Ulysses observations

Available online at www.sciencedirect.com

Planetary and Space Science 52 (2004) 561 – 572

www.elsevier.com/locate/pss

On a systematic spectral variation of energetic ions in the Jovian outer magnetosphere: HI-SCALE/Ulysses observations P.K. Marhavilas∗ , G.C. Anagnostopoulos, E.T. Sarris Space Research Laboratory, Department of Electrical and Computer Engineering, Demokritos University of Thrace, Vas. So!as 12 St., Xanthi 671 00, Greece Received 29 January 2003; accepted 15 July 2003

Abstract An important 8nding of Ulysses 9yby of Jupiter is the discovery of a large-scale energetic particle layer adjacent to the magnetopause. Previous studies discussed the particle 9ux and the anisotropy characteristics of this layer. This study examines the spectral characteristics of the large-scale magnetopause boundary layer of energetic particles. Examination of Ulysses’ observations in the Jovian outer magnetosphere reveals the following picture of energetic ions: (1) The ion 9ux increases and the spectrum hardens in the direction from the magnetosheath/magnetopause toward the outer magnetosphere. (2) At middle latitudes, the energy spectrum is continuous and is well described by a power law (dj=dE = kE − ), with a spectral index  ranging between ∼2:0 and ∼2:5 over the whole energy range (∼60 to ∼4000 keV). (3) Close to the magnetopause the spectrum is described by a power law, but with a di@erent slope at low (¡ ∼400 keV) and higher (¿ ∼400 keV) energies. (4) The ion spectrum near the magnetopause is in general softer than far from the magnetopause ( ∼ = ∼2:6 to ∼3:4 at energies ¿ ∼400 keV). (5) The spectral shape varies with a period of ∼5 or 10 h. The observations are explained in terms of a periodic motion of the s/c within the large-scale boundary layer of energetic ions in the outer magnetosphere. This boundary was more evident in the duskside south magnetosphere and was found to extend in radial distances from ∼49Rj to 83Rj in this area. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Jupiter; Magnetosphere; Magnetopause; Boundary layer; Spectral variations

1. Introduction 1.1. Major regions in the Jovian magnetosphere Ulysses explored the huge and amazing plasma laboratory of the Jovian magnetosphere for the 8fth time, during its voyage to the solar poles (in February of year 1992), and extended our knowledge on the structure and the dynamics of the Jovian magnetosphere gained by the discoveries and the results of the four previous missions (Pioneers 10 and 11, Voyagers 1 and 2). An important consequence of Ulysses’ trajectory was the exploration, for the 8rst time, of the dusk high-latitude magnetosphere and of an extended energetic particle boundary layer within it. Magnetic 8eld observations from the Pioneers 10 and 11 and the Voyager 1 and 2 missions had provided the basis for the identi8cation of three regions of the Jovian magnetosphere based on magnetic 8eld measurements: the outer magnetosphere, the middle magnetosphere, and the inner one ∗

Corresponding author. Tel.: +30-25410-79405; fax: +30-25410-79454. E-mail address: [email protected] (P.K. Marhavilas).

0032-0633/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2003.07.006

(Smith et al., 1976; AcuHna et al., 1983; Hawkins et al., 1998). The inner magnetosphere is dominated by the planetary dipole and lies within ∼15 planetary radii (RJ ). The middle magnetosphere is dominated by the equatorial current sheet and the magnetic 8eld lines tend to be radial and oppositely directed (outward–inward) in the two sides of the magnetodisk current sheet. The outer magnetosphere extends from the end of the current sheet to the magnetopause. This last region is highly dynamic, as the outward pressure of the magnetosphere is balanced by the highly variable solar wind, and is characterized by an irregular magnetic 8eld, which in general points in the southward direction at low latitudes. Pioneer 11 was the 8rst spacecraft that explored the Jovian magnetosphere at high (northern) latitudes. It measured enhanced middle- and high-energy particle 9uxes during its outbound trajectory, which were above those expected from a disk model. Many investigators have noted that the particle 9uxes on Pioneer 11 outbound were comparable with the peak 9uxes observed inbound and that this seems to be inconsistent with particle con8nement to a thin disk (Sentman et al., 1975; Kennel and Coroniti, 1979).

562

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

Measurements from various experiments collected by Ulysses throughout the dayside and, in particular, the duskside high-latitude magnetosphere support the existence of a large-scale boundary layer of energetic particles at middle-high latitudes and suggest that we should change our picture on the major energetic particle regions in the Jovian magnetosphere. For instance, Cowley et al. (1996) analyzed ∼1 MeV proton 9uxes from the Anisotropy Telescopes (ATs) instrument of the COSPIN experiment and 8rst inferred that, beyond the inner magnetosphere, Ulysses sampled two large ion regions, on its outbound pass of the Jovian magnetosphere: (a) the well-known magnetodisk plasma sheet in the middle magnetosphere and (b) a large-scale ion region in the outer magnetosphere spreading from moderate latitudes to the boundary layer (their Figs. 3 and 5). Later on, several researchers con8rmed a signi8cant contribution of 9ux gradients to the anisotropies in all the magnetospheric regions of Jupiter, by using measurements from di@erent experiments onboard Ulysses. In the middle magnetosphere, the major gradient components were latitudinal and directed toward the equatorial plane, consistent with the 9ux maxima observed near/at the current sheet (Cowley et al., 1996; Staines et al., 1996; Laxton et al., 1997; Hawkins et al., 1998). On the contrary, the ion observations in the outer high-latitude magnetosphere outbound (between the end of day 41 and the middle of day 43) revealed intensity gradients with positive latitudinal components, suggesting 9ux increasing in the direction away from the equator toward the southern pole (Laxton et al., 1997; Hawkins et al., 1998). This 9ux increase is consistent with the hypothesis of a separate large-scale ion layer in the high-latitude outer magnetosphere. Krupp et al. (1999) having analyzed in detail Ulysses/EPAC energetic ion measurements in the duskside magnetosphere outbound, reported a ∼5 h modulation in ion 9uxes (in 0:57 MeV protons and 0.59 –0:66 MeV=N helium ions) and the spectral slope of energetic protons at high latitudes. In order to explain the presence of ∼5 h variations, Krupp et al. suggested the existence of a boundary layer of energetic particles, which Ulysses encounters periodically, twice per rotation (∼10 h), due to the boundary’s in–out oscillation. In addition, Anagnostopoulos et al. (2001) presented an analysis of long-lasting (∼2–3 h) energetic (¿ ∼60 keV) ion and (¿ ∼40 keV) electron events observed by the Ulysses/HI-SCALE instrument in the high-latitude Jovian magnetosphere, and inferred that the high-latitude events (in both the prenoon dayside and the southern duskside magnetosphere) are di@erent from the magnetodisk plasma sheet ion events. The HI-SCALE intensity, composition, spectral and pitch angle distribution measurements data were found to be, in general, consistent with the existence of large-scale layers of energetic ions and electrons in the high-latitude Jovian magnetosphere (Anagnostopoulos et al., 2001; Karanikola et al., 2003 (this issue)). In particular, the following observations were found to support a

large-scale boundary layer at high latitudes: (1) Oppositely directed intensity gradients suggesting an ion layer within the Jovian high-latitude magnetosphere, (2) Intensity minimum between the magnetodisk plasma sheet and the magnetopause, (3) Spectral index peaks at times of ∼10 h periodic approach of the spacecraft to the magnetopause, (4) Di@erent (rigidity-dependent dispersion of) 9ux-time pro8les at high latitudes from the magnetodisk crossings, (5) Di@erent spectral characteristics from magnetodisk approaches (softening, instead of hardening, of the ion spectrum at 9ux minima), (6) A phase shift of ∼7 h between spectral index peaks observed inbound and outbound, consistent with the detection of a boundary layer northward/southward of the magnetodisk inbound/outbound (Cowley et al., 1996; Hawkins et al., 1998; Laxton et al., 1997; Anagnostopoulos et al., 1998b, 2001). Fig. 7 depicts schematically the Jovian magnetosphere and shows two large-scale energetic particle regions far from the planet: (a) the well-known magnetodisk plasma sheet at low latitudes and (b) the large-scale boundary layer of energetic particles in the outer magnetosphere. Ulysses’ results suggest that the Jovian magnetosphere may now be better classi8ed into inner, middle and outer magnetosphere based on energetic particle measurements, supplementary to magnetic 8eld data. 1.2. Energetic particle ∼5=10 h periodicities in the high-latitude magnetosphere The most common variations characterizing the Jovian magnetosphere and its environment are the ∼10=5 h periodicities, which are evident in energetic particle (McKibben and Simpson, 1974; Krimigis et al., 1981; Anagnostopoulos and Karanikola, 2002 and references therein), magnetic 8eld (Lepping et al., 1981; Anagnostopoulos et al., 1998a), plasma (Canu et al., 1993 and references therein) and electromagnetic radiation (Kaiser et al., 1993 and references therein) measurements. For a long time the most comprehensible reason for the presence of ∼10=5 h particle periodicities in the Jovian magnetosphere has been the ∼10 h periodic motion of the inclined (∼10◦ ) plasma sheet with respect to the equatorial plane. This motion has, as a consequence, one or two spacecraft encounters with the magnetodisk current sheet within a ∼10 h period. For instance, Krimigis et al. (1981) analyzed data from Voyager 1 and 2 and found the existence of a ∼10 h quasi-periodic modulation in the spectral slope of energetic (∼0:5 and ∼2:0 MeV) ions, in the outer magnetosphere, with the spectrum being softer at equatorial crossings and harder o@ at the equator. Lanzerotti et al. (1992a, 1993), based on Ulysses energetic particle observations, reported an ∼10 h periodicity in the 9ux of the CNO

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

population, in the ion energy density and in plasma , with the highest values at times of plasma sheet crossings. However, the most famous periodicity of Jovian particles is the clocklike ∼10 h periodic rocking of the relativistic (¿ ∼1 MeV) electron spectrum (McKibben and Simpson, 1974; Chenette et al., 1974; Simpson et al., 1992), which is clear in the outer magnetosphere and in the interplanetary space (Eraker, 1982; Schardt and Goertz, 1983; Simpson et al., 1992). It is generally accepted that the ∼10 h variation of the relativistic electron spectrum is not produced by the periodic proximity of the Jovian magnetodisk to the spacecraft due to the rotation of the planet. Two models were proposed to explain the periodic variation of the relativistic electron spectrum: the “magnetic anomaly” (Hill et al., 1974; Dessler and Vasiliunas, 1979) and the “clock” model (Simpson et al., 1992). Later, Anagnostopoulos et al. (1998b) having used energetic particle data from the HI-SCALE/Ulysses instrument, observed a ∼10 h periodic softening in the ion spectrum that was almost in phase with the “clock” relativistic electron variation. They also found that the low-energy (¿ ∼50 keV) ion and electron spectrum inbound was in general related to 9ux decreases and not related to plasma sheet approaches, as in the case of relativistic electrons. They suggested that the concept of the high-latitude boundary layer can explain both the HI-SCALE low-energy ion/electron and the COSPIN relativistic electron 10=5 h spectral variations, since the two major particle regions in the middle and the outer magnetosphere, the plasma sheet and the high-latitude boundary layer, can produce a ∼5 h modulation in the particle data, due to their periodic approach to the spacecraft within the ∼10 h planetary rotation. Although this suggestion should be further checked, we note that the ∼10 h rocking of the relativistic electron spectrum was in general observed in the high-latitude magnetosphere (McKibben and Simpson, 1974; Simpson et al., 1992). The purpose of this paper is to (a) elaborate the spectral features of the large-scale energetic ion boundary layer at high latitudes, (b) provide additional information for its identi8cation and (c) provide further evidence in favor of its permanent appearance all over the outer magnetosphere. For this reason, we examine large-scale 9uxes and energy spectra graphs of energetic ions observed by the HI-SCALE instrument onboard Ulysses in the Jovian outer magnetosphere, which extend our previous results (Anagnostopoulos et al., 1998b, 2001; Anagnostopoulos and Karanikola, 2002). The analysis of the recorded particle measurements reveals the following depiction: (1) The ion 9ux increases and the spectrum shows a systematic hardening in the direction from the magnetosheath/magnetopause toward the outer magnetosphere. (2) At middle latitudes, near 9ux maximum, the energy spectrum is continuous and is well described by a power law over the whole energy range (∼60–∼4000 keV) covered by the Heliosphere Instrument for spectra, composition, and anisotropy at low energies

563

HI-SCALE instrument, while at higher latitudes, the spectrum is softer and is composed of two parts described by di@erent power laws. (3) The spectral shape varies periodically (∼10 h period) as Ulysses s/c approaches the magnetopause. The 10=5 h periodicities are explained in terms of a periodic motion of the s/c within the large-scale boundary layer of energetic ions in the outer magnetosphere. 2. ULYSSES mission and instrumentation The Heliosphere Instrument for spectra, composition, and anisotropy at low energies (HI-SCALE) onboard the ULYSSES spacecraft (Lanzerotti et al., 1992b) consists of 8ve apertures in two telescope assemblies mounted in a unit that contains the instrument electronics. To attain the lowest energy of response over a wide variety of particle species with appropriate geometrical factors and angular resolution, HI-SCALE utilizes three distinct silicon solid-state detector systems. These are low-energy magnetic/foil spectrometers (LEMS/LEFS) and composition aperture (CA). (CA; is sometimes called “WART”). The LEMS/LEFS systems provide pulse-height-analyzed single-detector measurements with active anticoincidence. The CA system uses a multiparameter detection technique to provide measurements of ion composition in an energy range similar to those of LEMS/LEFS. HI-SCALE is designed to obtain measurements of interplanetary ions and electrons. The ions (Ei ¿ 50 KeV) and electrons (Ee ¿ 30 KeV) are detected by 8ve separate solid-state detector telescopes, oriented to give essentially complete pitch-angle coverage from the spinning spacecraft. Ion elemental abundances are determined by a PE vs. E telescope using a thin (5 m) front detector element in a three-element telescope. Experiment operation is controlled by a microprocessor-based data system. In-9ight calibration is provided by radioactive sources mounted on closable telescope covers. Ion and electron spectral information is determined using both broad-energy-range rate channels and a pulse-height analyzer for more detailed spectra. Table 1 Table 1 Energy channels of HI-SCALE instrument

LEMS30 (4 Sectors)

LEMS120 (8 Sectors)

Name P1 P2 P3 P4 P5 P6 P7 P8

Description 56 –78 keV ions 78–130 keV ions 130 –214 keV ions 214 –337 keV ions 337–594 keV ions 594 –1073 keV ions 1073–1802 keV ions 1802–4752 keV ions

Name P’1 P’2 P’3 P’4 P’5 P’6 P’7 P’8

DE1 DE2 DE3 DE4

38–53 keV electrons 53–103 keV electrons 103–175 keV electrons 175 –315 keV electrons

Description 61–77 keV ions 77–127 keV ions 127–207 keV ions 207–336 keV ions 336 –601 keV ions 601–1123 keV ions 1123–1874 keV ions 1874 –4752 keV ions

564

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

lists the selected energy channels (for ions and electrons) of the HI-SCALE instrument. 3. Observations The large-scale energetic ion and electron magnetopause boundary layer was most clearly revealed in Ulysses 9ux observations during the spacecraft voyage in the south (duskside) magnetosphere outbound and in Fig. 1 we show the representative observations of the energetic particle boundary layer in this area. Fig. 1 reviews the HI-SCALE P’6 (601–1123 keV) and P’8 (1874 –4752 keV) energetic ion intensities and the W2 (0.95 –1:6 MeV) energetic proton intensities between days 40 and 43 of the year 1992; at the top of Fig. 1 the various parts of the magnetosphere (Phillips et al., 1993) and the spacecraft radial distance from the planet are shown. The ∼10 h periodic intensity variation seen in the part of the

graph marked as “middle” (magnetosphere) was thought to be an e@ect produced by the approach of the current sheet to the spacecraft (Lanzerotti et al., 1993; Smith and Wenzel, 1993; Anagnostopoulos et al., 2001). The distinct intensity enhancement from ∼20 : 00 UT on day 41 to near the 8rst exit from the magnetosphere, at ∼14 : 30 UT on day 43, was suggested as the large-scale energetic particle boundary layer (Cowley et al., 1996; Anagnostopoulos et al., 2001); this region was also identi8ed as the “outer” part of the south duskside magnetosphere (Cowley et al., 1996). During later expansions of the magnetopause on day 43 at ∼18 : 00 UT (Fig. 1) and on day 45 (data not shown here), the energetic particle layer in the duskside magnetosphere could still be observed by Ulysses (Anagnostopoulos et al., 2001). From Fig. 1, we note that in general higher intensities of 0.95 –1:6 MeV protons were observed in the boundary layer, at radial distances between ∼49RJ and ∼83RJ , than near the current sheet, at radial distances between ∼16RJ and 49RJ . Furthermore, we note that intensity minima (at ∼15 : 00 UT,

Ulysses/HI-SCALE 1min averaged data, outbound Jupiter MIDDLE 1E+9

20

40

30

BL

OUTER 50

60

BL

70

MP MP MSh

80

90

Distance from Jupiter (Rj) 1E+8 4 P'6(x10 ):601-1123keV ions

Fluxes [particles/ (cm^2 sec sr MeV)]

1E+7

1E+6

P'8(103):1874-4752keV ions

1E+5

1E+4

1E+3

1E+2

1E+1

1E+0

W2:0.95-1.6 MeV protons 1E-1

40

41

42

43

44

Day of the year 1992 Fig. 1. The HI-SCALE P’6 (601–1123 keV) and P’8 (1874 –4752 keV) energetic ion intensities and the W2 (0.95 –1:6 MeV) energetic proton intensities between days 40 and 43, 1992 during the outbound trajectory of Ulysses; the various parts of the magnetosphere and the spacecraft radial distance from the planet at those times are shown at the top of the 8gure. The ∼10 h periodic intensity variation seen in the part of the graph marked as “middle” (magnetosphere) is an e@ect produced by the approach of the current sheet. The distinct intensity enhancement from ∼49RJ to near the 8rst exit from the magnetosphere, at ∼83RJ , was suggested to be a large-scale energetic particle magnetopause boundary layer (Cowley et al., 1996; Anagnostopoulos et al., 2001).

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

565

ULYSSES/HI-SCALE INBOUND 100

90

80

OUTBOUND

20min averaged data 70

60

40

50

60

70

80

90

Distance from Jupiter (Rj)

S3

S2

S1

S4

1E+2

P'4/P'8

1E+2

MIDDLE

OUTER

MIDDLE

OUTER

1E+1

1E+1

(a)

(b)

7term-filtered

7term-filtered

P'1:ions 61-77keV

P'1

Fluxes [particles/ (cm^2 sec sr MeV)]

1E+5

1E+5

1E+4

1E+4

P'6(x10) 1E+3

1E+3

P'6(x10):ions 601-1123keV 1E+2

1E+2

1E+1

1E+1

1E+0

1E+0

P'8 1E-1 34

(c) 35

37 41

36

1E-1

P'8:ions 1874-4752keV

(d) 42

43

44

Day of 1992 Fig. 2. The 9ux ratio P’4/P’8 (used as spectral index) (panels a, b) and 9uxes from the channels P’1, P’6 and P’8 (panels c, d) measured in the low-latitude dayside (panels a, c) and the south duskside (panels b, d) outer magnetosphere compared. The observations suggest a mirror picture in spectral and 9ux data in the outer magnetosphere inbound and outbound, with a characteristic hardening of the ion spectrum in the direction from the magnetopause toward the outer magnetosphere. The bars marked S1; : : : S4 indicate the time periods for which ion spectra were evaluated and are indicated in Fig. 3.

day 42 and at ∼01 : 00 UT, day 43) were detected ∼20 and ∼10 h before Ulysses crossed the magnetopause (∼12 : 00 UT, day 43), which suggest that the 9ux variation in the duskside south magnetosphere was most probably a spatial e@ect and was controlled by the ∼10 h motion of the magnetospheric boundary. In the following we will concentrate on the spectral variations of energetic ions in the Jovian magnetosphere as detected by the HI-SCALE experiment onboard Ulysses. We will examine long time variations (of the time scale of a few days), due to the motion of the spacecraft within the large-scale energetic particle magnetopause boundary layer,

and short time variations (of the scale of some hours), due to the ∼10 h periodic motion of the magnetosphere. In Fig. 2 we compare the large-scale spectral variations in the dayside outer magnetosphere inbound (left column) with the spectral variations in the duskside south outer magnetosphere outbound (right column). More explicitly, Fig. 2 illustrates time pro8les of the 9ux ratio P’4/P’8 from the ion channels P’4 and P’8 of the LEMS120 detector (P’4 and P’8 also represent the 9ux from the corresponding channels), which is used as a spectral index (panels a, b) and the 9uxes of ions from the LEMS120 channels P’1, P’6, P’8 (panels c, d) during the days 34 –36 (left column) and

566

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

INBOUND

OUTBOUND

j [ particles/ (cm^2 sec sr MeV) ]

1E+6

1E+6

LEMS120

LEMS120

1E+5

1E+5

1E+4

E-2.1 E-2.4

1E+3

1E+4

E-2.3

E-2.0

1E+3

1E+2 1E+1

1E+2

(a)

E-3.2

S2

E-2.8

(b)

S3 1E+1

S4

1E+0

1E+0

S1

S1: 1992, 34, 03:00-10:00 S2: 1992, 35, 10:00-12:00

1E-1 1E+1

1E+2

1E+3

S3: 1992, 41, 22:00 - 41, 24:00 S4: 1992, 42, 12:00 - 43, 08:00

1E+4 1E+1

1E+2

1E-1 1E+3

1E+4

Energy [keV] Fig. 3. Representative ion spectra obtained from the HI-SCALE LEMS120 detector within the energetic particle magnetopause boundary layer during the inbound (a) and the outbound (b) Ulysses’ pass of the Jovian magnetosphere. Spectra are shown for times of (high energy) ion 9ux maxima (S2 and S3) and times of low ion intensities near the magnetopause (S1 and S4). Softer spectra were detected near the magnetopause than within the inside magnetosphere, in particular at higher energies ( ∼ = 3:2; 2:8 vs.  ∼ = 2:0; 2:3). The spectra are continuous (in the energy range ∼0:01–∼4:00 MeV) within the outer magnetosphere, but show a change in the slope (at ∼400 keV) near the magnetopause.

41– 43 (right column) of the year 1992 in the regions of interest: the pre-noon and the duskside outer magnetosphere, respectively. Firstly, we discuss the 9ux and spectral time pro8les in the right column, which allow a better insight into the characteristics of the boundary layer of energetic particles observed in the high-latitude (south duskside) magnetosphere (Fig. 1). From this column we see that as the spacecraft moved toward the magnetopause, between ∼49RJ and 83RJ , the 9ux of the low (0.061–0:077 MeV) energy ions remained almost constant, while the 9ux of the high-energy ions (∼1:87–4:75 MeV) decreased, and the spectrum, as a consequence, became softer. This two-day continuous softening toward the magnetopause observed by Ulysses suggests that the spacecraft observed a large-scale energetic ion structure with characteristic spectral strati8cation. The comparison of the left and the right column suggests a similar behavior for the 9ux and spectral observations in the dayside magnetosphere, at radial distances between ∼81RJ and 108RJ , with the duskside (south) Jovian magnetosphere. This comparison suggests an exciting mirror behavior of 9ux and spectral observations in the Jovian outer magnetosphere inbound and outbound during Ulysses 9yby of Jupiter. Since similar spectral characteristics were found inbound and outbound, at di@erent sites of the outer magnetosphere, and at di@erent times (days 34 –36 vs. days 41– 43), Ulysses’ observations most probably suggest that the spectral variations were a permanent (spatial) and not a temporal e@ect. The bars marked as S1 : : : S4 in Fig. 2 indicate the times for which energy spectra of ion intensities were evaluated. Fig. 3 displays representative ion spectra obtained from the HI-SCALE LEMS120 detector within the energetic particle boundary layer around the assumed internal edge of the

layer (S2 and S3) and near the magnetopause (S1 and S4), during the inbound (a) and the outbound (b) Ulysses’ pass of the Jovian magnetosphere; the spectra in the outer magnetosphere (S1, S4) were evaluated over long time intervals (03:00 –10 : 00 UT, day 34 and 12 : 00 UT, day 42—08 : 00 UT, day 43) to eliminate ∼5=10 h periodic variations and re9ect the (averaged) spectra at those positions. The spectra S2 and S3 are continuous and are well described by a power law (dj=dE =kE − ), with spectral indices 2 ∼ = 2:0 and 3 ∼ = 2:3, all the way from ∼100 keV to ∼2–3 MeV; the spectra S1 and S4 show a change in their slope at ∼400 keV, from 1 ∼ = 2:4 to 1 ∼ = 3:2 and from 4 ∼ = 2:1 to 4 ∼ = =2:8, respectively. It is also noted that softer spectra were detected near the magnetopause than within the inside magnetosphere, in particular at higher energies ( ∼ = 3:2; 2:8 vs.  ∼ = 2:0; 2:3). (We should note that we evaluated the energy spectra (not illustrated here) from ion measurements from the LEMS30 detector, printing in a di@erent direction (Lanzerotti et al., 1993), and we found similar spectral shapes.) Since we found a characteristic average spectral variation within large-scale spatial structures in the outer magnetosphere, inbound and outbound, which were observed by Ulysses persistently for ∼2 days and were organized by distance from the magnetopause, we anticipate that a similar behavior should be revealed within the scale of the planet rotation (∼10 h) due to the periodic motion of the magnetospheric boundaries relative to the spacecraft’s position. Fig. 4 displays the 9ux-time pro8le of P’8 (1874 – 4752 keV) ions detected by the LEMS120 detector of the HI-SCALE experiment onboard Ulysses in the outer high-latitude duskside magnetoshpere and the adjacent magnetosheath (panel d), and ion spectra at certain times of minima and maxima of P’8 9uxes (panels a–c). From Fig. 4 we see that the ion 9ux decreased as the space-

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

567

LEMS120

1E+4

E

j [ particles/(cm^2 sec sr MeV) ]

1E+5

1E+5

1E+5

1E+4

-2.4

E

1E+3

E

-2.4

E

1E+4

-2.5

-2.5

E

-2.2

1E+3

1E+3

-2.6

E

E

E

-3.0

1E+2

1E+2

1E+2

-2.5

E F

1E+1

1E+1

(a)

(b)

D

1992, 42, 17:50 - 18:10 1992, 42, 15:20 - 15:50

1E-1 1E+1

1E+2

1E-1 1E+3

1E+4

I (c)

G 1E+0

1E+0

1E+0

-3.4

E

1E+1

1992, 43, 12:30 - 13:00 1992, 43, 10:30 - 11:30

1992, 42, 21:45 - 22:15 1992, 43, 01:30 - 02:10

1E+1

1E+2

H

1E-1 1E+3

1E+4

1E+1

1E+2

1E+3

1E+4

E n e r g y [keV] BL

BL

P'8: 1874-4752 keV ions E

j [ particles/ (cm^2 sec sr MeV) ]

I 1E+1

F

1E+0

D

G H

(d) 1E-1

42

43

44

Fig. 4. Ion spectra (a–c) and 9ux-time pro8le of 1874 –4752 keV ions (d) as measured by the HI-SCALE experiment onboard Ulysses in the south duskside magnetosphere and the adjacent magnetosheath. The energy spectra at times of 9ux maxima (E, F, I) are continuous and are well described by a power law all the way from ∼100 keV to ∼4 MeV. The ion 9ux decreased when Ulysses approached or crossed the magnetopause quasi-periodically (every ∼10 h); the spectra at those times were softer and presented a change in the slope at ∼400 keV (D, G and H).

craft approached or crossed the magnetospheric boundaries quasi-periodically (times marked D, G and H), with the period (∼10 h) of the planet rotation (panel d). Furthermore, we observe that the spectrum changes in the course of the periodic approaches of the spacecraft to the magnetopause in the same sense as during the long time periods examined in Figs. 2 and 3: (a) the spectra within the magnetosphere (E, F, I) are continuous and are well described by power laws (dj=dE = kE − ; E ∼ = 2:2; F ∼ = 2:5; I ∼ = 2:5) all the way from ∼100 keV to ∼4 MeV, whereas the spectra

at/near the magnetopause (D, G, H) showing a change in the slope at ∼400 keV (D : from 2.4 to 2.6, G : from 2.4 to 3.0, H : 2.5 to 3.4), (b) lower intensities and softer spectra ( ∼ = 2:6–3:4 vs. 2.2–2.5, in the energy range ∼0:4–3 MeV) were detected near the magnetopause than well within the magnetosphere. The middle Jovian magnetosphere is in general characterized as the region in9uenced by the periodic approach of the magnetodisk (Smith and Wenzel, 1993). Anagnostopoulos et al. (1998b, 2001) pointed out that Ulysses observed

568

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

O U T B O U N D - Outer MSp

I N B O U N D - middle MSp

BL MSh

BL

70

60

50

40 Distance (Rj)

60 70 Distance from Jupiter (Rj)

OMagnetopause Boundary Layer Events 3

80

MSh

90

MP

2

y(t)=0.18t +3.90t +11.61t +41.2 20min averaged data

1E+2

P'4/P'8

P'4/P'8

1E+2

1E+1

1E+1

(a)

100

P'1(x10):61-77 keV ions

1E+4 1E+3

10

1E+2 1E+1

Fluxes [ Particles/ (cm^2 sec sr MeV) ]

Fluxes [ particles/ (cm^2 sec sr MeV)]

1E+6 1E+5

20min averaged data

(d) 100.0

1E+6 1E+5

P'1(x10) 10.0

1E+4 1E+3 1E+2

1.0

1E+1

P'8

1

1E+0

P'8:1874-4752keV ions 1E-1

1E+0 0.1

(e)

(b)

1E-1

0<0

u10f5A_C.grf

20

44

ABS (theta)

Theta

39 10

34

29

(f)

(c)

0

24 36

37

38

Day of 1992

42

43

44

Day of 1992

Fig. 5. Spectral and 9ux observations of energetic ions from the HI-SCALE instrument along with the magnetic dipole latitude of the Ulysses spacecraft in the prenoon middle magnetosphere inbound (A) and the duskside southern magnetosphere outbound (B). The dotted normal lines in Fig. 5A mark the times of spectral index peaks, while the open and solid circles in Fig. 5B, mark times of 9ux minima and maxima (details in the text). From Figs. 5A and B, it is evident that the intensities of low (higher) energy ions remained almost constant (decreased) and the ion spectrum became softer (dashed curves in the 8gure) as Ulysses moved toward higher latitudes closer to the magnetopause.

spectral peaks within the middle magnetosphere with a di@erent signature from that of the magnetodisk crossings. Figs. 5A and B compare the spectral and 9ux observations of energetic ions from the HI-SCALE experiment

onboard Ulysses in the middle magnetosphere inbound and the outer magnetosphere outbound (in contrast to Figs. 2a and b which compared observations in the outer magnetosphere inbound and the outer magnetosphere outbound).

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

4. Discussion–conclusions 4.1. A third major particle region in the Jovian magnetosphere: spectral characteristics of the large-scale magnetopause boundary layer of energetic particles During Ulysses 9yby of Jupiter in 1992, many experiments detected a plasma, energetic particle and

Day of the Year 1992 40

41

42

43

44

-5

-5

-15 -20

-25 -25 -35

-30 -35

-45 -40 -55

-45

Magnetic Latitude (deg)

-10 -15

Z d i s t a n c e (Rj)

More speci8cally, Figs. 5A and B show time pro8les of: (i) the 9ux ratio P’4/P’8, where P’4 and P’8 are the 9uxes from the corresponding ion channels (panels a,d), (ii) the 9uxes [particles=(cm2 s sr MeV)] of the ion channels P’1 (61–77 keV) and P’8 (1874 –4752 keV) (panels b,e), and (iii) the O4 magnetic dipole latitude (AcuHna et al., 1983) of Ulysses (panels c,f), during days 36 –37 and 42– 43 of year 1992, respectively. In Fig. 5A, the shaded parts of the intensity pro8les indicate times of spectral softening identi8ed by Anagnostopoulos et al. (1998b, 2001) as particle events occurred by the approach of the high-latitude magnetopause boundary layer. The observations in Fig. 2B were all obtained in the duskside south magnetopause particle layer and in the magnetosheath. In both 8gures, the open and solid circles mark characteristic times throughout the high-latitude energetic ion boundary layer events. In Fig. 5A the dotted normal lines mark the times of P’4/P’8 9ux ratio peaks during the high-latitude magnetopause boundary layer events identi8ed by Anagnostopoulos et al. (1998b, 2001). The open circles in Fig. 5B mark three P’8 (and P’4) 9ux minima separated by ∼10 h intervals (15 : 30 UT on day 42, 00:30 and 11 : 00 UT on day 43) and two other minima between them (Krupp et al., 1999; Anagnostopoulos et al., 2001) as well as the P’4/P’8 9ux ratio peaks observed at (or near) the P’8 9ux minima; the solid circles in Fig. 5B mark the times of 9ux maxima in the four intervals de8ned by the 8ve minima. The dashed lines in both 8gures indicate the trend of 9ux and spectral variation. From these 8gures (Figs. 5A, B), we see that the intensities of low-energy ions remain almost constant, while the high-energy ion intensities in the magnetopause boundary layer show a radial dependence (higher values far from the planet) in the middle magnetosphere inbound, but anti-radial dependence (higher values closer to the planet) in the outer magnetosphere outbound. However, if we take into account the increase of the spacecraft magnetic latitude during both periods (dashed lines indicate the increasing latitude trend in the bottom panels), we see that the ion spectrum became softer (dashed curves in the top panels) as the spacecraft moved towards the magnetopause, at higher latitudes, in the middle magnetosphere inbound (A) and the outer magnetosphere outbound (B). Thus, from the comparison of Figs. 5A and B we can infer that similar 9ux and spectral variations were observed in the middle pre-noon and the outer duskside magnetosphere, if the approach to the magnetopause, due to latitudinal removals of the spacecraft, is taken into account.

569

-50 -65 -55 -75

-60 -65 20

30

40

50

60

70

80

90

Distance from Jupiter (Rj)

Fig. 6. The declination of the Ulysses spacecraft from the magnetodisk (z in RJ and  in degrees) as a function of the radial distance from Jupiter, during the outbound pass of the Jovian magnetosphere. The segments of the spacecraft trajectory: (a) when Ulysses crossed the boundary layer, at radial distances between R = 49RJ and 83RJ from the planet; and (b) when Ulysses approached the boundary layer quasi-periodically at distances R ¡ 49RJ are indicated.

electromagnetic environment in the high-latitude duskside magnetosphere (Stone et al., 1992, their Fig. 1A; Phillips et al., 1993, their Fig. 12; Krupp et al., 1999; Anagnostopoulos et al., 2001), which is not expected from a magnetodisk model. In particular, the observations manifest the existence of a third major particle regime beyond the inner magnetosphere and the magnetodisk plasma sheet: the large-scale magnetopause boundary layer of energetic particles, more evident at high latitudes (Cowley et al., 1996; Laxton et al., 1997; Anagnostopoulos et al., 1998b, 2001; Karanikola et al., 2003; Krupp et al., 1999). Fig. 6 shows the trajectory of Ulysses in the Jovian magnetosphere outbound. More explicitly, Fig. 6 displays the declination of Ulysses from the magnetodisk (z in RJ and  in degrees) as a function of the radial distance from Jupiter. In this 8gure, the parts of the Ulysses’ trajectory when the spacecraft was found in the magnetopause energetic particle boundary layer are highlighted in order to emphasize the large dimensions of the magnetopause boundary layer of energetic ions. Fig. 6 demonstrates the segments of the spacecraft trajectory when Ulysses crossed the boundary layer, at radial distances between R = 49RJ and 83RJ from the planet, and when it approached the boundary layer quasi-periodically at distances R ¡ 49RJ . In this work, we mostly studied the spectral characteristics of energetic (¿ ∼50 keV) ions in the outer Jovian magnetosphere, and in particular, its high-latitude portion. In our investigation, we used measurements from the HI-SCALE experiment onboard Ulysses. The spectral observations in the large-scale magnetopause boundary layer were examined during three characteristic time periods, with di@erent magnetic con8gurations: (A) Day 34, 00 : 00 UT–day 35, 09 : 00 UT (Fig. 2), when the spacecraft traversed the (dayside) low-latitude

570

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

Fig. 7. Schematic indicating a model for the outer magnetosphere. Two major particle regions can be seen far from Jupiter: (1) the magnetodisk, at low latitudes; and (2) the large-scale energetic particle magnetopause boundary layer, more evident at high latitudes. The arrows indicate the trajectory of Ulysses (dashed) and the direction of spectral hardening (solid) inferred from the observations obtained during three long time periods (A, B, C) examined in the paper. A systematic hardening of the spectrum in the direction from the magnetopause toward the inside boundary layer was revealed in HI-SCALE ion measurements.

outer magnetosphere inbound, before it encountered enhanced particle intensities closer to the middle magnetosphere (Cowley et al., 1996; Anagnostopoulos et al., 1998b, 2001). (B) Days 36 –37. During this period Ulysses entered/approached the northern magnetopause boundary layer and the magnetodisk plasma sheet successively for several times inbound (Fig. 5A). (C) Days 42– 43 (Figs. 1, 2, 4 and 5B). At this interval Ulysses explored the (duskside) south magnetosphere outbound. Fig. 7 depicts a schematic of the large-scale energetic particle magnetopause boundary layer as inferred from the Ulysses observations. Fig. 7 suggests the existence of two large-scale energetic particle regions outside the radiation belts: (1) the well-known magnetodisk plasma sheet at low latitudes and (2) the recently con8rmed high-latitude energetic particle layer in the outer magnetosphere. In order to better explain the spectral variations we found in the magnetopause boundary layer, in Fig. 7, we represent with dashed and solid arrows the trajectory of Ulysses and the inferred general direction of the high-energy particle intensity gradient vectors and of spectral hardening at the corresponding sites of the magnetosphere we studied during periods A–C; in all the cases a hardening of the spectrum is suggested from the direction of the magnetopause toward the inside magnetosphere. During periods A and C, Ulysses observed, in the boundary layer, ion intensity gradients and hardening of the spectrum in the direction from the magnetopause toward the inside magnetosphere (Figs. 1, 2b, 4 and 5B), consistent with an anti-radial intensity dependence in the outer mag-

netosphere. During period B, Ulysses observed an intensity decrease and a general softening of the spectrum as it moved outside the magnetodisk toward the planet (Fig. 5A); this kind of variation can be interpreted as a latitudinal e@ect re9ecting a hardening of the spectrum in the direction from the magnetopause toward the inside magnetosphere, as in the previous cases (A and C). The examination of Ulysses’ observations (9ux–time pro8les, spectral indices, energy distributions) during the spacecraft 9yby of Jupiter reveals the following spectral characteristics for the boundary layer of energetic ions in the outer magnetosphere: (1) The low-energy (∼50 keV) ion 9ux remains at almost a constant level throughout the magnetopause boundary layer in all of the three cases examined (Figs. 2 and 5). At higher energies, the ion 9ux decreases and the spectrum becomes harder (displaying inverse energy dispersion) in the direction from the magnetosheath/magnetopause toward the inside magnetosphere (dashed curves in the top panels of Figs. 2 and 5) both during the inbound (days 36 –38) and outbound (days 42– 44) trajectory. More intense decreases were detected for ions of higher energies. (2) At middle latitudes, near 9ux maximum, the energy spectrum is continuous and well described by a power law (dj=dE = kE − ), over the whole energy range (∼60 to ∼4000 keV) covered by the HI-SCALE instrument. Near the magnetopause, the spectrum is composed of two parts described by di@erent power laws (Figs. 3, 4a–c). (3) The ion spectra soften at times of 9ux minima, when the spacecraft approaches the magnetopause (Figs. 1–5). (4) The spectral shape varies periodically with a ∼5 or 10 h period (Figs. 1, 2b, 3d). 4.2. Implications of the energetic particle layer for the structure and the dynamics of the Jovian magnetosphere The presence of a distinct energetic particle layer in the outer Jovian magnetosphere is a major 8nding of Ulysses during its 9yby of Jupiter. The magnetopause boundary layer was detected in the prenoon magnetosphere inbound and the duskside magnetosphere outbound and its signature was more evident at high latitudes. Pioneer 11, a spacecraft that also visited the Jovian magnetosphere at high latitudes, had observed a layer of enhanced proton and electron intensities outside the magnetodisk as well (Sentman et al., 1975; Kennel and Coroniti, 1979). The detection of the large-scale magnetopause boundary layer of energetic particles from both spacecraft which visited the high-latitude Jovian magnetosphere suggests that most probably this major particle region is a permanent characteristic of the Jovian magnetosphere and not a temporal e@ect (Cowley et al., 1996). Previous studies con8rmed that the high-latitude energetic particle boundary layer is characterized by a decrease in the strength of the streaming 9ux with increasing distance from the planet. The broadening in pitch angle distributions

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

was observed both as a general decrease in the value of the anisotropy index as long as the spacecraft approaches the magnetopause (in a time scale of 2–3 days during Ulysses outbound pass) and as a ∼10 h quasi-periodic variation of the anisotropy index related to the approach of the magnetopause (Sentman et al., 1975; Lanzerotti et al., 1993; Seidel et al., 1997; Anagnostopoulos et al., 2001; Anagnostopoulos and Karanikola, 2002). The large-scale softening of the spectrum suggested by the HI-SCALE/Ulysses data analysis of the present study and the large-scale broadening of the particle distributions suggested by previous studies (Sentman et al., 1975; Lanzerotti et al., 1993; Seidel et al., 1997) in the direction from the inside magnetosphere toward the magnetopause identify a large-scale region with permanent general characteristics of energetic particles. We suggest therefore that the Jovian magnetosphere may now be better classi8ed into inner, middle and outer magnetosphere based on energetic particle characteristics, supplementary to the magnetic 8eld features. The spectral pattern we suggested for the magnetopause boundary layer based on the HI-SCALE/Ulysses observations has important implications for the ∼10 h periodic spectral variations known in the high-latitude magnetosphere from previous missions. The Ulysses observations were explained in terms of a large-scale boundary ion layer with peak 9ux at some distance from the magnetopause and energy-dependent 9ux decrease (causing a softening of the spectrum) toward the magnetospheric boundary. A periodic motion of the spacecraft in this region, due to the ∼10 h rotation of the Jovian magnetosphere, results in: (1) a 9ux decrease as the spacecraft approaches the magnetopause every 10 h, and (2) softening of the spectrum at the same times. Periodicities of ∼10 or 5 h can be observed dependent on the distance of the spacecraft from the magnetopause and the dimensions of the layer. Obviously, the proposed mechanism explains the ∼10=5 h ion periodicities in the outer high-latitude magnetosphere as a spatial e@ect. It is worth noting that the famous ∼10 h rocking of the relativistic electron spectrum can be explained in a similar way, i.e. as a spatial e@ect of a boundary layer of relativistic electrons, since relativistic electrons show similar variations with the HI-SCALE energetic particles; (for instance 9ux minima and softening of the spectrum as the spacecraft moved far from the magnetodisk toward the magnetopause (Anagnostopoulos et al., 1998b). The importance of the boundary layer in the high-latitude outer magnetosphere in providing the magnetosheath and the interplanetary space with energetic ions and electrons has been noted in the scienti8c literature (McKibben et al., 1993; Zhang et al., 1993; Marhavilas et al., 2001; Anagnostopoulos et al., 2001; Anagnostopoulos and Karanikola, 2002). Moreover, the spectral characteristics of the large-scale magnetopause boundary layer studied in this paper explain the crucial characteristics of magnetospheric upstream ion events and this hypothesis will be discussed in a future paper.

571

Acknowledgements We acknowledge helpful discussions with the HI-SCALE Team members and constructive suggestions of the two referees. This work has been supported by contract PRENED 1999-KE882 of the Secretariat for Research of the Demokritos University of Thrace. References AcuHna, M.H., Behannon, K.W., Connerney, J.E., 1983. Jupiter’s magnetic 8eld and magnetosphere. In: Dessler, A.J. (Ed.), Physics of the Jovian Magnetosphere. Cambridge Planetary Science Series, Cambridge University Press, New York, pp. 1–50. Anagnostopoulos, G.C., Karanikola, I., 2002. Energetic ions observed by Ulysses in the Jovian magnetosheath. Planet. Space Sci. 50, 637–654. Anagnostopoulos, G.C., Balogh, A., Marhavilas, P.K., Rigas, A., Sarris, E.T., Krimigis, S.M., Trochoutsos, P.C., 1998a. Continual presence of ∼10=5 h period alfven waves and plasma modulation observed by Ulysses upstream from Jupiter’s bow shock. Geophys. Res. Lett. 25 (9), 1553. Anagnostopoulos, G.C., Marhavilas, P.K., Sarris, E.T., Karanikola, I., Balogh, A., 1998b. Energetic ion populations and periodicities near Jupiter. J. Geophys. Res. 102, 20,055. Anagnostopoulos, G.C., Karanikola, I., Marhavilas, P.K., 2001. Large-scale energetic particle layers in the high latitude Jovian magnetosphere. Planet. Space Sci. 49 (10 –11), 1049–1065. Canu, P., Cornilleau-Wehrlin, N., de Villedary, C., Lellogg, P.J., Harney, C.C., MacDowall, R.J., 1993. Observations of electron plasma waves upstream of the Jovian bow shock. Planet. Space Sci. 41, 811–822. Chenette, D.L., Conlon, T.F., Simpson, J.A., 1974. Bursts of relativistic electrons from Jupiter observed in interplanetary space with the time variation of the planetary rotation period. J. Geophys. Res. 79, 3551. Cowley, S.W.H., Balogh, A., Dougherty, M.K., Dunlop, M.W., Edwards, T.M., Forsyth, R.J., Hynds, R.J., Laxton, N.F., Staines, K., 1996. Plasma low in the Jovian magnetosphere and related magnetic e@ects: Ulysses observations. J. Geophys. Res. 101, 15,197–15,210. Dessler, A.J., Vasiliunas, V.M., 1979. The magnetic anomaly model of the Jovian magnetosphere: predictions for Voyager. Geophys. Res. Lett. 6, 1. Eraker, J.H., 1982. Origin of the low energy relativistic interplanetary electrons. Astrophys. J. 257, 862. Hawkins, S.E., Cheng, A.F., Lanzerotti, L.J., 1998. Bulk 9ows of hot plasma in the Jovian magnetosphere: a model of anisotropic 9uxes of energetic ions. J. Geophys. Res. 103, 20,031–20,054. Hill, T.W., Carbary, J.F., Dessler, A.J., 1974. Periodic escape of relativistic electrons from the Jovian magnetosphere. Geophys. Res. Lett. 8, 333. Kaiser, M.L., Desch, M.D., Farrell, W.M., 1993. Clock-like behavior of Jovian continuum radiation. Planet. Space Sci. 41, 1073. Karanikola, I., Athanasiou, M., Anagnostopoulos, G.C., Pavlos, G.P., Preka-Papadema, P., 2003. Quasi-periodic emissions (15-80 min) from the poles of Jupiter as a principal source of the large scale high latitude magnetopause boundary layer of energetic particle. Planet. Space Sci., doi:10.1016/j.pss.2003.10.002. Kennel, C.F., Coroniti, F.V., 1979. Jupiter’s magnetosphere and radiation belts. In: Kennel, C.F., Lanzerotti, L.J., Parker, E.N. (Eds.), Solar System Plasma Physics, Vol. III. North-Holland Publishing Company, Amsterdam. Krimigis, S.M., Carbary, J.F., Keath, E.P., Bostrom, C.O., Axford, W.I., Gloeckler, G., Lanzerotti, L.J., Armstrong, T.P., 1981. Characteristics of hot plasma in the Jovian magnetosphere: results from the Voyager spececraft. J. Geophys. Res. 86, 8227. Krupp, N., Dougherty, M.K., Woch, J., Seidel, R., Keppler, E., 1999. Energetic particles in the duskside Jovian magnetosphere. J. Geophys. Res. 104, 14,767.

572

P.K. Marhavilas et al. / Planetary and Space Science 52 (2004) 561 – 572

Lanzerotti, L.J., Armstrong, T.P., Gold, R.E., Anderson, K.A., Krimigis, S.M., Lin, R.P., Pick, M., Roelof, E.C., Sarris, E.T., Simnett, G.M., Maclennan, C.G., Choo, H.T., Tappin, S.J., 1992a. The hot plasma environment at Jupiter: Ulysses results. Science 257, 1518. Lanzerotti, L.J., Gold, R.E., Anderson, K.A., Armstrong, T.P., Lin, R.P., Krimigis, S.M., Pick, M., Roelof, E.C., Sarris, E.T., Simnett, G.M., Frain, W.E., 1992b. Heliosphere instrument for spectra, composition and anisotropy at low energies. Astron. Astrophys. Suppl. Ser. 92, 349–363. Lanzerotti, L.J., Armstrong, T.P., Maclennan, C.G., Simnett, G.M., Cheng, A.F., Gold, R.E., Thomson, D.J., Krimigis, S.M., Anderson, K.A., Hawkins III, S.E., Pick, M., Roelof, E.C., Sarris, E.T., Tappin, S.J., 1993. Measurements of hot plasmas in the magnetosphere of Jupiter. Planet. Space Sci. 41, 893. Laxton, N.F., Balogh, A., Cowley, S.W.H., Dunlop, M.W., Forsyth, R.J., Hynds, R.J., Staines, K., 1997. Origins of the 8rst-order anisotropy of ∼1 MeV protons in the Jovian magnetosphere during the Ulysses 9yby: 9ux gradients and plasma 9ows. Planet. Space Sci. 45, 1143–1170. Lepping, R.P., Burlaga, L.F., Klein, L.W., Jesseu, J.M., Goodrich, C.C., 1981. Observations of the magnetic 8eld and plasma 9ow in Jupiter’s magnetosheath. J. Geophys. Res. 86, 8141. Marhavilas, P.K., Anagnostopoulos, G.C., Sarris, E.T., 2001. Periodic signals in Ulysses’ energetic particle events upstream and downstream from the Jovian bow shock. Planet. Space Sci. 49 (10 –11), 1031– 1047. McKibben, R.B., Simpson, J.A., 1974. Evidence from charged particle studies for the distortion of the Jovian magnetosphere. J. Geophys. Res. 79, 3545. McKibben, R.B., Simpson, J.A., Zhang, M., 1993. Impulsive bursts of relativistic electrons discovered during Ulysses’ traversal of Jupiter’s duskside magnetosphere. Planet. Space Sci. 41, 1041. Phillips, J.L., Bame, S.J., Barraclough, B.L., McComas, D.J., Forsyth, R.J., Canu, P., Kellogg, P.J., 1993. Ulysses plasma electron observations in the Jovian magnetosphere. Planet. Space Sci. 41, 877.

Schardt, A.W., Goertz, C.K., 1983. High-energy particles. In: Dessler, A.J. (Ed.), Physics of the Jovian Magnetosphere. Cambridge University Press, Cambridge, p. 157. Seidel, R., Keppler, E., Krupp, N., Woch, J., Hawkins, S.E., Lanzerotti, L.J., Dougherty, M.K., 1997. Energetic electron beams in the duskside Jovian magnetosphere: Ulysses EPAC and HI-SCALE measurements. Planet. Space Sci. 45, 1473–1480. Sentman, D.D., Van Allen, J.A., Goertz, C.K., 1975. Recirculation of energetic particles in Jupiter’s magnetosphere. Geophys. Res. Lett. 2 (10), 465–468. Simpson, J.A., Anglin, J.D., Balogh, A., Burrows, J.R., Cowley, S.W., Ferrando, P., Heber, B., Hynds, R.J., Kunow, H., Marsden, R.G., McKibben, R.B., Muller-Mellin, R., Page, D.E., Raviart, A., Sanderson, T.R., Staines, K., Wenzel, K.P., Wilson, M.D., Zhang, M., 1992. Energetic charged-particle phenomena in the Jovian magnetosphere: 8rst results from COSPIN collaboration. Science 257, 1543. Smith, E.J., Davis, L., Jones Jr., D.E., 1976. Jupiter’s magnetism: observations and theory. In: Gehrels, T. (Ed.), Jupiter. University of Arizona Press, Tucson, pp. 788–829. Smith, E.J., Wenzel, K-P., 1993. Introduction to the Ulysses encounter with Jupiter. J. Geophy. Res. 98, 21,111–21,127. Staines, K., Balogh, A., Cowley, S.W.H., Edwards, T.M., Forsyth, R.J., Hynds, T.J., Laxton, N.F., 1996. An overview of the anisotropy telescope observations of MeV ions during the Ulysses Jupiter encounter. Planet. Space Sci. 44, 341–369. Stone, R.G., Pedersen, B.M., Harvey, C.C., Canu, P., Cornilleau-Wehrlin, N., Desch, M.D., de Villedary, C., Fainberg, J., Farrell, W.M., Goetz, K., Hess, R.A., Hoang, S., Kaiser, M.L., Kellogg, P.J., Lecacheux, A., Lin, N., MacDowall, R.J., Manning, R., Meetre, C.A., Meyer-Vernet, N., Moncuquet, M., Osherovich, V., Reiner, M.J., Tekle, A., Thiessen, J., Zarka, P., 1992. Ulysses radio and plasma wave observations in the Jupiter environment. Science 257, 1524. Zhang, M., Simpson, J.A., McKibben, R.B., 1993. Relativistic electron 9ux anisotropies in the duskside Jovian magnetosphere: a test for source location and escape mechanism. Planet. Space Sci. 41, 1029.