2 resembles that of ACR particles, for which the peaking at this location is due to the separation between the geomagnetic cutoff and the trapping limit [Selesnick et al., 1995]. [33] It has also been suggested that EMIC [Hudson et al., 1998], hydromagnetic [Dragt, 1961], or lower-frequency [McIlwain, 1965] wave activity may cause loss of radiation belt protons. The appearance of a slot in the proton population at L 2.5 is suggestive of cyclotron resonant pitch angle scattering, as seen for the electron slot region. However, no satellite wave data was available during these events and examination of ground-based wave data was beyond the scope of this paper. Future work will include determining what type of waves, if any, were present during these events. [34] In summary, the August and September 1998 events seem most consistent with radial diffusion of solar particles by a slow mechanism such as ULF waves. The April 2000 event seems more likely to be caused by a rearrangement of existing inner zone particles by pitch angle scattering. The July 2000 event is the only event that may be associated with SSC acceleration on a drift period timescale. However, many questions remain to be answered, and it may be that more than one mechanism plays a role in each event. [35] Acknowledgments. We thank the members of the SAMPEX and Polar instrument teams, who have contributed to the success of these missions. The work at Aerospace has been supported through NASA grant NAS and by cooperative agreement Z between the University of Maryland and the Aerospace Corporation, funded through NASA grant NAG Storm sudden commencement data were obtained from the World Data Center in Copenhagen web site. Provisional and final Dst were obtained from the World Data Center at Kyoto web site. Solar Proton Data were obtained from the NOAA space environment center. [36] Janet G. Luhmann thanks Mary K. Hudson and another referee for their assistance in evaluating this paper. References Anderson, B. J., R. B. Decker, N. P. Paschalidis, and T. Sarris, Onset of nonadiabatic particle motion in the near-earth magnetotail, J. Geophys. Res., 102, 17,553, Baker, D. 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