Interchange reconnection as the source of the fast solar wind within coronal holes

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called 'coronal holes'. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating^1,2 and interchange reconnection^3-5. The coronal magnetic field near the solar surface is structured on scales associated with 'supergranulation' convection cells, whereby descending flows create intense fields. The energy density in these 'network' magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft⁶ that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic 'switchbacks'^7,8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts.

Compensating for gyroradius effects in beamlines with small Helmholtz coils

Measurements of lighter, low-energy charged particles in a laboratory beamline are complicated due to the influence of Earth's magnetic field. Rather than nulling out the Earth's magnetic field over the entire facility, we present a new way to correct particle trajectories using much more spatially limited Helmholtz coils. This approach is versatile and easy to incorporate in a wide range of facilities, including the existing ones, enabling measurements of low-energy charged particles in a laboratory beamline.

Closed Fluxtubes and Dispersive Proton Conics at Jupiter's Polar Cap

Two distinct proton populations are observed over Jupiter's southern polar cap: a ∼1 keV core population and ∼1-300 keV dispersive conic population at 6-7 R_J planetocentric distance. We find the 1 keV core protons are likely the seed population for the higher-energy dispersive conics, which are accelerated from a distance of ∼3-5 R_J. Transient wave-particle heating in a "pressure-cooker" process is likely responsible for this proton acceleration. The plasma characteristics and composition during this period show Jupiter's polar-most field lines can be topologically closed, with conjugate magnetic footpoints connected to both hemispheres. Finally, these observations demonstrate energetic protons can be accelerated into Jupiter's magnetotail via wave-particle coupling.

Water-Group Pickup Ions From Europa-Genic Neutrals Orbiting Jupiter

Water-group gas continuously escapes from Jupiter's icy moons to form co-orbiting populations of particles or neutral toroidal clouds. These clouds provide insights into their source moons as they reveal loss processes and compositions of their parent bodies, alter local plasma composition, and act as sources and sinks for magnetospheric particles. We report the first observations of H₂ ⁺ pickup ions in Jupiter's magnetosphere from 13 to 18 Jovian radii and find a density ratio of H₂ ⁺/H⁺ = 8 ± 4%, confirming the presence of a neutral H₂ toroidal cloud. Pickup ion densities monotonically decrease radially beyond 13 R _J consistent with an advecting Europa-genic toroidal cloud source. From these observations, we derive a total H₂ neutral loss rate from Europa of 1.2 ± 0.7 kg s^-1. This provides the most direct estimate of Europa's H₂ neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding satellite-magnetosphere interactions.

In Situ Observations of Interstellar Pickup Ions from 1 au to the Outer Heliosphere

Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIs compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs' mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon's Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to ∼ 47 au from the Sun or halfway to the heliospheric termination shock.

Probing the Magnetosheath Boundaries Using Interstellar Boundary Explorer (IBEX) Orbital Encounters

Inside the magnetosheath, the IBEX-Hi energetic neutral atom (ENA) imager measures a distinct background count rate that is more than 10 times the typical heliospheric ENA emissions observed when IBEX is outside the magnetosheath. The source of this enhancement is magnetosheath ions of solar wind (SW) origin that deflect around the Earth's magnetopause (MP), scatter and neutralize from the anti-sunward part of the IBEX-Hi sunshade, and continue into the instrument as neutral atoms, behaving indistinguishably from ENAs emitted from distant plasma sources. While this background pollutes observations of outer heliospheric ENAs, it provides a clear signature of IBEX crossings over the magnetospheric boundaries. In this study, we investigate IBEX encounters with the magnetosheath boundaries using ∼8 yr of orbital data, and we determine the MP and bow shock (BS) locations derived from this background signal. We find 280 BS crossings from X _GSE ∼ 11 R_e to X _GSE ∼ -36 R_e and 241 MP crossings from X _GSE ∼ 6 R_e to X _GSE ∼ -48 R_e. We compare IBEX BS and MP crossing locations to those from IMP-8, Geotail, Cluster, Magion-4, ISEE, and Magnetospheric Multiscale Mission, and we find that IBEX crossing locations overlap with the BS and MP locations inferred from these other data sets. In this paper, we demonstrate how IBEX can be used to identify magnetosheath crossings, and extend boundary observations well past the terminator, thus further constraining future models of magnetosheath boundaries. Furthermore, we use the IBEX data set to show observational evidence of near-Earth magnetotail squeezing during periods of strong interplanetary magnetic field B _y.

Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause

Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ~0.45 km s^-1 and heated by ~1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun's relative speed of 25.85 km s^-1 and temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a source of the differential cross sections for elastic collisions with helium atoms.

Neutral Atom Imaging of the Solar Wind-Magnetosphere-Exosphere Interaction Near the Subsolar Magnetopause

Energetic neutral atoms (ENAs) created by charge-exchange of ions with the Earth's hydrogen exosphere near the subsolar magnetopause yield information on the distribution of plasma in the outer magnetosphere and magnetosheath. ENA observations from the Interstellar Boundary Explorer (IBEX) are used to image magnetosheath plasma and, for the first time, low-energy magnetospheric plasma near the magnetopause. These images show that magnetosheath plasma is distributed fairly evenly near the subsolar magnetopause; however, low-energy magnetospheric plasma is not distributed evenly in the outer magnetosphere. Simultaneous images and in situ observations from the Magnetospheric Multiscale (MMS) spacecraft from November 2015 (during the solar cycle declining phase) are used to derive the exospheric density. The ~11-17 cm-3 density at 10 RE is similar to that obtained previously for solar minimum. Thus, these combined results indicate that the exospheric density 10 RE from the Earth may have a weak dependence on solar cycle.

First Global Images of Ion Energization in the Terrestrial Foreshock by the Interstellar Boundary Explorer

The Interstellar Boundary Explorer (IBEX) mission provides global energetic neutral atom (ENA) observations from the heliosphere and the Earth's magnetosphere, including spatial, temporal, and energy information. IBEX views the magnetosphere from the sides and almost always perpendicular to noon-midnight plane. We report the first ENA images of the energization process in the Earth's ion foreshock and magnetosheath regions. We show ENA flux and spectral images of the dayside magnetosphere with significant energization of ENA plasma sources (above ~2.7 keV) in the region magnetically connected to the Earth's bow shock (BS) in its quasi-parallel configuration of the interplanetary magnetic field (IMF). We also show that the ion energization increases gradually with decreasing IMF-BS angle, suggesting more efficient suprathermal ion acceleration deeper in the quasi-parallel foreshock.

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU₆₉). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU₆₉) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin.

Turbulence in the Local Interstellar Medium and the IBEX Ribbon

The effects of turbulence in the very local interstellar medium (VLISM) have been proposed by Giacalone & Jokipii (2015) to be important in determining the structure of the Interstellar Boundary Explorer (IBEX) ribbon via particle trapping by magnetic mirroring. We further explore this effect by simulating the motion of charged particles in a turbulent magnetic field superposed on a large-scale mean field, which we consider to be either spatially-uniform or a draped field derived from a 3D MHD simulation. We find that the ribbon is not double-peaked, in contrast to Giacalone & Jokipii (2015). However, the magnetic mirror force still plays an important role in trapping particles. Furthermore, the ribbon's thickness is considerably larger if the large-scale mean field is draped around the heliosphere. Voyager 1 observations in the VLISM show a turbulent field component that is stronger than previously thought, which we test in our simulation. We find that the inclusion of turbulent fluctuations at scales ≳100 au and power consistent with Voyager 1 observations produces a ribbon whose large-scale structure is inconsistent with IBEX observations. However, restricting fluctuations to <100 au produces a smoother ribbon structure similar to IBEX observations. Different turbulence realizations produce different small-scale features (≲10°) in the ribbon, but its large-scale structure is robust if the maximum fluctuation size is ≲50 au. This suggests that the magnetic field structure at scales ≲50 au is determined by the heliosphere-VLISM interaction and cannot entirely be represented by pristine interstellar turbulence.

Alfvénic velocity spikes and rotational flows in the near-Sun solar wind

The prediction of a supersonic solar wind¹ was first confirmed by spacecraft near Earth^2,3 and later by spacecraft at heliocentric distances as small as 62 solar radii⁴. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy^5-7. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams⁸. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii^9-11, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second-considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age^12-14.

Probing the energetic particle environment near the Sun

NASA's Parker Solar Probe mission1 recently plunged through the inner heliosphere of the Sun to its perihelia, about 24 million kilometres from the Sun. Previous studies farther from the Sun (performed mostly at a distance of 1 astronomical unit) indicate that solar energetic particles are accelerated from a few kiloelectronvolts up to near-relativistic energies via at least two processes: 'impulsive' events, which are usually associated with magnetic reconnection in solar flares and are typically enriched in electrons, helium-3 and heavier ions2, and 'gradual' events3,4, which are typically associated with large coronal-mass-ejection-driven shocks and compressions moving through the corona and inner solar wind and are the dominant source of protons with energies between 1 and 10 megaelectronvolts. However, some events show aspects of both processes and the electron-proton ratio is not bimodally distributed, as would be expected if there were only two possible processes5. These processes have been very difficult to resolve from prior observations, owing to the various transport effects that affect the energetic particle population en route to more distant spacecraft6. Here we report observations of the near-Sun energetic particle radiation environment over the first two orbits of the probe. We find a variety of energetic particle events accelerated both locally and remotely including by corotating interaction regions, impulsive events driven by acceleration near the Sun, and an event related to a coronal mass ejection. We provide direct observations of the energetic particle radiation environment in the region just above the corona of the Sun and directly explore the physics of particle acceleration and transport.

Parallax of the IBEX Ribbon Indicates a Spatially-Retained Source

In 2009, the Interstellar Boundary Explorer (IBEX) discovered the existence of a narrow "ribbon" of intense energetic neutral atom (ENA) emission projecting approximately a circle in the sky. It is believed that the ribbon originates from outside of the heliopause in radial directions ( r ) perpendicular to the local interstellar magnetic field (ISMF), B , i.e., B∙ r = 0. Swaczyna et al. (2016a) estimated the distance to the IBEX ribbon via the parallax method comparing the ribbon position observed from the opposite sides of the Sun. They found a parallax angle of 0.41° ± 0.15°, yielding a distance of140 - 38 + 84 au to a portion of the ribbon at high ecliptic latitudes. In this study, we demonstrate how the apparent shift of the ribbon in the sky, and thus the apparent distance to the ribbon's source found via the parallax, depends on the transport effects of energetic ions outside the heliopause. We find that the apparent shift of the ribbon based on the "spatial retention" model with ion enhancement near B∙ r = 0, as proposed by Schwadron & McComas (2013), agrees with the parallax of the source region. Parallax is also accurate for a homogeneously-distributed emission source. However, if there is weak pitch angle scattering and ions propagate freely along the ISMF, the apparent shift is significantly smaller than the expected parallax because of the highly anisotropic source. In light of the results from Swaczyna et al. (2016a), our results indicate that the IBEX ribbon source is spatially confined.

Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU₆₉, a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU₆₉ is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU₆₉'s origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.

Strong Scattering of ~keV Pickup Ions in the Local Interstellar Magnetic Field Draped Around Our Heliosphere: Implications for the IBEX Ribbon's Source and IMAP

The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere and, after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas (2013) in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially-retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbon's intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbon's source as well as an asymmetry in the ribbon's shape, i.e., radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbon's shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbon's structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe (IMAP) may observe the ribbon.

Strong Scattering of ~keV Pickup Ions in the Local Interstellar Magnetic Field Draped Around Our Heliosphere: Implications for the IBEX Ribbon's Source and IMAP

The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere and, after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas (2013) in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially-retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbon's intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbon's source as well as an asymmetry in the ribbon's shape, i.e., radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbon's shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbon's structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe (IMAP) may observe the ribbon.

Comparing Electron Energetics and UV Brightness in Jupiter's Northern Polar Region During Juno Perijove 5

We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m2, respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions.

Comparing Electron Energetics and UV Brightness in Jupiter's Northern Polar Region During Juno Perijove 5

We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m², respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions.

In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary Shock

Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed that PUIs should be preferentially heated at quasiperpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H^{+}) PUIs at an interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at ∼34  au from the Sun. At this shock, H^{+} PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H^{+} SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H^{+} SWIs lose ∼85% of their energy flux across the shock and H^{+} PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H^{+} PUIs is approximately six times that of H^{+} SWIs. We find that H^{+} PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.

Jupiter's magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits

The Juno spacecraft acquired direct observations of the jovian magnetosphere and auroral emissions from a vantage point above the poles. Juno's capture orbit spanned the jovian magnetosphere from bow shock to the planet, providing magnetic field, charged particle, and wave phenomena context for Juno's passage over the poles and traverse of Jupiter's hazardous inner radiation belts. Juno's energetic particle and plasma detectors measured electrons precipitating in the polar regions, exciting intense aurorae, observed simultaneously by the ultraviolet and infrared imaging spectrographs. Juno transited beneath the most intense parts of the radiation belts, passed about 4000 kilometers above the cloud tops at closest approach, well inside the jovian rings, and recorded the electrical signatures of high-velocity impacts with small particles as it traversed the equator.

Juno-UVS approach observations of Jupiter's auroras

Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Prior to the bow shock crossing on 24 June 2016, the Juno approach provided a rare opportunity to correlate local solar wind conditions with Jovian auroral emissions. Some of Jupiter's auroral emissions are expected to be controlled or modified by local solar wind conditions. Here we compare synoptic Juno-UVS observations of Jupiter's auroral emissions, acquired during 3-29 June 2016, with in situ solar wind observations, and related Jupiter observations from Earth. Four large auroral brightening events are evident in the synoptic data, in which the total emitted auroral power increases by a factor of 3-4 for a few hours. Only one of these brightening events correlates well with large transient increases in solar wind ram pressure. The brightening events which are not associated with the solar wind generally have a risetime of ~2 h and a decay time of ~5 h.

The FIELDS Instrument Suite for Solar Probe Plus: Measuring the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence, and Radio Signatures of Solar Transients

NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

Pluto's interaction with its space environment: Solar wind, energetic particles, and dust

The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region's surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons' closest approach, indicating an upper limit of <4.6 kilometers(-3) for the dust density in the Pluto system.

An integrated time-of-flight versus residual energy subsystem for a compact dual ion composition experiment for space plasmas

We have developed a novel concept for a Compact Dual Ion Composition Experiment (CoDICE) that simultaneously provides high quality plasma and energetic ion composition measurements over 6 decades in ion energy in a wide variety of space plasma environments. CoDICE measures the two critical ion populations in space plasmas: (1) mass and ionic charge state composition and 3D velocity and angular distributions of ∼10 eV/q-40 keV/q plasma ions—CoDICE-Lo and (2) mass composition, energy spectra, and angular distributions of ∼30 keV-10 MeV energetic ions—CoDICE-Hi. CoDICE uses a common, integrated Time-of-Flight (TOF) versus residual energy (E) subsystem for measuring the two distinct ion populations. This paper describes the CoDICE design concept, and presents results of the laboratory tests of the TOF portion of the TOF vs. E subsystem, focusing specifically on (1) investigation of spill-over and contamination rates on the start and stop microchannel plate (MCP) anodes vs. secondary electron steering and focusing voltages, scanned around their corresponding model-optimized values, (2) TOF measurements and resolution and angular resolution, and (3) cross-contamination of the start and stop MCPs' singles rates from CoDICE-Lo and -Hi, and (4) energy resolution of avalanche photodiodes near the lower end of the CoDICE-Lo energy range. We also discuss physical effects that could impact the performance of the TOF vs. E subsystem in a flight instrument. Finally, we discuss advantages of the CoDICE design concept by comparing with capabilities and resources of existing flight instruments.

The SupraThermal Ion Monitor for space weather predictions

Measurement of suprathermal energy ions in the heliosphere has always been challenging because (1) these ions are situated in the energy regime only a few times higher than the solar wind plasma, where intensities are orders of magnitude higher and (2) ion energies are below or close to the threshold of state-of-art solid-state detectors. Suprathermal ions accelerated at coronal mass ejection-driven shocks propagate out ahead of the shocks. These shocks can cause geomagnetic storms in the Earth's magnetosphere that can affect spacecraft and ground-based power and communication systems. An instrument with sufficient sensitivity to measure these ions can be used to predict the arrival of the shocks and provide an advance warning for potentially geo-effective space weather. In this paper, we present a novel energy analyzer concept, the Suprathermal Ion Monitor (STIM) that is designed to measure suprathermal ions with high sensitivity. We show results from a laboratory prototype and demonstrate the feasibility of the concept. A list of key performances is given, as well as a discussion of various possible detectors at the back end. STIM is an ideal candidate for a future space weather monitor in orbit upstream of the near-earth environment, for example, around L1. A scaled-down version is suitable for a CubeSat mission. Such a platform allows proofing the concept and demonstrating its performance in the space environment.

Decades-long changes of the interstellar wind through our solar system

The journey of the Sun through the dynamically active local interstellar medium creates an evolving heliosphere environment. This motion drives a wind of interstellar material through the heliosphere that has been measured with Earth-orbiting and interplanetary spacecraft for 40 years. Recent results obtained by NASA's Interstellar Boundary Explorer mission during 2009-2010 suggest that neutral interstellar atoms flow into the solar system from a different direction than found previously. These prior measurements represent data collected from Ulysses and other spacecraft during 1992-2002 and a variety of older measurements acquired during 1972-1978. Consideration of all data types and their published results and uncertainties, over the three epochs of observations, indicates that the trend for the interstellar flow ecliptic longitude to increase linearly with time is statistically significant.

Response in electrostatic analyzers due to backscattered electrons: case study analysis with the Juno Jovian Auroral Distribution Experiment-Electron instrument

In this study, we introduce a model to characterize electron scattering in an electrostatic analyzer. We show that electrons between 0.5 and 30 keV scatter from internal surfaces to produce a response up to ~20% of the ideal, unscattered response. We compare our model results to laboratory data from the Jovian Auroral Distribution Experiment-Electron sensor onboard the NASA Juno mission. Our model reproduces the measured energy-angle response of the instrument well. Understanding and quantifying this scattering process is beneficial to the analysis of scientific data as well as future instrument optimization.

Reflections of ions in electrostatic analyzers: a case study with New Horizons/Solar Wind Around Pluto

Electrostatic analyzers (ESAs), in various forms, are used to measure plasma in a range of applications. In this article, we describe how ions reflect from the interior surfaces of an ESA, the detection of which constitutes a fundamentally nonideal response of ESAs. We demonstrate this effect by comparing laboratory data from a real ESA-based space instrument, the Solar Wind Around Pluto (SWAP) instrument, aboard the NASA New Horizons spacecraft, to results from a model based on quantum mechanical simulations of particles reflected from the instrument's surfaces combined with simulations of particle trajectories through the instrument's applied electrostatic fields. Thus, we show, for the first time, how reflected ions in ESAs lead to nonideal effects that have important implications for understanding the data returned by these instruments, as well as for designing new low-background ESA-based instruments. Specifically, we show that the response of SWAP widens considerably below a level of 10(-3) of the peak response. Thus, a direct measurement of a plasma distribution with SWAP will have an energy-dependent background on the order of ≤10(-3) of the peak of the signal due to that distribution. We predict that this order of magnitude estimate for the background applies to a large number of ESA-based instruments because ESAs operate using a common principle. However, the exact shape of the energy-dependent response will be different for different instruments. The principle of operation is that ions outside the ideal range of energy-per-charge are deflected into the walls of the ESA. Therefore, we propose that a new design paradigm is necessary to mitigate the effect of ion reflections and thus accurately and directly measure the energy spectrum of a plasma using ESAs. In this article, we build a framework for minimizing the effect of ion reflections in the design of new ESAs. Through the use of existing computer simulation software, a design team can use our method to quantify the amount of reflections in their instrument and iteratively change design parameters before fabrication, conserving resources. A possible direction for the new design paradigm is having nonsolid walls of the ESA, already used in some applications.

Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX)

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.

Width and variation of the ENA flux ribbon observed by the Interstellar Boundary Explorer

The dominant feature in Interstellar Boundary Explorer (IBEX) sky maps of heliospheric energetic neutral atom (ENA) flux is a ribbon of enhanced flux that extends over a broad range of ecliptic latitudes and longitudes. It is narrow (approximately 20 degrees average width) but long (extending over 300 degrees in the sky) and is observed at energies from 0.2 to 6 kilo-electron volts. We demonstrate that the flux in the ribbon is a factor of 2 to 3 times higher than that of the more diffuse, globally distributed heliospheric ENA flux. The ribbon is most pronounced at approximately 1 kilo-electron volt. The average width of the ribbon is nearly constant, independent of energy. The ribbon is likely the result of an enhancement in the combined solar wind and pickup ion populations in the heliosheath.

Comparison of Interstellar Boundary Explorer observations with 3D global heliospheric models

Simulations of energetic neutral atom (ENA) maps predict flux magnitudes that are, in some cases, similar to those observed by the Interstellar Boundary Explorer (IBEX) spacecraft, but they miss the ribbon. Our model of the heliosphere indicates that the local interstellar medium (LISM) magnetic field (B(LISM)) is transverse to the line of sight (LOS) along the ribbon, suggesting that the ribbon may carry its imprint. The force-per-unit area on the heliopause from field line draping and the LISM ram pressure is comparable with the ribbon pressure if the LOS approximately 30 to 60 astronomical units and B(LISM) approximately 2.5 microgauss. Although various models have advantages in accounting for some of the observations, no model can explain all the dominant features, which probably requires a substantial change in our understanding of the processes that shape our heliosphere.

Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX)

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.

Direct observations of interstellar H, He, and O by the Interstellar Boundary Explorer

Neutral gas of the local interstellar medium flows through the inner solar system while being deflected by solar gravity and depleted by ionization. The dominating feature in the energetic neutral atom Interstellar Boundary Explorer (IBEX) all-sky maps at low energies is the hydrogen, helium, and oxygen interstellar gas flow. The He and O flow peaked around 8 February 2009 in accordance with gravitational deflection, whereas H dominated after 26 March 2009, consistent with approximate balance of gravitational attraction by solar radiation pressure. The flow distributions arrive from a few degrees above the ecliptic plane and show the same temperature for He and O. An asymmetric O distribution in ecliptic latitude points to a secondary component from the outer heliosheath.

Imaging the interaction of the heliosphere with the interstellar medium from Saturn with Cassini

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 degrees ecliptic longitude extending from north to south and looping back through approximately 80 degrees, 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.

Structures and spectral variations of the outer heliosphere in IBEX energetic neutral atom maps

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 lambda, latitude beta) = (221 degrees, 39 degrees), 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.