[show abstract][hide abstract] ABSTRACT: We investigate variations of ion flux over the ionosphere and in the plasma sheet when storm-time substorms are initiated, using simultaneous observations of neutral atoms in the energy range of up to a few keV measured by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite and energetic (9-210 keV/e) ion flux measured by the Energetic Particles and Ion Composition/Suprathermal Ion Composition Spectrometer (EPIC/STICS) on board the Geotail satellite. We examined three storm intervals during which the IMAGE satellite was located near its apogee and the Geotail satellite was in the plasma sheet on the nightside. Low-energy neutral atoms traveling from the direction of the Earth can be created by outflowing ionospheric ions through charge exchange processes. The observed neutral atom flux enhancement at storm-time substorms indicates that substorms can cause an immediate increase of low-energy ion flux over the ionosphere by a factor of 3-10. In the plasma sheet, the flux ratio of O+/H+ is rapidly enhanced at storm-time substorms and then increased gradually or stayed at a constant level in a timescale of
[show abstract][hide abstract] ABSTRACT: Shock accelerated nonthermal particles are thought to contribute to modify the shock structure. Here we present two such cases at two strong interplanetary shocks in 1994 and 2003, and try to see how the nonlinear feature depend on the shock parameters, such as Mach number and shock angle.
[show abstract][hide abstract] ABSTRACT: We examine the electric field observations made by the Geotail spacecraft in the magnetotail during the intense magnetic storm of November 20–21, 2003. During this storm, Geotail traversed the magnetotail at R ∼ 9–12 Re from dusk to dawn through the midnight sector at ∼17 UT on November 20 and observed the convection electric field in the near-Earth plasma sheet for the main phase and early recovery phase. While the electric field in the plasma sheet during the storm interval shows highly turbulent characteristics in a wide range of local time, the average intensity of the duskward electric field is not so different in magnitude from those observed during other small to moderate level storms. A comparison with those calculated using the empirical models reveals that the independent behavior of the tail convection intensity with regard to the storm driver intensity still holds for an intense storm case. These results give a further support to the hypothesis suggested by our previous study that the tail convection supplies the ring current particles rather constantly regardless of the solar wind condition.
Geophysical Research Letters - GEOPHYS RES LETT. 01/2006; 33(21).
[show abstract][hide abstract] ABSTRACT: The Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft observed the saturnian magnetosphere from January 2004 until Saturn orbit insertion (SOI) on 1 July 2004. The MIMI sensors observed frequent energetic particle activity in interplanetary space for several months before SOI. When the imaging sensor was switched to its energetic neutral atom (ENA) operating mode on 20 February 2004, at approximately 10(3) times Saturn's radius RS (0.43 astronomical units), a weak but persistent signal was observed from the magnetosphere. About 10 days before SOI, the magnetosphere exhibited a day-night asymmetry that varied with an approximately 11-hour periodicity. Once Cassini entered the magnetosphere, in situ measurements showed high concentrations of H+, H2+, O+, OH+, and H2O+ and low concentrations of N+. The radial dependence of ion intensity profiles implies neutral gas densities sufficient to produce high loss rates of trapped ions from the middle and inner magnetosphere. ENA imaging has revealed a radiation belt that resides inward of the D ring and is probably the result of double charge exchange between the main radiation belt and the upper layers of Saturn's exosphere.
[show abstract][hide abstract] ABSTRACT: We statistically and quantitatively examine the outflow of energetic ions from the magnetosphere during magnetic storms. We also evaluate the contribution of the outflow to the decay of the ring current. We use energetic ion (9–210 keV) data obtained by the energetic particle and ion composition (EPIC) instrument and magnetic field data obtained by the magnetic field measurements (MGF) system, both on board the Geotail spacecraft. The outflowing energy flux, that is, the energy flux lost by ring current ions flowing through the magnetopause, is defined as the energy flux normal to the magnetopause and is calculated based on measurements made adjacent to the earthward side of the low-latitude boundary layer. Our statistics show that the outflowing energy flux is about 105–108 keV/(cm2s) during both the main phase and the recovery phase. It is higher on the afternoonside than on the morningside. It is better correlated with the square root of the dynamic pressure of the solar wind than the electric field of the solar wind, which is a proxy for the strength of the convection electric field. The contribution of the outflow to the rapid decay of the ring current is estimated to be at least 23% and could be much higher than 23% for the 23 September 2001 storm, based on an underestimated leakage area which is determined from magnetic field measurements. We suggest that the drift governing the ion outflow mainly is the $\nabla$B drift which has a radial component that arises from a day-night gradient of the magnetic field in the magnetosphere caused by the solar wind compression. We conclude that the ion outflow contributes significantly to the rapid decay of the ring current, even in the case of a sudden northward turning of the interplanetary magnetic field which causes a sudden decrease in the convection electric field.
Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recent studies have statistically shown that the magnetic reconnection site at substorm expansion onset is located in the magnetotail at X ∼ −20 RE on average. For a substorm event that occurred at ∼0153 UT on 2 July 1996, however, Geotail observed a series of tailward but slow flows with southward magnetic fields fairly close to the Earth at (X, Y) ∼ (−7, 9) RE. The flows had enhancements of the total pressure and the total magnetic field as well as bidirectional field-aligned low-energy electrons in their central part. We interpret these as signatures for tailward moving small plasmoids with scales of ∼0.5–3 RE. Considering that GOES-8 observed a dipolarization at (X, Y) ∼ (−4, 5) RE after the expansion onset, we estimate that the magnetic reconnection occurred between the Geotail and GOES-8 positions. UVI auroral images from Polar and ground magnetic field data show that this substorm, initiated at ∼20 hours MLT and ∼64° magnetic latitude, was not very intense, and the period examined was not during an intense storm. The southward interplanetary magnetic field (IMF) was not very large, while the large duskward IMF persisted for more than 12 hours before the onset as well as the somewhat large solar wind dynamic pressure. It seems likely that the global ionospheric convection was not very strong. Locally enhanced convection and auroral oval expansion due to the large IMF By and the solar wind dynamic pressure might lead to the initiation of the magnetic reconnection much closer to the Earth than usual.
Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: We studied dynamics of O+ ions during the superstorm that occurred on 29–31 October 2003, using energetic (9–210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of XGSM ∼ −6 RE to −8.5 RE, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals (1–10 keV cm−3), while the O+ energy density increased from 0.05–3 keV cm−3 during the quiet intervals to ∼100 keV cm−3 during the superstorm. The O+/H+ energy density ratio reached 10–20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/H+ energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/H+ energy density ratio as large as 10–20 for the October 2003 superstorm.
Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have examined the electric field observations made by the Geotail spacecraft in the near-Earth magnetotail during magnetic storms to study enhanced convection and the associated electric field that is thought to be key to causing the injection of particles into the ring current. Several recent modeling studies of the storm-time ring current suggest that an enhanced convection electric field in the magnetosphere, which is induced by a continuous southward interplanetary magnetic field (IMF), is responsible for steady particle transport into the inner magnetosphere during the storm main phase. The enhanced convection is envisioned to weaken, or cease after the main phase in response to the decrease of southward IMF, leading to the formation of a relatively symmetric ring current around the Earth during the recovery phase. However, surprisingly, our present study has not found clear evidence for the existence of such a large, steady earthward convection during either the storm main phase or the recovery phase. The observed electric field properties in the near-Earth plasma sheet are basically classified into two categories: One is characterized by intermittent bursts of fluctuating duskward electric fields associated with substorm expansions, and the other is dominated by a relatively steady, weak duskward electric field. The weak strength of the convection electric field in the latter category is seen even during storm main phase. The statistical study on this relatively steady, weak field shows that it has a weak duskward component of ~0.3 mV/m on average during both the main and recovery phases, which is almost comparable to that observed during quiet times. Their comparison with the solar wind parameters and the polar cap potential drop calculated using the Boyle model and Weimer model reveals that the weak duskward electric field tends to show poor correlation with these parameters, suggesting that storm-time convection electric field in the plasma sheet is not directly driven by either of them. These results imply that in the near-Earth plasma sheet beyond geosynchronous distance, particle injection for the storm-time ring current is not governed by enhanced convection induced by the solar wind, contrary to conclusions based on simulation studies of the storm-time ring current. The present study suggests the importance of re-examining the contribution to the ring current from the near-Earth plasma sheet for both substorm and nonsubstorm time intervals on the basis of observations made in the magnetosphere.
Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Two coronal mass ejections associated with the X17 and X10 solar flares reached the Earth's environment at very high speeds on 29 and 30 October 2003, respectively, causing very intense geomagnetic storms (Dst ~ -400 nT). The present study focused on the main phase of the 30 October storm during which the Geotail spacecraft was within the near-Earth magnetotail at X ~ -8 RE. A number of extremely intense substorms occurred during this period. In one of them, the intensity of the westward auroral electrojet exceeded 3000 nT, which was one of the largest magnitudes ever observed. The energetic particle observations from the low-altitude, polar-orbiting NOAA satellites indicate that the auroral oval shifted equatorward to magnetic latitudes much lower than usual, as low as 50°. Throughout the interval, the magnetic field in the near-Earth magnetotail, and possibly the plasma density, was much larger than usual, indicating a considerable degree of energy accumulation in the lobe region and compression of the plasma sheet and very intense cross-tail currents. The dense plasma may be responsible for the intense auroral electrojet and the intense ring current. Very large, rapid dipolarizations occurred in relation to the intense substorms. High-energy particle fluxes were an order of magnitude higher than usual, and their increases took place immediately after the dipolarizations. Fast tailward flows with large southward magnetic fields as well as fluxes of energetic heavy ions (oxygen) were also observed, suggesting that the magnetic reconnection took place in the near-Earth magnetotail, associated with the very intense substorms. This location is much closer to the Earth than usual, probably as close to the Earth as ever reported. These magnetotail and auroral observations as well as other results reported previously suggest that the entire magnetosphere was considerably distorted during the storm.
Journal of Geophysical Research 01/2005; 110. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager l spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling’’ and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R
S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge–energy–mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ∼ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5∘ full width half maximum) over the range 7 keV/nuc E < 3="" mev/nuc.="" chems="" uses="" electrostatic="" deflection,="" tof,="" and="" energy="" measurement="" to="" determine="" ion="" energy,="" charge="" state,="" mass,="" and="" 3-d="" anisotropy="" in="" the="" range="" 3="" ≤="">E ≤ 220 keV/e with good (∼0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E≤ 0.884 MeV in the forward direction (G ∼ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ∼ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-α measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ∼60 R
S every 2–3 h (every ∼10 min from ∼20 R
S). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be < 0.1="" s,="" which="" is="" more="" than="" adequate="" for="" microsignature="" studies.="" data="" obtained="" during="" venus-2="" flyby="" and="" earth="" swingby="" in="" june="" and="" august="" 1999,="" respectively,="" and="" jupiter="" flyby="" in="" december="" 2000="" to="" january="" 2001="" show="" that="" the="" instrument="" is="" performing="" well,="" has="" made="" important="" and="" heretofore="" unobtainable="" measurements="" in="" interplanetary="" space="" at="" jupiter,="" and="" will="" likely="" obtain="" high-quality="" data="" throughout="" each="" orbit="" of="" the="" cassini="" mission="" at="" saturn.="" sample="" data="" from="" each="" of="" the="" three="" sensors="" during="" the="" august="" 18="" earth="" swingby="" are="" shown,="" including="" the="" first="" ena="" image="" of="" part="" of="" the="" ring="" current="" obtained="" by="" an="" instrument="" specifically="" designed="" for="" this="" purpose.="" similarily,="" measurements="" in="" cis-jovian="" space="" include="" the="" first="" detailed="" charge="" state="" determination="" of="" iogenic="" ions="" and="" several="" ena="" images="" of="" that="" planet’s="">
Space Science Reviews 08/2004; 114(1):233-329. · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: In order to identify acceleration sites of energetic ions upstream of the Earth's bow shock and in the dayside magnetosheath, we investigated energetic ion events observed by the Energetic Particles and Ion Composition (EPIC) instrument on board the Geotail spacecraft from 1995 to 2001. Energetic ion data obtained by the Ion Composition System (ICS) sensor were used to select ``events,'' that is, intervals when the 77-107 keV ion flux increased by more than two orders of magnitude within 10 min, and examine their spatial distribution and its geomagnetic activity dependence. Heavy ions with energy range of 9-210 keV/e detected by the Suprathermal Ion Composition Spectrometer (STICS) sensor were chosen to calculate the percentage of low-charge-state heavy ions (PLCS = (O+ + N+)/(O+ + N+ + O6+ + O7+)) in each event, and the events were classified into three groups: high-charge-state (HCS) events (PLCS = 75%). The HCS and LCS events occurred at almost the same rate in geomagnetically quiet conditions (SYM-H >= 0 nT), while the LCS events were observed more frequently than the HCS events in geomagnetically disturbed conditions (SYM-H
Journal of Geophysical Research 01/2004; 109. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Spectra, integral moments, and composition (H, He, O, S) of energetic ions (50 keV to 50 MeV) are presented for selected Jupiter magnetospheric positions near the equator between radial distances of ~6 to ~46 Jupiter radii (RJ), as revealed by analysis of the Galileo Energetic Particle Detector data. These characteristics are then used as the basis of interpreting and modeling reported signatures of energetic ion/neutral gas interactions within Jupiter's inner magnetosphere, particularly energetic neutral atom emissions measured during the Cassini spacecraft flyby of Jupiter. Key findings include the following: (1) sulfur ions significantly dominate the energetic (>=50 keV) ion density and pressure at all radial distances >7 RJ; (2) protons dominate integral number and energy intensity planetward of 20-25 RJ; (3) a distinct signature of local, equatorial acceleration of energetic protons is revealed between Io (5.9 RJ) and Europa (9.4 RJ); (4) significant spectral and compositional signatures of neutral gas interactions are also revealed between the orbits of Io and Europa; (5) a previously reported significant depletion of ring current ion populations between Io and Europa during the early-phase operation of Galileo (~1995), as compared with observations obtained during the Voyager epoch (1979), has persisted and probably deepened during later Galileo phases (1999); and (6) detailed energetic neutral atom emission modeling, based on the in situ results reported here, further constrains recent estimates of the contents of the neutral gas torus of Europa.
Journal of Geophysical Research, v.109 (2004). 01/2004;
[show abstract][hide abstract] ABSTRACT: We found a drifting population with a fairly long (∼several tens of minutes) dispersion at a geocentric distance >9 RE, indicating that energetic particles can lie on a closed drift path around the Earth much farther than the geosynchronous distance. In this event, Geotail was situated in the plasma sheet in the post-midnight sector at a distance of ∼10 RE and observed a drift echo of energetic protons with energies of several tens to hundreds of keV. This drifting population showed an azimuthal sector anisotropy in particle distribution as it passed by the spacecraft. The drift echo showed a lack of duskward-directed particle fluxes in its leading portion, became isotropic in the middle of the event, and finally showed enhanced duskward-directed fluxes in its trailing portion. Since the magnetic field was dominated by the Bz component during this event, this sequence of anisotropy can be considered in terms of a density gradient structure of the drifting population with a scale of the order of proton gyroradius (∼0.5 RE) in the equatorial plane. Because the duskward-directed flux at the spacecraft can be attributed to protons with their guiding centers sunward of the observation point, it is suggested that the drifting proton echo has a spatial structure such that protons are localized tailward of the spacecraft at the leading portion, while a sunward density gradient exists around the trailing portion. Thus the present study shows that a drift echo is not an uniform population but has a complicated structure with a sharp density gradient in the radial direction, which may reflect some characteristics of its source site as well as its drift path.
Geophysical Research Letters - GEOPHYS RES LETT. 01/2003; 30(6).
[show abstract][hide abstract] ABSTRACT: The present study aims to investigate how and where ions of ionospheric origin are accelerated to the ring current energy (a few tens to a few hundreds of keV) and how they are supplied to the ring current. We examined the plasma sheet ion composition during magnetic storm development, using energetic (9–210 keV) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. We selected two magnetic storms, that is, the 16–17 May 2000 storm and the 25 December 1998 storm, for which the energy density ratios of O+/H+ and He+/H+ in the plasma sheet were calculated from the EPIC/STICS data. These magnetic storms had a minimum of the SYM-H index (the 1-min Dst index) less than −50 nT and a duration of the main phase shorter than 6 hours. We obtained the following results: (1) Both the O+/H+ and He+/H+ energy density ratios were anticorrelated with the SYM-H index (∣r∣ = 0.73–0.88); (2) The O+/H+ energy density ratio was rather constant at ∼0.1 before storms, but reached 0.3–1.0 at the storm maximum; and (3) The He+/H+ energy density ratio increased from 0.01–0.02 before storms to 0.04–0.1 at the storm maximum. These ion composition changes are comparable to those in the ring current, which have been reported by previous studies, indicating that ions of ionospheric origin are possibly convected to the ring current via the plasma sheet. A close inspection of ion energy spectra revealed that the observed ion composition changes can be attributed to the mass-dependent acceleration of ions by the dawn-to-dusk electric field in the current sheet and the additional transport of ionospheric ions into the plasma sheet.
Journal of Geophysical Research 01/2003; 108. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Voyager and Galileo data taken near Io are used to calculate ion and electron energy spectra to study the interaction of Io with its space environment. Ion spectra for energetic protons, helium, oxygen and sulfur are presented for the first time. We find intensities of ions above 100 keV are more than an order of magnitude smaller than their counterparts at Europa. Oxygen ions appear to have net losses as they diffuse inward from Europa far exceeding losses of protons or sulfur ions. Estimates of surface sputtering rates by these particles suggest only a small fraction of the neutral gas supplied by Io comes from this process. Radiation dose rates into the surface are heavily dominated by electrons in less than ~1% of the surface area.
Geophysical Research Letters - GEOPHYS RES LETT. 01/2003; 30(18).
[show abstract][hide abstract] ABSTRACT: We present a description of and results from our ongoing development of miniature time-of-flight mass spectrometers (TOF-MS) for future landed planetary missions. A new, flexible TOF-MS instrument has been developed to permit the analysis of elemental, isotopic, and molecular composition in solid samples. The new instrument incorporates normal-incidence ultraviolet pulsed laser desorption, a gridless ion source, and a novel curved-field reflectron to maximize sensitivity and resolution over wide abundance and mass ranges. Results with unprepared sample materials are compared with previous work based on infrared laser ablation. Flight instruments based on these technologies can potentially be as small as a soda can without sacrificing their ability to detect low levels of atomic and organic species at grain scales.
[show abstract][hide abstract] ABSTRACT: A CMOS time-of-flight (TOF) system-on-a-chip (SoC) for precise time-interval measurement with low power and high rate has been developed. Microchannel plate electron multipliers or photomultiplier tubes (PMTs) typically produce the start-stop delta-Ts of radiation events to be processed. The TOF chip includes two constant fraction discriminators (CFDs) and a time-to-digital converter (TDC). The CFDs interface to start and stop anodes through two simple preamplifiers and perform the signal processing for time walk compensation. The TDC digitizes the time difference with reference to an off-chip precise oscillator. The design is full custom in both the analog and the digital sections. A first version of the TOF chip developed in a 0.8-μm CMOS process achieved CFD time walk of ∼350 ps for an input amplitude dynamic range of 40 dB and a combined CFD+TDC time jitter of ∼180 ps. The total power dissipation was ∼20 mW at an event rate of 100 K/s and ∼30 mW at a rate of 1 Meg/s. This chip is part of the high-energy neutral atom instrument onboard the NASA/IMAGE spacecraft launched in March 2000. It is selected for many other instruments, including the particle sensor, the fast plasma sensor, and the laser altimeter of Messenger for the Mercury exploration to be launched in 2004.
IEEE Transactions on Nuclear Science 07/2002; · 1.22 Impact Factor
[show abstract][hide abstract] ABSTRACT: Several planetary missions have reported the presence of substantial numbers of energetic ions and electrons surrounding Jupiter; relativistic electrons are observable up to several astronomical units (au) from the planet. A population of energetic (>30[?]keV) neutral particles also has been reported, but the instrumentation was not able to determine the mass or charge state of the particles, which were subsequently labelled energetic neutral atoms. Although images showing the presence of the trace element sodium were obtained, the source and identity of the neutral atoms---and their overall significance relative to the loss of charged particles from Jupiter's magnetosphere---were unknown. Here we report the discovery by the Cassini spacecraft of a fast (>103[?]km[?]s-1) and hot magnetospheric neutral wind extending more than 0.5[?]au from Jupiter, and the presence of energetic neutral atoms (both hot and cold) that have been accelerated by the electric field in the solar wind. We suggest that these atoms originate in volcanic gases from Io, undergo significant evolution through various electromagnetic interactions, escape Jupiter's magnetosphere and then populate the environment around the planet. Thus a 'nebula' is created that extends outwards over hundreds of jovian radii.
[show abstract][hide abstract] ABSTRACT: O+1 and N+1 are important ring current ions in great magnetic storms. We study ~10-210 keV/e O+1 and N+1 in dayside outer ring current (ORC) at ~9-15 RE using Geotail's EPIC/STICS ion spectrometer. We find: average N+1/O+1 (GammaN/O) varies by ~2 over a solar cycle, ~40% (~20%) at solar minimum (maximum); individual GammaN/O values range from ~0.15 (moderate solar maximum storm) to ~1 (18-hr solar minimum superquiet interval); and N+1 is third in importance after H+ and O+1 during two moderate storms (Dstmin ~ -80 nT, ~25% great storm intensity), one at minimum, one maximum. High-latitude topside ionospheric AE-D/MIMS ion composition data form a baseline reference used to argue that ORC GammaN/O variations generally reflect and may be partially explained by topside ionospheric density GammaN/O spatial variations.
Geophysical Research Letters, v.29 (2002). 01/2002;
[show abstract][hide abstract] ABSTRACT: For the first time, two spacecraft, Galileo and Cassini, observed Jupiter's magnetosphere simultaneously for nearly half a year between October 2000 and March 2001. This provided an unprecedented opportunity to disentangle spatial and temporal aspects of the dynamics of the Jovian magnetosphere. In this paper we report new results on the source of the leakage of energetic particles (electrons with energy 15 keV to several MeV and ions with energy > 30 keV) from the dusk side of the magnetosphere. The dual spacecraft measurements show clearly that magnetospheric particles leak directly into the interplanetary medium from the closed magnetosphere, and are the source for the “upstream” particle events [Baker et al., 1996; Zwickl et al., 1981; Krimigis, 1992; Haggerty and Armstrong, 1999; Anagnostopoulos et al., 1998] that have been reported from instruments during prior single spacecraft encounters with the planet. These events, consisting of high-energy particles of Jovian origin, have been observed throughout the heliosphere [Baker and Van Allen, 1976] and their propagation has recently been modelled [Fichtner et al., 2000; Ferreira et al., 2001]. Jupiter then is an important contributor to the interplanetary charged particle fluxes, especially within an astronomical unit of the planet.
Geophysical Research Letters, v.29 (2002). 01/2002;