Ulysses Low Energy Telescope (LET) homepage address http://helio.estec.esa.nl/let.html
The Division has continued to cooperate, under the leadership of J.A. Simpson of the University of Chicago (USA) as PI, in the COSPIN (Cosmic ray and Solar Charged Particle Investigation) consortium for Ulysses. SSD built one of five COSPIN particle detectors, the Low Energy Telescope (LET), and, together with Imperial College (UK), the Data Processing Unit. LET measures the flux, spectra and composition of nuclei at 1-20 MeV/nucleon. SSD staff involved in the data analysis were V. Bothmer, M. Desai, R.G. Marsden, T.R. Sanderson and K.-H. Trattner.
Recent studies (T. Sanderson) include the analysis of the period when the Ulysses spacecraft passed from the south polar to the north polar region. This period has been called the 'fast latitude scan', since the spacecraft crossed from 80°S to 80°N in a period of only a few months. Fig. 4.2.3.1/1 shows proton intensities measured at 1.8-3.8 MeV (red trace, top panel) and 8-19 MeV (green trace) for the 4-year period centred around this fast latitude scan, together with, in the lower panel, the heliographic latitude of the spacecraft. During the ascent in latitude in 1994 and early 1995, a single peak per solar rotation was observed in the lower energy proton intensity, the amplitude decreasing with time and increasing latitude. While at low latitudes the peak was associated with Corotating Interaction Regions (CIRs) with forward and reverse shocks, at intermediate latitudes (up to ~35°) it was associated only with reverse shocks. At higher latitudes (up to ~70°), peaks were seen with no shocks at all. Beyond 70°, the intensity dropped to the background cosmic ray level, and no recurrent peaks were being observed in both polar regions.
Figure 4.2.3.1/1: Energetic ion intensity observed during the
4-year period centred around the fast latitude scan. The top
panel shows the intensity at 1.8-3.8 MeV (red trace) and 8-19 MeV
(green trace), while the bottom panels shows the heliographic
latitude.
During the fast latitude scan, two peaks per rotation were observed as the spacecraft re-encountered and crossed the heliospheric current sheet (HCS). The appearance of a 4-sector structure was due to a change in shape of the current sheet.
The descent from the north pole to low latitudes has been eagerly awaited to see if the same pattern of events would be found as during the initial ascent in latitude. The first significant peak in the 1.8-3.8 MeV proton intensity was seen at around 55°N, with larger peaks at intervals thereafter. The observed peaks are somewhat sporadic in nature, and do not exhibit the same regularity as those seen in 1993 and 1994. Although not immediately apparent from the Fig., there are two irregular peaks per rotation. The HCS was crossed for the first time at ~20°N, giving rise to a large peak, which was associated (with the observation by the magnetometer) of a reverse shock. During 1993 and 1994, there was only one broad and regular excursion of the HCS into the southern hemisphere, which gave rise to the single peak per rotation. During 1996, the HCS was more irregular and, as solar minimum was passed, continually evolving, giving rise to an irregular two peaks per rotation signature of the CIRs.
Other studies underway (M. Desai and T. R. Sanderson) include a statistical analysis of the relation between the intensity and composition of the ions associated with the CIRs and the various parameters that affect them, such as latitude, radial distance and shock strength. Preliminary results (shown in Fig. 4.2.3.1/2) indicate a good correlation for CIRs observed in the latitude range 30-40°S between the intensity and shock strength, but not between intensity and shock normal angle. We find no similar correlation of other latitude bands. This lack of correlation elsewhere is attributed to the high level of solar activity prevalent during the time Ulysses traversed the low latitude regions. It is thought that solar activity supplies additional seed particles for the CIR-related shocks to accelerate. Given the variability of such activity, this effect could produce fluctuations in the low energy seed particle intensity that are significant enough to suppress effectively any correlation between the proton intensity and the shock strength. These results are consistent with predictions of the first order Fermi acceleration mechanism. The correlation between the proton intensity and the shock normal angle is observed to be weak at all latitudes, and is probably due to the combined effects of the gradient drift and the diffusive acceleration mechanisms.
Figure 4.2.3.1/2: Correlation between the ~1 MeV pro-ton
intensity measured at corotating reverse shocks in 30-40°S
and two parameters that characterise the strength of the shock.
The correlation coefficient, r, is shown in each panel.
Significant progress has been made in the study of the latitudinal gradient of anomalous cosmic rays (K.H. Trattner and R.G. Marsden). With data covering almost four complete equator- to-pole scans now available, it has been possible to investigate in detail the behaviour of these particles, which originate as interstellar neutrals and are subsequently accelerated at the boundary of the heliosphere.
Data from COSPIN/LET have been used, together with reference measurements from SAMPEX (in collaboration with B. Klecker, MPE Garching), to derive latitudinal gradients for the He, O, N and Ne species over a range of energies. Fig. 4.2.3.2 shows a summary of the gradients derived for N, O and Ne plotted versus energy per nucleon. The measured gradients are small and positive (typically 1-2% per degree), confirming the initial results derived from a more limited data set.
Figure 4.2.3.2: Summary of the gradients derived for N, O and
Ne.
Reference
Trattner, K.H. et al. (1996). A&A 316, 519.
Wind 3DP experiment homepage http://helio.estec.esa.nl/wind_3dp_sst.html
The Division is cooperating, under the leadership of R. P. Lin (UC Berkeley), in the 3-D Plasma and Energetic Particle Experiment on NASA's Wind spacecraft (Lin et al., 1995). Other partners are the University of Washington and CESR in Toulouse. The SSD lead investigator is T. Sanderson. Since its launch in 1994, Wind has followed a complicated path that includes travelling from the Earth to the upstream libration point (215 RE from Earth) and back, several excursions far into the upstream region, and a number of passes through the magnetosphere. Upstream particles have been observed all the way out to the libration point. Our previous observations on ISEE 3 with the low energy proton experiment (DFH) had shown that these upstream events had durations of only a few minutes or tens of minutes. At their onset, beams were often observed, together with low frequency waves with periods of around 30 s. From resonance conditions, we predicted that these waves were most likely produced by protons with energies of around 1 keV. On ISEE 3, there was no instrumentation suitable to observe such particles, coming from the direction of the Earth.
Fig. 4.2.3.3 shows the upstream event of 13 August 1995, when Wind was about 80 RE upstream of Earth. The plot displays the composite spectrum obtained from the 15 channels (~100 eV to ~27 keV) of the Proton Electrostatic Analyser (PESA), together with the spectrum from the nine channels (~20 keV to ~4 MeV) of the Solid State Telescopes of the Wind experiment. The solid traces show the spectra observed during the event, while the dashed lines give the spectra observed before the event. For PESA, different spectra are shown for four different viewing directions, i.e. towards the Sun, dawn, Earth and dusk. We believe this was the first time that these upstream protons have been observed over such a large range of energies far upstream from Earth. The spectrum observed looking towards the Sun, dawn and dusk has evidence of a turnover at around 1 keV, as expected from our previous studies.
Figure 4.2.3.3: Proton spectrum observed during the upstream
event of 13 August 1995, when the Wind spacecraft was ~80
RE upstream from Earth.
References
Lin, R.P. et al. (incl. T.R. Sanderson). (1996). Space
Sci. Rev. 71, 125.
Sanderson, T.R., Henrion, J.P.G., Wenzel, K.-P. et al.
(1996). Geophys. Res. Lett. 23, 1215.
We have combined data from several sources to examine the state of the interplanetary medium during 1995. One of the main reasons for this was to provide a baseline for the Geotail mission. Fig. 4.2.3.4 shows a composite plot made of data from several instruments, taken from the Wind key parameters. This plot can be found as a full size A0 Postscript poster on our homepage ( http://helio.estec.esa.nl) under the Wind entries. Other derived quantities shown here have been taken from the various other homepages, the references for which can also be found in the same location.
Figure 4.2.3.4: Wind summary parameters for 1995.
The following parameters and derived quantities are included here:
Many features of interplanetary conditions observed on Wind that are relevant to the Geotail mission can be seen on this plot. For example, the underlying recurrent sector structure and its relation to the current sheet. A regular 4-sector structure with two high speed streams at the start of 1995 turns into a much more irregular structure towards the end of 1995. The CIRs related to these high speed streams can be seen in the magnetic field magnitude trace. Accelerated electrons and protons from these CIRs can be seen in the energetic particle intensity traces.
Transient events occur at random intervals, the most notable being the October CME event, which gave rise to the largest electron and proton increase in the interplanetary medium of 1995. The Bz trace shows the times of southward interplanetary field relevant for the substorms observed by Geotail. This poster also demonstrates the use of modern printing technology in combining large quantities data from several data sources.
Analysis of the detector stacks of the Dublin-SSD Ultra-Heavy (UH) Cosmic Ray Experiment, retrieved in 1990 on NASA's Long Duration Exposure Facility (LDEF) after almost 6 years of space exposure, has continued at the Dublin Institute for Advanced Studies, SSD's partner in the development of the instrument. The experiment collected cosmic ray nuclei of Z>65 and E>1.5 GeV/n. Almost 40% of the sample has been analysed to date, providing a sample of 16 actinides. The main objective of this analysis is to search for signatures of nucleosynthesis processes in the relative abundances of certain elements or groups of elements in this charge region, thereby gaining insight into the origin and propagation of these rare nuclei. For instance, comparison of the abundances of Os, Ir and Pt, which represent the heaviest stable r-process nuclei, with that of Rb (considered to be at least 50% of s-process origin) provides information on the relative importance of these two basic processes.
Next generation quality data have emerged. More than 900 nuclei of Z>60 and which have traversed a complete stack have been located to date. Fig. 4.2.3.5 shows the observed charge spectrum for the data sample after applying final selection criteria. The main points of interest here are the pronounced peak in the distribution centred in the platinum (Pt) region, (74less than or equal to Z less than or equal to 80), the depleted peak in the lead (Pb) region (81 less than or equal to Z less than or equal to 83) and the significant number of actinide elements (Z greater than or equal to 88) present. The measured Pb/Pt ratio of 9.25±0.03 compares well with previously obtained data (HEAO 3 and Ariel), which had a much higher uncertainty. The actinide/ subactinide ratios (Z greater than or equal to 88/74 less than or equal to Z less than or equal to 87) to date is 0.923 ±0.006, which is high compared to a propagated solar system sample, suggesting r- process enhancement at the source. Future analysis on the UHCosmic Ray Experiment is committed to the improvement of charge resolution and to increasing the sample total in the actinide region. Special attention is being paid to the nuclei fragmentation in the detector stacks. It appears that the actinide/subactinide ratio is not affected significantly by this effect (Keane et al., 1997; O'Sullivan et al, 1996).
Figure 4.2.3.5: Charge spectrum for ultra-heavy cosmic ray
nuclei with Z>65 based on ~780 nuclei (~40% of total sample)
from the Dublin/SSD LDEF experiment.
Former members of the Division (F. Jansen, T. Röwf) continued to work on a new source and propagation model for cosmic rays, the halo diffusion model. Initial results indicate reasonable agreement between the observed and calculated abundances. The latest calculations include radioactive decay of the UH nuclei at the source and during their propagation through space (Jansen et al., 1997).
References
Jansen, F. et al. (1997). Adv. Space Res. Submitted.
Keane, A.J. et al. (incl. K.-P. Wenzel). (1997). Proc.
18th Int. Conf. on Nucl. Tracks in Solids, in press.
O'Sullivan, D. et al. (incl. K.-P. Wenzel). (1996).
Radiation Measurements 26, (6), 889.
The use of ion emitters for spacecraft potential control is aimed at improving the accuracy of quasi-static electric field measurements with the double probe technique. GEOS 2 and ISEE results had already indicated that the disturbing effect of photoelectrons around the probes and spacecraft would have been significantly reduced if the spacecraft potential had been kept at a level close to that of the ambient plasma. A novel type of ion emitter based on field emission and ionisation of indium atoms was developed for the Japanese Geotail and Russian Interball/Aurora satellites. Geotail was launched in July 1992 and the capability of the instrument to discharge the spacecraft was readily demonstrated, on a routine basis, in different regions of the magnetosphere. It was shown that an ion beam of 10 mA can reduce potentials larger than +40 V down to +4 V with respect to the ambient plasma (Schmidt et al., 1995). Interball/Aurora was launched on 29 August 1996; SPEX, the active potential control instrument, was operated for the first time jointly with the electric field instrument on 3 October. A quick look at the data has confirmed that the instrument works well and is able to control the spacecraft potential.
The electric charging of the German Equator-S spacecraft, to be launched in late 1997, will be controlled with a similar instrument. The Space Propulsion Division of ESTEC has selected the ion emitter designed in our laboratories as a potential system for future attitude control and propulsion.
It was expected that questions related to plasma/spacecraft interactions would have been solved during the Cluster mission. Each spacecraft carried an ion emitter and would have provided information about cold electrons and ions with good time and energy resolutions. In preparation for the Cluster mission, with the support of the Geotail results and in collaboration with IGPP/ UCLA, kinetic plasma codes have been used to simulate beam/plasma instabilities. The results suggest that ion acoustic waves can be triggered in the plasma sheet boundary layer if the angle between the injected beam and the local magnetic field is close to 90°. The verification of this prediction with Geotail has not yet succeeded; the wave instrument on Cluster would have been better suited to this task.
Models of the photoelectron sheath around the spacecraft and of the potential barrier as a function of surface potential, combined with Geotail data, show that a controlled spacecraft potential of +2 V with respect to the ambient plasma is associated with a barrier of about the same level, located at a distance of about ten times the spacecraft radius. A further reduction of the spacecraft potential increases the height of the barrier and cold electrons with energies below the barrier height cannot reach the spacecraft. Thus, active potential control should aim at a surface potential of about 2 V above the ambient plasma (Zhao et al., 1996). This hypothesis might be verified during the reflight of the Cluster mission if the ion emitter is operated during the release of the wire booms.
References
Schmidt, R., Arends, H., Pedersen, A. et al. (1995). J.
Geophys. Res. 100, 17253.
Zhao, H., Escoubet, C.E., Schmidt, R. et al. (1996). J.
Geophys. Res. 101, 15653.
The Polar spacecraft was launched on 24 February 1996 into a highly elliptical polar orbit (apogee 9RE , perigee 2 RE). Polar and Wind form the Global Geospace Science Program (GGS), which is the principal US contribution to the International Solar Terrestrial Physics Programme (ISTP). The Electric Field Instrument on Polar (EFI; PI F. Mozer, University of California, Berkeley) measures quasi-static electric fields in outer magnetospheric regions such as the cusp, entry layer and plasma mantle. EFI data, combined with those of the Magnetic Field Experiment (MFE; PI C. Russell, University of California, Los Angeles), yield important derived parameters, such as plasma drift velocity, electric current and energy flux.
Simultaneous EFI and MFE measurements are plotted in Fig. 4.2.3.7, as functions of universal time, geocentric distance (measured in RE) and magnetic coordinates (local time, latitude and L-shell parameter). The following physical quantities are displayed, from top to bottom: negative value of the spacecraft potential, the three components of the electric field and those of the magnetic field; the quasi-static fields are both measured in an orthogonal despun coordinate system linked to the spacecraft. The Polar spacecraft crosses the magnetopause on the upleg of its trajectory at around 03.20 and reenters the magnetosphere at 07.00.
Figure 4.2.3.7: Quasi-static electric and magnetic fields
measured near apogee with the EFI and MFE instruments on Polar.
The combination of in situ magnetospheric measurements with ionospheric observations along the geomagnetic field lines (optical, magnetic and radar), near the satellite footpoint, is of particular interest for studies of energy transfer processes at the dayside magnetopause and associated low altitude phenomena. Polar's observations are supplemented by those of several additional ISTP missions, in other regions of the magnetosphere and in the solar wind (Wind, IMP 8, Geotail, Interball); they provide highly valuable insights into the capability of future multi-spacecraft missions (Cluster II/Phoenix). The scientists involved in this study are A. Pedersen, R. Grard and B. Jacobsen.
A method has been developed for mapping electric fields from any location on a closed magnetic field line in the magnetosphere, down to the ionosphere or up to the magnetic equator in the distant magnetotail (Mälkki & Pedersen, 1996). This approach is based on the Tsyganenko T89 model and assumes magnetic flux conservation and negligible parallel electric fields, which implies that the magnetic field lines are equipotential.
At local times other than midnight, dawn or dusk, one observes a strong distortion of the flux tube, in addition to different stretchings of the east-west and north-south dimensions. This is schematically illustrated in Fig. 4.2.3.8. The magnetic meridians and parallels, perpendicular to each other in the ionosphere, make a rather small angle when mapped to the magnetic equator of the tail. On the duskside magnetotail, the ionospheric west and south directions in the northern hemisphere map to almost anti- parallel directions, whereas on the dawnside the mapped directions are nearly parallel. This illustrates the caution to be exercised in using electric fields measured in one region to describe the situation in another part of the magnetosphere. For example, a strong south-westerly electric field in the dusk ionosphere (or a north-westerly one in the dawn ionosphere) corresponds to a very small field in the distant magnetic equator. This technique has been used to map electric fields observed on ISEE 1 to the ionosphere and magnetic equator.
Figure 4.2.3.8: Schematic mapping of an ionospheric magnetic
field cross section (A, B, C, D)to a magnetotail equator cross
section (A',B',C',D') assuming closed field lines. The unit
vectors w and s give the ionospheric westward and southward
directions in the northern hemisphere, respectively, and the
associated mapped directions in the magnetotail.
Reference
Mälkki, A. & Pedersen, A. (1996). J. Geomagn.
Geoelectr. 48, 897.
The electric potential of a spacecraft depends primarily upon the ambient plasma density. Conversely, the spacecraft potential, usually measured with an electric sensor, yields information about the ambient medium. Using potential measurements of the ISEE 1 spacecraft performed between 1977 and 1984, images of the plasma density in the Earth environment have been obtained in 3-D. Fig. 4.2.3.9 shows a slice through the magnetosphere in the noon-midnight meridional plane with the Earth at the centre (this plane contains the Earth-Sun direction and the magnetic dipole axis). Two cases are considered: low activity with auroral electrojet index (AE) in the range 0-200 nT (panel a) and high activity with AE in the range 200-2000 nT (panel b). The dayside magnetosphere is highly compressed during high activity: the magneto-sheath (wide red band) is observed about 2 RE closer to the Earth than during low activity. When AE is large, the plasmasphere is flattened in the XY plane and the density is larger in the distant magnetotail (X<-6 RE ).
Figure 4.2.3.9: Meridional view of the average plasma density
in the Earth environment during (a) quiet and (b) disturbed
periods. The bow shock (BS) and magnetopause (MP) are shown in
nominal positions. Distances are given in Earth radii
(RE).
References
Escoubet, C.P., Pedersen, A., Schmidt, R. et al. (1997).
J. Geophys. Res. Submitted.
The reflight of the Tethered Satellite (TSS 1R) took place during the STS-75 Shuttle mission, launched on 22 February 1996. This mission was a joint venture between NASA and the Italian Space Agency (ASI). The Solar System Division provided the plasma sensors and control electronics for RETE (Research on Electrodynamic Tether Effects, PI M. Dobrowolny, ASI), one of the experiment onboard the satellite. Other collaborating institutions were IFSI/CNR Frascati (I), Universita di Roma (I) and the Observatoire de Paris, Meudon (F). The SSD activities were led by J.-P. Lebreton, supported by A. Butler (electronics) and A. Toni (software).
The main elements of the system are the 20.7 km-long electrically conducting tether, the Shuttle-based deployer and the upward-deployed satellite. When deployed, the conducting tether cuts through the Earth magnetic field lines. An emf of up to 0.25 V/m is induced along the tether. The positively-charged upper end of the tether was in electrical contact with the satellite skin. The negatively charged lower end could be connected through various loads to the Shuttle ground. When a current is established in the system, electrons are collected at the satellite and a current circulates down the tether; an equivalent ion current is collected at the Shuttle end, or, alternatively, an electron current is re-injected into the ionosphere by Shuttle-based electron accelerators. The understanding of the current closure path in the external part of the circuit, i.e. in the ionosphere, was one of the major objectives of the mission.
Tether deployment was supposed to last about 6 h for a full length of 20.7 km. It was to be followed by an on-station phase of about 20 h, and then by the 12 h retrieval phase with an intermediate stop at 2.7 km. Unfortunately, the tether broke during deployment when a length of 19.7 km was reached. At that time, all switches were open and an emf of 3500 V was available at the lower end of the tether. More than 0.5 A of current was drawn for a few minutes before the tether broke. During the tether break event, a current of >1 A was achieved, and the satellite charged at about 2000 V. During that event, gas trapped inside the tether leaked out and a hollow-cathode type of discharge was initiated around the disconnected end of the tether.
After the tether break, NASA established a global network of ground stations. The battery-operated satellite, with its 20 km- long tether hanging along the local vertical, was re-acquired for about 3 days. This unexpected configuration was used to perform ad hoc experiments. About 3 days after separation, the Shuttle and the satellite drew relatively close (50 km), and this configuration was used for electron beam propagation experiments between the Shuttle and the satellite. Attempts were then made to use the tether as a long receiving antenna.
The as-run mission profile, illustrated in Fig. 4.2.3.10/1, provided three distinct science phases: 1: satellite in Shuttle cargo bay (2 days); 2: tether deployment (5 h 30 min); 3: satellite free-flying with a 20 km tether antenna attached (73 days).
Figure 4.2.3.10/1: The as-run mission timelines for both
Tethered Satellite TSS 1 (top) and TSS 1R (bottom) missions are
illustrated against the planned mission timeline (centre). OST1:
on-station (phase 1); RET1: retrieval (phase 1); OST2: on-station
(phase 2); RET2: retrieval (phase 2).
So far, the data analysis has concentrated on results obtained during the tether deployment phase. An excellent data set was acquired to allow the study of the current-voltage (I-V) characteristics of the system, which was one of the main scientific objectives (Fig. 4.2.3.10/2). A set of 18 I-V characteristics was acquired for satellite voltages ranging up to 1200 V (left panel). The complete set of data was normalised (the parameters depend upon the local ionospheric plasma density and temperature) and compared to the isotropic model (Alpert, right panel), and to the Parker-Murphy (P-M) model (centre panel), which assumes magnetic confinement of the current collection. The current collected exceeds by a factor of 2-4 the P-M model, which was the accepted model before the mission. The apparent agreement with the Alpert model is somewhat unexpected as that model does not consider the role of the magnetic field. The preliminary findings confirm the results of the first mission (Dobrowolny et al., 1995).
Figure 4.2.3.10/2: (left) The 18 I-V characteristics acquired
during the TSS 1R tether deployment phase are individually
plotted. Each set of three curves was acquired within 4 min under
essentially the same plasma conditions. Centre: the normalised
current (I/I0 ) is plotted against the normalised
satellite potential (Vsat/V0), where
I0=pRs2Neqe
Vthe is the thermal current and
V0=B2Rs2qe
/2me is the Parker-Murphy current collection cross
section perpendicular to B. The solid line represents the P-M
model given by
I/I0=½+(Vsat/V0)½.
Curves corresponding to 2, 3 and 4 times the P-M model
current are also plotted with a dotted line. Right: the
normalised current I/I0 is plotted against the
normalised satellite potential Tetha*=(qeVsat/kTe)/(l/Rs)¾.
The curved line represents the Alpert model.
Strong wave activity was measured by the RETE wave sensor when there was a current flow at the satellite (Fig. 4.2.3.10/3). The level of wave activity seemed to depend on the current level and the satellite potential. It increased significantly in the lower hybrid range (4-6 kHz) when the satellite potential value was greater than the energy of the ram O+ ions (about 5 eV). Wave amplitudes of >10 V/m were observed during the 4 min event starting GMT 1996: 56:23:48. The wave amplitude was maximum in the ram direction where the reflection of the ram ion was most efficient.
Figure 4.2.3.10/3: Sample of 13 min of Tether RETE data,
showing the Ex (top) and the Bx (bottom) components during three
different satellite charging conditions. The satellite potential
(green), the current (red) and the response of the ion ram sensor
(blue) are plotted in the bottom panel. Wave and plasma sensors
are located on opposite sides of the satellite. Maximum current
in the ion probe indicates that the plasma sensor looks in the
ram direction, hence that the wave sensors are in the wake. The
strength of the E and B signal depends upon the tether current
and the satellite potential. The amplitude of the waves is
maximum in the ram direction. The scale is uncalibrated dB.
None of the existing pre-TSS 1R models correctly predicted the efficiency and the scaling of current collection by a charged body inside a collisionless magnetoplasma. Comprehension of the underlying plasma physics and of the scaling laws is of paramount importance for understanding the high charging of orbiting bodies and the plasma-interaction of planetary satellites, such as Jupiter's Io, with the magnetospheric plasma.
References
Dobrowolny, M., Giuidoni, U., Lebreton, J.-P. et al. (1995).
J. Geophys. Res. 100, 23953.
Thompson, D.C., Lebreton, J.-P. et al. (1997). Geophys.
Res. Lett. Submitted.
Vannaroni, G., Lebreton, J.-P. et al. (1997). Geophys.
Res. Lett. Submitted.
The plasma density and drift velocity derived from the PWS (Plasma Wave System) and ASPERA instruments on the Phobos 2 Mars orbiter have been combined to yield the solar wind dynamic pressure. The plasma density was obtained by combining electron plasma oscillation and spacecraft potential observations from PWS with ion density measurements from ASPERA. Fig. 4.2.3.11 illustrates the good agreement of the two data sets. It had been demonstrated that the bow shock and planetopause locations are not affected by the solar wind ram pressure, which is consistent with the hypothesis that Mars has no significant intrinsic magnetic field.
Figure 4.2.3.11: Solar wind density near Mars. Each average
value is represented by a dot centred on a vertical bar with a
length equal to twice the standard deviation.
It had been hoped that the investigation of the Martian plasma and wave environment would continue when the data collected with the ELISMA instrument aboard the Mars-96 orbiter became available. ELISMA was developed by this Division within a consortium including several laboratories from Bulgaria, France, Poland, Russia, UK, Ukraine and USA. Unfortunately, Mars-96, launched on 16 November 1996, never made it out of Earth orbit and reentered the atmosphere 4.5 h after lift-off.
References
Trotignon, J.G., Grard, R. et al. (1996). Planet. Space
Sci. 44, 117.
Trotignon, J.G., Dubinin, E., Grard, R. et al. (1996). J.
Geophys. Res.101, 24965.
Despite the fact that it is known only through three flybys, its radio emissions and a few telescopic observations, Saturn's magnetosphere is recognised as a very interesting object, somehow intermediate between the Jovian and terrestrial magnetospheres. Its remarkable properties include the unique diversity of its plasma sources, its fast rotation rate and the very important role played by its interaction with Titan. As part of an ongoing magnetospheric effort for the Cassini/Huygens mission to Saturn and Titan, Fig. 4.2.3.12 illustrates the first global 3-D model of the Kronian (Saturnian) magnetic field structure, which is valid within ~24 RS of the planet surface (1 RS=60 330 km).
Figure 4.2.3.12: Modelled shape of Saturn's magneto-pause.
Each tick interval represents 10 RS.
As in the cases of Earth and Jupiter, the net magnetic field at Saturn results from the superposition of three distinct contributions in regions exterior to the magnetotail: a field intrinsic to the planet (well approximated by a dipole of momentum 0.21 gauss RS3, aligned with the planet axis); a field resulting from the various plasma currents inside the magnetospheric cavity (represented by an azimuthal equatorial ring current extending 8-15.5 RS ); and a contribution from the interaction of the magnetosphere with the solar wind.
Reference
Maurice, S. et al. (1996). The geometry of Saturn's
magnetosphere. J. Geophys. Res. 101, 27053.
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