- Email: [email protected]
The Global Significance of CEP Events
THE GLOBAL SIGNIFICANCE
OF C E P E V E N T S
Jiasheng Chen and Theodore A. Fritz 1
1Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
ABSTRACT Recently, a new magnetospheric phenomenon,CEPevent has been discovered. This discovery has created a great avenue for the Sun-Earth connection investigations. CEP events were detected in the high-altitude cusp for hours at a time, and each of the events is associated with a dramatic decrease and large fluctuations in the local magnetic field strength. Associated with these cavities are ions with energies from 40 keV up to 8 MeV that are more typical of the trapped ring current and radiation belt populations than the solar wind. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers and these cusp particles will form an energetic particle layer on the magnetopause. The intensities of the energetic ions were observed to increase by as large as four orders of the magnitudes during the cusp crossings, indicating the dayside high-altitude cusp is a key region for transferring the solar wind energy, mass, and momentum into the Earth's magnetosphere. The charge state distribution of these cusp ions is indicative of their seed populations being a mixture of the ionospheric and the solar wind particles. These CEP events together with the cusp's connectivity have significant global impacts on the geospace environment research and have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the ring current and in the upstream ion events. New observational facts demonstrate that these cusp energetic ions drift into the nightside plasma sheet along closed field lines, populate the dayside magnetopause boundary layer along closed/open field lines, and escape into the upstream and downstream regions along open field lines. INTRODUCTION The origin of the energetic particles in the nightside plasma sheet, in the ring current, and in the upstream ion events has been a long-standing and significant problem since the discovery of the Earth's radiation belt by Van Allen and his colleagues in 1958 (Van Allen and Frank, 1959; Yoshida et al., 1960), and how the much lower energy solar wind ions are energized locally in geospace has remained unkown. These energized ions control the Earth's ring current and probably contribute to most of its other current system. Understanding how the geospace environment is affected locally by the Sun- to specify and forecast space weather is the main goal of the space weather program. Without this basic understanding, no valid models can be developed. Recently, a new magnetospheric phenomenon called the Cusp Energetic Particle (CEP) event was discovered (Chen et al, 1997, 1998; Fritz et al., 1999a). This phenomenon has been reported to NASA Headquarters as one of the most important discoveries (http://www-istp.gsfc.nasa.gov/istp/polar/2001jan.html) of the POLAR project. About 93% of the POLAR cusp crossings are CEP events, indicating that the occurrence of the CEP events is a common feature of the cusp (Fritz et al., 1999b). These CEP events have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the magnetosphere and in the upstream ion events, and they may hold the key for understanding how the solar wind energy, mass, and momentum transfer into the Earth's magnetosphere.
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J.S. Chen and Th. A. Fritz
HIGH-ALTITUDE CUSP: THE MOST DYNAMIC REGION IN GEOSPACE The cusp, by definition, is near zero magnetic field magnitude and a funnel-shaped volume between magnetic field lines that map to the dayside and nightside of the magnetopause surface. Due to reconnections of the geomagnetic field with the interplanetary magnetic field (IMF) as well as solar wind pressure, the cusps become open funnels for direct entry of magnetosheath plasma into the magnetosphere (e.g., Reiff et al., 1977; Crooker, 1979; Meng, 1982; Newell and Meng, 1987). In the traditional view, the cusp was only a sink, and no significant energetic ion intensities were expected to be observed there. Observationally, the cusp regions are identified by a combination of low magnetic field strength and high plasma intensity (Fung et al., 1997; Chen et al., 1998). If ions with energy above 40 keV present in the cusp, it is called a CEP event. As an example, the bottom two panels of Figure 1 shows what the cusp looks like from measured magnetic field and lower energy ion data, where the data were obtained from the POLAR spacecraft. These two panels suggest that POLAR was in the cusp from 12 to 18:18 UT on 4/20/99. Figure 1 shows that the cusp magnetic field (CMF) may feature strong diamagnetic cavities with large fluctuations (bottom panel) and the 55-300 keV/e He ++ flux may increase (dotted line in top panel). Middle panel is the plot of the time profiles of the lower energy 0 >+3 (1-10 keV/e) and He ++ (1-18 keV/e). The cusp helium time profiles at higher energies (55-300 keV/e) are different from that at lower energies (1-18 keV/e), and 1 keV/e and 55 keV/e ions occupy distinct and overlapping regions of the cusp. Of particular interest is the top panel, in which an unexpected energetic 0 <+2 (70-200 keV/e) population (solid line) was observed in the high-altitude dayside cusp. IOO 7
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Fig. 1. CEP events on April 20, 1999. The panels show the variation of the 70-200 keV/e 0 -<+2 (solid line) and the 55-300 keV/e He++ (dotted line) fluxes (top panel), the 1-10 keV/e 0 >+3 (solid line) and 1-18 keV/e He++ (dotted line) fluxes (middle), and the magnetic field (bottom) versus time, respectively. The distance of POLAR from the Earth (in RE), the magnetic latitude (MLAT), and the magnetic local time (MLT) are shown at the bottom.
The ion charge states can be used to determine the seed populations of the cusp energetic ions. Since the 0 <+2 ions are of ionospheric origin and the 0 >+3 ions are of solar origin (Gloeckler et al., 1986), Figure 1 reveals that the seed populations of the high-altitude cusp ions were a mixture of the ionospheric and the solar particles. The typical energies for both the solar wind/magnetosheath ions and ionospheric ions are much
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The Global Significance of the CEP Events less than 40 keV, the observed cusp energetic ions should be energized somewhere within the magnetosphere. In fact, the energetic ions could be energized in the cusp diamagnetic cavities by the resonate interactions of these ions with the turbulent ultra-low frequency (ULF) electromagnetic power (Chen and Fritz, 1998). The observation of the energetic 0 <+2 ions of ionospheric origin in the CEP event can be easily understood from cusp acceleration; i.e., lower energy ionospheric oxygen ions are transported via the cusp ion fountain into the cusp and are energized there. Figure 1 shows the distance from the Earth (in RE), the magnetic latitude (MLAT), and the magnetic local time (MLT) of POLAR. It indicates that this cusp has a size of about 6 RE, much larger than expected. Figure 2 is the time-intensity profiles of the 20-200 keV electrons (top panel) and the 0.52-1.15 MeV helium (middle panel) as well as the associated CMF strength (bottom panel) during the 9/18/96 CEP event period. The figure indicates that energetic electrons also present in the high-altitude cusp. However, (1) the energetic helium ion intensity increased by about three orders of magnitudes above background, while the energetic electron intensity increased by only about a factor of 10, and (2) the number of the electron CEP events is less than the number of ion CEP events. These two features are expected by the resonant acceleration mechanism since the proton gyro-frequency in the CEP events has a value of about 1 Hz, while electron gyro-frequency in the CEP events is about 1000 Hz. Chen et al. (1998) reported that the power density of the magnetic plasma wave at 1000 Hz is more than two orders of magnitudes lower than that at 6 Hz. 3.5
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Figure 3 displays a measured CEP helium energy spectrum (solid circles) at 8:30-8:48 UT on 8/27/96. For comparison, a Maxwellian distribution curve peaked at 1 keV/e is also plotted in the figure. This Maxwellian curve represents approximately the thermalized solar wind plasma energy distribution with a typical energy of 1 keV/e. The figure shows that the helium ions in this CEP event have energy up to 4 MeV/e (or 8 MeV) and that.the higher the helium energy the larger the difference of the helium flux from the Maxwellian distribution.
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J.S. Chen and Th. A. Fritz
Around solar minimum from POLAR launch through the end of 1997, there were about 300 cusp crossings, in which 279 or 93% of the crossings were identified as CEP events (Fritz et al., 1999b). In April 1999 when closer to solar maximum, 31 cusp crossings were identified as CEP events (even though there was a 4 day data gap for 4/5/99-4/8/99). (One of these was shown in Fig. 1). Therefore, the occurrence of CEP events is a common feature in the cusp. The significant increase of the energetic ion fluxes and the large turbulence in the cusp diamagnetic cavities suggest that the high-altitude cusp is the most dynamic region in geospace. 10 6 7 q)
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GLOBAL SIGNIFICANCE Because the cusp magnetic field lines are connected to almost everywhere in geospace, the cusp energetic particles as a source have great impacts on the geospace environment when they move away from the cusp along the field lines. Energetic Particles in Nightside Plasma Sheet One well-known character of the energetic charged particles in the nightside plasma sheet is the butterfly pitch angle distribution (PAD) (West, 1965; Serlemitsos, 1966; Pfitzer et al., 1969; Haskell, 1969). Charged particles differing in pitch angles follow different drift paths due to drift-shell splitting as they drift around from the nightside to the dayside magnetosphere. Conservation of the first and second adiabatic invariants causes equatorially mirroring particles to drift on contours of constant magnetic field. Charged particles with small pitch angles, which mirror away from the equator, drift on more or less circular drift paths. On the other hand, at L > 7, quatorially mirroring particles are preferentially lost to the magnetopause on open drift paths as the contours of constant geomagnetic field are further out on the dayside than on the nightside, due to the day-night asymmetry. A minimum in the observed PAD develops for equatorially mirroring particles beyond L - 7. This is known as a butterfly distribution. Figure 2 is typical for other CEP events. The three facts that have been observed in the CEP events are:
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The Global Significance of the CEP Events
(1) extremely high ion fluxes from 1 keV up to 8 MeV are observed around the cusp, (2) the enhancement of the energetic electron fluxes are much less than that of energetic ion fluxes, and (3) the number of the electron CEP events is less than the number of the ion CEP events. When these energetic charged particles drift out the cusp along close field lines into the nightside on drift paths, they have significant impact on the nightside plasma sheet. Specifically, the cusp as an active source of the magnetospheric energetic particles has some very clear predictions about particle butterfly PADs; i.e., it predicts that (1) for the same energy range the pitch angle anisotropy for the butterfly PADs should greater in the case of the electrons than in the case of the protons, (2) for the same butterfly event the protons should be less anisotropic than the electrons, (3) the butterfly PADs should be less anisotropic in the lower energy channels than in the higher energy channels, and (4) the butterfly PADs should be more frequent in the electrons than in the protons over the entire nightside. Recent studies involving measurements of 22.5-1200 keV electrons and 24-2081 keV protons made by the ISEE satellites near the equatorial plane on the nightside of the magnetosphere have provided new evidence supporting the above four predictions (Fritz et al., 2000). Energetic Particles in Ourter Radiation Belt
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Figure 4 compares the proton fluxes measured in the two different regions over the energy range of 1 keV to 2 MeV. In Figure 4, the solid circles represent the proton flux observed by the Applications Technology Satellite (ATS-6) (Fritz and Cessna, 1975) at L = 6.6 near midnight for equatorially mirroring particles during a substorm period on July 20, 1974 (Fritz et al., 1977), and the open symbols (triangles and squares) represent the proton flux from a combination of two POLAR sensors during the CEP event period on 10/14/96. This is the same ATS-6 spectrum that was used by Spjeldvik and Fritz (1978) as their source spectrum in their radial diffusion modeling. It is noticed that the two fluxes were measured at different periods with year 1996 being solar minimum and year 1974 being two years before solar minimum which was expected to have higher flux than that in the solar minimum. Another point is that the substorm may increase the proton flux at energy less than 200 keV. The interesting features are: (1) at energy < 4 keV, the proton fluxes in both regions are comparable; (2) at energy within 4-150 keV, radiation belt has higher
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J.S. Chen and Th. A. Fritz
flux than that of the CEP event, which may be due to longer trapping time in the belt; and (3) at energy > 200 keV, the CEP event has higher flux. When the charged particles start in the cusp they have almost complete access to the equatorial plasma sheet and outer magnetosphere. Energetic Particles in Ring Current One possibility for producing the energetic ion fluxes in the ring current is the dipolarization process during substorms (Lezniak and Winckler, 1970; Quinn and Southwood, 1982; Aggson et al., 1983). Those particles may reach drift paths that are connected to the polar cusp through the Shabansky orbit due to the field minimum in the cusp geometry (Mead, 1964; Shabansky, 1971; Antonova and Shabansky, 1975; Daly and Fritz, 1982). However, Delcourt and Sauvaud (1999) showed that such an orbit exists only for a narrow range of radial distances when the charged particles are started from the equatorial plane in the geomagnetic tail around 7 RE and will start to drift northward from equator to the cusp at about 16 hours MLT. At closer distances the particles execute normal drift motion confined to the equatorial plane and show higher flux because of longer trapping time; at larger distances these particles reach the dusk flank in the equatorial plane and are lost to the interplanetary medium. A higher ion phase space density is a necessary condition for the source region, and a lack of such a maximum does establish that the proposed region cannot be the source of higher fluxes observed in other regions. Since the ions drift westward at the equatorial plane, higher flux is expected in the duskside than in the dawnside if nightside substorm is the source. .
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From 7:30-16:00 UT on 8/27/96, POLAR was in an inbound orbit. POLAR was in the ring current near the equatorial plane at about 16 hours MLT at 14:45-15:30 UT with L -- 4.92-7.27, and was in the cusp at 8:00-10:36 UT (Chen and Fritz, 2001). The oxygen ions measured in the ring current can be compared with that measured in the cusp at a different time when the ion fluxes in the ring current at the two different time periods are similar. On 8/27/96, the Dst index had values of-7, -1, -5 nT at 8, 15, and 16 UT, respectively, and had an averaged value of about -5 nT at 8-16 UT, which suggests no geomagnetic storm activity during the time. Since the Dst is used for monitoring ring current variations, the flat Dst values suggest no large changes of the ring current. The 50-400 keV ion fluxes, measured by both the LANL1991_080 and the LANL1994_084 geostationary satellites, varied by a factor of less than 2 during 8-16 UT, consistent with the Dst measurements. Figure 5 compares the magnetic moment spectra of the energetic 0 <+2 at the cusp (solid circles) with that in the ring current (open squares). The 0 <+2 ion fluxes were dominated by singly ionized
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The Global Significance of the CEP Events oxygen ions. Both magnetic moment spectra of the energetic 0 <+2 ions are fitted to a power-law form, Flux=A(M/Mo) -~, where M is the magnetic moment, M0 = 1 keV/e/nT, A is the spectral amplitude, and 7 is the spectral index. The resulting spectral amplitude and index are 58.6 particles/cm2-sr-s-keV/e and 3.4 for the cusp 0 <+2 ions and are 0.55 particles/cm2-sr-s-keV/e and 3.8 for the ring current ions. The two solid lines in Figure 5 are the least-squares fit to this form. The spectral amplitude is, in fact, the energetic 0 <+2 ion flux at M0. The amplitude ratio of the cusp to the ring current is about 106.5. When the cusp energetic ions drift away from the cusp along the closed field lines, some of them can enter the ring current. Energetic Particles in Upstream Ion Events On September 11, 1996 at 22:06-22:30 UT, GEOTAIL was under normal IMF condition and was in the foreshock region where a harder energy spectrum was expected (Anagnostopoulos et al., 1986; Paschalidis et al., 1994), and POLAR was in the cusp. Figure 6 compares the > 60 keV ion flux observed by the GEOTAIL to that observed by the POLAR during this period. It shows that the ion flux in the CEP event at 100 keV was more than four orders of magnitudes higher than that in the foreshock region and that the shape of the ion energy spectra in the CEP events was different from that in the upstream from the bow shock.
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Another example is the 5/4/98 ion event (Chen and Fritz, 1999). At about 11:00 UT, the WIND spacecraft was close to the forward libration point, about 213 RE upstream from the Earth. Figure 7 shows the locations of three other spacecraft (INTERBALL and POLAR at 11:00-11:45 UT, and GEOTAIL at 11:0011:24 UT) on May 4, 1998 relative to the magnetopause and the bow shock in geocentric solar ecliptic (GSE) coordinates. The magnetopause is obtained from the model of Shue et al. (1998) and the bow shock is from the model of Formisano (1979). The directions of the measured magnetic fields associated with each individual spacecraft are plotted as arrows of arbitary length beginning at that spacecraft. The left panel (X-Y plane) indicates a positive IMF By-component at the time, which would move the cusp duskward; the right panel (X-Z plane) indicates a negative IMF Bz-component that would move the cusp equatorward. POLAR was just above the cusp, INTERBALL was upstream from and connected magnetically to the quasiparallel bow shock, and GEOTAIL was in the equatorial plane in the post dusk magnetosheath. These three
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J.S. Chen and Th. A. Fritz
satellites were just in the right locations that connected magnetically to the possible source regions. This unique multi-spacecraft data set provided a first-ever test of various sources in an energetic ion event. If ring current leakage was the dominant source mechanism, GEOTAIL should observe the highest energetic ion flux, and it should also observe the shocked solar wind flow due to positive By. If the quasi-parallel bow shock was the dominant source mechanism, the INTERBALL should observe the highest ion flux since it was directly connected to the quasi-parallel bow shock by the IMF and was very close to the bow shock (Fig. 7). If the cusp was the dominant source mechanism, the P O L A R should observe the highest ion flux because it was just above the cusp along magnetic field lines connecting to the cusp. ,5/4/98
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The simultaneous ion observations by the four spacecraft (WIND, dashed line; GEOTAIL, dotted line; POLAR, solid line; and INTERBALL, thick dot-line) over an energy range of ~ 100-400 keV, are compared in Figure 8 during 10:48-12:30 UT on May 4, 1998. P O L A R observed an increase of energetic ion flux at about 10:51 UT. There was an ion data gap for INTERBALL before 10:58 UT. From 10:58 to 12:00 UT, INTERBALL looked at t h e - X direction and observed an upstream ion event, in which the 116-416 keV ion flux was two orders of magnitudes higher than that measured by WIND. During the interval 10:58-11:45 UT, POLAR looked along the local magnetic field direction and observed the highest ion flux which was one order of magnitude larger t h a n that measured by both GEOTAIL and INTERBALL. From 10:59 to 11:24 UT, both GEOTAIL and INTERBALL observed similar ion intensities to within a factor of two, and the ion energy spectra, measured by these three spacecraft, had a similar spectral shape over 40-1500 keV. With a closer inspection on Figure 8, one finds that both POLAR and I N T E R B A L L observed similar ion flux variations, suggesting a common energetic ion source. Furthermore, I N T E R B A L L responded to the ion variations later than POLAR; that is, POLAR detected the peak and the valley (minimum) of the 120-420 keV ion flux before INTERBALL did, which suggests that P O L A R was closer to the energetic ion source region than INTERBALL. Further calculations indicate that the energetic ion flux measured by INTERBALL was independent of whether the bow shock was quasi-parallel or quasi-perpendicular. The facts that POLAR observed the highest energetic ion intensities and responded to the energetic ion variations earlier argue strongly that the main source for these energetic ions was neither the bow shock nor the leakage from the ring current, but most likely in the cusp. Some of the cusp energetic ions may escape into the solar
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The Global Significance of the CEP Events
wind through open field lines when the cusp diamagnetic cavities collapse.
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SUMMARY AND CONCLUSIONS CEP (cusp energetic particle) event,as a new magnetospheric phenomenon, has been was detected in the high-altitude cusp for hours at a time, and is associated with a dramatic decrease and large fluctuations in the local magnetic field strength. The cusp ions have energies from 40 keV up to 8 MeV, more typical of the ring current than the solar wind. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers and these cusp particles will form an energetic particle layer on the magnetopause. The intensities of the energetic ions were observed to increase by as large as four orders of the magnitudes during the cusp crossings, indicating the dayside high-altitude cusp is a key region for transferring the solar wind energy, mass, and momentum into the Earth's magnetosphere. The charge state distribution of these cusp ions was indicative of their seed populations being a mixture of the ionospheric and the solar wind particles. The multi-spacecraft observations during the May 4, 1998 ion event clearly demonstrates that the energetic ions (40 keV to MeV) in the magnetosheath and upstream of the bow shock were not accelerated at the bow shock but were most probably from the cusp. These CEP events together with the cusp's connectivity have significant global impacts on the geospace environment research and have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the ring current and in the upstream ion events. New observational facts demonstrate that these cusp energetic ions drift into nightside plasma sheet along closed field lines, populate the dayside magnetopause boundary layer along closed/open field lines, and escape into the upstream and downstream regions along open field lines. A CKN OWLED G EM ENTS We thank S. P. Christon, T. E. Eastman, J. Fennell, P. T. Newell, M. Schultz, R. B. Sheldon, D. Sibeck, G. Siscoe, H. E. Spence, and J. D. Sullivan for useful discussions. We are grateful to C. T. Russell for providing
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J.S. Chen and Th. A. Fritz us thePOLAR magnetic field data, D. J. Williams and R. McEntire for the GEOTAIL ion data, and K. Kudela for the INTERBALL energetic ion data. This research was supported by NASA grants NAG5-2578, NAG5-7677, NAG5-7841, and NAG5-9562. REFERENCES Aggson, T. L., J. P. Heppner, and N. C. Maynard, Observations of large magnetospheric electric fields during the onset phase of a substorm, J. Geophys. Res., 88, 3981-3990, 1983. Anagnostopoulos, G. C., Sarris, E. T., Krimigis, S. M., Magnetospheric origin of energetic (E > 50 keV) ions upstream of the bow shock: The October 31, 1977, event. J. Geophys. Res., 91, 3020-3028, 1986. Antonova, A. E., and V. P. Shabansky, Particle and magnetic field in the outer dayside geomagnetosphere, Geomangn. Aeron., 15, 2, 297-302, 1975. Chen, J., and T. A. Fritz, Correlation of cusp MeV helium with turbulent ULF power spectra and its implications. Geophys. Res. Lett., 25, 4113-4116, 1998. Chen, J., and T. A. Fritz, May 4, 1998 storm: observations of energetic ion composition by POLAR. Geophys. Res. Lett., 26, 2921-2924, 1999. Chen, J., and T. A. Fritz, Energetic oxygen ions of ionospheric origin observed in the cusp, Geophys. Res. Lett., 28, 1459, 2001. Chen, J., T. A. Fritz, R. B. Sheldonl H. E. Spence, W. N. Spjeldvik, et al., A New, Temporarily Confined Population in the Polar Cap During the August 27, 1996 Geomagnetic Field Distortion Period. Geophys. Res. Lett., 24, 1447-1450, 1997. Chen, J., T. A. Fritz, R. B. Sheldon, H. E. Spence, W. N. Spjeldvik, et al., Cusp Energetic Particle Events: Implications for a Major Acceleration Region of the Magnetosphere. J. Geophys. Res., 103, 69-78, 1998. Crooker, N. U., Dayside merging and cusp geometry, J. Geophys. Res., 84, 951-959, 1979. Daly, P. W., and T. A. Fritz, Trapped electron distributions on open field lines, J. Geophys. Res., 87, 6081, 1982. Delcourt, D. C., and J.-A. Sauvaud, Populating of the cusp and boundary layers by energetic (hundreds of keV) equatorial particles, J. Geophys. Res., 104, 22635, 1999. Formisano, V., Orientation and shape of the Earth's bow shock in three dimensions, Planet. Space Sci., 27, 1151-1161, 1979. Fritz, T. A., and J. R. Cessna, ATS-6 NOAA low energy proton experiment, IEEE Trans. Aerospace Electron. Sys., AES-11, (6), 1145, 1975. Fritz, T. A., J. Chen, and R. B. Sheldon, The role of the cusp as a source for magnetospheric particles: A new paradigm? Adv. Space Res., 25, No. 7-8, 1445-1457, 2000. Fritz, T. A. et al., Significant initial results from the environmental measurements experiment on ATS-6, NASA Technical Paper 1101, NASA Scientific and Technical Information Office, Washington, DC, 34 p. 1977. Fritz, T. A., J. Chen, R. B. Sheldon, H..E. Spence, J. F. Fennell, S. Livi, C. T. Russell, and J. S. Pickett, Cusp energetic particle events measured by POLAR spacecraft, Phys. Chain. Earth (C), 24, No. 1-3, 135-140, 1999a. Fritz, T. A., M. Karra, B. Finkemeyer, and J. Chen, Statistical studies with Polar of energetic particles near the dayside cusp, EOS Trans. AGU, 80, No. 46, F862, 1999b. Fung, S. F., T. E. Eastman, S. A. Boardsen, and S.-H. Chen, High-altitude cusp positions sampled by the Hawkeye satellite, Phys. Chem. Earth, 22, No. 7-8, 653, 1997. Gloeckler, G. et al., Solar Wind Carbon, Nitrogen and Oxygen Abundances Measured in the Earth's Magnetosheath with AMPTE/CCE. Geophys. Res. Lett., 13, 793-796, 1986. Haskell, G. P., Anisotropic fluxes of energetic particles in the outer magnetosphere, J. Geophys. Res., 74, 1740, 1969. Lezniak, T. W., and J. R. Winckler, Experimental study of magnetospheric motion and the acceleration of energetic electrons during substorms, J. Geophys. Res., 75, 7075, 1970. Mead, G. D., Deformation of geomagnetic field by the solar wind, J. Geophys. Res., 69, 1181-1195, 1964. Meng, C.-I., Latitudinal variation of the polar cusp during a geomagnetic storm, Geophys. Res. Lett., 9,
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The Global Significance of the CEP Events 60, 1982. Newell, P. T., and C.-I. Meng, Cusp width and Bz: Observations and conceptual model, J. Geophys. Res., 92, 13,673-13,678, 1987. Paschalidis, N. P., E. T. Sarris, S. M. Krimigis, R. W. McEntire, M. D. Levine, I. A.. Daglis, and G. C. Anagnostopoulos, Energetic ion distributions on both sides of the Earth's magnetopause, J. Geophys. Res., 99, 8687-8703, 1994. Pfitzer, K., T. W. Lezniak, and J. R. Winckler, Experimental verification of drift shell splitting in the distorted magnetosphere, J. Geophys. Res., 74, 4687, 1969. Quinn, J. M., and D. J. Southwood, Observation of parallel ion energization in the equatorial region, J. Geophys. Res., 87, 10536, 1982. Reiff, P. H., T. W. Hill, and J. L. Burch, Solar wind plasma injection at the dayside magnetospheric cusp, J. Geophys. Res., 82, 479, 1977. Serlemitsos, P., Low energy electrons in the dark magnetosphere, J. Geophys. Res., 71, 61-77, 1966. Shabansky, V. P., Some processes in the magnetosphere, Space Sci. Rev., 12, 299-418, 1971. Spjeldvik, W. N. and T. A. Fritz, Composition of hot plasmas in the inner magnetosphere: Observations and theoretical analysis of protons, helium ions, and oxygen ions, In Space Research VIII, ed. by M. J. Rycroft and A. C. Strickland, Oxford, Eng., Pergamon Press, 317, 1978. Van Allen, J. A., and L. A. Frank, Radiation around the Earth to a radial distance of 107,400 km, Nature, 183, 430, 1959. West, H. I., Jr., Some observations of the trapped electrons produced by Russian high altitude nuclear detonation of October 28, 1962, in Radiation Trapped in the Earth's Magnetic Field, edited by B. M. McCormac, 634, D. Reidel, Dordrecht, 1965. Yoshida, S., G. H. Ludwig, and J. A. Van Allen, Distribution of trapped radiation in the geomagnetic field, J. Geophys. Res., 65, 807, 1960. Shue, J.-H. et al., Magnetopause location under extreme solar wind conditions, J. Geophys. Res., 103, 17691, 1998.
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OF C E P E V E N T S
Jiasheng Chen and Theodore A. Fritz 1
1Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
ABSTRACT Recently, a new magnetospheric phenomenon,CEPevent has been discovered. This discovery has created a great avenue for the Sun-Earth connection investigations. CEP events were detected in the high-altitude cusp for hours at a time, and each of the events is associated with a dramatic decrease and large fluctuations in the local magnetic field strength. Associated with these cavities are ions with energies from 40 keV up to 8 MeV that are more typical of the trapped ring current and radiation belt populations than the solar wind. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers and these cusp particles will form an energetic particle layer on the magnetopause. The intensities of the energetic ions were observed to increase by as large as four orders of the magnitudes during the cusp crossings, indicating the dayside high-altitude cusp is a key region for transferring the solar wind energy, mass, and momentum into the Earth's magnetosphere. The charge state distribution of these cusp ions is indicative of their seed populations being a mixture of the ionospheric and the solar wind particles. These CEP events together with the cusp's connectivity have significant global impacts on the geospace environment research and have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the ring current and in the upstream ion events. New observational facts demonstrate that these cusp energetic ions drift into the nightside plasma sheet along closed field lines, populate the dayside magnetopause boundary layer along closed/open field lines, and escape into the upstream and downstream regions along open field lines. INTRODUCTION The origin of the energetic particles in the nightside plasma sheet, in the ring current, and in the upstream ion events has been a long-standing and significant problem since the discovery of the Earth's radiation belt by Van Allen and his colleagues in 1958 (Van Allen and Frank, 1959; Yoshida et al., 1960), and how the much lower energy solar wind ions are energized locally in geospace has remained unkown. These energized ions control the Earth's ring current and probably contribute to most of its other current system. Understanding how the geospace environment is affected locally by the Sun- to specify and forecast space weather is the main goal of the space weather program. Without this basic understanding, no valid models can be developed. Recently, a new magnetospheric phenomenon called the Cusp Energetic Particle (CEP) event was discovered (Chen et al, 1997, 1998; Fritz et al., 1999a). This phenomenon has been reported to NASA Headquarters as one of the most important discoveries (http://www-istp.gsfc.nasa.gov/istp/polar/2001jan.html) of the POLAR project. About 93% of the POLAR cusp crossings are CEP events, indicating that the occurrence of the CEP events is a common feature of the cusp (Fritz et al., 1999b). These CEP events have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the magnetosphere and in the upstream ion events, and they may hold the key for understanding how the solar wind energy, mass, and momentum transfer into the Earth's magnetosphere.
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J.S. Chen and Th. A. Fritz
HIGH-ALTITUDE CUSP: THE MOST DYNAMIC REGION IN GEOSPACE The cusp, by definition, is near zero magnetic field magnitude and a funnel-shaped volume between magnetic field lines that map to the dayside and nightside of the magnetopause surface. Due to reconnections of the geomagnetic field with the interplanetary magnetic field (IMF) as well as solar wind pressure, the cusps become open funnels for direct entry of magnetosheath plasma into the magnetosphere (e.g., Reiff et al., 1977; Crooker, 1979; Meng, 1982; Newell and Meng, 1987). In the traditional view, the cusp was only a sink, and no significant energetic ion intensities were expected to be observed there. Observationally, the cusp regions are identified by a combination of low magnetic field strength and high plasma intensity (Fung et al., 1997; Chen et al., 1998). If ions with energy above 40 keV present in the cusp, it is called a CEP event. As an example, the bottom two panels of Figure 1 shows what the cusp looks like from measured magnetic field and lower energy ion data, where the data were obtained from the POLAR spacecraft. These two panels suggest that POLAR was in the cusp from 12 to 18:18 UT on 4/20/99. Figure 1 shows that the cusp magnetic field (CMF) may feature strong diamagnetic cavities with large fluctuations (bottom panel) and the 55-300 keV/e He ++ flux may increase (dotted line in top panel). Middle panel is the plot of the time profiles of the lower energy 0 >+3 (1-10 keV/e) and He ++ (1-18 keV/e). The cusp helium time profiles at higher energies (55-300 keV/e) are different from that at lower energies (1-18 keV/e), and 1 keV/e and 55 keV/e ions occupy distinct and overlapping regions of the cusp. Of particular interest is the top panel, in which an unexpected energetic 0 <+2 (70-200 keV/e) population (solid line) was observed in the high-altitude dayside cusp. IOO 7
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The ion charge states can be used to determine the seed populations of the cusp energetic ions. Since the 0 <+2 ions are of ionospheric origin and the 0 >+3 ions are of solar origin (Gloeckler et al., 1986), Figure 1 reveals that the seed populations of the high-altitude cusp ions were a mixture of the ionospheric and the solar particles. The typical energies for both the solar wind/magnetosheath ions and ionospheric ions are much
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The Global Significance of the CEP Events less than 40 keV, the observed cusp energetic ions should be energized somewhere within the magnetosphere. In fact, the energetic ions could be energized in the cusp diamagnetic cavities by the resonate interactions of these ions with the turbulent ultra-low frequency (ULF) electromagnetic power (Chen and Fritz, 1998). The observation of the energetic 0 <+2 ions of ionospheric origin in the CEP event can be easily understood from cusp acceleration; i.e., lower energy ionospheric oxygen ions are transported via the cusp ion fountain into the cusp and are energized there. Figure 1 shows the distance from the Earth (in RE), the magnetic latitude (MLAT), and the magnetic local time (MLT) of POLAR. It indicates that this cusp has a size of about 6 RE, much larger than expected. Figure 2 is the time-intensity profiles of the 20-200 keV electrons (top panel) and the 0.52-1.15 MeV helium (middle panel) as well as the associated CMF strength (bottom panel) during the 9/18/96 CEP event period. The figure indicates that energetic electrons also present in the high-altitude cusp. However, (1) the energetic helium ion intensity increased by about three orders of magnitudes above background, while the energetic electron intensity increased by only about a factor of 10, and (2) the number of the electron CEP events is less than the number of ion CEP events. These two features are expected by the resonant acceleration mechanism since the proton gyro-frequency in the CEP events has a value of about 1 Hz, while electron gyro-frequency in the CEP events is about 1000 Hz. Chen et al. (1998) reported that the power density of the magnetic plasma wave at 1000 Hz is more than two orders of magnitudes lower than that at 6 Hz. 3.5
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Figure 3 displays a measured CEP helium energy spectrum (solid circles) at 8:30-8:48 UT on 8/27/96. For comparison, a Maxwellian distribution curve peaked at 1 keV/e is also plotted in the figure. This Maxwellian curve represents approximately the thermalized solar wind plasma energy distribution with a typical energy of 1 keV/e. The figure shows that the helium ions in this CEP event have energy up to 4 MeV/e (or 8 MeV) and that.the higher the helium energy the larger the difference of the helium flux from the Maxwellian distribution.
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J.S. Chen and Th. A. Fritz
Around solar minimum from POLAR launch through the end of 1997, there were about 300 cusp crossings, in which 279 or 93% of the crossings were identified as CEP events (Fritz et al., 1999b). In April 1999 when closer to solar maximum, 31 cusp crossings were identified as CEP events (even though there was a 4 day data gap for 4/5/99-4/8/99). (One of these was shown in Fig. 1). Therefore, the occurrence of CEP events is a common feature in the cusp. The significant increase of the energetic ion fluxes and the large turbulence in the cusp diamagnetic cavities suggest that the high-altitude cusp is the most dynamic region in geospace. 10 6 7 q)
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GLOBAL SIGNIFICANCE Because the cusp magnetic field lines are connected to almost everywhere in geospace, the cusp energetic particles as a source have great impacts on the geospace environment when they move away from the cusp along the field lines. Energetic Particles in Nightside Plasma Sheet One well-known character of the energetic charged particles in the nightside plasma sheet is the butterfly pitch angle distribution (PAD) (West, 1965; Serlemitsos, 1966; Pfitzer et al., 1969; Haskell, 1969). Charged particles differing in pitch angles follow different drift paths due to drift-shell splitting as they drift around from the nightside to the dayside magnetosphere. Conservation of the first and second adiabatic invariants causes equatorially mirroring particles to drift on contours of constant magnetic field. Charged particles with small pitch angles, which mirror away from the equator, drift on more or less circular drift paths. On the other hand, at L > 7, quatorially mirroring particles are preferentially lost to the magnetopause on open drift paths as the contours of constant geomagnetic field are further out on the dayside than on the nightside, due to the day-night asymmetry. A minimum in the observed PAD develops for equatorially mirroring particles beyond L - 7. This is known as a butterfly distribution. Figure 2 is typical for other CEP events. The three facts that have been observed in the CEP events are:
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The Global Significance of the CEP Events
(1) extremely high ion fluxes from 1 keV up to 8 MeV are observed around the cusp, (2) the enhancement of the energetic electron fluxes are much less than that of energetic ion fluxes, and (3) the number of the electron CEP events is less than the number of the ion CEP events. When these energetic charged particles drift out the cusp along close field lines into the nightside on drift paths, they have significant impact on the nightside plasma sheet. Specifically, the cusp as an active source of the magnetospheric energetic particles has some very clear predictions about particle butterfly PADs; i.e., it predicts that (1) for the same energy range the pitch angle anisotropy for the butterfly PADs should greater in the case of the electrons than in the case of the protons, (2) for the same butterfly event the protons should be less anisotropic than the electrons, (3) the butterfly PADs should be less anisotropic in the lower energy channels than in the higher energy channels, and (4) the butterfly PADs should be more frequent in the electrons than in the protons over the entire nightside. Recent studies involving measurements of 22.5-1200 keV electrons and 24-2081 keV protons made by the ISEE satellites near the equatorial plane on the nightside of the magnetosphere have provided new evidence supporting the above four predictions (Fritz et al., 2000). Energetic Particles in Ourter Radiation Belt
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Figure 4 compares the proton fluxes measured in the two different regions over the energy range of 1 keV to 2 MeV. In Figure 4, the solid circles represent the proton flux observed by the Applications Technology Satellite (ATS-6) (Fritz and Cessna, 1975) at L = 6.6 near midnight for equatorially mirroring particles during a substorm period on July 20, 1974 (Fritz et al., 1977), and the open symbols (triangles and squares) represent the proton flux from a combination of two POLAR sensors during the CEP event period on 10/14/96. This is the same ATS-6 spectrum that was used by Spjeldvik and Fritz (1978) as their source spectrum in their radial diffusion modeling. It is noticed that the two fluxes were measured at different periods with year 1996 being solar minimum and year 1974 being two years before solar minimum which was expected to have higher flux than that in the solar minimum. Another point is that the substorm may increase the proton flux at energy less than 200 keV. The interesting features are: (1) at energy < 4 keV, the proton fluxes in both regions are comparable; (2) at energy within 4-150 keV, radiation belt has higher
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J.S. Chen and Th. A. Fritz
flux than that of the CEP event, which may be due to longer trapping time in the belt; and (3) at energy > 200 keV, the CEP event has higher flux. When the charged particles start in the cusp they have almost complete access to the equatorial plasma sheet and outer magnetosphere. Energetic Particles in Ring Current One possibility for producing the energetic ion fluxes in the ring current is the dipolarization process during substorms (Lezniak and Winckler, 1970; Quinn and Southwood, 1982; Aggson et al., 1983). Those particles may reach drift paths that are connected to the polar cusp through the Shabansky orbit due to the field minimum in the cusp geometry (Mead, 1964; Shabansky, 1971; Antonova and Shabansky, 1975; Daly and Fritz, 1982). However, Delcourt and Sauvaud (1999) showed that such an orbit exists only for a narrow range of radial distances when the charged particles are started from the equatorial plane in the geomagnetic tail around 7 RE and will start to drift northward from equator to the cusp at about 16 hours MLT. At closer distances the particles execute normal drift motion confined to the equatorial plane and show higher flux because of longer trapping time; at larger distances these particles reach the dusk flank in the equatorial plane and are lost to the interplanetary medium. A higher ion phase space density is a necessary condition for the source region, and a lack of such a maximum does establish that the proposed region cannot be the source of higher fluxes observed in other regions. Since the ions drift westward at the equatorial plane, higher flux is expected in the duskside than in the dawnside if nightside substorm is the source. .
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From 7:30-16:00 UT on 8/27/96, POLAR was in an inbound orbit. POLAR was in the ring current near the equatorial plane at about 16 hours MLT at 14:45-15:30 UT with L -- 4.92-7.27, and was in the cusp at 8:00-10:36 UT (Chen and Fritz, 2001). The oxygen ions measured in the ring current can be compared with that measured in the cusp at a different time when the ion fluxes in the ring current at the two different time periods are similar. On 8/27/96, the Dst index had values of-7, -1, -5 nT at 8, 15, and 16 UT, respectively, and had an averaged value of about -5 nT at 8-16 UT, which suggests no geomagnetic storm activity during the time. Since the Dst is used for monitoring ring current variations, the flat Dst values suggest no large changes of the ring current. The 50-400 keV ion fluxes, measured by both the LANL1991_080 and the LANL1994_084 geostationary satellites, varied by a factor of less than 2 during 8-16 UT, consistent with the Dst measurements. Figure 5 compares the magnetic moment spectra of the energetic 0 <+2 at the cusp (solid circles) with that in the ring current (open squares). The 0 <+2 ion fluxes were dominated by singly ionized
- 244 -
The Global Significance of the CEP Events oxygen ions. Both magnetic moment spectra of the energetic 0 <+2 ions are fitted to a power-law form, Flux=A(M/Mo) -~, where M is the magnetic moment, M0 = 1 keV/e/nT, A is the spectral amplitude, and 7 is the spectral index. The resulting spectral amplitude and index are 58.6 particles/cm2-sr-s-keV/e and 3.4 for the cusp 0 <+2 ions and are 0.55 particles/cm2-sr-s-keV/e and 3.8 for the ring current ions. The two solid lines in Figure 5 are the least-squares fit to this form. The spectral amplitude is, in fact, the energetic 0 <+2 ion flux at M0. The amplitude ratio of the cusp to the ring current is about 106.5. When the cusp energetic ions drift away from the cusp along the closed field lines, some of them can enter the ring current. Energetic Particles in Upstream Ion Events On September 11, 1996 at 22:06-22:30 UT, GEOTAIL was under normal IMF condition and was in the foreshock region where a harder energy spectrum was expected (Anagnostopoulos et al., 1986; Paschalidis et al., 1994), and POLAR was in the cusp. Figure 6 compares the > 60 keV ion flux observed by the GEOTAIL to that observed by the POLAR during this period. It shows that the ion flux in the CEP event at 100 keV was more than four orders of magnitudes higher than that in the foreshock region and that the shape of the ion energy spectra in the CEP events was different from that in the upstream from the bow shock.
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Another example is the 5/4/98 ion event (Chen and Fritz, 1999). At about 11:00 UT, the WIND spacecraft was close to the forward libration point, about 213 RE upstream from the Earth. Figure 7 shows the locations of three other spacecraft (INTERBALL and POLAR at 11:00-11:45 UT, and GEOTAIL at 11:0011:24 UT) on May 4, 1998 relative to the magnetopause and the bow shock in geocentric solar ecliptic (GSE) coordinates. The magnetopause is obtained from the model of Shue et al. (1998) and the bow shock is from the model of Formisano (1979). The directions of the measured magnetic fields associated with each individual spacecraft are plotted as arrows of arbitary length beginning at that spacecraft. The left panel (X-Y plane) indicates a positive IMF By-component at the time, which would move the cusp duskward; the right panel (X-Z plane) indicates a negative IMF Bz-component that would move the cusp equatorward. POLAR was just above the cusp, INTERBALL was upstream from and connected magnetically to the quasiparallel bow shock, and GEOTAIL was in the equatorial plane in the post dusk magnetosheath. These three
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J.S. Chen and Th. A. Fritz
satellites were just in the right locations that connected magnetically to the possible source regions. This unique multi-spacecraft data set provided a first-ever test of various sources in an energetic ion event. If ring current leakage was the dominant source mechanism, GEOTAIL should observe the highest energetic ion flux, and it should also observe the shocked solar wind flow due to positive By. If the quasi-parallel bow shock was the dominant source mechanism, the INTERBALL should observe the highest ion flux since it was directly connected to the quasi-parallel bow shock by the IMF and was very close to the bow shock (Fig. 7). If the cusp was the dominant source mechanism, the P O L A R should observe the highest ion flux because it was just above the cusp along magnetic field lines connecting to the cusp. ,5/4/98
11:00-11:45
Y(Re) [ /
UT Z(Re) ] ~
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MP
MP
-10
X(Re) t . . . . l,
,I
1. . . . I . . . .
,,I,H , , , , r .... l . . . . ! . . . . J,, ,I,L, I X(Re)
Fig. 7. Bow shock (BS), the magnetopause (MP), the orbits of GEOTAIL (at 11:00-ii:24 UT), INTERBALL and POLAR (at 11:00-11:45 UT), and the magnetic fields associated with each individual spacecraft projected into the X-Y plane (left panel) and X-Z plane (right panel) in GSE coordinates on May 4, 1998.
The simultaneous ion observations by the four spacecraft (WIND, dashed line; GEOTAIL, dotted line; POLAR, solid line; and INTERBALL, thick dot-line) over an energy range of ~ 100-400 keV, are compared in Figure 8 during 10:48-12:30 UT on May 4, 1998. P O L A R observed an increase of energetic ion flux at about 10:51 UT. There was an ion data gap for INTERBALL before 10:58 UT. From 10:58 to 12:00 UT, INTERBALL looked at t h e - X direction and observed an upstream ion event, in which the 116-416 keV ion flux was two orders of magnitudes higher than that measured by WIND. During the interval 10:58-11:45 UT, POLAR looked along the local magnetic field direction and observed the highest ion flux which was one order of magnitude larger t h a n that measured by both GEOTAIL and INTERBALL. From 10:59 to 11:24 UT, both GEOTAIL and INTERBALL observed similar ion intensities to within a factor of two, and the ion energy spectra, measured by these three spacecraft, had a similar spectral shape over 40-1500 keV. With a closer inspection on Figure 8, one finds that both POLAR and I N T E R B A L L observed similar ion flux variations, suggesting a common energetic ion source. Furthermore, I N T E R B A L L responded to the ion variations later than POLAR; that is, POLAR detected the peak and the valley (minimum) of the 120-420 keV ion flux before INTERBALL did, which suggests that P O L A R was closer to the energetic ion source region than INTERBALL. Further calculations indicate that the energetic ion flux measured by INTERBALL was independent of whether the bow shock was quasi-parallel or quasi-perpendicular. The facts that POLAR observed the highest energetic ion intensities and responded to the energetic ion variations earlier argue strongly that the main source for these energetic ions was neither the bow shock nor the leakage from the ring current, but most likely in the cusp. Some of the cusp energetic ions may escape into the solar
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The Global Significance of the CEP Events
wind through open field lines when the cusp diamagnetic cavities collapse.
. 1000t I
....
....
....
100
(~
X
D
1
-':.
10
_J b_ Z
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T h ~ : 1 1 6 - 4 t 6 keY, INTERBALL Dotted line: 1 1 0 - 3 6 3 keV, GEOTAIL Doshed line: 1 1 5 - 4 0 0 keV, WIND
1 IF 1
1 1.0
,
,
,
,
1
~
,
,
1 1.5 UT ( h o u r s )
,
I
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,
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,
,
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Fig. 8. Time profiles of the ,-., 110-400 keV ion fluxes measured by the WIND (dashed line), the GEOTAIL (dotted line), the INTERBALL (thick dot-line), and the POLAR (solid line) spacecraft during 10:48-12:30 UT on May 4, 1998.
SUMMARY AND CONCLUSIONS CEP (cusp energetic particle) event,as a new magnetospheric phenomenon, has been was detected in the high-altitude cusp for hours at a time, and is associated with a dramatic decrease and large fluctuations in the local magnetic field strength. The cusp ions have energies from 40 keV up to 8 MeV, more typical of the ring current than the solar wind. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers and these cusp particles will form an energetic particle layer on the magnetopause. The intensities of the energetic ions were observed to increase by as large as four orders of the magnitudes during the cusp crossings, indicating the dayside high-altitude cusp is a key region for transferring the solar wind energy, mass, and momentum into the Earth's magnetosphere. The charge state distribution of these cusp ions was indicative of their seed populations being a mixture of the ionospheric and the solar wind particles. The multi-spacecraft observations during the May 4, 1998 ion event clearly demonstrates that the energetic ions (40 keV to MeV) in the magnetosheath and upstream of the bow shock were not accelerated at the bow shock but were most probably from the cusp. These CEP events together with the cusp's connectivity have significant global impacts on the geospace environment research and have been shedding light on the long-standing unsolved fundamental issue about the origins of the energetic particles in the ring current and in the upstream ion events. New observational facts demonstrate that these cusp energetic ions drift into nightside plasma sheet along closed field lines, populate the dayside magnetopause boundary layer along closed/open field lines, and escape into the upstream and downstream regions along open field lines. A CKN OWLED G EM ENTS We thank S. P. Christon, T. E. Eastman, J. Fennell, P. T. Newell, M. Schultz, R. B. Sheldon, D. Sibeck, G. Siscoe, H. E. Spence, and J. D. Sullivan for useful discussions. We are grateful to C. T. Russell for providing
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J.S. Chen and Th. A. Fritz us thePOLAR magnetic field data, D. J. Williams and R. McEntire for the GEOTAIL ion data, and K. Kudela for the INTERBALL energetic ion data. This research was supported by NASA grants NAG5-2578, NAG5-7677, NAG5-7841, and NAG5-9562. REFERENCES Aggson, T. L., J. P. Heppner, and N. C. Maynard, Observations of large magnetospheric electric fields during the onset phase of a substorm, J. Geophys. Res., 88, 3981-3990, 1983. Anagnostopoulos, G. C., Sarris, E. T., Krimigis, S. M., Magnetospheric origin of energetic (E > 50 keV) ions upstream of the bow shock: The October 31, 1977, event. J. Geophys. Res., 91, 3020-3028, 1986. Antonova, A. E., and V. P. Shabansky, Particle and magnetic field in the outer dayside geomagnetosphere, Geomangn. Aeron., 15, 2, 297-302, 1975. Chen, J., and T. A. Fritz, Correlation of cusp MeV helium with turbulent ULF power spectra and its implications. Geophys. Res. Lett., 25, 4113-4116, 1998. Chen, J., and T. A. Fritz, May 4, 1998 storm: observations of energetic ion composition by POLAR. Geophys. Res. Lett., 26, 2921-2924, 1999. Chen, J., and T. A. Fritz, Energetic oxygen ions of ionospheric origin observed in the cusp, Geophys. Res. Lett., 28, 1459, 2001. Chen, J., T. A. Fritz, R. B. Sheldonl H. E. Spence, W. N. Spjeldvik, et al., A New, Temporarily Confined Population in the Polar Cap During the August 27, 1996 Geomagnetic Field Distortion Period. Geophys. Res. Lett., 24, 1447-1450, 1997. Chen, J., T. A. Fritz, R. B. Sheldon, H. E. Spence, W. N. Spjeldvik, et al., Cusp Energetic Particle Events: Implications for a Major Acceleration Region of the Magnetosphere. J. Geophys. Res., 103, 69-78, 1998. Crooker, N. U., Dayside merging and cusp geometry, J. Geophys. Res., 84, 951-959, 1979. Daly, P. W., and T. A. Fritz, Trapped electron distributions on open field lines, J. Geophys. Res., 87, 6081, 1982. Delcourt, D. C., and J.-A. Sauvaud, Populating of the cusp and boundary layers by energetic (hundreds of keV) equatorial particles, J. Geophys. Res., 104, 22635, 1999. Formisano, V., Orientation and shape of the Earth's bow shock in three dimensions, Planet. Space Sci., 27, 1151-1161, 1979. Fritz, T. A., and J. R. Cessna, ATS-6 NOAA low energy proton experiment, IEEE Trans. Aerospace Electron. Sys., AES-11, (6), 1145, 1975. Fritz, T. A., J. Chen, and R. B. Sheldon, The role of the cusp as a source for magnetospheric particles: A new paradigm? Adv. Space Res., 25, No. 7-8, 1445-1457, 2000. Fritz, T. A. et al., Significant initial results from the environmental measurements experiment on ATS-6, NASA Technical Paper 1101, NASA Scientific and Technical Information Office, Washington, DC, 34 p. 1977. Fritz, T. A., J. Chen, R. B. Sheldon, H..E. Spence, J. F. Fennell, S. Livi, C. T. Russell, and J. S. Pickett, Cusp energetic particle events measured by POLAR spacecraft, Phys. Chain. Earth (C), 24, No. 1-3, 135-140, 1999a. Fritz, T. A., M. Karra, B. Finkemeyer, and J. Chen, Statistical studies with Polar of energetic particles near the dayside cusp, EOS Trans. AGU, 80, No. 46, F862, 1999b. Fung, S. F., T. E. Eastman, S. A. Boardsen, and S.-H. Chen, High-altitude cusp positions sampled by the Hawkeye satellite, Phys. Chem. Earth, 22, No. 7-8, 653, 1997. Gloeckler, G. et al., Solar Wind Carbon, Nitrogen and Oxygen Abundances Measured in the Earth's Magnetosheath with AMPTE/CCE. Geophys. Res. Lett., 13, 793-796, 1986. Haskell, G. P., Anisotropic fluxes of energetic particles in the outer magnetosphere, J. Geophys. Res., 74, 1740, 1969. Lezniak, T. W., and J. R. Winckler, Experimental study of magnetospheric motion and the acceleration of energetic electrons during substorms, J. Geophys. Res., 75, 7075, 1970. Mead, G. D., Deformation of geomagnetic field by the solar wind, J. Geophys. Res., 69, 1181-1195, 1964. Meng, C.-I., Latitudinal variation of the polar cusp during a geomagnetic storm, Geophys. Res. Lett., 9,
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