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The cusp as a source of magnetospheric particles1
Radiation Measurements 30 (1999) 599±608
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The cusp as a source of magnetospheric particles1 Theodore A. Fritz*, Jiasheng Chen Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA Received 10 July 1998; received in revised form 17 June 1999
Abstract Observations made by the Polar satellite have shown that plasma of solar wind/magnetosheath origin is rammed into the high altitude polar cusp creating a diamagnetic cavity of large dimensions. The Earth's dipole ®eld can be excluded from this region in a turbulent manner with the magnitude of the ®eld strength reaching close to zero nT at times. At such times energetic particles are produced in this region in intensities which exceed those measured in the trapping regions of the magnetosphere beyond L = 6.5. These particles can then ¯ow back out of the cusp along ®eld lines that form the magnetopause. A fraction of these particles can enter the magnetosphere along the magnetopause on the dusk and dawn ¯anks. Due to existing gradients in the geomagnetic ®eld, cusp accelerated ions can enter the magnetosphere along the dawn ¯ank and electrons along the dusk ¯ank. For those particles entering near the geomagnetic equatorial plane with pitch angles close to ninety degrees, they will drift along contours of constant magnetic ®eld strength reaching deep into the nightside inner magnetosphere. From observations made by the Polar, ATS-6, and ISEE satellites it is argued that this cusp source appears to be capable of providing energetic ions to the magnetosphere and possibly energetic electrons which form the source population of the subsequent radial diusion and formation of the radiation belts. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
* Corresponding author. 1 This paper was presented at the Workshop on ``Space Radiation Environment Modeling: New Phenomena and Approaches'', held in Moscow, Russia, 7±9 October 1997. Section I of this workshop was entitled ``New Experimental Data and Radiation Belt Modeling'' and was dedicated to the work and memory of Professor B. A. Tverskoy. Professor Tverskoy made a signi®cant contribution to the ®eld of magnetospheric physics in a number of areas. It was the intention of one of the authors [TAF] to draw attention to some of Professor Tverskoy's work on radial diusion but in the process of preparing this paper the authors became aware of his work on a model of the inner magnetosphere which may be strongly supported by the conclusions drawn from the new experimental results presented in this paper.
There are many processes occurring in the inner magnetosphere that result in time variations of the trapped radiation belt particle ¯uxes measured at the equator and at low altitudes. The relative importance of each of these processes in under active investigation as a result of the many satellite missions being conducted during the time period of the International Solar Terrestrial Physics (ISTP) program. Still the fundamental energy distribution and radial structure of energetic ions and electrons in the radiation belts can be explained by the process of radial diusion with accompanying losses associated with charge exchange and coulomb collisions. This radial transport of charged particles by diusion processes is driven by ¯uctuations in the geomagnetic ®eld or in the geoelectric ®eld and draws upon a reservoir of particles in the
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sociated with the high altitude polar cusps for energetic particles which has the potential to produce the source spectrum of the outer magnetosphere. 2. Polar observations
Fig. 1. The predicted and measured position of the peak of the radial pro®le of ions (protons and alpha particles) as a function of their energy and position in L (from Fritz and Spjeldvik, 1981).
outer magnetosphere. This process has been studied by many authors (e.g. Tverskoy, 1965; Cornwall, 1971; Spjeldvik and Fritz, 1978). Such transport occurs while conserving the ®rst and second adiabatic invariants of the particles. Published work usually begins with a measured energy spectrum of the ions and/or electrons in the range of L > 7 as the input source of the subsequent radial diusion (Tverskoy, 1965; Spjeldvik and Fritz, 1978). In fact Tverskoy (1965) made a prediction of the location in L of the peak of a radial pro®le of a given ion species of a given energy based on a balance of the radial diusion time scales and time scales associated with the given loss processes. The results of these predictions from his work and subsequent ion pro®le measurements are shown in Fig. 1 from Fritz and Spjeldvik (1981). Li et al. (1997) have shown that the particles that form the source spectrum used in the radial diusion models must be processed by an acceleration mechanism internal to the magnetosphere since there are insuf®cient densities at constant ®rst adiabatic invariant (magnetic moment) in the solar wind to produce the measured spectrum at L > 7. It is generally believed that this intermediate acceleration process is associated with magnetospheric substorms and companion mechanisms in the magnetotail of the Earth. In this paper observations from the Polar and ISEE satellites will be presented and discussed which oer a new source as-
Using observations from the Polar satellite, Chen et al. (1997, 1998) and Chen and Fritz (1998) have shown that energetic particles of recent solar wind/magnetosheath origin are energized in situ in the high altitude polar cusp. Polar was launched on 24 February 1996 into a 1.8 RE by 9 RE polar orbit which provided an excellent and unique opportunity to investigate energetic particles in the high altitude polar cusp region. An example of such a cusp energetic particle (or CEP) event is shown in Fig. 2. On 27 August 1996, the Polar satellite was inbound from its apogee of 9 Earth radii (RE) situated directly over the northern polar cap region of the Earth. The data presented are of the measured intensity of helium ions with energy between 0.5 and 1.2 MeV and we see that ¯uxes of these very energetic ions are found in a region which we associate with the high altitude, polar magnetic ®eld cusp region. The CEP event lasts for two hours from 8.5 to 10.5 UT in this case. The intensity of energetic 1±200 keV/e helium ions demonstrate strong increases at the same time that large decreases in the locally measured magnetic ®eld are found for this same CEP event. When the charge state composition is examined for this event only those ion charge states associated with the solar wind (e.g. He++ and O>+2) are measured. No singly ionized helium or oxygen ions are found in these CEP events. Chen and Fritz (1998) have shown that there are no equivalent intensities of such ions present in the upstream interplanetary medium. Fig. 3 shows a comparison of the measured ¯uxes for the same 27 August 1996 CEP event shown in Fig. 2 and the simultaneous ¯uxes measured upstream from the magnetopause by the Geotail satellite. These issues have been further discussed in a Comment (Trattner et al., 1999) and a Reply (Fritz and Chen, 1999). During the ®rst 8 months of Polar operations about 70 CEP events were observed. Their distribution in local time and magnetic latitude is presented in Fig. 4. The results shown are consistent with such events being observed basically whenever Polar was in the appropriate cusp location to observe such events. CEP events do not appear to be correlated with any conditions in the upstream interplanetary medium although the IMF appears to have had a very strong preference for orientations such that the Bx-component was in the negative direction during all of these CEP events. The intensity of CEP particle ¯uxes are comparable to those observed in the outer magnetosphere. This can be seen by constructing the energy spectrum
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Fig. 2. An example of the CEP event on 27 August 1996. The panels from top to bottom show the counting rate for O<+3, O>+2, He+, He++ vs time, the corresponding variation of the local geomagnetic ®eld, and the ¯ux of the 0.52±1.15 MeV helium, respectively, where the vertical dashed lines mark the four dierent regions comprising individual CEP events. The distance of Polar from the Earth (in RE), the magnetic latitude (MLAT), and the magnetic local time (MLT) are shown at the bottom of the ®gure (from Fritz et al., 1999b).
of protons from a combination of Polar sensors over a very extended energy range (Fig. 5) and comparing this spectrum to one measured by the geostationary satellite ATS-6 near the midnight meridian for equatorially mirroring particles (Fritz et al., 1977). This is the same ATS-6 spectrum that was used by Spjeldvik and Fritz (1978) as their source spectrum. It therefore appears that energetic particles are being produced in the high altitude cusp with intensities comparable to those at the geostationary orbit in the inner magnetosphere. These particles are apparently contained within a diamagnetic cavity which can only contain them for a ®nite time after which they leak/ ¯ow along the open ®eld lines forming an energetic particle layer on the magnetopause. Are there other energetic particle observations which support the existence of particle entry into the magnetosphere consistent with that outlined above ?
3. ISEE observations When the ISEE-1 and 2 satellites were near the equatorial plane on the night side of the magnetosphere beyond L = 6.5 butter¯y-type pitch angle distributions were often observed in measurements of energetic ions and electrons. A butter¯y-type distribution is associated with a minimum in the particle ¯ux at pitch angles of 908. An example is shown in Fig. 6 for electrons with energy from 189 keV to 1.2 MeV. The radial pro®les of the intensity of electrons and ions for pitch angles of 908 and 308 recorded during the ISEE-1 inbound orbit from which the data of Fig. 6 were extracted are shown in Figs. 7 and 8 (Bhattacharjya and Fritz, 1998; Fritz et al., 1999a). The shaded region indicates the region in which the butter¯y distributions were observed. Note in Fig. 7 that the electron pro®les all show the presence of such
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Fig. 3. Simultaneously measured spectra of the CEP event of Fig. 2 in the cusp and in the upstream region in front of the magnetopause by the Geotail satellite. The inset shows the location of Geotail with respect to the location of the magnetopause and bow shock that have been determined by Fair®eld (1971). The IMF vector shown is the average values of the GSE Bx and By components during the period shown. The actual IMF values were about 6 nT and variations were such that Bz was consistently positive and approximately equal to vBxv. Bx was consistently negative and By varied from ÿ1.6 nT to +vBxv.
butter¯y distributions for the energy range covered (22.5 keV to 1.2 MeV) with the maximum dierence between the 908 and 308 pitch angle intensity increasing with increasing energy. West et al. (1973) ®rst observed these butter¯y distributions in the electron pitch angle distributions in
measurements made by the OGO-5 satellite. The distributions were observed all of the time when the satellite was close to the magnetic equator and beyond the geostationary orbit on the night side of the magnetosphere. Those authors explained these distributions as being produced by the azimuthal drift of electrons conserving their ®rst two adiabatic invariants. The drift trajectories for these two dierent pitch angle particles are shown in Fig. 9 drawn from Lyons and Williams (1984). Note that the small pitch angle particles will drift around the Earth at all radial distances associated with where the butter¯y distributions were observed in Figs. 7 and 8. This is not the case for the particles which mirror at the equator. These near-equatorial particles originating near the midnight meridian will follow drift paths which take them to the magnetopause over the region beyond 6 earth radii. Hence, such particles will most likely escape the magnetosphere and be lost. Both electrons and ions can potentially be lost in this manner, electrons drifting into the dawn magnetopause and ions drifting into dusk magnetopause. For such equatorially mirroring particles this will occur in an manner independent of energy. Why then do Figs. 7 and 8 show an energy dependence to the ¯uxes of both electrons and ions. In fact the lowest energy ion channel (24±44.5 keV) actually shows an enhancement of the intensity of such ions in the region where other channels are demonstrating butter¯y distributions. The most straightforward explanation of these observations is that the region just outside the magnetopause can act as a source of these ions and possibly electrons. Are there other observations consistent with a source of energetic particles in the dayside cusp?
Fig. 4. The event-averaged positions of the CEP events in the magnetosphere with MLT vs MLAT (a) and MLAT vs R (b) in polar coordinates. In (a), the four dashed circles from inside to outside indicated the MLAT position from 808 to 508, respectively, while in (b), the dashed circles represent the distance of Polar from the Earth (in RE) (from Chen et al., 1998).
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Fig. 5. Composite ®gure showing the proton energy spectrum measured by the Polar satellite in the high altitude cusp during a CEP event and by the ATS-6 satellite at the geostationary orbit on the night side of the Earth.
4. Other observations Fritz (1970) using energetic (>40 keV) electron measurements made at low altitudes concluded that there must be a source of such particles near local noon based on the frequent occurrence of the condition of isotropy over the upward hemisphere. Recently, Blanchard et al. (1998) have found ¯uxes of MeV electrons near local noon at high latitude welldetached from the trapping boundary using the lowaltitude SAMPEX satellite but argued for a dierent source of these particles rather than the more obvious dayside source. There have been a number of observations of energetic particles in the cusp. Antonova (1996) reported the observation of signi®cant ¯uxes of energetic (>100 keV) protons on two consecutive orbits of the Prognoz 9 satellite when it intersected the dayside polar cusp region. There was no simultaneous measurement of the magnetic ®eld available and the results were modeled as the result of particle trapping in the high altitude trapping region described by Antonova and Shabansky (1975). There have also been many reports of an energetic particle layer in the region between the magnetopause and the bow shock. Sarris et al. (1976) reported that bursts of energetic particles of non-thermal origin both
within and outside of the magnetotail are a semipermanent feature with energies ranging beyond 4.5 MeV for ions and 1 MeV for electrons. Baker and Stone (1978) observed a layer of energetic electrons (E > 200 keV) to be persistently present (97% of crossing, >70% of the time) in the range ÿ10 RE > XSM > ÿ40 RE. The electrons were streaming tailward along the local magnetosheath ®eld lines. This layer appeared to completely envelope the magnetopause and had a roughly annular cross-section. Baker and Stone found that ¯uxes of these particles in the plasma sheet were lower than those in the adjacent magnetopause energetic particle layer. Mitchell et al. (1987) analyzed magnetopause crossings of the ISEE-1 satellite along the dusk and dawn ¯anks and found that the occurrence of ¯uxes of >25 keV ions which they ascribed to a boundary layer increased with increasing distance away from the subsolar region becoming a tailward ¯ow with the particles appearing on both sides of the ``magnetopause''. Since ISEE-1 was a satellite with a low inclination orbit, this is what would be anticipated from a cusp source at high latitudes.
5. Conclusion The polar cusp appears to produce energetic par-
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Fig. 6. An example of detailed electron pitch angle distributions measured by the ISEE-1 satellite on 3 April 1978 close to the geomagnetic equator near the midnight meridian. The energy channels presented are 189±302 keV (E5), 302±477 keV (E6), 477±756 keV (E7), 756±1200 keV (E8) respectively (from Fritz et al., 1999).
ticles from a thermalized solar wind population which could then become the source of a layer of energetic ions and electrons at the magnetopause. This energetic particle population should have access to the inner magnetosphere along paths of constant magnetic ®eld magnitude due to their gradient and curvature drifts, ions entering along the dawn ¯ank of the magnetopause and electrons possibly entering along the dusk ¯ank. This population can then become the source population for subsequent radial diusion and the populating of the radiation belts in the manner described by Tverskoy (1965), Cornwall (1971) and Spjeldvik and Fritz (1978). Tverskoy (1970) has postulated the entry of such a population of energetic ions along the dusk ¯ank and has developed a complete theory and description of the functioning of the magnetospheric processes based on this initial condition.
Acknowledgements The authors have bene®ted from the eort of and discussions with the following individuals in the preparation of this manuscript: Harlan Spence, Robert Sheldon, Mohamed Alothman, Gina Gugliotti, Jyotirmoyee Bhattacharjya (all at Boston University), Stefano Livi (Max Planck Institute for Aeronomy), and Joe Fennell (Aerospace Corp.). Appreciation is expressed to Jack Scudder (University of Iowa) and Chris Russell (UCLA) for the use of the various Polar data sets and products and to Donald Williams (JHU/ APL) for the use of Geotail energetic particle data. The authors want to acknowledge the contribution to the Polar CAMMICE sensor eort of Bryan Laubscher, Robert Hedges, Rose Vigil, and Gina Lujan at Los Alamos National Laboratory, Stefano Livi, Hartmut Sommer, and Eckhart Steinmetz at the
Fig. 7. (a) Radial pro®les of electrons at two selected pitch angles of 908 and 308 measured by the inbound ISEE-1 satellite close to the geomagnetic equator near the midnight meridian on 2±3 April 1978. The energy channels presented are 22.5±39 keV (E1), 39± 75 keV (E2), 75±120 keV (E3), 120±189 keV (E4) respectively (from Fritz et al., 1999). (b) Same as (a) but for the electron energy passbands presented in Fig. 5 (from Fritz et al., 1999).
Fig. 8. (a) Same as Fig. 7(a) but for the ion energy channels with the following passbands for protons: 24±44.5 keV (P1), 44.5±65.3 keV (P2), 65.3±95.5 keV (P3), 95.5±142 keV (P4) respectively (from Fritz et al., 1999). (b) Same as Fig. 7(a) but for the ion energy channels with the following passbands for protons: 142±210 keV (P5), 210±333 keV (P6), 333±849 keV (P7), 849±2081 keV (P8) respectively (from Fritz et al., 1999).
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Fig. 9. Average magnetic ®eld characteristics projected to the geomagnetic equatorial plane: (a) contours of constant magnetic ®eld intensity: (b) contours in the equatorial plane of the latitude and local time of the point of intersection of a ®eld line with earth's surface. An energetic particle with an equatorial pitch angle of 908 will following the constant B-®eld contours of (a) whereas a particle with a small equatorial pitch angle mirroring just above the atmosphere of the earth will essentially follow the solid contours in (b) as each of these particle types drift in azimuth (from Lyons and Williams, 1984).
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Max Planck Institute for Aeronomy, Manuel Grande and his colleagues at the Rutherford Appleton Laboratory in the United Kingdom, Joe Fennell, Rocky Koga, Patty Lew, Norm Katz, Jim Roeder, and Bill Crain at the Aerospace Corporation. Much of this research was supported by NASA grants NAG5-2578, NAG5-7677 and NAG5-7841. References Antonova, A.E., 1996. High-latitude particle traps and related phenomena. Radiation Measurements 26 (3), 409±411. Antonova, A.E., Shabansky, 1975. Particles and magnetic ®eld in the outer dayside geomagnetosphere. Geomangn. Aeron. 15 (2), 297±302. Baker, D.N., Stone, E.C., 1978. The magnetopause energetic electron layer 1. Observations along the distant magnetotail. J. Geophys. Res. 83 (A9), 4327±4338. Bhattacharjya, J., Fritz, T.A., 1998. Statistics of observed butter¯y distributions in the outer nightside. EOS, Transaction of AGU SM51A-13. Blanchard, G.T., Lyons, L.R., Blake, J.B., Rich, F.J., 1998. SAMPEX observations of energetic electron precipitation in the dayside low-altitude boundary layer. J. Geophys. Res. 103 (A1), 191±198. Chen, J., Fritz, T.A., 1998. Correlation of cusp MeV helium with turbulent ULF power spectra. Geophys. Res. Lett. 25, 4113±4116. Chen, J., Fritz, T.A., Sheldon, R.B., Spence, H.E., Spjeldvik, W.N., Fennell, J.F., Livi, S., 1997. A new, temporarily con®ned population in the polar cap during the August 27, 1996 geomagnetic ®eld distortion period. Geophys. Res. Lett. 24, 1447±1450. Chen, J., Fritz, T.A., Sheldon, R.B., Spence, H.E., Spjeldvik, W.N., Fennell, J.F., Livi, S., Russell, C.T., Pickett, J.S., Gurnett, D.A., 1998. Cusp energetic particle events: Implication for a major acceleration region of the magnetosphere. J. Geophys. Res. 103 (A1), 69±78. Cornwall, J.M., 1971. Transport and loss processes for magnetospheric helium. J. Geophys. Res. 76, 264. Fair®eld, D.H., 1971. Average and unusual locations of the Earth's magnetopause and bow shock. J. Geophys. Res. 76, 6700. Fritz, T.A., 1970. Study of the high-latitude, outer-zone boundary region for >40 keV electrons with satellite Injun 3. J. Geophys. Res. 75 (28), 5387±5400. Fritz, T.A., Chen, J., 1999. Reply to ``Comment on: `Correlation of cusp MeV helium with turbulent ULF power spectra and its implications''' by K. J. Trattner, S. A. Fuselier, W. K. Peterson, and S.-W. Chang. Geophys. Res. Lett. 26 (10), 1363±1364. Fritz, T.A., Spjeldvik, W.N., 1981. Steady-state observations
of geomagnetically trapped energetic heavy ions and their implications for theory. Planet. Space Sci. 29 (11), 1169± 1193. Fritz, T.A., Chen, J., Sheldon, R.B., 1999a The role of the cusp as a source for magnetospheric particles: a new paradigm? Advances in Space Research (In press). Fritz, T.A., Chen, J., Sheldon, R.B., Spence, H.E., Fennell, J.F., Livi, S., Russell, C.T., Pickett, J.S., 1999b. Cusp energetic particle events measured by the Polar spacecraft. Phys. Chem. Earth (C) 24, 135±140. Fritz, T.A., Arthur, C.W., Blake, J.B., Coleman, P.J., Corrigan, J.P., Cummings, W.D., DeForest, S.E., Erickson, K.N., Konradi, A., Lennartsson, W., Masley, A.J., Mauk, B.H., McIlwain, C.E., McPherron, R.L., Paulikas, G.A., P®tzer, K.A., Reasoner, D.L., Satterblom, P.R., Su, S.Y., Walker, R.J., Whipple, E.C., Wilken, B., Winckler, J.R., 1977. Signi®cant initial results from the environmental measurements experiment on ATS-6, NASA Technical Paper 1101. NASA Scienti®c and Technical Information Oce, Washington, DC. Li, X., Baker, D.N., Temerin, M., Larson, D., Lin, R.P., Reeves, G.D., Looper, M., Kanekal, S.G., Mewaldt, R.A., 1997. Are energetic electrons in the solar wind the source of the outer radiation belts? Geophys. Res. Lett. 24 (8), 923±926. Lyons, L.R., Williams, D.J., 1984. Quantitative Aspect of the Magnetospheric Physics. Reidel, Dordrecht. Mitchell, D.G., Kutchko, F., Williams, D.J., Eastman, T.E., Frank, L.A., Russell, C.T., 1987. An extended study of the low-latitude boundary layer on the dawn and dusk ¯anks of the magnetosphere. J. Geophys. Res. 92 A7, 7394±7404. Sarris, E.T., Krimigis, S.M., Armstrong, T.P., 1976. Observation of magnetospheric bursts of high-energy protons and electrons at 035 R with IMP 7. J. Geophys. Res. 81 (A13), 2341±2355. Spjeldvik, W.N., Fritz, T.A., 1978. Composition of hot plasmas in the inner magnetosphere: observations and theoretical analysis of protons, helium ions, and oxygen ions. In: Rycroft, M.J., Strickland, A.C. (Eds.), Space Research VIII. Pergamon Press, Oxford, pp. 317±320. Trattner, K.J., Fuselier, S.A., Peterson, W.K., Chang, S-W., 1999. Comment on: `Correlation of cusp MeV helium with turbulent ULF power spectra and its implications'. Geophys. Res. Lett. 26 (10), 1361±1362. Tverskoy, B.A., 1965. Transport and acceleration of particles in the earth's magnetosphere. Geomangn. Aeron. 5, 617. Tverskoy, B.A., 1970. Electric ®elds in the magnetosphere and the origin of trapped radiation. In: Dyer, A. (Ed.), Solar Terrestrial Physics. Riedel, Dordrecht, pp. 297±317. West Jr, H.I., Buck, R.M., Walton, J.R., 1973. Electron pitch angle distributions of energetic electrons throughout the magnetosphere as observed by OGO-5. J. Geophys. Res. 78, 1064±1081.
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The cusp as a source of magnetospheric particles1 Theodore A. Fritz*, Jiasheng Chen Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA Received 10 July 1998; received in revised form 17 June 1999
Abstract Observations made by the Polar satellite have shown that plasma of solar wind/magnetosheath origin is rammed into the high altitude polar cusp creating a diamagnetic cavity of large dimensions. The Earth's dipole ®eld can be excluded from this region in a turbulent manner with the magnitude of the ®eld strength reaching close to zero nT at times. At such times energetic particles are produced in this region in intensities which exceed those measured in the trapping regions of the magnetosphere beyond L = 6.5. These particles can then ¯ow back out of the cusp along ®eld lines that form the magnetopause. A fraction of these particles can enter the magnetosphere along the magnetopause on the dusk and dawn ¯anks. Due to existing gradients in the geomagnetic ®eld, cusp accelerated ions can enter the magnetosphere along the dawn ¯ank and electrons along the dusk ¯ank. For those particles entering near the geomagnetic equatorial plane with pitch angles close to ninety degrees, they will drift along contours of constant magnetic ®eld strength reaching deep into the nightside inner magnetosphere. From observations made by the Polar, ATS-6, and ISEE satellites it is argued that this cusp source appears to be capable of providing energetic ions to the magnetosphere and possibly energetic electrons which form the source population of the subsequent radial diusion and formation of the radiation belts. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
* Corresponding author. 1 This paper was presented at the Workshop on ``Space Radiation Environment Modeling: New Phenomena and Approaches'', held in Moscow, Russia, 7±9 October 1997. Section I of this workshop was entitled ``New Experimental Data and Radiation Belt Modeling'' and was dedicated to the work and memory of Professor B. A. Tverskoy. Professor Tverskoy made a signi®cant contribution to the ®eld of magnetospheric physics in a number of areas. It was the intention of one of the authors [TAF] to draw attention to some of Professor Tverskoy's work on radial diusion but in the process of preparing this paper the authors became aware of his work on a model of the inner magnetosphere which may be strongly supported by the conclusions drawn from the new experimental results presented in this paper.
There are many processes occurring in the inner magnetosphere that result in time variations of the trapped radiation belt particle ¯uxes measured at the equator and at low altitudes. The relative importance of each of these processes in under active investigation as a result of the many satellite missions being conducted during the time period of the International Solar Terrestrial Physics (ISTP) program. Still the fundamental energy distribution and radial structure of energetic ions and electrons in the radiation belts can be explained by the process of radial diusion with accompanying losses associated with charge exchange and coulomb collisions. This radial transport of charged particles by diusion processes is driven by ¯uctuations in the geomagnetic ®eld or in the geoelectric ®eld and draws upon a reservoir of particles in the
1350-4487/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 9 9 ) 0 0 2 3 9 - 5
600
T.A. Fritz, J. Chen / Radiation Measurements 30 (1999) 599±608
sociated with the high altitude polar cusps for energetic particles which has the potential to produce the source spectrum of the outer magnetosphere. 2. Polar observations
Fig. 1. The predicted and measured position of the peak of the radial pro®le of ions (protons and alpha particles) as a function of their energy and position in L (from Fritz and Spjeldvik, 1981).
outer magnetosphere. This process has been studied by many authors (e.g. Tverskoy, 1965; Cornwall, 1971; Spjeldvik and Fritz, 1978). Such transport occurs while conserving the ®rst and second adiabatic invariants of the particles. Published work usually begins with a measured energy spectrum of the ions and/or electrons in the range of L > 7 as the input source of the subsequent radial diusion (Tverskoy, 1965; Spjeldvik and Fritz, 1978). In fact Tverskoy (1965) made a prediction of the location in L of the peak of a radial pro®le of a given ion species of a given energy based on a balance of the radial diusion time scales and time scales associated with the given loss processes. The results of these predictions from his work and subsequent ion pro®le measurements are shown in Fig. 1 from Fritz and Spjeldvik (1981). Li et al. (1997) have shown that the particles that form the source spectrum used in the radial diusion models must be processed by an acceleration mechanism internal to the magnetosphere since there are insuf®cient densities at constant ®rst adiabatic invariant (magnetic moment) in the solar wind to produce the measured spectrum at L > 7. It is generally believed that this intermediate acceleration process is associated with magnetospheric substorms and companion mechanisms in the magnetotail of the Earth. In this paper observations from the Polar and ISEE satellites will be presented and discussed which oer a new source as-
Using observations from the Polar satellite, Chen et al. (1997, 1998) and Chen and Fritz (1998) have shown that energetic particles of recent solar wind/magnetosheath origin are energized in situ in the high altitude polar cusp. Polar was launched on 24 February 1996 into a 1.8 RE by 9 RE polar orbit which provided an excellent and unique opportunity to investigate energetic particles in the high altitude polar cusp region. An example of such a cusp energetic particle (or CEP) event is shown in Fig. 2. On 27 August 1996, the Polar satellite was inbound from its apogee of 9 Earth radii (RE) situated directly over the northern polar cap region of the Earth. The data presented are of the measured intensity of helium ions with energy between 0.5 and 1.2 MeV and we see that ¯uxes of these very energetic ions are found in a region which we associate with the high altitude, polar magnetic ®eld cusp region. The CEP event lasts for two hours from 8.5 to 10.5 UT in this case. The intensity of energetic 1±200 keV/e helium ions demonstrate strong increases at the same time that large decreases in the locally measured magnetic ®eld are found for this same CEP event. When the charge state composition is examined for this event only those ion charge states associated with the solar wind (e.g. He++ and O>+2) are measured. No singly ionized helium or oxygen ions are found in these CEP events. Chen and Fritz (1998) have shown that there are no equivalent intensities of such ions present in the upstream interplanetary medium. Fig. 3 shows a comparison of the measured ¯uxes for the same 27 August 1996 CEP event shown in Fig. 2 and the simultaneous ¯uxes measured upstream from the magnetopause by the Geotail satellite. These issues have been further discussed in a Comment (Trattner et al., 1999) and a Reply (Fritz and Chen, 1999). During the ®rst 8 months of Polar operations about 70 CEP events were observed. Their distribution in local time and magnetic latitude is presented in Fig. 4. The results shown are consistent with such events being observed basically whenever Polar was in the appropriate cusp location to observe such events. CEP events do not appear to be correlated with any conditions in the upstream interplanetary medium although the IMF appears to have had a very strong preference for orientations such that the Bx-component was in the negative direction during all of these CEP events. The intensity of CEP particle ¯uxes are comparable to those observed in the outer magnetosphere. This can be seen by constructing the energy spectrum
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Fig. 2. An example of the CEP event on 27 August 1996. The panels from top to bottom show the counting rate for O<+3, O>+2, He+, He++ vs time, the corresponding variation of the local geomagnetic ®eld, and the ¯ux of the 0.52±1.15 MeV helium, respectively, where the vertical dashed lines mark the four dierent regions comprising individual CEP events. The distance of Polar from the Earth (in RE), the magnetic latitude (MLAT), and the magnetic local time (MLT) are shown at the bottom of the ®gure (from Fritz et al., 1999b).
of protons from a combination of Polar sensors over a very extended energy range (Fig. 5) and comparing this spectrum to one measured by the geostationary satellite ATS-6 near the midnight meridian for equatorially mirroring particles (Fritz et al., 1977). This is the same ATS-6 spectrum that was used by Spjeldvik and Fritz (1978) as their source spectrum. It therefore appears that energetic particles are being produced in the high altitude cusp with intensities comparable to those at the geostationary orbit in the inner magnetosphere. These particles are apparently contained within a diamagnetic cavity which can only contain them for a ®nite time after which they leak/ ¯ow along the open ®eld lines forming an energetic particle layer on the magnetopause. Are there other energetic particle observations which support the existence of particle entry into the magnetosphere consistent with that outlined above ?
3. ISEE observations When the ISEE-1 and 2 satellites were near the equatorial plane on the night side of the magnetosphere beyond L = 6.5 butter¯y-type pitch angle distributions were often observed in measurements of energetic ions and electrons. A butter¯y-type distribution is associated with a minimum in the particle ¯ux at pitch angles of 908. An example is shown in Fig. 6 for electrons with energy from 189 keV to 1.2 MeV. The radial pro®les of the intensity of electrons and ions for pitch angles of 908 and 308 recorded during the ISEE-1 inbound orbit from which the data of Fig. 6 were extracted are shown in Figs. 7 and 8 (Bhattacharjya and Fritz, 1998; Fritz et al., 1999a). The shaded region indicates the region in which the butter¯y distributions were observed. Note in Fig. 7 that the electron pro®les all show the presence of such
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Fig. 3. Simultaneously measured spectra of the CEP event of Fig. 2 in the cusp and in the upstream region in front of the magnetopause by the Geotail satellite. The inset shows the location of Geotail with respect to the location of the magnetopause and bow shock that have been determined by Fair®eld (1971). The IMF vector shown is the average values of the GSE Bx and By components during the period shown. The actual IMF values were about 6 nT and variations were such that Bz was consistently positive and approximately equal to vBxv. Bx was consistently negative and By varied from ÿ1.6 nT to +vBxv.
butter¯y distributions for the energy range covered (22.5 keV to 1.2 MeV) with the maximum dierence between the 908 and 308 pitch angle intensity increasing with increasing energy. West et al. (1973) ®rst observed these butter¯y distributions in the electron pitch angle distributions in
measurements made by the OGO-5 satellite. The distributions were observed all of the time when the satellite was close to the magnetic equator and beyond the geostationary orbit on the night side of the magnetosphere. Those authors explained these distributions as being produced by the azimuthal drift of electrons conserving their ®rst two adiabatic invariants. The drift trajectories for these two dierent pitch angle particles are shown in Fig. 9 drawn from Lyons and Williams (1984). Note that the small pitch angle particles will drift around the Earth at all radial distances associated with where the butter¯y distributions were observed in Figs. 7 and 8. This is not the case for the particles which mirror at the equator. These near-equatorial particles originating near the midnight meridian will follow drift paths which take them to the magnetopause over the region beyond 6 earth radii. Hence, such particles will most likely escape the magnetosphere and be lost. Both electrons and ions can potentially be lost in this manner, electrons drifting into the dawn magnetopause and ions drifting into dusk magnetopause. For such equatorially mirroring particles this will occur in an manner independent of energy. Why then do Figs. 7 and 8 show an energy dependence to the ¯uxes of both electrons and ions. In fact the lowest energy ion channel (24±44.5 keV) actually shows an enhancement of the intensity of such ions in the region where other channels are demonstrating butter¯y distributions. The most straightforward explanation of these observations is that the region just outside the magnetopause can act as a source of these ions and possibly electrons. Are there other observations consistent with a source of energetic particles in the dayside cusp?
Fig. 4. The event-averaged positions of the CEP events in the magnetosphere with MLT vs MLAT (a) and MLAT vs R (b) in polar coordinates. In (a), the four dashed circles from inside to outside indicated the MLAT position from 808 to 508, respectively, while in (b), the dashed circles represent the distance of Polar from the Earth (in RE) (from Chen et al., 1998).
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Fig. 5. Composite ®gure showing the proton energy spectrum measured by the Polar satellite in the high altitude cusp during a CEP event and by the ATS-6 satellite at the geostationary orbit on the night side of the Earth.
4. Other observations Fritz (1970) using energetic (>40 keV) electron measurements made at low altitudes concluded that there must be a source of such particles near local noon based on the frequent occurrence of the condition of isotropy over the upward hemisphere. Recently, Blanchard et al. (1998) have found ¯uxes of MeV electrons near local noon at high latitude welldetached from the trapping boundary using the lowaltitude SAMPEX satellite but argued for a dierent source of these particles rather than the more obvious dayside source. There have been a number of observations of energetic particles in the cusp. Antonova (1996) reported the observation of signi®cant ¯uxes of energetic (>100 keV) protons on two consecutive orbits of the Prognoz 9 satellite when it intersected the dayside polar cusp region. There was no simultaneous measurement of the magnetic ®eld available and the results were modeled as the result of particle trapping in the high altitude trapping region described by Antonova and Shabansky (1975). There have also been many reports of an energetic particle layer in the region between the magnetopause and the bow shock. Sarris et al. (1976) reported that bursts of energetic particles of non-thermal origin both
within and outside of the magnetotail are a semipermanent feature with energies ranging beyond 4.5 MeV for ions and 1 MeV for electrons. Baker and Stone (1978) observed a layer of energetic electrons (E > 200 keV) to be persistently present (97% of crossing, >70% of the time) in the range ÿ10 RE > XSM > ÿ40 RE. The electrons were streaming tailward along the local magnetosheath ®eld lines. This layer appeared to completely envelope the magnetopause and had a roughly annular cross-section. Baker and Stone found that ¯uxes of these particles in the plasma sheet were lower than those in the adjacent magnetopause energetic particle layer. Mitchell et al. (1987) analyzed magnetopause crossings of the ISEE-1 satellite along the dusk and dawn ¯anks and found that the occurrence of ¯uxes of >25 keV ions which they ascribed to a boundary layer increased with increasing distance away from the subsolar region becoming a tailward ¯ow with the particles appearing on both sides of the ``magnetopause''. Since ISEE-1 was a satellite with a low inclination orbit, this is what would be anticipated from a cusp source at high latitudes.
5. Conclusion The polar cusp appears to produce energetic par-
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Fig. 6. An example of detailed electron pitch angle distributions measured by the ISEE-1 satellite on 3 April 1978 close to the geomagnetic equator near the midnight meridian. The energy channels presented are 189±302 keV (E5), 302±477 keV (E6), 477±756 keV (E7), 756±1200 keV (E8) respectively (from Fritz et al., 1999).
ticles from a thermalized solar wind population which could then become the source of a layer of energetic ions and electrons at the magnetopause. This energetic particle population should have access to the inner magnetosphere along paths of constant magnetic ®eld magnitude due to their gradient and curvature drifts, ions entering along the dawn ¯ank of the magnetopause and electrons possibly entering along the dusk ¯ank. This population can then become the source population for subsequent radial diusion and the populating of the radiation belts in the manner described by Tverskoy (1965), Cornwall (1971) and Spjeldvik and Fritz (1978). Tverskoy (1970) has postulated the entry of such a population of energetic ions along the dusk ¯ank and has developed a complete theory and description of the functioning of the magnetospheric processes based on this initial condition.
Acknowledgements The authors have bene®ted from the eort of and discussions with the following individuals in the preparation of this manuscript: Harlan Spence, Robert Sheldon, Mohamed Alothman, Gina Gugliotti, Jyotirmoyee Bhattacharjya (all at Boston University), Stefano Livi (Max Planck Institute for Aeronomy), and Joe Fennell (Aerospace Corp.). Appreciation is expressed to Jack Scudder (University of Iowa) and Chris Russell (UCLA) for the use of the various Polar data sets and products and to Donald Williams (JHU/ APL) for the use of Geotail energetic particle data. The authors want to acknowledge the contribution to the Polar CAMMICE sensor eort of Bryan Laubscher, Robert Hedges, Rose Vigil, and Gina Lujan at Los Alamos National Laboratory, Stefano Livi, Hartmut Sommer, and Eckhart Steinmetz at the
Fig. 7. (a) Radial pro®les of electrons at two selected pitch angles of 908 and 308 measured by the inbound ISEE-1 satellite close to the geomagnetic equator near the midnight meridian on 2±3 April 1978. The energy channels presented are 22.5±39 keV (E1), 39± 75 keV (E2), 75±120 keV (E3), 120±189 keV (E4) respectively (from Fritz et al., 1999). (b) Same as (a) but for the electron energy passbands presented in Fig. 5 (from Fritz et al., 1999).
Fig. 8. (a) Same as Fig. 7(a) but for the ion energy channels with the following passbands for protons: 24±44.5 keV (P1), 44.5±65.3 keV (P2), 65.3±95.5 keV (P3), 95.5±142 keV (P4) respectively (from Fritz et al., 1999). (b) Same as Fig. 7(a) but for the ion energy channels with the following passbands for protons: 142±210 keV (P5), 210±333 keV (P6), 333±849 keV (P7), 849±2081 keV (P8) respectively (from Fritz et al., 1999).
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Fig. 9. Average magnetic ®eld characteristics projected to the geomagnetic equatorial plane: (a) contours of constant magnetic ®eld intensity: (b) contours in the equatorial plane of the latitude and local time of the point of intersection of a ®eld line with earth's surface. An energetic particle with an equatorial pitch angle of 908 will following the constant B-®eld contours of (a) whereas a particle with a small equatorial pitch angle mirroring just above the atmosphere of the earth will essentially follow the solid contours in (b) as each of these particle types drift in azimuth (from Lyons and Williams, 1984).
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Max Planck Institute for Aeronomy, Manuel Grande and his colleagues at the Rutherford Appleton Laboratory in the United Kingdom, Joe Fennell, Rocky Koga, Patty Lew, Norm Katz, Jim Roeder, and Bill Crain at the Aerospace Corporation. Much of this research was supported by NASA grants NAG5-2578, NAG5-7677 and NAG5-7841. References Antonova, A.E., 1996. High-latitude particle traps and related phenomena. Radiation Measurements 26 (3), 409±411. Antonova, A.E., Shabansky, 1975. Particles and magnetic ®eld in the outer dayside geomagnetosphere. Geomangn. Aeron. 15 (2), 297±302. Baker, D.N., Stone, E.C., 1978. The magnetopause energetic electron layer 1. Observations along the distant magnetotail. J. Geophys. Res. 83 (A9), 4327±4338. Bhattacharjya, J., Fritz, T.A., 1998. Statistics of observed butter¯y distributions in the outer nightside. EOS, Transaction of AGU SM51A-13. Blanchard, G.T., Lyons, L.R., Blake, J.B., Rich, F.J., 1998. SAMPEX observations of energetic electron precipitation in the dayside low-altitude boundary layer. J. Geophys. Res. 103 (A1), 191±198. Chen, J., Fritz, T.A., 1998. Correlation of cusp MeV helium with turbulent ULF power spectra. Geophys. Res. Lett. 25, 4113±4116. Chen, J., Fritz, T.A., Sheldon, R.B., Spence, H.E., Spjeldvik, W.N., Fennell, J.F., Livi, S., 1997. A new, temporarily con®ned population in the polar cap during the August 27, 1996 geomagnetic ®eld distortion period. Geophys. Res. Lett. 24, 1447±1450. Chen, J., Fritz, T.A., Sheldon, R.B., Spence, H.E., Spjeldvik, W.N., Fennell, J.F., Livi, S., Russell, C.T., Pickett, J.S., Gurnett, D.A., 1998. Cusp energetic particle events: Implication for a major acceleration region of the magnetosphere. J. Geophys. Res. 103 (A1), 69±78. Cornwall, J.M., 1971. Transport and loss processes for magnetospheric helium. J. Geophys. Res. 76, 264. Fair®eld, D.H., 1971. Average and unusual locations of the Earth's magnetopause and bow shock. J. Geophys. Res. 76, 6700. Fritz, T.A., 1970. Study of the high-latitude, outer-zone boundary region for >40 keV electrons with satellite Injun 3. J. Geophys. Res. 75 (28), 5387±5400. Fritz, T.A., Chen, J., 1999. Reply to ``Comment on: `Correlation of cusp MeV helium with turbulent ULF power spectra and its implications''' by K. J. Trattner, S. A. Fuselier, W. K. Peterson, and S.-W. Chang. Geophys. Res. Lett. 26 (10), 1363±1364. Fritz, T.A., Spjeldvik, W.N., 1981. Steady-state observations
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