Characteristics of 2 6 MeV electrons in the slot region and inner radiation belt

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2006ja011748, 2006 Characteristics of 2 6 MeV electrons in the slot region and inner radiation belt Yihua Zheng, 1 Anthony T. Y. Lui, 1 Xinlin Li, 2 and Mei-Ching Fok 3 Received 27 March 2006; revised 9 June 2006; accepted 11 July 2006; published 7 October [1] In this paper, the long-term relativistic electron behavior in the often overlooked slot region and the inner belt was studied using the 2 6 MeV electron data from the SAMPEX satellite during In particular, we investigated the penetration of the 2 6 MeV electrons to the slot region, their flux enhancement in the inner belt, and how these two processes are related to magnetospheric activity levels and solar wind parameters. It is found that the 2 6 MeV electron penetration (L < 3) usually takes place after those intense storms whose Dst min is less than 130 nt. The penetration distance has a good correlation with the daily Dst minimum delayed by 3 days. However, it has little or no correlation with any of the individual daily solar wind parameters. A superposed epoch analysis shows a similar time lag between the peak values of the solar wind parameters/dst index and the flux peak in the slot region at L = 2.5. Although the inner belt has fewer variations compared to the outer belt, it has its own response to different conditions. Analysis of the inner belt indicates that the flux enhancement is associated with high solar wind speed (>550 km s 1 ) as well as low solar wind density (4.4 cm 3 ). Either a couple of very extreme storms or recurrent intense storms will create conditions favorable for the MeV electron enhancement in the inner belt. However, without high-speed solar wind on average, a series of intense storms alone are not sufficient to create inner belt electron flux enhancement. Citation: Zheng, Y., A. T. Y. Lui, X. Li, and M.-C. Fok (2006), Characteristics of 2 6 MeV electrons in the slot region and inner radiation belt, J. Geophys. Res., 111,, doi: /2006ja Introduction [2] Understanding the dynamics of relativistic electrons in the inner magnetosphere is of great importance from both a practical and space physics point of view. While it is known that relativistic electrons can have deleterious effects on space assets and humans in space, the highly complex behavior and the delicate balance between acceleration and loss processes of radiation belt electrons have not been fully resolved. [3] Despite the challenges and difficulties, considerable progress has been made in radiation belt electron research. It is now known that the radiation belt electrons in the outer zone (3 < L < 7) exhibit highly dynamic variations both in space and time, especially during geomagnetic active times. Relativistic electron flux typically decreases by 2 or 3 orders of magnitude during the main phase of geomagnetic storms and recovers to or increases beyond the prestorm level during the recovery phase, with a timescale of a few days [Dessler and Karplus, 1961; Baker et al., 1994; Li et al., 1997; Kim and Chan, 1997; Reeves et al., 1998; 1 Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA. 2 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA. 3 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. Copyright 2006 by the American Geophysical Union /06/2006JA Miyoshi et al., 2003]. However, not all geomagnetic storms create a net increase of relativistic electron flux in the outer zone [Reeves et al., 2003]. In fact, electron fluxes actually decrease below their prestorm levels during some storms [e.g., Onsager et al., 2002; O Brien et al., 2001]. The association of solar wind speed and the appearance of relativistic electrons in the outer zone has long been known [Paulikas and Blake, 1979]. Recently, Li et al. [2005] showed that both nonrelativistic and relativistic electrons in the energy range of 50 kev to 6 Mev at geosynchronous orbit are strongly modulated by the solar wind speed along with the polarity of the interplanetary magnetic field (IMF). The long-term relativistic electron measurements show a strong seasonal and solar cycle variation [Li et al., 2001] with its flux being most intense, on average, during the declining phase of the sunspot cycle, weakest near the sunspot minimum, and becoming more intense again during the ascending phase of the solar cycle. However, the flux is not most intense near or at the sunspot maximum. The outer belt electrons are most intense and also penetrate deepest around the equinoxes [e.g., Baker et al., 1999], exhibiting a semiannual variation. [4] On the basis of years of studies of the outer belt electrons, various mechanisms have been proposed to account for their dynamic variations [e.g., Friedel et al., 2002]. The existing acceleration mechanisms include inward radial diffusion [e.g., Lanzerotti et al., 1970], recirculation [Fujimoto and Nishida, 1990], resonant acceleration 1of11

2 Figure 1. Logarithmic value of the 2 6 MeV daily averaged electron fluxes (in cm 2 s 1 sr 1 MeV 1) from 6 July 1992 to the end of the 2003 measured by the SAMPEX spacecraft. Data are plotted according to the color bar to the right. The vertical axis is the L shell parameter. The Dst index during the same period (in white) is overplotted. by whistler mode waves [e.g., Horne and Thorne, 1998; Summers et al., 1998, 2004], and enhanced radial transport or acceleration by the ULF waves [e.g., Elkington et al., 1999; Liu et al., 1999; Ukhorskiy et al., 2005]. The loss mechanisms include adiabatic motion due to changing magnetic field topology, outward radial diffusion, fast loss processes due to pitch angle scattering by plasmaspheric hiss or EMIC waves and the particle loss from drifting out of the magnetopause. However, the relative importance of these mechanisms remains to be quantified. [5] However, so far the majority of the studies on the relativistic electrons, no matter whether observational, theoretical or based on modeling, have focused on the physics of the outer radiation belt. Only very recently, Li et al. [2006] showed that there is a remarkable correlation between the inner edge of the outer radiation belt electrons and the innermost plasmapause location (LPP) and they interpreted this correlation as an indication that the depth to which the outer radiation belt electrons can penetrate into the inner magnetosphere is closely tied to LPP. In this paper, we aim to study the long-term relativistic electron behavior in the often overlooked slot region (L 2 3) and the inner belt (L < 2) by using the daily-averaged 2 6 MeV electron fluxes obtained from the SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) satellite during In particular, we want to study the penetration of the MeV electrons at this energy range to the slot region and their flux enhancement in the inner belt and how these two processes are related to geomagnetic activity levels (such as Dst index, used often as a measure of storms) and some parameters in the solar wind. Our goal here is not to seek a detailed explanation of single events, but to find the common characteristics which are similar among a large number of events. We aim to establish the conditions in the solar wind and ring current that lead to the variations of MeV electrons in the slot region and the inner belt, and therefore to improve our understanding and predicting ability with regard to relativistic electrons in the entire magnetosphere. 2. Data Description [6] The daily electron flux at 2 6 MeV for almost 12 years (from the launch of the spacecraft on 3 July 1992 to March 2004) from the SAMPEX mission (see Baker et al. [1993] for spacecraft description) is used for this study. The SAMPEX spacecraft measures energetic electrons from a low-altitude ( km) 82-degree inclined orbit. [7] The solar wind data used in this study are same as those used by Temerin and Li [2006]. They were, at first, from the Wind spacecraft [Lin et al., 1995; Lepping et al., 1995] and then combined with the data from ACE spacecraft [McComas et al., 1998; Smith et al., 1998] after its launch in 1998 to fill gaps in the Wind data. The solar wind data since 2000 are exclusively from ACE. All solar wind data were propagated to the magnetosphere (see Temerin and Li [2006] for details). The time resolution is 10 min. [8] Hourly final Dst indices for years and provisional Dst values for 2003 are obtained from the Kyoto World Data Center ( dstdir/index.html). 3. Observations and Discussion 3.1. Penetration of 2 6 MeV Electrons to the Slot Region [9] The slot region (L 2 3), between the inner belt (L < 2) and the outer belt (L = 3 7), is usually a region devoid 2 of 11

3 Figure 2. Detailed display of 2 6 MeV electron data for the year 2001 in the same format as Figure 1. Notice the correlation of the penetration of 2 6 MeV electrons with the Dst minimum during intense storms. of energetic electrons and is therefore favored as a location for spacecraft operation because of its benign environment [Baker et al., 2004, and references therein]. The formation of the slot region is usually attributed to the strong pitch angle scattering and loss of the particles to the Earth s upper atmosphere due to waves such as plasmaspheric hiss [e.g., Thorne et al., 1973]. However, during enhanced geomagnetic activities, the penetration of the outer belt energetic electrons can take place, partially or fully filling the slot region, one dramatic example of which is during 24 March 1991 storm [Blake et al., 1992; Li et al., 1993]. [10] Figure 1 shows the logarithmic value of the 2 6 MeV daily averaged electron fluxes (in cm 2 s 1 sr 1 MeV 1) from 6 July 1992 to the end of the 2003 measured by the SAMPEX spacecraft, with the color bar to the right. The vertical axis is the L shell parameter. Overplotted is the Dst index during the same period (in white). From Figure 1 we can see that the penetration of electrons usually takes place when the negative value of Dst is large. It is notable that the penetration seems to correlate well with the occurrence of intense storms (Dst < 100 nt, classified according to Gonzalez et al. [1994]). The detailed display of electron data for the year 2001 in Figure 2, in the same format as Figure 1, further confirms the correlation of the penetration of 2 6 MeV electrons with the Dst minimum during intense storms. [11] In order to study the penetration of the 2 6 MeV electrons in a quantitative fashion, we define the inner edge of the outer radiation belt as the L value where the electron flux is 20% of the maximum flux on the same day. The inner edge of the outer radiation belt (Loutinner_edge) determined using this method is shown in Figure 3 for the year 2001, along with Dst indices. Figure 3 shows that this inner edge usually moves much closer to the Earth during a series of intense magnetic storms. The degree of penetration and its duration seem to depend both on the intensity of individual storms and on the number of intense storms occurring during a certain period. The penetration distance is defined as the difference between this edge and L = 3.0 (the nominal inner extent of the outer zone) as in DL ¼ 3:0 Loutinner edge ð1þ Figure 3. Inner edge of the outer radiation belt (Loutinner_edge, see the text for definition) for the year 2001, along with Dst indices. This inner edge is show to usually move much closer to the Earth during intense magnetic storms. 3 of 11

4 Figure 4. Histogram of number of events, which were selected if the daily Dst minimum is less than or equal to 130 nt, as a function of year during The black trace is the yearly average of daily sunspot number. The correlation coefficient between the event occurrence and the sunspot number is By studying DL from equation (1) and the corresponding Dst indices for about 11 years ( ), we notice that a salient penetration almost always takes place when the Dst value is below 130 nt. Therefore we examined the events that have the daily Dst minimum less than or equal to 130 nt. Figure 4 shows the histogram of number of events as a function of year during The black trace is the yearly average of daily sunspot number. The correlation coefficient (r) between the event occurrence and the sunspot number is Figure 4 indicates that the occurrence rate of these intense storms (Dst min 130) correlates well with the solar cycle, with more intense storms occurring during solar maximum than solar minimum period. [12] To determine what factors contribute to the penetration, we correlate the penetration distance with several parameters, such as Dst, solar wind density n SW, IMF jbj, B z, solar wind dynamics pressure Pdyn, and solar wind speed V SW. Since the solar wind data starts from 1995, the correlation analysis is applied to the events during Among all these parameters, only Dst has a relatively good correlation with the penetration distance DL. All the other individual parameters show poor or no correlation. It is also found that the highest correlation coefficient occurred when Dst (t) correlates with DL(t + 3 days), i.e., the deepest penetration takes roughly about 3 days (the resolution of DL is 1 day, same as the electron flux) to develop on average. Figure 5 shows the correlation results between DL(t + 3 days) and Dst (t), with daily minimum values of Dst (r = 0.76) (Figure 5, top) and daily average values of Dst (r = 0.48) (Figure 5, bottom). The penetration distance was found to have either poor (with r below 0.5) or no correlation with any of the single solar wind parameters (n SW, IMF jbj, B z, Pdyn, V SW ) regardless of different lag times. Note that this does not contradict with the Dst model results as shown by Temerin and Li [2006]. A complex function of solar wind parameters may have a good correlation with the penetration depth, but searching for that combination is beyond the scope of this paper. The correlation analyses show that any individual parameter of solar wind and IMF does not have a direct control of the penetration depth, but the resulting magnetospheric activity condition (Dst can be used as a measure of it) due to solar wind magnetosphere coupling does have effects on the penetration. [13] In addition to the correlation study, superposed epoch analyses are employed to study the penetration of relativistic electrons to the slot region from two different perspectives. [14] First, we selected the set of days on which 2 6 MeV electron flux has a sudden clear increase in the slot region at L = 2.5. This set of days (22 events in total during ) is taken as zero epoch times (t0) for a superposed epoch analysis. Data for each event are taken between t0 9 days and t days. Figure 6 shows the results. From top to bottom, the parameters are V SW, Pdyn, n SW, IMF B z, IMF jbj, the inner edge of the outer radiation belt Lout inner_edge, Kp (with Kp in solid and its daily maximum in dotted line), Dst (with Dst in solid and its daily minimum in dotted line), and the log of the fluxes at different L shells in the slot region (L = 2.7 in dotted line, L = 2.5 in solid line, L = 2.2 in dashed line and L = 2.0 in dot-dashed line). The vertical dashed line is used to mark the zero epoch time. We can see that the negative peak of the averaged Dst occurred about 3 days Figure 5. Correlation results between DL(t + 3 days) and Dst (t), with (top) daily minimum values of Dst (r = 0.76) and (bottom) daily average values of Dst (r = 0.48). 4of11

5 can see that the penetration starts to take place about 3 4 days before the flux peak, reaching its maximum depth around the same time of the flux peak. The 3-day time lag displayed in Figure 6 is also consistent with the correlation analysis result between Dst and the penetration distance. Figure 6 reveals the common characteristics in solar wind and Dst associated with the penetration and the flux enhancement in the slot region. It is worth noting from the log flux plot in Figure 6 that the flux reaches its peak almost simultaneously at L = 2.7 and L = 2.5, while it takes a longer time (about 1 2 days longer) at L = 2.2 and an even longer time for the flux at L = 2.0. [15] Second, we did a superposed epoch analysis of the intense storms whose Dst is less than or equal to 130 nt. Figure 6. A superposed epoch analysis of 22 events. The zero epoch time (t0) of an event is chosen when the 2 6 MeV electron flux has a sudden clear increase in the slot region at L = 2.5. Data for each event are taken between t0 9 days and t days. From top to bottom, the parameters are V SW, Pdyn, n SW, IMF B z, IMF jbj, the inner edge of the outer radiation belt Lout inner_edge, Kp (with Kp in solid and its daily maximum in dotted line), Dst (with Dst in solid and its daily minimum in dotted line), and the log of the fluxes at different L shells in the slot region (L = 2.7 in dotted line, L = 2.5 in solid line, L = 2.2 in dashed line and L = 2.0 in dot-dashed line). The vertical dashed line is used to mark the zero epoch time. prior to the flux peak at L = 2.5 (A similar time lag is seen in maximum solar wind speed, maximum of IMF jbj and Kp). The negative peak of IMF B z occurred about 4 days (3 days for the positive peak) prior to the flux peak in the slot region. Notice that there are multiple peaks in n SW, Pdyn, and IMF jbj 3 6 days prior to the flux peak. The timing difference of negative peaks in Dst and Dst min is probably due to different time resolution with the former at 1 hour and the latter at 1 day. From the Lout inner_edge plot, we Figure 7. Superposed epoch analysis results in the same format as Figure 6 except that the log of the electron flux at L = 3.5 is also shown. This analysis is based on the events selected using a different criterion. The zero epoch time is set when Dst of those storms (Dst min 130) reaches its minimum (t0). The time range from t0 3 days to t0 + 7 days is used. The same set of parameters as in Figure 6 is considered for the analysis. 5of11

6 Figure 8. Superposed epoch analysis result for the years of Notice the similar quasiperiodic oscillations (about 4 times a year) shown in the inner belt flux and the solar wind dynamic pressure. The other parameters, however, display a semiannual variation. A total of 34 events are found that meet this criterion during The zero epoch time is set when Dst of the storms reaches its minimum (t0). The time range from t0 3 days to t0 + 7 days is used. The same set of parameters as in Figure 6 is considered for the analysis. Figure 7 shows the superposed epoch analysis results in the same format as Figure 6 except that the log of the electron flux at L = 3.5 is also shown in Figure 7. We can see that the average value of the minimum Dst is about 200 nt. The solar wind parameters all display a peak either at the Dst minimum (V SW, Kp) or just slightly earlier (IMF jbj, B z, n SW, Pdyn) than that. Figure 7 shows the common features of solar wind and IMF for these very intense storms, with IMF jbj reaching 25 nt, IMF B z having its minimum about 18 nt, n SW reaching 16 cm 3, the peak of the solar wind dynamic pressure of 10 npa, and a maximum solar wind speed of 610 km s 1. Lout inner_edge and the log flux at L = 2.5 from Figure 7 again display a 2 3 days time lag. The penetration reaches its maximum about 2 3 days after the Dst minimum and the flux peak at L = 2.7 and L = 2.5 happens about 2 3 days later after zero epoch time. Another interesting feature is that the flux evolution in the outer zone at L = 3.5 during these intense storms is quite different from that in the slot region. The flux at L = 3.5 immediately decreases following the Dst minimum and lasts for about 2 days and then increases later on The 2 6 MeV Electron Behavior in the Inner Belt [16] The inner radiation belt (L < 2), centering at L = 1.5, is usually thought to be quite stable, with fluxes varying only during the most intense geomagnetic disturbances. The trapped electrons found in this region are believed to be populated primarily by diffusive transport processes [Boscher et al., 1997] Quasiperiodic Oscillations in Inner Belt Electron Flux [17] With the advantage of the long-term electron data collected by the SAMPEX satellite, we studied the behavior of 2 6 MeV electrons in the inner belt region. An interesting feature emerging from our analysis is that the inner belt electron flux displays a quasiperiodic oscillation: about 4 times a year during the years of (around solar minimum). The superposed epoch analysis result is shown in Figure 8 for these years. As shown by Li et al. [2001], there is a semiannual variation in the outer belt electron flux, Dst (sixth panel) and solar wind speed (first panel) during The semiannual variation is also shown in IMF jbj, and IMFB z. However, the inner belt electron flux exhibits higher frequency variability (4 cycles per year) than that of the outer belt flux (2 cycles per year). The same kind of periodicity as the inner belt flux appears in the solar wind dynamic pressure and how these two quantities are related is not well understood. [18] A superposed epoch analysis of the same set of parameters during years (near solar maximum), however, does not show any quasiperiodic oscillations (see Figure 9). We speculate that the larger geomagnetic disturbances (more frequent occurrence of very intense storms) during these three years mask out the periodicity shown during relatively quiet times Flux Enhancement of the Inner Belt Electrons [19] From Figure 1, we can see that the inner belt electron flux displays a certain degree of variability during the years from 1992 to The inner belt electron flux was relatively strong until 1996 probably due to the combined effects of the aftermath of the 24 March 1991 storm and the high-speed solar wind streams during these years. The flux almost diminished during the year 1997 (solar minimum) and then gradually increased after It reached its peak after the 2003 Halloween storm and the November 2003 superstorm [Baker et al., 2004] which have had and probably will have a long-lasting effect on the flux enhancement of the inner belt electrons. [20] Four intervals of noticeable flux enhancement in the inner belt are found during the period of , with the first one (case 1) starting on 14 July 2000, the second one (case 2) starting on 9 April 2001, the third one (case 3) starting on 30 May 2003, and the fourth one (case 4) starting on 30 October Dates were chosen on which there is an increase in the log of the averaged flux in the region of L = 1.3 to L = 1.8. Each enhancement interval lasted for quite a long period (more than 4 months), yet each interval has its unique features either in Dst or in solar wind parameters when we plotted all the four cases with the zero 6of11

7 292, 221 on DOY = 274, 295, 301, 310, 328, respectively, for the occurrence of the intense storms). By plotting the two cases from t0 30 days to t days, we notice that the averaged solar wind speed for these two cases was relatively low compared to the four cases where there is a flux enhancement. [22] Correlation analyses were performed among the six events mentioned above. Figure 10 shows the results between the correlation of the log value of the flux in the inner belt region (L = ) and the Dst minimum (Figure 10, top), the correlation of the log flux with the solar wind speed (Figure 10, middle), and the correlation of the inner belt flux with the combined quantity Dst min * V SW (Figure 10, bottom). Each point on the plots represents one event as the quantities were averaged over the whole event Figure 9. Superposed epoch analysis of the same set of parameters during years (near solar maximum). There are no quasiperiodic oscillations. time set on the days (t0) mentioned above from t0 30 days to t days. For the first case, the enhancement is coincident with 5 very intense magnetic storms with Dst min < 130 nt. For the second case, the two large storms with their minimum Dst of 387 nt (31 March 2001) and 271 nt (12 April 2001) probably contributed most to the flux enhancement. For the third case, there were no very intense storms during the interval (only three storms whose Dst min < 130 nt, with Dst min = 131, 145, 168 on DOY = 150, 169, and 230, respectively), but the solar wind speed on average was high. The fourth case is the extreme case where the Halloween storm (30 October 2003, Dst min = 401 nt) and the consequent super storm on 20 November 2003 (Dst min = 472 nt) took place, creating a filled slot region and greatly enhanced inner belt. [21] In order to find the common characteristics associated with the inner belt flux enhancement, two counter cases were also selected, where there were a series of intense magnetic storms (Dst min < 100 nt) and yet there was no flux enhancement. The zero time for the first case (E1) is 6 August 1998 (Dst min = 138, 155, 207, 149, 131 on DOY = 218, 239, 267, 312, 317, respectively, for the occurrence of the intense storms) and 3 October 2001 (E2) for the other (Dst min = 148, 177, 157, Figure 10. Results between the correlation of the log value of the flux in the inner belt region (L = ) and (top) the Dst minimum, (middle) the correlation of the log flux with the solar wind speed, and (bottom) the correlation of the inner belt flux with the combined quantity Dst min * V SW. 7of11

8 terms of Dst min ) shows that ring current plays an important role in controlling the dynamics of energetic electrons in the radiation belt, even in the slot region. On the basis of outer zone radiation belt observations, Tverskaya et al. [2003, and references therein] showed that the L position (L max ) of the peak intensity of the outer belt relativistic electron flux depends on the magnetic storm amplitude (jdstj max = Dst min ) and they have the following relationship (for jdstj up to 400 nt): 2: jdstj max ¼ L 4 max ð2þ Figure 11. Correlation result of the inner belt log flux with the averaged solar wind density. The two have a good anticorrelation (r = 0.94). interval (150 days). The log value of the inner belt flux has the same correlation coefficient (r = 0.59) with either the Dst minimum or the averaged solar wind speed. The correlation coefficient (r = 0.73) becomes higher when the log flux of the inner belt correlates with the combined quantity Dst min * V SW. Surprisingly, the log flux has a very high anticorrelation (r = 0.94) with the averaged solar wind density (see Figure 11). [23] Even though the correlation analyses were based on six events only (that is all we have), the results may have important implications: the inner belt flux enhancement is probably controlled by both the averaged Dst value and solar wind speed, and the inner belt electron flux enhancement seems to happen when the average solar wind density is small. [24] Another way of studying the MeV electron flux enhancement in the inner belt was employed. We calculated two types of averaged values (one is from the interval of t0 10 days to t0 days (pre) and the other is from t0 to t days (pos)) of these parameters (the log of the inner belt electron flux (LogFlux), Dst min, Dst ave, V SW, IMF jbj, and IMF B z, n SW and Pdyn, AE and Kp indices) for the four events with flux enhancement and the two events without enhancement. Since the AE indices are only available up to the end of 2001, we cannot calculate the values for cases 3 and 4. The results are tabulated in Table 1. [25] Table 1 again shows that the inner belt electron enhancement is usually associated with high-speed solar wind (V SW > 550 km s 1, V SW = 597 km s 1 for the four events with flux enhancement) and low solar wind density (n SW = 4.4 cm 3 ), in agreement with the results from the above correlation analysis. A large negative Dst is not always needed for the inner belt flux enhancement when the average solar wind speed is high, for example, case 3. On the other hand, one or two superstorm(s) (Dst min < 300 nt) can keep the inner belt MeV electron flux elevated for a long time, such as in cases 2 and 4. [26] The inner belt flux enhancement is also coexistent with higher Kp (with the averaged Kp > 3 and the maximum Kp > 5) and AE (>300 nt) values Discussion [27] The association of the penetration of energetic electrons to the slot region and intense magnetic storms (in This relation shows that the peak intensity of the electron flux appears at lower L shells with increasing magnetic storm levels. The correlation between the penetration distance DL and jdstj max from our analysis is consistent with the statistical behavior expressed in equation (2). The smaller L location of the peak flux intensity during intense magnetic storms (L max ) will favor the penetration of the relativistic electrons to the slot region either through radial diffusion or storm time enhanced wave-particle interactions. Both our results and those of Tverskaya show that magnetic storms do have important impacts on the dynamic evolution of the radiation belt electrons, but the effects may be only felt at limited L shells. This may also help explain why satellites sitting at a fixed L shell (for example, at Table 1. Averaged Values of Several Parameters During Pre- and Post-10 Days of the Events No With Enhancement Enhancement Parameters Case1 Case 2 Case 3 Case 4 E1 E2 LogFlux (l = ) Pre (10 days) Pos (10 days) Dst min Pre Pos Dst ave Pre Pos V SWPre Pos IMF B Pre Pos IMF B z Pre Pos N SWPre Pos Pdyn Pre Pos AE Pre Pos Kp_max Pre Pos Kp_ave Pre Pos of11

9 geosynchronous orbit) see different relativistic electron flux characteristics during different magnetic storms [e.g., Reeves et al., 2003]. [28] As noted by O Brien et al. [2003], the relationship between the plasmapause location (L PP ) and the minimum of Dst developed by O Brien and Moldwin [2003] has some resemblance as the relationship between the location of relativistic electron peak intensity (L max ) and the minimum of Dst in Equation (2). The relationship between the inner edge of the outer radiation belt and the plasmapause location has also been reported [Baker et al., 2004; Goldstein et al., 2005]. Therefore the complicated interactions among the plasmasphere, ring current and radiation belt remain to be further investigated. The roles of various kinds of waves in the vicinity of the plasmapause on the radiation belt dynamics must still be clarified [Summers, 2005]. [29] With regard to the dynamics of the slot region filling, the rare and unusual event that occurred between 29 October and 4 November 2003, widely known as the Halloween storm, has provided unique opportunities for testing different theories of particle acceleration and has been studied quite extensively [Baker et al., 2004; Horne et al., 2005; Spasojevic and Inan, 2005; Shprits et al., 2006; Loto aniu et al., 2006]. Two leading acceleration mechanisms for radiation belt electrons include inward radial diffusion driven by ULF waves [e.g., Elkington et al., 1999] and local acceleration driven by VLF waves [e.g., Summers et al., 1998; Horne et al., 2005]. According to the radial diffusion theory, electrons are diffused across the magnetic field via global-scale fluctuations in the Earth s magnetic and electric fields at frequencies that closely match the electron drift frequencies of a few millihertz (mhz). The process operates more efficiently when the ultralow-frequency (ULF) waves at a few mhz are enhanced. Whistler mode chorus emission, which is excited throughout the low-density region exterior to the plasmasphere and over a broad range of local times ( MLT), is capable of causing local acceleration. Normally whistler mode chorus diffusion can be an important mechanism for accelerating electrons in the region 3 < L < 5 outside the plasmasphere. However, during very intense storms or superstorms, the extreme compression of the plasmaspause will allow the local acceleration process to be operative down to L = 2 [Horne et al., 2005; Shprits et al., 2006]. On the basis of the observational facts and numerical results, Horne et al. [2005] and Shprits et al. [2006] showed that the formation of new radiation belt in the slot region during the recovery phase of the Halloween storm (from 1 November 2003 onward) cannot be attributed to radial diffusion, instead the energy diffusion driven by whistler mode waves is sufficient to explain the enhancement of the electron flux and the new belt with the observed timescale. Using groundbased magnetometer data of ULF waves and mapping the observed ULF wave power on the ground to the equatorial region using a guided Alfvén model, Loto aniu et al. [2006] showed that the relativistic flux increases seen in the slot region on 29 October 2003 (during the main phase of the storm) can be accounted for by the radial diffusion caused by the ULF wave drift-resonant mechanism. It should be noted that the studies mentioned above that emphasize one mechanism over the other are not opposed to each other; rather, they were looking at different stages of the storm. The dominance of two acceleration mechanisms can vary according to different storms or different phases of a storm. [30] Our statistical study on the slot region dynamics does not aim to distinguish the relative importance of different acceleration mechanisms. Rather it reveals some common characteristics of the slot region filling. The 2 3 day delay between the Dst min and the flux peak in the slot region shows that during intense storms (Dst min < 130 nt) it takes 2 3 days on average after Dst min for relativistic electrons to penetrate to the slot region regardless of physical processes (including losses too) involved. Our study and existing literature [e.g., Goldstein et al., 2005; Li et al., 2006] show that both the ring current and plasmasphere play very important roles in the slot region filling. The effects of the ring current and plasmasphere on the penetration of radiation belt electrons to the slot region are inseparable under the influences of the solar wind. Intense storms (a strong ring current) not only distort the magnetic field configuration in the inner magnetosphere, but also cause compression of the plasmasphere and foster the growth of various types of waves near the plasmapause, which can facilitate the penetration of the relativistic electrons to the slot region. [31] Although the 2 6 MeV electron flux in the inner belt displays fewer variations than that of the outer belt region, it waxes and wanes according to different solar wind and magnetospheric conditions. The interesting four cycle oscillations per year during of the inner flux are worth noting. Similar oscillations in solar wind dynamic pressure were also found. However, more data during similar solar activities are needed to further confirm this. [32] The dependence of inner belt flux on both the intensity of the magnetic storms and the solar wind speed indicates that even though the electron inner belt is often disconnected from the electron outer belt by the slot region, it shares some common characteristics with the outer belt. The relationship between the inner belt flux enhancement and the solar wind density n SW can find some similarities with the geosynchronous relativistic electron flux behavior. Interestingly, Lyons et al. [2005] reported that the flux increases at geosynchronous orbit appear to not begin until n SW drops below 5 cm 3, even if the solar wind speed increases before that time. However, the mechanisms for the anticorrealtion may not be the same. [33] As the slot region is often used as a favorable location for low Earth orbit (LEO) and medium Earth orbit (MEO) satellite operations, understanding of the dynamic evolution of this region is critical practically in terms of satellite safety. Studies of the slot region and the inner belt are also important for understanding the physics of the coupling of three important regions in the inner magnetosphere (the plasmasphere, ring current and radiation belt) and for facilitating the development of a better forecasting model of the radiation belt. It is found that the NASA AE8 radiation belt models predict slot fluxes that are orders of magnitude less than that observed by TSX5/CEASE [Brautigam et al., 2004]. As part of our future work, we plan to study the relativistic electron penetration using a kinetic model of the radiation belt [Fok et al., 2001; Zheng et al., 2003] by comparing model results with 9of11

10 observations to improve both our understanding and modeling capabilities of the radiation belt. 4. Summary [34] We investigated the penetration of the 2 6 MeV electrons to the slot region and their dynamic behavior of the inner belt using data taken during from the SAMPEX satellite. Summaries of our results are as follows: [35] 1. The electron penetration usually occurs in association with intense magnetic storms. [36] 2. The penetration distance DL has a good correlation with Dst min with a lag time of 3 days, but it has no or little correlation with any individual the solar wind parameter. [37] 3. The penetration distance DL usually reaches its maximum about 3 days (on average) after the Dst min of an intense storm. [38] 4. A superposed epoch analysis of those very intense storms (Dst min 130) shows that their occurrence corresponds to extreme solar wind conditions, such as large IMF jbj, very large negative IMF B z, large solar wind speed, density, dynamic pressure, etc. (see Figure 7). [39] 5. During solar minimum years ( ), the inner belt 2 6 MeV electrons exhibit a 4-cycle oscillation per year. A similar oscillation is also found in the solar wind dynamic pressure. No obvious oscillation is observed during the solar maximum years (such as ). [40] 6. The inner belt electron flux enhancement depends both on a high solar wind speed and a large negative Dst averaged over an interval. The flux enhancement is found to be coincident with low solar wind density, high Kp and AE values. Either a couple of very extreme storms or recurrent intense storms will create conditions favorable for the MeV electron enhancement in the inner belt. 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