Chapter 29Energetic Particles in the Cusp:A Cluster/RAPID View

T. Asikainen

Abstract Energetic particles have been persistently observed in the exterior cuspby different satellite missions such as POLAR, Cluster-II, Viking, ISEE etc. Yet thesource and the acceleration mechanism of these particles have remained unclear.In this paper I review our studies of energetic particles in the cusp and the nearbyhigh-latitude region of closed magnetospheric field lines (HLPS, high-latitude day-side plasma sheet) using the data obtained by the RAPID instrument onboard theCluster-II satellites. We conducted a large scale statistical study to examine the de-pendence of the energetic particle fluxes in the cusp and HLPS on solar wind/IMFconditions as well as on geomagnetic activity. The study showed that energetic ionfluxes in the HLPS correlate strongly with substorm activity and electron fluxes withsolar wind speed and geomagnetic activity. In the exterior cusp a clear correlationbetween lower energy ions (E < 75 keV) and IMF jBy j was found while the moreenergetic particles in the cusp (E > 75 keV) correlated with substorm activity. Ourcase studies have shown that when IMF By dominates reconnection can take placenear the cusp and release energetic particles from closed field lines to the cusp.Coupled with these detailed observations the statistical results imply that the en-ergetic particles in the HLPS and the cusp originate in the near-Earth magnetotailfrom where they can drift to the HLPS region. From the HLPS the higher energyparticles diffuse more or less directly into the cusp while the lower energy particlesare released into the cusp by reconnection. These observations provide a consis-tent explanation for the cusp energetic particles without a need for significant localacceleration of shocked solar wind plasma to MeV energies. While some energytransfer from the electromagnetic waves to plasma particles is known to occur in thecusp it cannot explain the observations discussed here.

T. Asikainen (�)Department of Physics Centre of Excellence in Research, P.O. Box 3000, FIN-90014, Universityof Oulu, Finlande-mail: timo.asikainen@oulu.fi

H. Laakso et al. (eds.), The Cluster Active Archive, Astrophysics and Space ScienceProceedings, DOI 10.1007/978-90-481-3499-1 29,c� Springer Science+Business Media B.V. 2010

415

416 T. Asikainen

29.1 Introduction

As the solar wind blows past the Earth and confines the geomagnetic field into themagnetospheric cavity two null points of magnetic field are formed at high latitudeson the dayside magnetopause. The field lines around these points form a funnel,a cusp, that extends all the way from the magnetopause into the ionosphere. Thecusps have a central role in the magnetospheric dynamics as most of the solar windplasma that enters the magnetosphere during dayside magnetospheric reconnectiontravels through them (e.g., Reiff et al. [14], Wing et al. [22], Onsager and Lockwood[12]). When reconnection occurs on the magnetopause shocked solar wind plasmafrom the magnetosheath gains access to the cusp and forms a turbulent diamagneticcavity. Accordingly, the high altitude cusp is typically seen as a region of enhancedplasma density with a fluctuating and depressed magnetic field relative to the sur-roundings.

In the classical model of the magnetosphere, the high-latitude dayside magneto-sphere and the adjacent cusp regions cannot trap particles stably [16] because theyare swept to the dayside magnetopause by the convection before they can executea complete drift orbit. Still, a number of studies concerning the high altitude cusp[1, 2, 5, 6, 9, 10, 23, 24] have reported persistent and significant fluxes of energeticparticles in this region. Numerical studies based on realistic magnetic field modelshave also revealed that the high-latitude dayside regions around the cusp can quasi-stably trap energetic particles whose motion is primarily governed by the magneticgradient-curvature drift rather than the convection [19]. It has also been shown thatthe regions around the cusp are connected to the night side magnetotail by so calledShabansky drift orbits [7, 18].

Despite the vast observational evidence of energetic particles in the cusp andrelated numerical modeling, the origin of these particles has remained controversial.Three different scenarios have been suggested to account for these particles: (1)local acceleration in the cusp [5], (2) inner magnetospheric source and transportfrom there to the cusp [1], (3) a bow shock source [4]. Chen et al. [5] and Fritzet al. [8] have reported a correlation between the energetic ion fluxes in the cuspand the power of the magnetic fluctuations in the cusp diamagnetic cavity. Based onthis and other suggestive observations they have argued that the energetic particlesare accelerated locally in the cusp by electromagnetic waves. Chang et al. [4] andTrattner et al. [20] have argued that the lower energy particles in the cusp maybe accelerated at the quasi-parallel bow shock, entering from there into the cuspalong field lines connecting the two regions. They also speculate that higher energyparticles observed in the cusp may diffuse there from within the magnetosphere.Kremser et al. [10] studied the low-altitude cusp and showed that ions with energyabove about 50 keV in the cusp are of magnetospheric origin while a separate lowerenergy ion population in the cusp (E < 15 keV) is of magnetosheath origin.

Nearly all the work done on the energetic particles in the cusp has beenconcentrated purely on the particles observed within the cusp itself while muchless attention have been given to the surrounding closed field line regions of the

29 Energetic Particles in the Cusp: A Cluster/RAPID View 417

magnetosphere which can efficiently trap energetic particles. In our work we havestudied the energetic particle fluxes in the cusp and also on the adjacent closed fieldline region equatorward of the cusp (which we call the high-latitude dayside plasmasheet or HLPS) as well as possible particle transport between these regions. In thispaper I review our work on the subject and present statistical and detailed evidencefor magnetospheric origin of energetic particles in the cusp.

29.2 Instrumentation

Much of the work on cusp energetic particles dates to the era of POLAR satellite(launched in February 1996) whose orbit was ideal for such studies. To provide analternate view of the same region we have used the Cluster-II satellites whose or-bital configuration allows the study of high-latitude dayside regions like the cusp,magnetopause and the high-latitude dayside plasma sheet during the northern hemi-sphere vernal equinox (January to May season). Each Cluster spacecraft includesa versatile set of instruments for measuring electromagnetic fields at different fre-quencies and charged particles at different energies. In this work we have utilizedmagnetic field data from the FGM instrument [3] and particle data from the CISplasma instrument which measures low energy ion species from spacecraft poten-tial up to 40 keV/q [15]. However, our most important source of energetic particledata is the RAPID instrument which measures electrons from 20 to 400 keV and ions(protons, helium ions and CNO group ions) at the energy range 28–1,500 keV forprotons, 138–1,500 keV for He ions and 90–1,500 keV for CNO-group ions. TheRAPID is capable of measuring the full 3D distribution of electrons at spacecraftspin resolution (4 s) and the 3D distribution of ions at 32 spin resolution (128 s).However, the corruption of the central ion detector heads has rendered the 3D iondistributions almost useless. A detailed description of the RAPID instrument isgiven by Wilken et al. [21] and an updated description of RAPID instrument andall the other Cluster instruments is available at the Cluster Active Archive website(http://caa.estec.esa.int/caa/instr doc.xml).

29.3 The High-Latitude Dayside Regions

During vernal equinoxes the apogee of the Cluster orbit is at about 19.6 RE at thedayside, i.e., well outside the magnetosphere in the solar wind. On such an orbitthe satellites traverse from the nightside ring current over the auroral regions andthe polar cap into the high latitude dayside regions. Depending on the solar windconditions (dynamic pressure, IMF etc.) and the geodipole tilt angle, which affectthe orientation of the whole dayside magnetosphere relative to the Cluster orbit, thesatellites can enter different regions on the dayside magnetosphere.

418 T. Asikainen

Fig. 29.1 Cluster-1 data from 17 February 2003. Panels from top to bottom are magnetic fieldintensity (FGM), plasma flow velocity (CIS), low energy ion spectrum (CIS/CODIF), energeticproton spectrum (RAPID) and energetic electron spectrum (RAPID)

Figure 29.1 shows a collection of data from Cluster 1 during a typical Clusterdayside pass on 17 February 2003. The panels from top to bottom depict magneticfield intensity, plasma flow velocity, low energy proton spectrum (CIS), high energyproton spectrum (RAPID) and electron spectrum (RAPID). At the beginning of thedepicted time interval the satellite was flying in the northern tail lobe over the po-lar cap towards the dayside. At about 03:00 UT the plasma density and energeticparticle fluxes were greatly enhanced indicating a mixture of magnetospheric andmagnetosheath plasma. By 04:00 UT the satellite had moved into a region whereonly magnetospheric energetic particles were present, the plasma density was lowand the plasma flow velocity was small. At the same time the magnetic field inten-sity behaved rather smoothly without any high frequency fluctuations or turbulence.This region, which is equatorward of the cusp funnel and inside the magnetopausewith high fluxes of energetic particles, is the high-latitude dayside plasma sheet,HLPS, or the high latitude trapping region. In the HLPS significant energetic parti-cle fluxes are persistently present as will be discussed later on. At about 07:10 UTthe satellite crossed the dayside magnetopause into the magnetosheath, which isseen as an abrupt change in all the depicted parameters. Most notably the magneticfield intensity dropped and became very turbulent, the flow velocity increased to asteady level, plasma density increased and energetic particles all but disappeared(except for the few enhancements mostly seen in energetic protons).

The satellites do not observe the HLPS region on every orbit. If the orientationof the dayside magnetosphere relative to the Cluster orbit is suitable the satellites

29 Energetic Particles in the Cusp: A Cluster/RAPID View 419

may enter the exterior cusp. A collection of data from such an orbit is shownin Fig. 29.2. Again the panels from top to bottom depict magnetic field intensity,plasma flow velocity, low energy proton spectrum (CIS), high energy proton spec-trum (RAPID) and electron spectrum (RAPID). At the beginning of the depictedtime interval the satellite flied over the polar cap in the northern tail lobe and beganto see plasma of increasing density as it approached the high-latitude magnetopause.After about 22:00 UT the satellite observed a region of decreased and highly fluctu-ating magnetic field with increased plasma density and small and turbulent plasmaflow velocity. This is the cusp diamagnetic cavity. High fluxes of energetic particles(both electrons and ions) were present in the cusp as can be seen from the RAPIDelectron and ion spectra in Fig. 29.2. Such elevated fluxes of energetic particles areseen nearly every time the satellite observes the cusp although the flux level dependson a variety of things as will be discussed later. The satellite left the cusp around23:00 UT skimming the dayside magnetopause and entering the magnetosheath. Asthe satellite exited the cusp it observed a number of sharp fluctuations in the mag-netic field intensity and associated strong peaks in energetic particle fluxes fromabout 22:50 to 23:30 UT. Such signatures are not a feature particular to this orbitalone but are actually observed often as the satellite exits the cusp and the IMF By

component dominates (like in the event under discussion).The two types of orbits discussed above are the most typical ones during ver-

nal equinoxes. As mentioned above, whether the satellites observe the cusp or theHLPS mainly depends on the orientation of the whole dayside magnetosphere rel-ative to the Cluster orbit. It is well known that the dipole tilt angle affects the cuspposition greatly. The greater the dipole tilt angle the more equatorward the cusp is.In addition it has been shown that the cusp position is affected by IMF direction[11, 13, 17] being generally more equatorward during negative IMF Bz. It is thusexpected that Cluster sees the HLPS region more preferentially when the dipole istilted away from the Sun and the IMF is northward, while the cusp would prefer-entially be observed when the dipole is tilted towards the Sun and the IMF beingsouthward. However, also the inclination of the Cluster orbit has an effect. In sevenyears since the launch of Cluster-II late in 2000 the inclination of the orbit has de-cayed from the initial 0ı to more than 30ı below the ecliptic. The orbital decay hasled to the fact that the northern cusp is now rarely seen by the Clusters while in thebeginning of the mission the northern cusp was seen more often. The southern cuspin contrast is now seen more often than at the beginning of the mission.

29.4 Reconnection Near the Cusp

Asikainen and Mursula [2] studied the event shown in Fig. 29.2 in detail and showedthat the sharp fluctuations in the magnetic field and associated energetic parti-cle peaks seen at the cusp edge were associated with reconnection near the cusp.Figure 29.3 shows a close-up of the time interval from 22:50 to 23:30 UT from Clus-ter 4 satellite. The panels from top to bottom depict the magnetic field components

420 T. Asikainen

Fig. 29.2 Cluster-1 data from 2–3 February 2003. Panels from top to bottom are magnetic fieldintensity (FGM), plasma flow velocity (CIS), low energy ion spectrum (CIS/CODIF), energeticproton spectrum (RAPID) and energetic electron spectrum (RAPID)

in a boundary normal (LMN) coordinate system, energetic proton spectrum and en-ergetic electron spectrum. One can see that each energetic particle enhancementoccurs during a bi-polar variation in the magnetic field normal component whichis a well known signature of transient reconnection at the magnetopause or a fluxtransfer event (FTE). In an FTE a reconnected flux tube moves along the magne-topause across the spacecraft giving rise to the bi-polar signature in the magneticfield normal component. During the time interval in Fig. 29.3 the IMF was rathersteady with By D 10 nT and Bz D �3 nT. During such an IMF one expects theantiparallel reconnection to occur near the dusk edge of the cusp funnel. Asikainenand Mursula [2] showed indeed that the observations are consistent with such a re-connection and that the energetic particles observed during the FTEs were particlesreleased from the magnetospheric closed field lines. The particles were streamingantiparallel to the reconnected flux tube towards the cusp. This work showed thatreconnection near the cusp can release particles from the closed field lines of HLPSand some of these particles have access into the cusp. As mentioned, such recon-nection events and similar (e.g., Zong et al. [25]) are often observed near the cuspmagnetopause when IMF By dominates suggesting that energetic particle fluxes inthe cusp might be dependent on IMF direction.

29 Energetic Particles in the Cusp: A Cluster/RAPID View 421

Fig. 29.3 Close-up of flux transfer events observed by Cluster-4 at 2 February 2003 22:50–23:30.Panels from top to bottom show magnetic field components in boundary normal coordinate sys-tem (BN is the normal component), energetic electron spectrum (RAPID) and energetic protonspectrum (RAPID)

29.5 Statistics of Energetic Particles in the HLPS and the Cusp

In order to understand the origin of the energetic particles in the cusp we have tostudy the energetic particle fluxes in the adjacent HLPS also. To this end, we per-formed an extensive statistical analysis of energetic particles in the HLSP and thecusp which was presented in Asikainen and Mursula [1]. Since then we have re-vised and extended our database to cover the January–April in 2002 and 2003. Forthis study on each Cluster orbit in this time range we identified the HLPS (22 obser-vations) and cusp (26 observations) regions. We calculated average energetic protonand electron fluxes measured by the RAPID at different energy channels. In addi-tion we calculated the simultaneous average solar wind and IMF parameters as wellas Kp and AE indices. For solar wind and IMF we used the high resolution (1 min)OMNI data from OMNIWeb (see http://omniweb.gsfc.nasa.gov/) in which the datahave been time shifted to the bow shock nose (for the details of the time shiftingprocedure please refer to the OMNIWeb website). Assuming a connection between

422 T. Asikainen

the near-Earth tail and the high-latitude dayside we have to take into account thepropagation time of energetic particles from the tail into the dayside. For 10s to100s of keV particles the drift time was estimated to be at most 3 h. When we calcu-lated the AE index averages we took the average from up to 3 h before the Clusterobservation time to account for the particle drift.

We found a good correlation between the energetic ion fluxes and AE/Kp in-dices in the HLPS at all energy channels. The correlation coefficients between ionfluxes and AE and Kp indices were between 0.52 and 0.75 at all energy channelsand they were all statistically significant. Electron fluxes in the HLPS were foundto correlate with solar wind speed with correlation coefficients at different energychannels ranging from 0.51 to 0.6. The electron fluxes also correlated with the mag-nitude of IMF By component (i.e., IMF jBy j). The correlation coefficients with IMFjBy j ranged from �0.55 to �0.6, thus the electron fluxes tend to be higher in theHLPS when IMF jBy j is small. These correlations suggest that the energetic parti-cles observed in the HLPS adjacent to the cusp come from the inner magnetosphereand/or near-Earth magnetotail where they are accelerated by substorms (thus theion flux-AE correlation) or processes responsible for the acceleration of radiationbelt electrons (thus the electron flux-solar wind speed correlation). The transport ofenergetic particles especially during substorms from the near-Earth magnetotail tothe HLPS/cusp has been shown possible by Delcourt and Sauvaud [7]. The negativecorrelation between IMF jBy j and the electron fluxes may be understood in termsof reconnection. When By dominates the reconnection has been observed to occurnear the cusp edges, i.e., at the closed HLPS field lines (see the discussion in theprevious section). At these times electrons which are faster than ions and bound tothe field lines more tightly than ions escape along the opened field lines more readilythan ions.

Turning on to the cusp we found significant differences in the behavior of en-ergetic particle fluxes compared to the HLPS region. We found that ion fluxes inthe cusp correlate mainly with IMF jBy j and AE index and that the correlation de-pends strongly on ion energy. Figure 29.4 shows the partial correlation coefficientsbetween energetic ion flux and log(AE) and IMF jBy j. Because partial correlationmeasures linear correlation we calculated the coefficient for log(AE) which displaysbetter linear relation with ion flux than just AE. One can see that at lower energiesthe fluxes correlate more with IMF jBy j and at higher energies more with AE index.The crossover takes place at the second energy channel which corresponds to about75 keV. These relationships show that the reconnection near the cusp when By dom-inates has a strong and statistically significant role in the release of energetic ionsinto the cusp from closed field lines. It seems that reconnection is the dominantfactor for ions below 75 keV. At higher energies the fluxes depend more on the AEindex which suggests that ions above 75 keV can more or less directly diffuse intothe cusp from closed field lines. Furthermore, transport of energetic particles with100s of keV energy from the near-Earth tail to the high-latitude dayside regions hasbeen shown to be enhanced during substorms [7].

Energetic electrons in the cusp behave totally differently from the ions. Theyshow correlation only with IMF Bz and the correlation coefficients are in the range

29 Energetic Particles in the Cusp: A Cluster/RAPID View 423

Fig. 29.4 Partial correlation coefficients between energetic ion flux in the cusp and log(AE) indexand IMF jBy j as a function of ion energy

0.51 to 0.65 at all energy channels. Thus the electron fluxes are higher in the cuspwhen IMF is northward. The electrons in the cusp do not show statistically sig-nificant dependence on AE or Kp indices or solar wind speed (as was observedin the HLPS region). To understand the correlation with IMF Bz let’s consider thecusp geometry during northward and southward IMF. During northward IMF thereconnection preferentially occurs poleward of the cusp producing a more closedfield line topology than during southward IMF when subsolar reconnection opensthe cusp field lines. At 10s to 100s of keV energy electrons are relativistic and inan open field line topology they are lost within seconds. A more closed field linetopology is expected to trap them for a longer time and more efficiently. Thus re-gardless of the strength of the source producing the electrons, their fluxes in thecusp are mainly determined by how efficiently they can be confined there, i.e., howclosed the cusp field line topology is. To reveal a possible dependence of the elec-tron fluxes on a source process inside the magnetosphere we separated from thedata those events where IMF was northward. Figure 29.5 shows the lowest energychannel electron flux (energy about 40 keV) with respect to AE index. One can seethat taking all the observations (left hand side of Fig. 29.5) there is no correlation.However, taking only events with northward IMF (right hand side of Fig. 29.5) whenthe cusp electron trapping is expected to be more efficient, a weak but statisticallysignificant correlation of 0.41 between the electron fluxes and AE index is revealed.

424 T. Asikainen

0 200 400 600 800100

101

102

103

AE index [nT]

Correlation coefficient 0.15

0 200 400 600 800101

102

103

AE index [nT]

Correlation coefficient 0.41el

ectr

on fl

ux. [

1/cm

2 s s

r ke

V]

elec

tron

flux

. [1/

cm2 s

sr

keV

]Fig. 29.5 Relationship between energetic electron flux (energy approx. 40 keV) in the cusp andAE index. Left hand side: All events included, right hand side: only events with northward IMFincluded

This suggests that also electrons in the cusp come from within the magnetosphereand are transported into the cusp from the near-Earth magnetotail especially duringsubstorms.

29.6 Conclusions

The results reviewed above suggest that the origin of energetic particles in thehigh-latitude dayside regions, both the HLPS and the cusp is in the near-Earthmagnetotail. During substorms the transport of energetic particles from the tail isenhanced and this is reflected in the positive correlation between the energetic ionfluxes and AE index in the HLPS and the cusp. Detailed case studies and largescale statistics showed that two mechanisms operate in transporting energetic par-ticles into the cusp: release by reconnection from the adjacent closed field linesand direct diffusion. For ions below �75 keV energy release by reconnection seemsto dominate and for more energetic ions direct diffusion dominates. Electrons inthe cusp are also transported there from within the magnetosphere but their flux ismostly determined by cusp magnetic field topology, i.e., whether cusp field lines areopen (southward IMF) or closed (northward IMF). Electron fluxes are higher whenthe cusp field lines are closed. These observations provide a consistent explanationfor the cusp energetic particles without a need for significant local acceleration ofshocked solar wind plasma to MeV energies. While some energy transfer from theelectromagnetic waves and turbulence does occur in the cusp it cannot explain theobservations presented here.

29 Energetic Particles in the Cusp: A Cluster/RAPID View 425

Acknowledgements The author would like to thank K. Mursula for fruitful co-operation andP. Daly for useful discussions regarding the RAPID instrument. The Cluster Active Archive, FGM,CIS and RAPID PI-groups are greatly acknowledged for providing high quality data used in thispaper.

References

1. Asikainen, T., Mursula, K.: Energetic particle fluxes in the exterior cusp and the high-latitudedayside magnetosphere: Statistical results from the Cluster/RAPID instrument, Annales Geo-physicae, 23, 2217–2230 (2005)

2. Asikainen, T., Mursula, K.: Reconnection and energetic particles at the edge of the exteriorcusp, Annales Geophysicae, 24, 1949–1956 (2006)

3. Balogh, A., Dunlop, M. W., Cowley, S. W. H., Southwood, D. J., Thomlinson, J. G., et al.: TheCluster Magnetic Field Investigation, Space Science Reviews, 79, 65–91 (1997)

4. Chang, S.-W., Scudder, J. D., Fuselier, S. A., Fennell, J. F., Trattner, K. J., Pickett, J. S., Spence,H. E., Menietti, J. D., Peterson, W. K., Lepping, R. P., Friedel, R.: Cusp energetic ions: A bowshock source, Geophys. Res. Lett., 25, 3729–3732 (1998)

5. 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.: Cusp energetic particle events: Implications for amajor acceleration region of the magnetosphere, J. Geophys. Res., 103, 69–78 (1998)

6. Chen, J., Fritz, T. A., Sheldon, R., Pickett, J., Russell, C.: The discovery of a new accelerationand possible trapping region of the magnetosphere, Adv. Space Res., 27, 1417–1422 (2001)

7. Delcourt, D., Sauvaud, J.-A.: Populating of Cusp and Boundary Layers by energetic (hudredsof keV) equatorial particles, J. Geophys. Res., 104, 22 635–22 648 (1999)

8. Fritz, T. A., Chen, J., Sheldon, R., Spence, H., Fennell, J., Livi, S., Russell, C., Pickett, J.: CuspEnergetic Particle Events Measured by POLAR Spacecraft, Phys. Chem. Earth, 24, 135–140(1999)

9. Fritz, T. A., Chen, J., Siscoe, G.: Energetic ions, large diamagnetic cavities and Chapman-Ferraro cusp, J. Geophys. Res., 108, 1028, (2003)

10. Kremser, G., Woch, J., Mursula, K., Tanskanen, P., Wilken, B., Lundin, R.: Origin of energeticions in the polar cusp inferred from ion composition measurements by the Viking satellite,Ann. Geophys., 13, 595–607 (1995)

11. Meng, C.-I., Case studies of the storm time variation of the polar cusp, J. Geophys. Res., 88,137 (1983)

12. Onsager, T. G., and Lockwood, M.: High-latitude particle precipitation and its relationship tomagnetospheric source regions, Space Sci. Rev., 80, 77–107 (1997)

13. Palmroth, M., Laakso, H., Pulkkinen, T., Location of high-altitude cusp during steady solarwind conditions, J. Geophys. Res., 106, 21109–21122 (2001)

14. Reiff, P. H., Hill, T. W., Burch, J. L.: Solar wind plasma injection at the dayside magnetosphericcusp, J. Geophys. Res., 82, 479–491 (1977)

15. Reme, H., Bosqued, J. M., Sauvaud, J. A., Cros, A., Dandouras, J., Aoustin, C., et al.: TheCluster Ion Spectrometry (CIS) Experiment, Space Sci. Rev., 79, 303–350 (1997)

16. Roederer, J. G., Dynamics of geomagnetically trapped radiation, Springer-Verlag BerlinHeidelberg New York, pp. 66 (1970)

17. Russell, C., POLAR eyes the cusp. In R. Harris (Ed.), Proceedings of the Cluster-II Workshop:Multi-scale/Multipoint Plasma Measurements, ESA SP-44, vol. 90, pp. 47–55, European SpaceAgency, Noordwijk, 2000

18. Shabansky, V. P.: Some processes in the magnetosphere, Space Sci. Rev., 12, 299–418 (1971)19. Sheldon, R., Spence, H., Sullivan, J., Fritz, T. A., Chen, J.: The discovery of trapped energetic

electrons in the outer cusp, Geophys. Res. Lett., 25, 1825–1828 (1998)20. Trattner, K., Fuselier, S., Peterson, W., Chang, S.-W., Friedel, R., Aellig, M.: Origins of ener-

getic ions in the cusp, J. Geophys. Res., 106, 5967–5976 (2001)

426 T. Asikainen

21. Wilken, B., Axford, W. I., Daglis, I., Daly, P., Guttler, W., Ip, W. H., Korth, A., et al.: RAPID:The Imaging Energetic Particle Spectrometer on Cluster, Space Science Reviews, 79, 399–473(1997)

22. Wing, S., Newell, P. T., Onsager, T. G.: Modeling the entry of magnetosheath electrons into thedayside ionosphere, J. Geophys. Res., 101, 13155–13167, (1996)

23. Zhang, H., Fritz, T. A., Zong, Q.-G., Daly, P. W.: Stagnant exterior cusp region as viewedby energetic electrons and ions: A statistical study using Cluster Research with AdaptiveParticle Imaging Detectors (RAPID) data, J. Geophys. Res., 110, A05211, doi:10.1029/2004JA010562, (2005)

24. Zong, Q.-G., Fritz, T. A., Korth, A., et al.: Energetic electrons as a field line topology tracer inthe high latitude boundary/cusp region: Cluster RAPID observations, Surveys in Geophysics,26, 215–240, (2005a)

25. Zong, Q.-G., Fritz, T. A., Spence, H., Zhang, H., Huang, Z. Y., Pu, Z. Y., Glassmeier, K.-H.,Korth, A., Daly, P. W., Balogh, A., Reme, H.: Plasmoid in the high latitude boundary/cuspregion observed by Cluster, Geophys. Res. Lett., 32, doi:10.1029/2004GL020960, (2005b)