JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A09S10, doi:10.1029/2003JA010111, 2004
Energetic particle observations in the vicinity of Jupiter: Cassini MIMI//LEMMS results N. Krupp,1 J. Woch,1 A. Lagg,1 S. Livi,2 D. G. Mitchell,2 S. M. Krimigis,2 M. K. Dougherty,3 P. G. Hanlon,3 T. P. Armstrong,4 and S. A. Espinosa5 Received 29 June 2003; revised 20 November 2003; accepted 2 December 2003; published 3 August 2004.
[1] We report on energetic particle measurements from the Low Energy Magnetospheric
Measurement System (LEMMS) on board the Cassini spacecraft during the Jupiter flyby (October 2000 to April 2001). Cassini passed Jupiter on its way to Saturn on the dusk flank of the magnetosphere and explored for the first time the dusk-to-midnight magnetosheath of the planet. The flyby period can be divided into three distinct regimes where energetic particle parameter changes are observed. The first period (October 2000 to 10 January 2001) covers the approach phase toward the planet, including bow shock crossings in and out of the magnetosheath as well as the closest approach. In this period, LEMMS responded on solar wind pressure pulses and recorded the passing of two interplanetary shocks. Hours before the first bow shock crossing, low-energy particle intensity increases were observed, possibly with Jovian origin. Before the closest approach approach to the planet two inbound and one outbound bow shock crossings have been observed. The second period (9–10 January) is the period where Cassini briefly entered the magnetosphere twice at about 200 RJ. LEMMS data are only available during a large portion of the second encounter. Inside the magnetosphere the energetic electrons showed a bidirectional pitch angle distribution along the magnetic field, indicative of a closed magnetic field configuration predominantly in the north-south direction. In addition, quasi-periodic electron intensity variations with periods of 40 min were observed. The third period (January to April 2001) covers the distances between 205 and 800 RJ. The spacecraft was skimming along the bow shock boundary in the dusk-to-midnight sector. More than 44 inbound and outbound bow shock crossings have been observed. The most interesting magnetosheath encounter occurred between day 19 and day 28 at distances between 300 and 400 RJ. In that period, LEMMS observed strong intensification of MeV-electron fluxes to values similar to those measured inside the magnetosphere as reported by Krupp et al. [2002]. Most of these increases in electrons were associated with sign changes in the north-south component of the magnetic field. We interpret these observations in terms of magnetic dayside reconnection and particle leakage through the INDEX TERMS: 2756 Magnetospheric Physics: Planetary magnetospheres duskside magnetosheath. (5443, 5737, 6030); 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2728 Magnetospheric Physics: Magnetosheath; 2748 Magnetospheric Physics: Magnetotail boundary layers; 2752 Magnetospheric Physics: MHD waves and instabilities; KEYWORDS: planetary magnetospheres, magnetospheres, Jupiter Citation: Krupp, N., J. Woch, A. Lagg, S. Livi, D. G. Mitchell, S. M. Krimigis, M. K. Dougherty, P. G. Hanlon, T. P. Armstrong, and S. A. Espinosa (2004), Energetic particle observations in the vicinity of Jupiter: Cassini MIMI/LEMMS results, J. Geophys. Res., 109, A09S10, doi:10.1029/2003JA010111.
1. Introduction 1
Max-Planck-Institut fu¨r Aeronomie, Katlenburg-Lindau, Germany. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA. 3 Imperial College, London, UK. 4 Fundamental Technologies, Lawrence, Kansas, USA. 5 University of Tokyo, Tokyo, Japan. 2
Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JA010111
[2] The NASA/ESA spacecraft Cassini, a mission to study the ring planet Saturn, was launched on 15 October 1997. To reach its final target, the spacecraft needed to gain energy through gravity assists from two Venus flybys in 1998, from an Earth flyby in 1999, and from a Jupiter flyby in 2000/2001. Besides Galileo, which orbited the largest planet in our solar system for almost 8 years (1995 – 2003), Cassini is the sixth spacecraft (after Pioneer 10 and 11,
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Voyager 1 and 2, and Ulysses) which flew by Jupiter. Ulysses provided the first measurements from the duskside high-latitude sector, and Galileo explored on its 28th and 29th orbit around the planet the equatorial dusk magnetosphere of Jupiter. However, these two spacecraft observed this section only for a few days due to their trajectory more or less radially away from Jupiter. In contrast, Cassini flew by Jupiter on the duskside at relatively large distances (closest approach at 138 RJ) more or less radially away from the Sun. This trajectory provided the first opportunity to extensively study the Jovian dusk-to-midnight magnetosheath out to distances of several hundred RJ. In this paper the results of energetic particle observations during the Jupiter flyby will be presented. In the time period between October 2000 and April 2001, Cassini was within 1000 planetary radii (RJ) from Jupiter. Initial results of the Cassini flyby including the dual spacecraft observations in the vicinity of Jupiter with the Galileo spacecraft have been published in Nature, volume 415. This paper focuses on observations obtained by the Low-Energy Magnetospheric Measurement System LEMMS, one sensor of the Magnetospheric Imaging Instrument (MIMI) as well as utilizing some of the magnetometer (MAG) data obtained during that period.
2. Instrumentation [3] The Low-Energy Magnetospheric Measurement System (LEMMS) is one out of three detector systems of the Magnetosphere Imaging Instrument (MIMI) on board the Cassini spacecraft. Together with the other two sensors CHEMS (Charge-Energy-Mass-Spectrometer) and INCA (Ion and Neutral Camera), MIMI is designed to study energetic neutral and charged particles in planetary magnetospheres and in interplanetary space. We concentrate here on a short description of LEMMS only. Details of the MIMI instrument may be found in the extensive instrument description by Krimigis et al. [2003]. [4] Energetic particles measured with LEMMS are analyzed by the particle type (ions or electrons), their incidence energy, and their incidence direction. The instrument is capable of detecting energetic ions with energies above 30 keV and electrons between 15 keV and several MeV. LEMMS consists of two telescope systems to measure low-energy and high-energy particles (low-energy and high-energy end). The opening angles of the two entrance apertures are 15 and 36 for the low-energy and highenergy end, respectively. The whole assembly is mounted on top of a programmable turntable rotating about the -yaxis of the spacecraft coordinate system (S/C) [Lagg et al., 2001]. This allows the determination of the incidence direction of particles within the x-z-plane of the S/C system. The analysis itself is based on the energy loss of these particles in a total of 11 semiconductor detectors at various positions inside the instrument. Electrons and ions incidenting the low-energy telescope are separated in the magnetic field of an internal magnet guiding electrons and ions to different detectors. The high-energy head consists of a stack of solid state detectors where ions and electrons are identified using logic conditions between several electronic thresholds of the detectors. A total of 57 different rate channels are distinguished by LEMMS. For three detectors,
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pulse height analysis is performed with 64 energy channels each. The spatial resolution of the instrument is determined by the speed of the instrument turntable. The data are normally subdivided into 16 different look directions (subsectors) during one rotation of the LEMMS sensor. In addition, for the so-called programmable priority counters (channels A0, A1, C0, and C1 during the Jupiter flyby), each subsector is divided into eight microsectors, providing 128 different look directions within one rotation. The same subsector is scanned every 86 s. Each subsector/microsector is sampled for 5.31 s/0.66 s, respectively. A full 4 p coverage providing measurements from directions in the three-dimensional space can only be achieved if in addition to the motion of the instrument turntable the spacecraft itself is spinning perpendicular to the LEMMS rotation axis. Full pitch angle coverage can only be achieved if the magnetic field vector lies within the scan plane (x-z-plane in the spacecraft coordinate system) of the instrument.
3. Energetic Particle Observations During the Cassini Jupiter Flyby [5] Figure 1 gives an overview of the observed energetic particle intensities for the flyby time period. For most of the time under study the LEMMS scan plane was perpendicular to the ecliptic with only a few exceptions when the spacecraft was spinning or when it was turning around the -y-axis to allow special observations for remote sensing instruments on board. The data shown are averaged over 16 LEMMS turntable rotations (23 min). Dashed vertical lines indicate time points of the first bow shock crossing (BS) and closest approach (CA). The spacecraft was periodically reoriented between basically two configurations: In the downlink configuration the main antenna (z-axis of the spacecraft) was pointed toward Earth while the x-axis was pointing north of the ecliptic plane (NEP). In the observation configuration the remote sensing instruments (-y-axis of the spacecraft) were positioned towards Jupiter while the secondary axes (x and z) were pointing to NEP (observation slit of remote sensing instruments parallel or perpendicular to the ecliptic). In addition a few other orientations of the spacecraft have been carried out for special purposes, i.e., -x-axis to the Sun (allowing the direct detection of the solar wind) and a few others. In this paper we will show the orientation of the spacecraft axes in the bottom panels of Figures 2, 5, 6, and 7 for reference. As mentioned before, LEMMS is scanning the x-z-plane, and therefore each reorientation of the spacecraft will cause changes in the particle distributions. [6] We divide the flyby period of Cassini at Jupiter in three different sections shown as grey-shaded areas: (1) 2000, day 335 – 2001, day 009: The inbound pass upstream of the planet, when Cassini was approaching the planet, including multiple bow shock crossings and closest approach (day 365, 1005 UT) until the brief entries into the magnetosphere. (2) 2001, day 009– 010: An interval when Cassini was briefly inside the Jovian magnetosphere twice at distances between 200 and 204 RJ from the planet. (3) 2001, day 010– 090: The outbound pass downstream of the planet, when Cassini was in the dusk-midnight magnetosheath at distances between 200 and 800 RJ. The timings of observed bow shock and magnetopause crossings
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Figure 1. Overview of ion and electron intensities (23 min averages) as measured by the Low-Energy Magnetospheric Measurement System (LEMMS) on board Cassini during the Jupiter flyby period. Dashed lines indicate first bow shock (BS) and closest approach (CA). Regions 1 – 3 are grey-shaded and separate the flyby period in inbound, inside magnetosphere, and outbound. during the flyby have been identified by signatures in the particle, fields, and wave instrument data on Cassini. These identified boundary crossings are summarized by W. Kurth et al. (Duskside boundaries of the Jovian magnetosphere: Observations by Cassini, submitted to Journal of Geophysical Research, 2004). The data gaps in the MIMI data during the flyby period are partly caused by problems on the Cassini spacecraft as well as caused by problems with the MIMI flight software during that time. 3.1. Measurements on the Inbound Pass [7] At the beginning of the time period (days 340 – 346), Cassini was about 400 RJ upstream of the planet. LEMMS measured the energetic particle response to a solar wind pressure pulse (day 342, 2315 UT) which was also seen by the Galileo spacecraft at distances of 120 RJ [Krupp et al., 2002]. The intensities of low-energy ions sharply increased at that time, while the intensity of low-energy electrons and MeV ions already changed 2 days earlier. The intensity of electrons with energies greater than 950 keV did not increase at that time. The passages of two interplanetary shocks (IP) (day 349, 1710 UT and day 354, 1512 UT) have been identified on Cassini in magnetometer and plasma data. The response of the second shock in energetic particle intensity changes has also been observed on board the Galileo spacecraft and is discussed in more detail by Hanlon et al. [2004]. After the passages of these two shocks, LEMMS recorded sporadic increases in low-energy ion intensities (days 354– 362). During that time, problems on board the Cassini spacecraft resulted in data gaps for all the instruments and prevented a more detailed analysis. So it
remains unclear if these increases were caused by upstreaming particles from Jupiter or not. Figure 2 shows a part of the inbound pass (2000 day 363 to 2001 day 003). About 20 hours before the first bow shock crossing on day 363, 0417 UT at about 142 RJ low-energy ion intensities start to increase in bursts by a factor of 3 – 5, while electrons did not show these changes (see Figure 2). The first of more than 44 bow shock crossings (inbound and outbound together) occurred on day 363 at a distance of 140 RJ from the planet. Inside the magnetosheath, low-energy ions showed large intensity fluctuations. None of them seemed to be related with spacecraft reorientations. The intensity of low-energy and high-energy electrons increased at the end of magnetosheath encounter 1 and inside magnetosheath encounters 2 and 3. High-energy intensity fluctuations are absent in magnetosheath 4. Especially in magnetosheath 3, low-energy ions and low-energy electrons showed different time profiles (for details see Krupp et al. [2002]). The intensity of high-energy ions remained close to background levels except on day 002 and 003 the levels raised by a factor of 3 – 5. 3.2. Measurements Inside the Jovian Magnetosphere [8] Cassini entered inside the Jovian magnetosphere in 2001 on day 009 (1250 – 2115) and 010 (0655 – 2035). LEMMS measurements are only available on day 010 between 1635 and 2200 due to problems in the MIMI flight software at that time (which was fixed after the flyby). The unexpected entrance into the magnetosphere occurred at distances of more than 200 RJ from the planet at about 1919 local time, hence at about 150 RJ downstream of the Galileo
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Figure 2. Intensities of ions (36 – 56 keV and 2370 – 4540 keV) and electrons (29 – 43 keV and E > 950 keV) as measured by the Low-Energy Magnetospheric Measurement System (LEMMS) on board Cassini during the inbound period of the Jupiter flyby. Solid lines indicate bow shock crossings (BS) (as derived from magnetometer, plasma, and wave data on board Cassini) and closest approach (CA). Gray-shaded areas mark the time periods when Cassini was inside the magnetosheath. The flags in the bottom panel indicate the orientation of the spacecraft axes. spacecraft on its 29th orbit radially away from the planet. Fortunately, during this period the magnetic field vector was in the LEMMS scan plane, and hence this configuration offered a nearly full pitch angle coverage in the magnetosphere. The observations of MIMI/LEMMS inside the magnetosphere are summarized in and Figures 3 and 4. Figures 3a and 3b show the measured ion and electron intensities on day 10, 1600 – 2200 UT inside the magnetosphere for several energy channels in the indicated energy ranges. It is clearly visible that the intensities of electrons inside the magnetosphere show quasi-periodic variations. A Fast Fourier frequency analysis of the electron data is shown in Figure 4. The main peak in the derived power spectrum is found around 40 min, a period which has been observed within the Jovian system in plasma wave [MacDowall et al., 1993], relativistic electron [McKibben et al., 1993], and x-ray data [Gladstone et al., 2002]. This period seems to be of global nature for the Jovian system. Figure 3c shows the observed pitch angle distributions for low-energy ions and electrons. The magnetopause crossing at 2035 UT is marked by a solid line. The intensity of the two energy channels is color-coded and normalized to the scan-averaged intensity. Grey at the top and bottom of the panels indicates lack of pitch angle coverage. It is clearly seen that most of the electrons were observed bidirectionally along the magnetic field direction (0 and 180). Cassini went back into the
magnetosheath by crossing the magnetopause at 2035 UT on day 10. Only 17 min later also Galileo left the magnetosphere at 2052 UT [see Krupp et al., 2002], indicative of a large-scale change in the size of the magnetosphere [Kurth et al., 2002]. The magnetic field connection between Cassini and the planet (0 pitch angle) remained for about 20 min with clear signatures of electrons streaming along the magnetic field but not at 180 because the other end of the field line lost their connection to the planet during the crossing and the bidirectional electron pitch angle distribution disappeared. In contrast the ions did not show this behavior during and after the magnetopause crossing. The pitch angle of their maximum intensity slightly changed from 90 and 150 and they isotropized right after the magnetopause crossing where no preferred pitch angle could be observed. After these 20 min of one-way connection to Jupiter the connection was lost resulting in a more or less isotropic distribution for electrons. 3.3. Measurements on the Outbound Pass [9] After the magnetopause crossing on day 10, Cassini spent at least 50 days in or close to the premidnight magnetosheath of the planet at distances between 200 and 800 RJ. During that phase of the encounter, energetic particle measurements showed a variety of enhancements, some of which were caused by changes in the spacecraft
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orientation and therefore changes in the orientation of the MIMI/LEMMS scan plane. However, the most prominent enhancements were seen in low-energy and high-energy electron channels between 17 January and 1 February 2001 (days 017 – 032) at distances between 280 and 450 RJ. Measurements during this time period are shown in Figure 5. Quasi-periodic increases in the intensities of electrons between 15 keV and several MeV could be observed peaking around day 24 and 27 with values as high as inside the magnetosphere on day 10 (see Figure 1).
Figure 4. Fast Fourier analysis power spectrum as derived from electron intensities measured by the MIMI/LEMMS instrument on board Cassini. The selected time period include all data shown in Figure 3 from day 10, 2001, 1600– 2200 UT. Low-energy ions, instead, show pronounced peaks only close to the bow shock boundary, and high-energy ions most of the time remained at background levels. The largest electron enhancements happened inside the magnetosheath and seem to be related to changes of the north-south component of the magnetic field. As an example we show the particle and field measurements from day 023. In Figure 6 the intensity of MeV-electrons is plotted together with the magnetic field components for that day. In most Figure 3. Intensities and pitch angle distributions as a function of time, distance and local time inside the Jovian magnetosphere as measured by the MIMI/LEMMS instrument on board Cassini on 10 January 2001, 1600– 2200 UT. The solid line indicates the magnetopause (MP) crossing on day 10, 2035 UT: (a) Intensities of ions with energies between 30 and 4000 keV in seven energy channels; (b) Intensities of electrons with energies between 15 keV and a few MeV in eight energy channels; (c) Normalized pitch angle distributions for ions (36 – 56 keV) and electrons (29 – 42 keV). The intensity at a certain pitch angle is colorcoded and normalized to the scan-averaged (=86 s) intensity for each LEMMS rotation. 5 of 10
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Figure 5. Ion and electron intensities as measured by the MIMI/LEMMS instrument on board Cassini during the outbound period of Jupiter flyby. Solid lines indicate bow shock crossings. Grey-shaded areas mark period where Cassini was inside the magnetosheath.
cases the electron intensity increases if the north-south component changes sign or is 0. [10] Some of these electron increases were observed periodically every 5 or 10 hours (indicative of their Jovian origin) as if they are released periodically at the half/full planet’s rotation rate from a source region in the Jovian system. An example of the periodicity is shown in Figure 7, supporting the relation of the electron intensity increases and the changes in the north-south direction of the magnetic field. Sometimes they occur when the field was southward for a period of time and happen just before it changes to northward, and on other occasions they occur after a change from a north to a southward field. This phenomenon requires careful analysis and work has begun on this. Table 1 summarize the identified increases in the electron intensity for the entire flyby. Their duration and amplitude is highly variable. The highest value has been observed on day 024 at about 350 RJ.
4. Summary and Discussion [11] This paper gives a summary of the highlights from energetic particle measurements during the flyby of the Cassini spacecraft at Jupiter in 2000/2001. The flyby period has been divided in an inbound part and an outbound part, and in a period where Cassini went inside the Jovian magnetosphere. During the inbound pass particles flowing upstream into the solar wind outside the bow shock could be identified by MIMI/LEMMS which are in good agreement
with previous measurements from earlier missions [see, i.e., Chenette et al., 1974; Zwickl et al., 1981; Krimigis et al., 1981; Haggerty and Armstrong, 1999; Anagnostopoulos et al., 2001; Krupp et al., 2002]. Whenever the field lines connect the magnetosphere and the spacecraft the particles can escape as already pointed out by Hill and Dessler [1976]. Cassini is able to confirm these findings and extend the local time and distance range where these particles have been observed. [12] The brief excursion of Cassini into the Jovian magnetosphere revealed two important observations in terms of energetic particles: [13] 1. MIMI/LEMMS observations could show that electrons are mostly traveling bidirectionally along the magnetic field lines in the duskside Jovian magnetosphere at 200 RJ. From this observation we conclude that even at those distances the magnetic field lines are closed and connected with the planet on both sides. This confirms Galileo energetic particle results at the same time but at a completely different distance and local time [Krupp et al., 2002]. As Cassini approached the magnetopause from the inside, it apparently entered a low-latitude boundary layer, in which the trapped ions (enhancement at 90 degree pitch angles in Figure 3) gain an additional anisotropy with a field-aligned component so that the pitch angle maximum appears at about 120– 130. This is likely a flow anisotropy, similar to the Earth’s LowLatitude Boundary Layer (LLBL) which flows antisunward near the magnetopause. The fact that it presents itself as a field-aligned component is simply an artifact of the field
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Figure 6. Electron intensity (>950 keV) and magnetic field components on day 023, 2001 when Cassini was inside the duskside magnetosheath of Jupiter. Solid lines mark increases in electron intensities and their relationship to sign changes of the north-south magnetic field component.
orientation in the LLBL; convection of a closed field region can appear as field-aligned flow [Mitchell et al., 1987]. We know from the electron measurements that this layer is a closed field region. The electrons also tell us where the transition to open field lines takes place, as they abruptly change from bidirectional to unidirectional at just after 2030 UT (Figure 3). Whether this boundary is the magnetopause is not entirely clear. Again by analogy to the Earth’s LLBL, there can exist a layer of open field lines, planetward of the magnetopause but adjacent to it, where magnetospheric electrons can be seen escaping the magnetosphere into the magnetosheath [Mitchell et al., 1987]. This same phenomenon appears to be acting at Jupiter. Although the precise location of the magnetopause is not well determined in the electron data, it is clear that the energetic electrons are moving along the field only in one direction
and so are being lost to the sheath. This is strong evidence for reconnection at the Jovian magnetopause, providing an open field topology. Such regions are very likely candidates for the source of the Jovian energetic electrons seen farther out in the sheath and in the interplanetary medium. Since the escape of energetic electrons appears to be unhindered (the return flux is absent), this implies that there is a very efficient mechanism for transporting the electron transverse to the field from closed field lines to open ones, inside the magnetosphere, so that the open field lines remain populated in spite of the fast leakage of the electrons parallel to the field. After the magnetopause crossing particles were leaking out of the magnetosphere until the connection was lost with the planet. [14] 2. A 40-min periodicity of energetic electrons is measured inside the Jovian magnetosphere which seems to be a characteristic period for the Jovian system. Such
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Table 1. Summary of Electron Intensity Increases as Measured by the MIMI/LEMMS Instrument During the Jupiter Flyby of the Cassini Spacecraft in 2000/2001 Year
doy
hhmm
Distance [RJ]
Local Time
2000 2001
365 006 – 007 013 019 020 020 – 021 021
0000 – 1200 0800 – 0400 0300 – 0900 1100 – 1900 1500 – 1600 2300 – 0100 0300 – 1000 1030 – 1330 1400 – 2200 0400 – 1500 2030
137 168 – 170 228 300 312 315 319 322 323 – 325 328 – 390 410 415 – 420 425 430 505 507 511 – 523 570 573 – 586 737 775 – 927
1555 1830 1940 2019 2023 2025 2026 2027 2028 2029 – 2045 2049 2050 2052 2053 2103 2103 2105 2109 2110 2119 2119 – 2123
022 – 027 028 029 030 036 037 041 042 055 058 – 070
0000 – 0300 0930 – 1330 1300 1645 1700 – 1800 0000 – 0100
the Jovian system can be measured in the dawn-to-noon sector (inbound passes of Pioneer 10, Pioneer 11, and Ulysses) and vice versa if the IMF points outward from the Sun then it is more favorable to observe electrons from Jupiter in the dusk-to-midnight sector of the magneto-
Figure 7. Periodic intensity variations of electrons in the Jovian dusk magnetosheath together with north-south magnetic field component. periodicity was observed by Simpson et al. [1992] during the Ulysses flyby at Jupiter in electron intensities and correlated with hot plasma bursts and radio emissions [MacDowall et al., 1993]. A possible explanation could be explosive merging processes taking place throughout the magnetosphere reaching to the magnetopause, indicative of a global phenomenon. MacDowall et al. [1993] reported the source of related radio emissions in high latitudes. One possible emission mechanism could be the electron cyclotron maser instability [Wu and Lee, 1979]. It is therefore possible that the 40-min oscillations of energetic particle intensities in the magnetosphere and close to the magnetopause are related to radio phenomena [MacDowall et al., 1993; Kaiser et al., 2001], magnetic field variations [Wilson and Dougherty, 2000], and possibly also to X-ray observations [Gladstone et al., 2002] with the same period. [15] The observations of energetic electrons downstream from Jupiter is a new piece of the puzzle to understand the dynamics of the Jovian magnetosphere. Tsuchiya et al. [1999], Morioka et al. [1997], and Morioka and Tsuchiya [1996] found that the release rate in the interplanetary medium is controlled by solar wind variations and by the polarity of the IMF in analyzing electron data from Pioneer 10, Pioneer 11, and Ulysses. They claim that if the polarity of the IMF is toward the Sun electrons from
Figure 8. Equatorial view of magnetic field lines dragged around the Jovian magnetosphere. In addition the trajectory of Cassini is shown.
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source regions for the Jovian electrons observed throughout the heliosphere. [17] Acknowledgments. The German contribution of the MIMI/ LEMMS Instrument was in part financed by the BMBF (Bundesministerium fu¨r Bildung und Forschung) through the DLR (Deutsches Zentrum fu¨r Luft- und Raumfahrt e.V.) under contracts 50 QJ 97032 and 50 OH 0103 and by the Max Planck Gesellschaft. Special thanks to Martha Kusterer and Steve Kellock for reducing the MIMI and MAG data sets, respectively. [18] Arthur Richmond thanks Michael L. Kaiser and R. Bruce McKibben for their assistance in evaluating this paper.
References
Figure 9. A north-south view of the Jovian magnetosphere from the dusk side towards the planet. Magnetic field lines with small or zero north-south components connect an equatorial source region with the spacecraft. sheath (outbound passes of Ulysses, Cassini, and Galileo). From our measurements we conclude more generally that released electrons could be observed whenever the source region and the spacecraft are magnetically connected. Figures 8 and 9 illustrate this possible scenario. The interplanetary magnetic field (IMF) is dragged around the obstacle into the duskside downstream magnetosheath. Under the assumption that a source region due to (patchy) continuous reconnection exists on the dayside some of the field lines can connect this source region with the spacecraft location. Electrons released from the source region traveling along the magnetic field lines could then be observed by Cassini. The fact that the energetic electrons were predominantly observed when the north-south component of the magnetic field was small or zero could then mean that the source region was restricted in latitude or more effective close to the Jovian equator as illustrated in Figure 9. From the fact that the intensities of the electrons in late January 2001 at distances around 350 – 450 RJ reached values similar to those inside the magnetosphere could be interpreted in two ways: On one hand this could mean that Cassini was very close to the magnetopause or even entered the magnetosphere briefly. Unfortunately, the pitch angle coverage was not good enough to observe bidirectional distributions at that time confirming another encounter with the magnetosphere. On the other hand these high values of MeV electrons at those distances could also mean that the connection to the very strong source at the equator was extremely good during that time so that nearly all electrons leaving the magnetosphere were detected. [16] We believe that the observed relativistic electrons downstream from the planet and shown in Figure 5 are at least one source of those particles observed throughout the heliosphere [see, i.e., Baker et al., 1996]. We think that after these measurements there is strong evidence that Jupiter permanently releases large MeV electron fluxes through the dusk-to-midnight magnetosheath. This would mean that Cassini flew directly through one of the main
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[email protected]) M. K. Dougherty and P. G. Hanlon, Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College London, Prince Consort
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Road, London SW7 2BW, UK. (
[email protected]; paul.hanlon@ ic.ac.uk) S. A. Espinosa, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan. (
[email protected]) S. M. Krimigis, S. Livi, and D. G. Mitchell, Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 207236099, USA. (
[email protected];
[email protected]; don.
[email protected]) N. Krupp, A. Lagg, and J. Woch, Max-Planck-Institut fu¨r Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany. (krupp@linmpi. mpg.de;
[email protected];
[email protected])
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