S. M. Krimigis

Global Observations of the Interstellar Interaction from the Interstellar Boundary Explorer (IBEX)

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. The Sun moves through the local interstellar medium, continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The recently launched Interstellar Boundary Explorer (IBEX) spacecraft has completed its first all-sky maps of the interstellar interaction at the edge of the heliosphere by imaging energetic neutral atoms (ENAs) emanating from this region. We found a bright ribbon of ENA emission, unpredicted by prior models or theories, that may be ordered by the local interstellar magnetic field interacting with the heliosphere. This ribbon is superposed on globally distributed flux variations ordered by both the solar wind structure and the direction of motion through the interstellar medium. Our results indicate that the external galactic environment strongly imprints the heliosphere.

A new form of Saturn's magnetopause using a dynamic pressure balance model, based on in situ, multi-instrument Cassini measurements

The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturns magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low-energy particle pressures from the Cassini plasma spectrometers electron spectrometer and high-energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size-pressure dependence described by a power law D-P(-1/5.0 +/- 0.8). This exponent is consistent with that derived from numerical magnetohydrodynamic simulations.

Low-energy charged particle environment at Jupiter: A first look

The low-energy charged particle instrument on Voyager was designed to measure the hot plasma (electron and ion energies ≳ 15 and ≳ 30 kiloelectron volts, respectively) component of the Jovian magnetosphere. Protons, heavier ions, and electrons at these energies were detected nearly a third of an astronomical unit before encounter with the planet. The hot plasma near the magnetosphere boundary is predominantly composed of protons, oxygen, and sulfur in comparable proportions and a nonthermal power-law tail; its temperature is about 3 x 108 K, density about 5 x 10–3 per cubic centimeter, and energy density comparable to that of the magnetic field. The plasma appears to be corotating throughout the magnetosphere; no hot plasma outflow, as suggested by planetary wind theories, is observed. The main constituents of the energetic particle population (≳200 kiloelectron volts per nucleon) are protons, helium, oxygen, sulfur, and some sodium observed throughout the outer magnetosphere; it is probable that the sulfur, sodium, and possibly oxygen originate at 1o. Fluxes in the outbound trajectory appear to be enhancedfrom ∼90� to ∼130� longitude (System III). Consistent low-energy particle flux periodicities were not observed on the inbound trajectory; both 5-and 10-hour periodicities were observed on the outbound trajectory. Partial absorption of > 10 million electron volts electrons is observed in the vicinity of the Io flux tube.

Electron Beams and Ion Composition Measured at Io and in Its Torus

Intense, magnetic field-aligned, bidirectional, energetic (>15 kiloelectron volts) electron beams were discovered by the Galileo energetic particles detector during the flyby of Io. These beams can carry sufficient energy flux into Jupiters atmosphere to produce a visible aurora at the footprint of the magnetic flux tube connecting Io to Jupiter. Composition measurements through the torus showed that the spatial distributions of protons, oxygen, and sulfur are different, with sulfur being the dominant energetic (>∼10 kiloelectron volts per nucleon) ion at closest approach.

Heavy-Ion Elemental Abundances in Large Solar Energetic Particle Events and Their Implications for the Seed Population

We have surveyed the ~0.1–10 MeV nucleon to the -1 abundances of heavy ions from 3He through Fe in 64 large solar energetic particle (LSEP) events observed on board the Advanced Composition Explorer from 1997 November through 2005 January. Our main results are (1) the 0.5–2.0 MeV nucleon to the -1 3He/ 4He ratio is enhanced between factors of ~2–150 over the solar wind value in 29 (~46%) events. (2) The Fe/O ratio in most LSEP events decreases with increasing energy up to ~60 MeV nucleon to the -1. (3) The Fe/O ratio is independent of CME speed, flare longitude, event size, the 3He/4He ratio, the pre-event Fe/O ratio, and solar activity. (4) The LSEP abundances exhibit unsystematic behavior as a function of M/Q ratio when compared with average solar wind values. (5) The survey-averaged abundances are enhanced with increasing M/Q ratio when compared with quiet coronal values and with average gradual SEP abundances obtained at 5–12 MeV nucleon to the -1. (6) The event-to-event variations in LSEP events are remarkably similar to those seen in CME-driven IP shocks and in 3He-rich SEP events. The above results cannot be explained by simply invoking the current paradigm for large gradual SEP events, i.e., that CME-driven shocks accelerate a seed population dominated by ambient coronal or solar wind ions. Instead, we suggest that the systematic M/Q-dependent enhancements in LSEP events are an inherent property of a highly variable suprathermal seed population, most of which is accelerated by mechanisms that produce heavy-ion abundances similar to those observed in impulsive SEP events. This heavy-ion-enriched material is subsequently accelerated at CME-driven shocks near the Sun by processes in which ions with higher M/Q ratios are accelerated less efficiently, thus causing the Fe/O ratios to decrease with increasing energy.

The Medium-Energy Particle Analyzer (MEPA) on the AMPTE CCE Spacecraft

The medium-energy particle analyzer (MEPA) was designed to measure the spectra and composition of magnetospheric particle populations from 10 keV per nucleon (for oxygen) to more than 6 MeV The instrument provides for high background rejection and a geometry tactor large enougn (10-2 cm2 * sr) to be sensitive to rare natural species and tracer ions beyond geosynchronous orbit, while having the ability to operate without saturation in the very high flux regions of the inner magnetosphere. The MEPA telescope measures time of flight and thus velocity of energetic ions from a thin front foil to a rear solid-state total-energy detector, determining the incident ion mass. The telescope is capable of isotopic resolution of hydrogen and helium, of elemental resolution up through oxygen, and can resolve major species and groups to beyond iron with 32-sector angular resolution and temporal resolution of 0.2-24 s. MEPA represents a new and extremely promising technology for space-particle instrumentation in that it has the capability of measuring low-energy heavy ions with high efficiency while discriminating against high natural fluxes of protons.

Energetic neutral atoms from a trans-Europa gas torus at Jupiter

The space environments—or magnetospheres—of magnetized planets emit copious quantities of energetic neutral atoms (ENAs) at energies between tens of electron volts to hundreds of kiloelectron volts (keV). These energetic atoms result from charge exchange between magnetically trapped energetic ions and cold neutral atoms, and they carry significant amounts of energy and mass from the magnetospheres. Imaging their distribution allows us to investigate the structure of planetary magnetospheres. Here we report the analysis of 50–80 keV ENA images of Jupiters magnetosphere, where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiters satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiters magnetosphere.

Hot Plasma and Energetic Particles in Neptune's Magnetosphere

The low-energy charged particle (LECP) instrument on Voyager 2 measured within the magnetosphere of Neptune energetic electrons (22 kiloelectron volts ≤ E ≤ 20 megaelectron volts) and ions (28 keV ≤ E ≤ 150 MeV) in several energy channels, including compositional information at higher (≥0.5 MeV per nucleon) energies, using an array of solid-state detectors in various configurations. The results obtained so far may be summarized as follows: (i) A variety of intensity, spectral, and anisotropy features suggest that the satellite Triton is important in controlling the outer regions of the Neptunian magnetosphere. These features include the absence of higher energy (≥150 keV) ions or electrons outside 14.4 RN (where RN = radius of Neptune), a relative peak in the spectral index of low-energy electrons at Tritons radial distance, and a change of the proton spectrum from a power law with γ ≥ 3.8 outside, to a hot Maxwellian (kT [unknown] 55 keV) inside the satellites orbit. (ii) Intensities decrease sharply at all energies near the time of closest approach, the decreases being most extended in time at the highest energies, reminiscent of a spacecrafts traversal of Earths polar regions at low altitudes; simultaneously, several spikes of spectrally soft electrons and protons were seen (power input ≈ 5 x 10-4 ergs cm-2 s-1) suggestive of auroral processes at Neptune. (iii) Composition measurements revealed the presence of H, H2, and He4, with relative abundances of 1300:1:0.1, suggesting a Neptunian ionospheric source for the trapped particle population. (iv) Plasma pressures at E ≥ 28 keV are maximum at the magnetic equator with β ≈ 0.2, suggestive of a relatively empty magnetosphere, similar to that of Uranus. (v) A potential signature of satellite 1989N1 was seen, both inbound and outbound; other possible signatures of the moons and rings are evident in the data but cannot be positively identified in the absence of an accurate magnetic-field model close to the planet. Other results indude the absence of upstream ion increases or energetic neutrals [particle intensity (j) < 2.8 x 10-3 cm-2 s-1 keV-1 near 35 keV, at ∼40 RN] implying an upper limit to the volume-averaged atomic H density at R ≤ 6 RN of ≤ 20 cm-3; and an estimate of the rate of darkening of methane ice at the location of 1989N1 ranging from ∼105 years (1-micrometer depth) to ∼2 x 106 years (10-micrometers depth). Finally, the electron fluxes at the orbit of Triton represent a power input of ∼109 W into its atmosphere, apparently accounting for the observed ultraviolet auroral emission; by contrast, the precipitating electron (>22 keV) input on Neptune is ∼3 x 107 W, surprisingly small when compared to energy input into the atmosphere of Jupiter, Saturn, and Uranus.

Radial and Longitudinal Dependence of Solar 4-13 MeV and 27-37 MeV Proton Peak Intensities and Fluences: Helios and IMP 8 Observations

We study the radial and longitudinal dependence of 4-13 and 27-37 MeV proton peak intensities and fluences measured within 1 AU of the Sun during intense solar energetic particle events. Data are from the IMP 8 and the two Helios spacecraft. We analyze 72 events and compute the total event fluence (F) and the peak intensity (J), distinguishing between the events absolute maximum intensity and that neglecting local increases associated with the passage of shocks or plasma structures. Simultaneous measurements of individual events by at least two spacecraft show that the dominant parameter determining J and F is the longitudinal separation () between the parent active region and the footpoint of the field line connecting each spacecraft with the Sun, rather than the spacecraft radial distance (R). We perform a multiparameter fit to the radial and longitudinal distributions of J and F for events with identified solar origin and that produce intensity enhancements in at least two spacecraft. This fit determines simultaneously the radial and longitudinal dependences of J and F. Radial distributions of events observed by at least two spacecraft show ensemble-averaged variations ranging from R-2.7 to R-1.9 for 4-13 and 27-37 MeV proton peak intensities, and R-2.1 to R-1.0 for 4-13 and 27-37 MeV proton event fluences, respectively. Longitudinal distributions of J and F are approximated by the form e, where 0 is the distribution centroid and k is found to vary between ~1.3 and ~1.0 rad-2. Radial dependences are less steep than both those deduced from diffusion transport models by Hamilton et al. in 1990 and those recommended by Shea et al. in 1988 to extrapolate J and F from R = 1 to R < 1 AU.

Hot ions in Jupiter's magnetodisc : a model for Voyager 2 Low-Energy Charged Particle measurements

The Low-Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy-dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two-species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest-energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near-Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton-Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower-energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.