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Voyager 1 (V1) began measuring precursor energetic ions and electrons from the heliospheric termination shock (TS) in July 2002. During the ensuing 2.5 years, average particle intensities rose as V1 penetrated deeper into the energetic particle foreshock of the TS. Throughout 2004, V1 observed even larger, fluctuating intensities of ions from 40 kiloelectron volts (keV) to [>/=]50 megaelectron volts per nucleon and of electrons from >26 keV to [>/=]350 keV. On day 350 of 2004 (2004/350), V1 observed an intensity spike of ions and electrons that was followed by a sustained factor of 10 increase at the lowest energies and lesser increases at higher energies, larger than any intensities since V1 was at 15 astronomical units in 1982. The estimated solar wind radial flow speed was positive (outward) at [approximately]+100 kilometers per second (km s⁻¹) from 2004/352 until 2005/018, when the radial flows became predominantly negative (sunward) and fluctuated between [approximately]-50 and 0 km s⁻¹ until about 2005/110; they then became more positive, with recent values (2005/179) of [approximately]+50 km s⁻¹. The energetic proton spectrum averaged over the postshock period is apparently dominated by strongly heated interstellar pickup ions. We interpret these observations as evidence that V1 was crossed by the TS on 2004/351 (during a tracking gap) at 94.0 astronomical units, evidently as the shock was moving radially inward in response to decreasing solar wind ram pressure, and that V1 has remained in the heliosheath until at least mid-2005.

The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo-electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock-heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude λ, latitude β) = (221°, 39°), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon's nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.

Leaving the heliosphere: Voyager 2 reports back On 30 August 2007 Voyager 2 began to cross the termination shock, a boundary produced by the inter-action of the Sun with the rest of the Galaxy, where the supersonic solar wind abruptly slows as it presses outward against the surrounding interstellar matter. Five Letters in this issue present the data that the probe sent back. The Voyager 2 crossings occurred about 1.5 billion kilometres closer to the Sun than those of Voyager 1, illustrating the asymmetry of the heliosphere. The results from the plasma experiment, low-energy particle, cosmic ray, magnetic field and plasma-wave detectors reveal a complex and dynamic shock, reforming itself in hours rather than days. The cover graphic of Voayer 2 on the brink of entering interstellar space is by Henry Kline of JPL. It may be decades before another probe crosses the termination shock but remote observations can now bridge the gap — as shown by Wang et al . who report measurements of energetic neutral atoms in the heliosheath from the STEREO A and B spacecraft that complement the Voyager in situ observations made at the same time. In News & Views, J R Jokipii puts the Voyager findings into context. For more on the on Voyager odyssey, see page 24, and the Author page, and go to the movie on http://www.nature.com/nature/videoarchive/voyager . Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably play key roles in mediating dynamical processes at the termination shock and in the heliosheath. Intensities of low-energy ions measured at Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. The acceleration of ions extracts a large fraction of bulk flow kinetic energy from the incident solar wind. Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by the spacecraft Voyager 1 (refs 1–5 ) and Voyager 2 (refs 3 , 6 ). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3–5 ). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August–1 September 2007 (refs 7–9 ). Unlike Voyager 1, Voyager 2 is making plasma measurements 7 . Data from the plasma 7 and magnetic field 8 instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure 8 and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind 7 .

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382 Saturn has a sufficiently strong dipole magnetic field to trap high-energy charged particles and form radiation belts, which have been observed outside its rings. Whether stable radiation belts exist near the planet and inward of the rings was previously unknown. The Cassini spacecraft’s Magnetosphere Imaging Instrument obtained measurements of a radiation belt that lies just above Saturn’s dense atmosphere and is decoupled from the rest of the magnetosphere by the planet’s A- to C-rings. The belt extends across the D-ring and comprises protons produced through cosmic ray albedo neutron decay and multiple charge-exchange reactions. These protons are lost to atmospheric neutrals and D-ring dust. Strong proton depletions that map onto features on the D-ring indicate a highly structured and diverse dust environment near Saturn.

We report an all-sky image of energetic neutral atoms (ENAs) >6 kilo-electron volts produced by energetic protons occupying the region (heliosheath) between the boundary of the extended solar atmosphere and the local interstellar medium (LISM). The map obtained by the Ion and Neutral Camera (INCA) onboard Cassini reveals a broad belt of energetic protons whose nonthermal pressure is comparable to that of the local interstellar magnetic field. The belt, centered at approximately 260° ecliptic longitude extending from north to south and looping back through approximately 80°, appears to be ordered by the local interstellar magnetic field. The shape revealed by the ENA image does not conform to current models, wherein the heliosphere resembles a cometlike figure aligned in the direction of Sun's travel through the LISM.

For more than five decades, the shape and interactions of the heliosphere with the local interstellar medium have been discussed in the context of two competing models, posited in 1961 1 : a magnetosphere-like heliotail and a more symmetric bubble shape. Although past models broadly assumed the magnetosphere-like concept, the accurate heliospheric configuration remained largely undetermined due to lack of measurements. In recent years, however, Voyagers 1 and 2 (V1 and V2) crossed the termination shock — the boundary where the solar wind drops — north and south of the ecliptic plane at 94 au 2 , 3 and 84 au 4 in 2004 and 2007, respectively, and discovered the reservoir of ions and electrons that constitute the heliosheath, while Cassini remotely imaged the heliosphere 5 for the first time in 2003. Here we report 5.2–55 keV energetic neutral atom (ENA) global images of the heliosphere obtained with the Cassini/Ion and Neutral Camera (INCA). We compare them with 28–53 keV ions measured within the heliosheath by the low-energy charged particle (LECP) experiment onboard V1 and V2 over an 11-year period (2003–2014). We show that the heliosheath ions are the source of ENA. These observations also demonstrate that the heliosphere responds promptly, within ~2–3 years, to outward propagating solar wind changes in both the nose and tail directions. These results, together with the V1 measurement of a ~0.5 nT interstellar magnetic field 6 and the enhanced ratio between particle pressure and magnetic pressure in the heliosheath 7 , strongly suggest a diamagnetic bubble-like heliosphere with few substantial tail-like features. Our results are consistent with recent modelling 8 – 11 . The authors put together measurements of ions and neutral atoms from Cassini and the two Voyagers and find that the heliosphere responds quickly (with a lag of 2–3 years) to the solar cycle and that it is bubble-shaped and not tail-shaped, as usually schematized.

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]10³ times Saturn's radius R[subscript S] (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.

The Magnetospheric Imaging Instrument on board Cassini has been providing measurements of energetic ion intensities, energy spectra, and ion composition, combining the Charge Energy Mass Spectrometer over the range 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System for ions in the range 0.024 to 18 MeV, and the Ion and Neutral Camera for ions and energetic neutral atoms in the range 3 to > 200 keV. Results of the energetic (E > 3 keV) particle pressure distribution throughout the Saturnian magnetosphere and comparison with in situ measurements of the magnetic pressure are presented. The study offers a comprehensive depiction of the average, steady state hot plasma environment of Saturn over the 3 years since orbit insertion on 1 July 2004, with emphasis on ring current characteristics. The results may be summarized as follows: (1) The Saturnian magnetosphere possesses a dynamic, high‐beta ring current located approximately between 8 and ∼15 RS, primarily composed of O+ ions, and characterized by suprathermal (E > 3 keV) particle pressure, with typical values of 10−9 dyne/cm2. (2) The planetary plasma sheet shows significant asymmetries, with the dayside region being broadened in latitude (±50°) and extending to the magnetopause, and the nightside appearing well confined, with a thickness of ∼10 RS and a northward tilt of some 10° with respect to the equatorial plane beyond ∼20 RS. (3) The average radial suprathermal pressure gradient appears sufficient to modify the radial force balance and subsequently the azimuthal currents. (4) The magnetic perturbation due to the trapped energetic particle population is ∼7 nT, similar to values from magnetic field–based studies (9 to 13 nT).

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) 1 , 2 . 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 1 , 3 , 4 , 5 , 6 . Here we report the analysis of 50–80 keV ENA images of Jupiter's magnetosphere 7 , where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiter's 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, Jupiter's magnetosphere.

The brightest energetic neutral atom (ENA) intensity viewed by spacecraft in high‐inclination Earth orbit is from low‐altitude emission (LAE). It is a prominent feature in the stereo ENA images obtained by cameras on the NASA TWINS 1/2 Mission of Opportunity. This emission is produced by energetic magnetospheric ions precipitating into the atomic oxygen exosphere at latitudes near the auroral zones at altitudes ∼300 km. The ions undergo multiple atomic collisions including charge exchange of ions and stripping of the resulting neutrals. Consequently, this is a “thick target” process. We introduce a “thick‐target” approximation that allows us to extract the shape of the spatially averaged spectra of the precipitating ions from the ENA spectra in the TWINS 1/2 pixels viewing the LAE from near their orbital apogees. These ENA‐extracted ion spectra are compared with in situ precipitating ion spectra measured concurrently by DMSP satellites (F15 and F16) at ∼825 km altitude, while they are passing directly over the LAE regions in the TWINS 1/2 images. We obtain good agreement between the shape of the ENA‐extracted and in situ ion spectra from three distinct precipitation regions over energies from 2 to 32 keV (assuming precipitating protons). The absolute normalization of the ENA‐extracted and in situ spectra depends upon the TWINS viewing geometry because the ENA LAE source is not resolved by the imager. None of the spectral shapes obtained is consistent with a simple thermal intensity spectrum with kT = 5 keV.