ENERGETIC ELECTRONS AS A FIELD LINE TOPOLOGY TRACER IN THE HIGH LATITUDE BOUNDARY / CUSP REGION: CLUSTER RAPID OBSERVATIONS

Transcription

1 ENERGETIC ELECTRONS AS A FIELD LINE TOPOLOGY TRACER IN THE HIGH LATITUDE BOUNDARY / CUSP REGION: CLUSTER RAPID OBSERVATIONS Q.-G. Zong, T. A. Fritz, H. E. Spence Center For Space Physics, Boston University, MA, USA A. Korth, P. W. Daly Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany. M. Dunlop, A. Balogh Space and Atmospheric Physics Group, Imperial College, London, UK J. F. Fennell Aerospace Corporation, Los Angeles, LA R. W. H. Friedel Los Alamos National Lab., Los Alamos, NM H. Reme CESR, Toulouse, France December 30, 2002 Abstract. Energetic electrons are unique in their capability to fully assess magnetic-fieldline topology and thus they should be able to clearly delineate regions of open and closed magnetic field lines in the high latitude regions and contributed crucially to understanding and resolving issues of topology in this region. The energetic electrons in the high latitude boundary regions (including cusp) have been examined in detail by using Cluster/RAPID data for four consecutive high latitude/ cusp crossings between 16 March to 19 March Energetic electrons with high and stable fluxes were observed in the time interval when the IMF had a predominately positive Bz component. These electrons appeared to be associated with a lower plasma density exhibiting no obvious tailward plasma flow (< 20 kev). On the other hand, no electrons or only spike -like electron events have been observed in the cusp region during southward IMF. At this time, the plasma density was as high as that in the magnetosheath and it was associated with a clear tailward flow. The fact that no stable energetic electron fluxes were observed during southward IMF indicates that the cusp has a general open field line geometry. The observations indicate that both the south and north high latitude magnetospheric boundary regions (including both north and south cusp) can be energetic particle trapping regions. The observation show that the observed electrons were locally trapped in a region which consisted of twisted closed field lines that could be formed by high latitude reconnection during IMF northward. The energetic electron observation provide new evidences to understand the dynamic cusp process. Finally, the trajectory tracing of test particles have been performed by using Tsyganenko 96 model which demonstrate that energetic particles could be indeed trapped in the high latitude magnetosphere for tens min to tens hours. Keywords: Magnetosphere, Cusp, Electron, Energetic Ions, Reconnection, Boundary Abbreviations: KAP Kluwer Academic Publishers; compuscript Electronically submitted article 2003 Kluwer Academic Publishers. Printed in the Netherlands. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.1

2 2 Q.-G. Zong et. al. 1. Introduction The boundaries of the magnetosphere including the polar cusp are key regions for the transfer of mass, momentum and energy from the solar wind into the magnetosphere when the IMF is southward or northward. The first identification of a thin layer of magnetosheath plasma located immediately inside the magnetopause was made by (Hones et al., 1972, ) who also introduced the term "Boundary Layer". Since then, the morphological characteristics as well as plasma properties of the magnetospheric boundary layer have been studied rather intensively (Rosenbauser, 1975; Eastman, 1976; Haerendel, 1978; Lundin, 1985; Lundin and Dubinn, 1985; Newell and Meng, 1988; Newell and Meng, 1998, ). The term "Low-latitude boundary layer", was apparently introduced by (Haerendel, 1978, ) to distinguish the very different properties observed at latitudes below about on the magnetopause surface. In addition there are three more boundary regions (in the high latitude) that are assumed to connect directly to the magnetosheath, they are the plasma mantle, the entry layer and the exterior cusp or stagnation region, seen in Figure 1. The plasma mantle which was first reported by (Rosenbauser, 1975) is located on the opened field lines where the injected magnetosheath plasma continues tailward. The plasma density in this region is less than but comparable the sheath density level and the β<<1. The entry layer (Paschmann et al., 1976, ) is located on the magnetospheric field lines just equatorward of the cusp. It is a region of diffusive, turbulent entry of magnetosheath plasma onto field lines that map to the low-altitude cusp. It has been so termed because it appears to be the region of dominant plasma entry into the magnetosphere. The transport mechanism is likely to be achieved through eddy convection which manifests itself in the irregular, low speed plasma flow, and may be excited by the turbulence in the adjacent exterior cusp (Haerendel, 1978). Localized reconnection has been proposed by (Haerendel, 1978), who envisioned it as an intermittent, small scale process related to eddy turbulence in the entry layer. This intermittent small-scale reconnection could be occurring in the the cusp region for both northward and southward IMF. In the entry layer, the plasma density is as high as or even higher than that in the magnetosheath, the temperature is very similar to that of the exterior cusp, and the plasma beta β 1. The exterior cusp / stagnation region is bounded on the inside by the cusp-like indentation of the magnetopause, and outside by the free-flow stream lines of the magnetosheath flow which (Sckopke et al., 1976; Sckopke et al., 1981, ) constitutes a pocket of hot and stagnant, possibly turbulent plasma. In fact, as earlier as 1960 s the stagnation region was already predicted by gas dynamics models (Spreiter and Summers, 1967; Spreiter and Stahara, 1980, ). Furthermore, this picture has been corroborated by HEOS 2 measurements electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.2

3 Filedline Tracer in the Cusp Region 3 Bow shock PM LLBL Diffusive entry Detached plasma MS Merging Eddy convection EL Inner magne t osphere Stagnation region Mid-altitude cusp (A) IMF southward Merging Bow shock MS PM? Stagnation region LLBL Diffusive entry Detached plasma High -altitude LLBL? Mid-altitude cusp Inner magne t osphere (B) IMF northward Figure 1. Sketch representation of the dayside boundary regions connection to the polar cusp field lines during southward (after haerendel, 1978) and northward IMF. MS, magnetosheath; PM, plasma mantle; LLBL, Low latitude boundary layer; EL, entry layer. (Sckopke et al., 1976; Sckopke et al., 1981, ). The stagnation region cannot be linked to the plasma mantle or LLBL in a simply way. A qualitative explanation was given by (Haerendel, 1978) who noted the similarity of the situation near the cusp to hydrodynamic flow around a corner, in which vortex formation and separation are known to occur and to initiate some level of turbulence (Figure 1). The exterior cusp region appears to be a steady high pressure center of "stagnant" magnetosheath plasma, the flow in this region is rather turbulent, both in magnitude and direction. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.3

4 4 Q.-G. Zong et. al. The mantle is generally thicker for southward than northward IMF B z (Sckopke et al., 1976, ). These original researchers believed that the plasma mantle is open, and the LLBL closed. It has been indicated that in the HLBL (Entry layer) the plasma density is almost as high as the magnetosheath but generally lacking the strong antisunward plasma flow. In fact sunward flow has even been reported by (Paschmann et al., 1976, ). (Lundin, 1985) suggest that a characteristic feature of the entry layer is a strong variability of magnetosheath plasma entry with frequent plasma injection. On the basis of the Defense Meteorological Satellite Program (DMSP) F2 data, (Newell and Meng, 1987) indeed observed that the cusp low-altitude latitudinal extent is narrower when B z is southward than when B z is northward. This has been interpreted that the enhanced convection flow is too rapid to allow the plasma to reach low altitudes. Furthermore, (Newell and Meng, 1987) indicate that there may not be simply slower tailward convection within the boundary layer but rather no tailward or even sunward convection when IMF B z is northward. In non-reconnection models the cusp position and extent are less sensitive to the IMF, but more strongly dependent on the solar wind ram pressure (Yamauchi and Lundin, 1998, ). It should be pointed out that indications of the existence of a boundary layer from the energetic particle measurements were provided even earlier. Energetic (> 25keV ) duskside particles are very often present on the magnetosheath field lines just outside the magnetosphere (Anderson et al., 1965; Haskell, 1969; West and Buck, 1976; Eccles and Fritz, 2002, ). Recently, observations by Geotail showed that the Earth s origin energetic ions (oxygen, helium, and hydrogen) leak out of the magnetosphere and can form layers in the equatorial magnetosheath (in the vicinity of the magnetopause) during intense storm activities. Energetic single charged oxygen ion enhancements are frequently detected in the magnetosheath and show occasionally a total duration of about 150 min (Zong and Wilken, 1998; Zong et al., 2001, ). In the high latitude region, layer-like energetic ions have been observed adjacent to the magnetopause outside the magnetosphere (in the magnetosheath) during IMF southward, whereas, layer-like energetic ions have been observed inside the magnetosphere during IMF northward. The energetic particles in this region are also highly anisotropic, exhibiting a clear sunward flow or bi-direction flow (sunward and anti-sunward) even if geomagnetic activity is very quiet (Zong et al., 2002, ). Energetic ions with energies from tens kev up to MeV have been observed near the magnetopause at the high latitude and the cusp region (Aparcio et al., 1991; Kremser et al., 1995; Chen et al., 1997; Chen et al., 1998; Chang et al., 1998; Fritz et al., 1999; Fritz, 2000; Chang et al., 2000; Fritz, 2001; Trattner et al., 2001, e.g.), although (Roederer, 1970) showed that the drift paths of energetic particles in the outer magnetosphere intersect the magnetopause which implies that there are no energetic particles stably trapped in electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.4

5 Filedline Tracer in the Cusp Region 5 the high latitude cusp region. The origin of the energetic particle in the cusp region has been a subject of controversy. (Chen et al., 1998) and (Fritz, 2000) argue that the particles observed in the cusp region are the result of a localized acceleration mechanism and should be one of the sources for magnetospheric energetic particles. On the other hand, (Chang et al., 1998; Trattner et al., 2001) suggested that the energetic particles in the cusp region are originated from either the bow shock or magnetosphere itself. In this case no local acceleration is needed. A possible clue for the solution to this puzzle has been suggested by (Sheldon et al., 1998) and (Delcourt and Sauvaud, 1998). They pointed out that a particle will drift on a closed path around the front of the magnetosphere and suggested that a possible stable trapping region may exist in the outer cusp. This kind of particle motion has been further explored by (Fritz, 2000) and has been applied to explain the origin of energetic particles in the high latitude boundary during very quiet geomagnetic conditions for both IMF north and southward conditions (Zong et al., 2002, ). Further measurements of energetic electrons are therefore essential for search the answers to those questions. A layer of energetic electrons (> 40keV ) lying primarily outside the magnetopause is found at high latitudes near the dusk-dawn meridional plane from the dayside to the distant magnetotail (Meng and Anderson, 1970; Meng and Anderson, 1975, ). The IMP-8 spacecraft had on-board sensors of large geometric factors that provided very sensitive measurements of 200 kev electrons at 35 R E geocentric distance. With these nearly continuous measurements, (Baker and Stone, 1977a; Baker and Stone, 1977b; Baker and Stone, 1977c) have shown that the energetic electron magnetopause layer is persistently present along the distant magnetotail at essentially all latitudes and that the 200keV electrons within the layer are strongly streaming tailward along the local magnetic field. Energetic electron layers were discovered carrying significant energy flows outside the magnetopause and this energy flux was seen to vary with the nature of the solar wind-magnetosphere interactivity. Furthermore, the energetic electron layer are observed most often in the dawn-side of the plasma sheet and protons in the dusk-side (Meng et al., 1981, ). The acceleration of ionospheric electrons to energies up to about 10 kev in the boundary region adjacent to the cusp was also reported by (Kremser and Lundin, 1990, ). Burst of energetic electrons (from >40 to 2000 kev) were observed in the magnetosheath and in the solar wind. A statistical study (Formisano, 1979, ) demonstrated that those electrons are of cusp origin: the flux intensity is highest in the exterior cusp region and decreases away from it. With increasing distance from this place the energy spectrum becomes harder (only energetic particles are able to go away). The measured anisotropy and magnetic field indicates that these particles are propagating away from the exterior cusp along the magnetic field lines. The observations of these electrons is also correlated with intense geomagnetic activity as electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.5

6 6 Q.-G. Zong et. al. measured by Dst. However, these studies may mix both the solar energetic electrons (Lin, 1985; Klassen et al., 2002) and the magnetospheric origin electrons because these studies were based on single satellite measurements. At that time there was no solar energetic electron monitor, like the ACE, Wind, or SOHO satellites in the upstream interplanetary medium. By using energetic electron observations, early work clearly delineated regions of open and closed magnetic field lines in the midtail region and thus contributed crucially to understanding the substorm dynamics. Subsequent work using the IMP-8 electron sensors (Bieber et al., 1982, ) demonstrated in a clear way that energetic electrons, and only energetic electrons, can be used to fully assess magnetic-field-line topology and thus distinguish between competing magnetotail dynamic models. This work remains definitive and similar methods were used extensively on ISEE-1 and -2 (Fritz et al., 1984, ) to assess where, when and how acceleration and transport processes occur in and around the magnetosphere. Bursty energetic electron events with energy of kev have been found in the cusp region by the CLUSTER spacecraft (Zong et al., 2003, ). These electron events are characterized by strong impulsive increases in the flux whereas energetic ions show no obvious changes. These bursts of electrons are well confined in a small scale flux rope structure and further could be explained as that energetic electrons leak out of the magnetosphere into the cusp through a transient reconnection in the cusp stagnation region. In this paper, the energetic electron behaviour in the high latitude magnetospheric regions has been examined in detail by using Cluster/RAPID data for four consecutive high latitude/ cusp crossings between 16 March to 19 March 2001 (two orbits). Both spike-like and stably trapped electrons were observed during different solar wind /IMF and geomagnetic activity conditions. Energetic electrons are uniquely qualified to fully assess magnetic-field-line topology and thus distinguish the open and close field lines in the high latitude boundary / cusp region. 2. Observations The data to be presented here were obtained by the RAPID instrument on board the Cluster spacecraft (Wilken et al., 1997, ). The energetic particle spectrometer RAPID in the CLUSTER payload features novel detection principles both for ions and electrons: For either species the instrument measures the vector velocity (V) and the energy (E). Each of the ion detector heads (IIMS) is composed of a time-of- flight (TOF) / energy (E) telescope with a solid state detector (SSD) as the back element. Species identification comes from the function ET 2 = A with E and T denoting the measured quantities electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.6

7 Filedline Tracer in the Cusp Region 7 energy and time-of-flight (equivalent to the particle velocity), A is the atomic mass of the particle. The energy range extends from 30 to 4000 kev. The advanced design of the telescope as a projection camera returns also information on the particle direction of incidence within 12 angular intervals over a field-of-view. A second detector system, the imaging electron spectrometer (IES), is dedicated to electrons between 20 to 400 kev. The IES head uses the optical principle of a pinhole camera with 9 angular intervals over Together with a sectored spin plane of the spacecraft, both systems cover the unit sphere in velocity space in a contiguous manner. Detailed information about the RAPID instrument has been given in (Wilken et al., 1997). Furthermore, we use magnetometer measurements from the fluxgate magnetometer (FGM) on board Cluster, which makes high resolution vector field measurements (Balogh et al., 1997, ) and plasma data from Cluster Ion Spectrometer (CIS) experiment (Reme et al., 1997, ). The criteria of the region identification in this paper is mainly based on the plasma data together with the energetic particle behaviour and magnetic field data. From March 16 to 19, 2001, the four Cluster satellites crossed the high latitude boundary and/or cusp regions four times (two in the southern hemisphere and the other two in the northern hemisphere) in two consecutive orbits, see Figure 2. Figure 3 shows the different regions south cusp, radiation belt and the north cusp as obtained by Cluster/RAPID during these two consecutive orbits. The geoactivity Dst index is shown in the top panel. The first two cusp crossings (1 and 2) happened during rather quiet geoactivity time period as reflected from the Dst index. In that time period, the Dst indices were small and positive. The later two crossings (3 and 4) occurred in the main phase of a strong magnetic storm. As we can see from Figure 3, the high latitude magnetosphere (both north and south cusp) regions are two of three locations that energetic particle are encountered. Both south and north high latitude boundary and/or cusp regions can be distinguished easily by the fact that (1) the ion flux increased sharply in all energy channels from 30 to 400 kev, whereas (2) energetic electron flux appeared in the quiet time and was not present in the disturbed time HIGH LATITUDE BOUNDARY /CUSP: QUIET TIME CROSSINGS (IMF NORTHWARD) In order to inspect energetic electron events in the high latitude boundary / cusp region in more detail, the fluxes of energetic electrons and ions together with plasma parameters and the local magnetic fields for two consecutive quiet time crossings are given in Figure 4. At about 17:06UT, March 16, 2001, the Cluster spacecrafts crossed the magnetopause from the magnetosheath into the high latitude boundary / cusp electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.7

8 8 Q.-G. Zong et. al. 18/ /3 Bow shock 19/ /3 1 16/3 17h00-19h10m South HLBL/Cusp quiet 2 17/3 06h00-09h20m North HLBL/Cusp quiet 3 19/3 01h00-02h30m South HLBL/Cusp activity 4 19/3 14h50-19h00m North HLBL/Cusp activity Figure 2. The Cluster trajectories through the Tsyganenko magnetic field model (Tsyganenko, 1996) for March 16 to 19, Only one Cluster satellite is shown since the separation is too small to be resolved. The Cluster satellites trajectory and the Tsyganenko magnetic field lines have been projected to GSE XZ planes. region and at around 19:07 entered the cusp region in the southern hemisphere, see Figure 4Left. As shown in Figure 4Right, on the Mar. 17, 2001, the Cluster spacecraft were travelling outbound from the north hemisphere. Starting in the northern lobe, the Cluster spacecraft crossed through the cusp from 04:48 UT to 06:00 UT into the high latitude boundary region and finally entered the magnetosheath at about 09:20UT. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.8

9 Proton ( (>30keV) South HLBL/Cusp 1 Data gap gap Data gap gap North HLBL/Cusp 4 Filedline Tracer in the Cusp Region Electron (>20 kev) Radation belt North HLBL/ HLBL/ Cusp Cusp 2 2 Bow shock & upstream South HLBL/Cusp HLBL/Cusp 3 3 Radation belt Dst index Figure 3. An overview of RAPID from 09:00 UT, March 16 to 21:00 UT, March 19, 2001 together with geomagnetic activity Dst index. From the top the panels show: Dst index; electron spectra from 20 to 400 kev; and proton spectra from 30 to 2000 kev. The marks indicate the different regions south HLBL/cusp, radiation belt and the north HLBL/cusp which Cluster experienced during its two consecutive orbits. The HLBL/Cusp here refers to all of the high latitude magnetospheric regions, the radiation belt here refers all inner magnetospheric region, the ion data gap is shaded in grey. The top panels (left and Right) in Figure 4 show energy-integrated ion and electron flux versus time profiles. The second panel shows the plasma density variation. The third and fourth panels show plasma velocity, Vx, electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.9

10 Electron Proton Electron Proton 10 Q.-G. Zong et. al. South Hemisphere Sheath HLBL Cluster / Rumba Mar Mantle Cusp 3 Cusp North hemisphere Mar. 17, 2001 Sheath HLBL 5 Density Density Vx Vx,Vy Vy,Vz Vz Bx Bx,By By reflection point By reflection point Bz reflection point By,Bz Bz Bz reflection point IMF Bz IMF Bz 04:00 08:00 12:00 Figure 4. Overview plots of RAPID, CIS and MGF data from 16:00 to 20:00 UT, March 16, 2001(Left) and from 04:00 to 12:00 UT, March 17, 2001(Right) during northward IMF. From the top the panels show: integral electron flux; proton flux; plasma density; plasma velocity (Vx, Vy and Vz) and magnetic field GSE components and magnitude (in nt). IMF Bz for March 17, 2001 is over-plotted in the panel 6, the time lag is adjusted. The vertical lines mark the different regions sheath, cusp, and the mantle which Cluster experienced. Vy and Vz. The last three panels in Figure 4 show the components of the magnetic field obtained by Cluster FGM instrument. As we can see from Figure 4, when the spacecraft moved into a region that we interpret as the mantle, electron and proton fluxes are only slightly higher than the background level, the plasma density is enhanced towards electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.10

11 Filedline Tracer in the Cusp Region 11 (1/cm s ster mev) 2 12:00 12:00 12:00 12:00 Figure 5. Differential energetic electron fluxes obtained by the ACE spacecraft at 1st Lagrange point upstream from the Earth for two energy ranges (38-53 kev and kev). magnetosheath density level, the intensity of the magnetic field is pretty high with a little fluctuation. The magnitude of the magnetic field in the lobe and mantle region is 50 to 100 nt. Both south (Figure 4Left) and north (Figure 4Right) high latitude boundary / cusp regions can be distinguished from simply entry into a high flux region of energetic electron and ions by the fact that (1) Associated plasma velocity Vx decreased sharply to around 0, whereas Vz showed surprisingly a significant enhancement at around -75 km/s (Figure 4); (2) Large and stable Bx, around 25 nt in the southern hemisphere (Figure 4Left) and -25 nt in the northern hemisphere (Figure 4Right). Both B y and B z changed the polarity during the spacecraft crossing the high latitude boundary / cusp region. B y changed its polarity first, then B z (Figure 4); (3) The plasma density in this region was higher than that in the lobe but lower than that in both the mantle and sheath. (4) The total magnetic field intensity was 25 nt with less fluctuation in Figure 4; (5) These regions had clear interfaces with the magnetosheath. During the above two quiet time high latitude boundary / cusp crossings, there were pronounced fluxes of electrons found in the high latitude boundary/cusp region, indicating closed field line geometry in the cusp region, a electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.11

12 12 Q.-G. Zong et. al. special open field line configuration could trap electrons very efficiently for a long time or a long-lived source supplying these electron to open field lines. These electrons lasted about 2 hours (1706 to 1907 UT, Mar 16, 2001) and 3 hours 20 min (0600 to 0920 UT, Mar 17, 2001) respectively. Furthermore, no obvious substorm injections were observed by the Los Almos satellites for both of the above quiet time high latitude boundary /cusp crossings (not shown here). Further, there were not energetic electron events observed by ACE in the upstream interplanetary space (Lagrangian 1 point) during Mar. 16 to 19, 2001 as documented in Figure 5. Thus, these observed electrons should not be solar energetic electron as described by (Lin, 1985; Klassen et al., 2002). The lack of substorm activity and high fluxes of electrons upstream at ACE indicated that the observed electron are locally trapped electron than substorm injected electron drifting to the high latitude region or solar flare related electrons HIGH LATITUDE BOUNDARY /CUSP: DISTURBED TIME CROSSINGS (IMF SOUTHWARD) In contrast with Figure 4, two consecutive HLBL/cusp crossings during geomagnetically disturbed times are shown in Figure 6. As we can see from Figure 6, there were no significant electrons found in the same place as in the previous Cluster orbit (see Figure 2) during the geomagnetically quiet time. The magnetic field data and the plasma density show no clear evidence to indicate where the magnetopause is located (Figure 6Left), although within the cusp and the boundary region there existed a lot of structure during the storm time (Figure 6Right, dst was around -40 nt). Only the energetic ions seem to provide some indication of the interface between the cusp and the magnetosheath. Both the southern cusp (Figure 6Left) and the northern cusp (Figure 6Right) can be distinguished by the fact that 1. The plasma density in the cusp is comparable with that in the magnetosheath. 2. In contrast with the quiet time cusp, the plasma velocity components, V x and V y are significant. 3. The magnetic field becomes more turbulent in the region labelled cusp than that in the magnetosheath as seen in the bottom panel of Figure The energetic proton flux increases dramatically, but there are no pronounced fluxes of energetic electrons present, indicating a probable open field line geometry in the cusp region during IMF southward or a lack of a source of these electrons. 5. Not only the magnetic field, but also the plasma density, and velocity are rather turbulent. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.12

13 Vz Filedline Tracer in the Cusp Region 13 Proton Sheath South hemisphere Cusp Cluster/ Rumba, Mar. 19, 2001 Mantle Lobe 4 Lobe Cusp North hemisphere Boundary layer Proton Electron Density Density Vx,Vy Vx,Vy Bx,By By IMF Bz Bz Bx Bx By Figure 6. RAPID, CIS and FGM summary plots from 00:00 to 04:00 UT, March 19, 2001 (Left) and from 0700 to 10:30 UT, March 19, 2001 (Right) during southward IMF. From the top the panels show: Integral electron flux; proton flux; plasma density; and GSE components and magnitude of the magnetic field (in nt). IMF Bz between 00:00 to 04:00 UT March 19, 2001 is over-plotted in the panel 6, the time lag is adjusted. The vertical lines mark the different regions lobe, mantel, cusp, and the sheath which Cluster experienced on March 19, Table I provide a synoptic view of these four cusp crossing during different geomagnetic activities and IMF orientation. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.13

14 14 Q.-G. Zong et. al. Table I. HLBL/Cusps Property Events HLBL/Cusp 1 HLBL/Cusp 2 HLBL/Cusp 3 HLBL/Cusp 4 Time 17:00-19:10 UT 06:00-09:20 UT 01:00-02:30 UT 14:50-16:00 UT Mar. 16, 2001 Mar. 17, 2001 Mar. 19, 2001 Mar. 19, 2001 Location south, 11.3 MLT north, 11.4 MLT south, 11.5 MLT north, 11.7 MLT Electron 10 4, stable 10 3, stable none multi-pulse Proton 5x10 5, stable 10 5, stable 10 5, stable 5 x 10 5, complex Local Plasma Vx no obvious no obvious significant -Vx significant -Vx Interface M clear clear clear clear (to the Mantle) Interface S clear clear not clear not clear (to the sheath) IMF, Solar Wind IMF Bz 0.5 nt 3.3 nt -4.4 nt -9.3 nt Dynamic Pressure 1.15 npa 1.01 npa 1.06 npa 5.03 npa Geoactivity quiet quiet activity storm Kp Dst 1nT 4nT -10 nt -40 nt 3. Interpretation and Discussion The Cluster separation distances were around 600 km during the time of the interest (Mar. 16 to 19, 2001). The remote-sensing ranges of the four spacecrafts for energetic ions overlap and the ion performance of the tetrahedron may be reduced to that of a single platform. However, the 30 kev electron gyroradii (even 100 kev) are always small compared to the size of the tetrahedron. The electron measurements obtained by the 4 different satellites can provide nearly instantaneous information about changes in the field configuration. The existing energetic electrons in the cusp region are vital in order to investigate the reconnection process because they are rather reliable indicator for closed and open field line topology in the cusp region, and to which the flux tube is connected. A 25 kev electron travels along field lines with a speed of nearly 14 R E /s, so, the swift electrons will trace-out field lines in a rather short time (a few seconds) in an open field line region. If an electron event exists for a few minutes which is much longer than their bounce time (about 1.5 s for 25 kev electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.14

15 Filedline Tracer in the Cusp Region 15 electrons at L=8), it means that a resupply process must exist. If no stable energetic electron fluxes have been observed this would indicate the cusp has a general open field line geometry, during IMF southward. The observed energetic electrons cannot be accelerated by the reconnection process in the adjacent magnetopause. The magnetic tension at the magnetopause applied by reconnection can only add a velocity on the order of the Alfvén velocity, associated with the change in B across the magnetopause. For a 50 nt field rotated by 180 0, B is 100 nt. Assuming a typical hydrogen plasma density of 20 cm 3, the Alfvén velocity is 490 km/s. For electrons this amounts to an energy gain of about 1 ev. However, the observed electrons have energies above 25 kev, which means that such an acceleration from reconnection is negligible. Also, although the it is claimed that Fermi mechanism in the quasi-parallel bow shock region could accelerate incident solar wind ions to energies up to about 200 kev, it can not accelerate electrons efficiently (Lee, 1982, ). Further, during the time period from Mar. 16 to 19, 2001, there are no solar energetic electron events (Lin, 1985; Klassen et al., 2002) observed by ACE at Lagrangian 1 point satellite in the interplanetary space (see Figure 5) THE INTERFACE TO THE MAGNETOSHEATH The variation of the energetic electron and ion fluxes for the boundary crossing observed by all Cluster spacecraft from 0919 to 0923 UT, Mar. 17, 2001 are given in Figure 7Left. As a reference, the plasma parameters and the magnetic field are also shown in Figure 7Left. A discontinuity between 0921 and 0922 UT can be seen in all panels of Figure 7Left, this occurred during the northward IMF (Figure 4). After 0922 UT, when the Cluster satellites crossed the magnetopause into the magnetosheath, the flux of energetic particle decrease quickly to the background level. This may indicate that the Cluster satellites moved far away from the magnetopause or that no particles escaped from the magnetosphere during the northward IMF. The spacecraft crossed the magnetopause at different times although the difference is quiet small. The numbers C1, C2, C3, and C4 represent the different Cluster spacecrafts: Rumba (1), Salsa (2), Samba (3), Tango (4). The arrows indicate the encounter time of the magnetopause. The encounter time of the proton and electron flux detected by four different satellites is slightly different. The maximum time difference between C3 (Samba, the earliest satellite to encounter the magnetopause) and C2 (Salsa, the last spacecraft) is 20 seconds for protons and 10 seconds for electrons. Using a technique described by (Dunlop and Woodward, 1998), the thickness of the magnetopause could be estimated to be d = 200km. In order to determine whether the magnetic field in this boundary layer is categorized as some MHD discontinuity, the magnetic field data was analelectron_tracer_kluwer.tex; 8/03/2003; 11:52; p.15

16 16 Q.-G. Zong et. al. Mar. 17, 2001 Mar.19, 2001 proton C1 C2 C3 C4 C1 C2 C3 C4 Proton Electron Electron Density Density Vx, Vy Vx, Vy Vz Vz Bx,By Bx,By Bz Bz Figure 7. Interface crossings from the cusp region to the magnetosheath at between 0919 and 0923 UT on March 17, 2001 during northward IMF(Left) and at between 0000 and 0100 UT on March 19, 2001 during southward IMF (Right). From the top panels: integral electron and proton flux; plasma ion density, plasma ion velocity Vx, Vy and Vz (in km/s) and superposed magnetic field Bx, By and Bz components. ysed by minimum variance analysis (MVA) (Sonnerup and Cahill, 1967; Sonnerup, 1976; Elphic and Russell, 1983; Hapgood, 1992, ). The analysis concentrated on variations involving a large and rapid change in both the magnetic field and energetic particle flux. This method is based on determining the directions of maximum and minimum variation for a given magnetic electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.16

17 Filedline Tracer in the Cusp Region 17 field vector data set. Choose a direction unit vector n, to let the value of m [(B i=1 i <B>) n] 2 (1) be the minimum and maximum. Here B i is the ith vector, < B > is the average field, denotes the sum over i. The minimization and maximization of Eq. 1 are accomplished by the method of Lagrange multiplies, and this in turn can be set up as a matrix diagonalization (or eigenvalue) problem. The solutions of the matrix M associated with the directions of maximum and minimum variance of the vector field B i. The minimum variance vector defines the boundary normal direction n. A third eigenvalue and eigenvector together with the maximum and minimum variance vectors completes the right-handed coordinate system. The boundary crossings occurred starting from 09:21:20 UT to 09:22:30 UT. The value of the normal component is -1.8 nt. The directions of the boundary normal coordinates system (i.e., the eigenvectors) in GSE are given as follows: (1) the minimum variation direction, Bx*=( ); (2) the intermediate variation direction, By*=( ); (3) the maximum variation direction, Bz*=( ). The ratio of the intermediate to minimum eigenvalue is 14.1; and the B z are This indicates the normal component for the boundary is well established (Sonnerup B and Cahill, 1967; Lepping and Behannon, 1980, ). The magnetopause boundaries with B n = 0 are referred to as rotational discontinuities (Landau and Lifshitz, 1960; Sonnerup and Ledley, 1979a, ). Thus, the magnetopause was a rotational discontinuity indicating that high latitude reconnection was ongoing. For a rotational discontinuity, a non-vanishing normal magnetic field component, B n, is allowed in a current sheet, which changes the physics of the current sheet dramatically. In a nonisotropic plasma, the pressure P = P and a change in the field magnitude on both side of the discontinuity can occur as a result of changes in the pressure anisotropy factor (Hudson, 1970; Hudson, 1971; Hudson, 1973; Sonnerup and Ledley, 1979b, ). The variation of the energetic electron and ion fluxes for boundary crossing obtained by all Cluster spacecraft from 0112 to 0116 UT, Mar. 19, 2001 are given in Figure 7Right. Note, this boundary crossing was made in the southern hemisphere during southward IMF (see Figure 2). From Figure 7Right, it can be seen that the flux of energetic ions increased after around 0114UT. After 0114 UT, the Cluster satellites crossed from the magnetosheath into the cusp region. This energetic particle layer was observed simultaneously by all Cluster satellites. In this event, the plasma and magnetic field data show no significant difference, there is no clear boundary layer that can be identified. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.17

18 18 Q.-G. Zong et. al nT 15 30nT 30 10nT 1.4nT 15nT nT nT nT 50nT 60nT 80nT 100nT nT nT nT ϕ = 0 ϕ =180 Figure 8. Meridional section noon-midnight for a modified two-dipole model (Antonova, 1968,Shabansky, 1968). The latitude φ is shown, on which the line of force with the given equatorial distance crosses the earth s surface. The narrow lines show the lines of B =const ENERGETIC ELECTRON AND IONS TRAPPED IN THE CUSP According to the tradition dipole field model, the dayside high latitude or cusp region cannot trap particles (Roederer, 1970; Roederer, 1977, ). The cusp region of the ideal dipole field is not an excluded region in the Sto mer theory(störmer, 1911, ). This means in the high latitude region, the particle can not be trapped much longer than the bounce time, the equal B will bring the particles away to the tail open field lines region. However, the dipole field electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.18

19 Filedline Tracer in the Cusp Region 19 can be modified fundamentally by the interaction with solar wind. The outer cusp regions where magnetic field lines either close in the dayside sector or extend into the night side sector over the polar cap caused by reconnection as proposed in the last section. This region is the locus of a weak magnetic field. This feature directly follows from interaction of the solar wind with the geomagnetic field and has been predicted as early as 1930 s by (Chapman and Ferraro, 1931) by using a simple image dipole and the early magnetic field model by (Antonova and Shabansky, 1968) (see Figure 8). This has also been supported by in situ magnetic field measurements (Zhou et al., 1997, ). Instead of a dipolar field, the cusp region appears to be quadrupolar. However, the importance of the existence of a B-minimum off equator in the outer cusp has been neglected or underestimated for a long period although it could be of extreme importance for understanding the energetic particles in the magnetosphere. In 1960 s (Antonova and Shabansky, 1968; Shabansky, 1968) already noted a minimum magnetic field existing off equator in the outer cusp region (Figure 8) and suggested that instead of drifting into the magnetopause, equatorial particles would branch off toward the magnetic minimum at high latitudes. Further, (Shabansky, 1971; Antonova and Shabansky, 1975) and (Antonova, 1996) provided observational evidences for trapping of energetic particles (several tens of kev up to a few hundreds of kev) in the high latitude region. (Sheldon et al., 1998) pointed out that an energetic electron will drift on a closed path around the front of the magnetosphere and found electrons could be trapped in the outer cusp, and such kind of particles motion orbit has been coined as "Sheldon orbit" (Fritz, 2000, ). (Delcourt and Sauvaud, 1998; Delcourt and Sauvaud, 1999) pointed out under the effect of the cuspward mirror force near the dayside magnetopause, energetic plasma sheet particles initially mirroring near the equator are expelled from low latitudes and subsequently swept into the boundary layer at high latitudes, this kind orbit has been coined as "Shabansky orbit" (Fritz, 2000, ). Figure 9 shows trajectories of test protons (1, 10, 100 kev) launched with 90 0 pitch angle from the cusp region. The trajectory tracing was performed using the Tsyganenko 96 model. In this tracing calculation, the full particles dynamics has been considered (not just the guding center computation) and it was performed using a fourth-order Runge-Kutta technique with a time step adjusted to some fraction of the particle gyration periods. It can be seen from Figure 9 that the test protons launched from the local minimum magnetic field region encircles the outer cusp region, all of the proton experience a more or less pronounced bouncing motion in the high latitude region which differs from mirroring motion at the equator (as ring current ions usually do). The selected parameters of the proton orbit are given in the Figure 10. electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.19

20 20 Q.-G. Zong et. al. 10 Green: 1 kev Blue: 10 kev Black:100 kev Figure 9. The trajectory of protons (1, 10 and 100 kev) with 900 pitch angle in the Tsyganenko 96 model. Those particles trapped in the high latitude region. In Figure 9 one can see the ion trajectories in the outer cusp region analogous to L shells of a dipolar magnetic field. The limiting second invariant of these trapped orbits occurs when the mirror point Bmin approaches the dayside equatorial field strength and local gradient drift away from the cusp. Those ion trajectories are formed two parts, one of them is located in the dayside which has the closed magnetic field lines, the other part is located in the mantel region with opened magnetic field lines. This behaviour follows from slight violation of the first adiabatic invariant during the drift path from electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.20

21 Filedline Tracer in the Cusp Region 21 Open Close open close Figure 10. Selected parameter of the proton with 50 kev orbits shown in Figure 9: (a) Pitch angle, (b)the first adiabatic invariant, (c) the total magnetic field experienced during its trajectory in the Tyganenko 96 model. the closed field lines region to the open field line region in the frontside magnetosphere. The particles behaviours with different energies are given in the Table II. It should be pointed out that an electron boundary layer just outside the magnetopause has been observed as early as 1970 s although the formation mechanism is still unclear. A layer of energetic electrons (> 40keV ) lying primarily outside the magnetopause is found at high latitudes near the duskdawn meridional plane from the dayside to the distant magnetotail (Meng and Anderson, 1970; Meng and Anderson, 1975, ). (Baker and Stone, 1977a; Baker and Stone, 1977b; Baker and Stone, 1977c) have shown that the energetic electron magnetopause layer is persistently present along the distant magnetotail at essentially all latitudes and that the 200keV electrons within the layer are strongly streaming tailward along the local magnetic field. The observations in this paper may offer an explanation for the presence of the electron boundary layer outside the magnetopause. Electron flux enhancements observed in the high latitude region could be trapped energetic elecelectron_tracer_kluwer.tex; 8/03/2003; 11:52; p.21

22 22 Q.-G. Zong et. al. Table II. Selected Particle Parameter of the Cusp s Trapped Motion Energy Period (T) T A (open) µ A (open) T B (close) µ B (close) Rigidity KeV min min kev / nt min kev / nt (nt Re) trons drifting in the "sheldon orbit" when the local minimum of the magnetic field is formed. 4. Summary and Conclusion The energetic electrons in the high latitude region (including cusp) has been examined in detail by using Cluster/RAPID data for four consecutive high latitude/ cusp crossings with energies of about kev between 16 March to 19 March The energetic electrons with high flux are observed in the time interval when the IMF had a predominate positive Bz component. No stable energetic electron flux were observed during southward IMF indicating that the cusp has a general open field line geometry. The high latitude magnetosphere (both north and south cusp) regions are able to trap energetic particle as observed by the Cluster RAPID. The existence of a B-minimum off equator in the outer cusp may allow energetic particles being trapped in the high latitude magnetospheric region for tens min to tens hours depending on their energy. The main observational facts can be summarized as: 1. Energetic ions were observed in the high latitude magnetospheric region for all four crossings, no matter IMF was south or northward; 2. Stably trapped energetic electrons were observed only during northward IMF. In contrast to the ions, there were not stable electron fluxes observed in the cusp region during southward IMF. The entry layer was composed of closed field lines which maintain energetic electron. Energetic electrons found in the high latitude regions were associated with lower plasma density and without an obvious tailward plasma flow. In the same location, electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.22

23 Filedline Tracer in the Cusp Region 23 during southward IMF, the electrons were not present in a region of high plasma density which was associated with a clear tailward flow. 3. There was a clear boundary between the cusp and magnetosheath during the northward IMF cases. The magnetopause has been identified as a rotational discontinuity indicating that high latitude reconnection is ongoing. The interface between the cusp and magnetosheath during southward IMF was not very clear with energetic ions exhibiting enhancement. 4. No stably trapped energetic electrons has been observed during southward IMF indicating the cusp had an open field line geometry. 5. The observation show that the observed electrons were locally trapped in a region which consist of twist closed field lines during northward IMF. The energetic electron observation provide new evidences to understand the dynamic cusp process. 6. The trajectory tracing of test particles have been performed by using Tsyganenko 96 model which demonstrate that energetic particles could be indeed trapped in the high latitude magnetospheric regions for tens min to tens hours. References Anderson, K. A., H. K. Harris, and R. J. Paoli: 1965, Energetic electron fluxes in and beyond the Earth s outer magnetosphere. J. Geophys. Res. 70, Antonova, A. E.: 1996, High-latitude particle traps and related phenomena. Radiat. Meas. 26(3), Antonova, A. E. and V. P. Shabansky: 1968, Structure of the geomagnetic field at great distance from the Earth. Geomagn. Aeron. 8, Antonova, A. E. and V. P. Shabansky: 1975, Particles and the magnetic field in outer noon magnetosphere of the Earth. Geomagn. Aeron. 15, Aparcio, B., B. Thelin, and R. Lundin: 1991, The polar cusp from a particle point of view: A statistical study based on the Viking data. J. Geophys. Res. 96, Baker, D. N. and E. C. Stone: 1977a, The Magnetopause Electron Layer Along the Distant Magnetotail. Geophys. Res. Lett. 4, Baker, D. N. and E. C. Stone: 1977b, The Magnetopause Electron Layer Along the Distant Magnetotail. Geophys. Res. Lett. 4, Baker, D. N. and E. C. Stone: 1977c, The Magnetopause Energetic Electron Layer 1. Observations Along the Distant Magnetotail. Geophys. Res. Lett. 4, Balogh, A., M. W. Dunlop, S. W. H. Cowley, D. J. Southwood, J. G. Thomlinson, K.-H. Glassmeier, G. Musmann, H. Luḧr, S. Buchert, M. H. Acuna, D. H. Fairfield, J. A. Slavin, W. Riedler, K. Sachwingenschuh, and M. G. Kivelson: 1997, The Cluster Magnetic Field Investigation. Space Sci. Rev. 79, Bieber, J. W., E. C. Stone, E. W. Hones, D. N. Baker, and S. J. Bame: 1982, Plasma behaviour during energetic electron streaming events: further evidence for substorm-associated magnetic reconnection. Geophys. Res. Lett. 9, Chang, S. W., J. D. Scudder, S. A. Fuselier, J. F. Fennell, K. J. Tratter,, J. S. Pickett, H. E. Spence, J. D. Menietti, W. K. Peterson, R. P. Lepping, and R. Friedel: 1998, Cusp energetic ions: A bow shock source. Geophys. Res. Lett. 25, electron_tracer_kluwer.tex; 8/03/2003; 11:52; p.23

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