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Solar Orbiter will observe the Sun and the inner heliosphere to study the connections between solar activity, coronal structure, and the origin of the solar wind. The plasma instruments on board Solar Orbiter will determine the three-dimensional velocity distribution functions of the plasma ions and electrons with high time resolution. The analysis of these distributions will determine the plasma bulk parameters, such as density, velocity, and temperature. This paper examines the effects of short-time-scale plasma variations on particle measurements and the estimated bulk parameters of the plasma. For the purpose of this study, we simulate the expected observations of solar wind protons, taking into account the performance of the Proton-Alpha Sensor (PAS) on board Solar Orbiter. We particularly examine the effects of Alfv\'enic and slow-mode-like fluctuations, commonly observed in the solar wind on timescales of milliseconds to hours, on the observations. We do this by constructing distribution functions from modeled observations and calculate their statistical moments in order to derive plasma bulk parameters. The comparison between the derived parameters with the known input allows us to estimate the expected accuracy of Solar Orbiter proton measurements in the solar wind under typical conditions. We find that the plasma fluctuations due to these turbulence effects have only minor effects on future SWA-PAS observations

In late 2016, the Interstellar Boundary Explorer (IBEX) observed an enhancement of hydrogen energetic neutral atom (ENA) flux in ∼20° south from the nose direction. This enhancement was a consequence of an abrupt increase of the solar wind (SW) dynamic pressure observed at 1 au in late 2014. In subsequent years, the increased flux of 4.3 keV ENAs was observed at higher latitudes filling in the heliosheath, in ENAs at lower energies, and the Ribbon flux. We observe that the rapid increase of SW pressure occurs every solar cycle (SC) from the beginning of the regular in situ SW measurements in the ecliptic plane. The SW pressure pulse happens about 4.7 yr from the beginning of each SC, it is during the maximum phase of solar activity, and repeats with a period of ∼10.2 yr. These repeating pulses of the SW pressure can cause periodic SC variations of the ENA production in the heliosheath. We follow McComas et al. results for the relation between SW pressure increase and ENA flux enhancement to investigate the periodic SW pressure increases and their consequences for the heliosphere. Our study of time delay between the cause (pressure pulse at 1 au) and the consequence (ENA enhancement) show that IBEX observed in 2009–2011 remnants of the SW pressure pulse that happened during the maximum of SC 23.

The highly varying plasma environment around comet 67P/Churyumov-Gerasimenko inspired an upgrade of the ion mass spectrometer (Rosetta Plasma Consortium Ion Composition Analyzer) with new operation ...

The Rosetta spacecraft studied the comet 67P/Churyumov–Gerasimenko for nearly two years. The Ion Composition Analyzer instrument on board Rosetta observed the positive ion distributions in the environment of the comet during the mission. A portion of the comet's neutral coma is expected to get ionized, depending on the comet's activity and position relative to the Sun, and the newly created ions are picked up and accelerated by the solar wind electric field, while the solar wind flow is deflected in the opposite direction. This interaction, known as the mass-loading mechanism, was previously studied by comparing the bulk flow direction of both the solar wind protons and the accelerated cometary ions with respect to the direction of the magnetic and the convective solar wind electric field. In this study, we show that energy–angle dispersion is occasionally observed. We report two types of dispersion: one where the observed motion is consistent with ions gyrating in the local magnetic field and another where the energy–angle dispersion is opposite to that expected from gyration in the local magnetic field. Given that the cometary ion gyro-radius in the undisturbed solar wind magnetic and electric field is expected to be too large to be detected in this way, our observations indicate that the local electric field might be significantly smaller than that of the undisturbed solar wind. We also discuss how the energy–angle dispersion, which is not consistent with gyration, may occur due to spatially inhomogeneous densities and electric fields.

We analyze plasma ion observations from the Solar Wind Around Pluto instrument on New Horizons as it traveled back through the dusk flank of the Jovian magnetotail from ~600 to more than 2500 Jovian radii behind the planet. We find that at all distances, light ions (mostly protons) dominate the heavy ions (S++ and O+) that are far more abundant in the near Jupiter plasma disk and that were expected to be the primary ions filling the Jovian magnetotail. This key new observation might indicate that heavy ions are confined closer to the equator than the spacecraft trajectory or a substantial addition of light ions via reconnection and/or mixing along the magnetopause boundary. However, because we find no evidence for acceleration of the tail plasma with distance, a more likely explanation seems to be that the heavy ions are preferentially released down the dawn flank of the magnetotail. Perhaps, this occurs as a part of the process where flux tubes, after expanding as they rotate across the near‐tail region, need to pull back inward in order to fit within the dawnside of the magnetopause. A second major finding of this study is that there are two dominant periods of the plasma structures in the Jovian magnetotail: 3.53 (0.18 full width at half maximum (FWHM)) and 5.35 (0.38 FWHM) days. Remarkably, the first of these is identical within the errors to Europa's orbital period (3.55 days). Both of these results should provide important new fodder for Jovian magnetospheric theories and lead to a better understanding of Jupiter's magnetosphere.

A Monte Carlo procedure is presented to calculate the solid angle subtended between a HPGe collimated detector and a PWR 17 x 17 fuel assembly. Self-shielding within the assembly is assessed in the case of gamma rays encountered in gamma spectroscopy of spent nuclear fuel. Self-shielding renders a non-uniform 3-D distribution of the radioactivity within the assembly whose extend depends on gamma ray energies of interest. Hence, the solid angle is reduced to an effective one for each of the different energies of interest.

Kinetic Alfven waves (KAWs) are the short-wavelength extension of the magnetohydrodynamics Alfven-wave branch in the case of highly oblique propagation with respect to the background magnetic field. Observations of space plasma show that small-scale turbulence is mainly KAW-like. We apply two theoretical approaches, a collisional two-fluid theory and a collisionless linear kinetic theory, to obtain predictions for the KAW polarizations depending on βp (the ratio of the proton thermal pressure to the magnetic pressure) at the ion gyroscale in terms of fluctuations in density, bulk velocity, and pressure. We perform a wavelet analysis of Magnetospheric Multiscale magnetosheath measurements and compare the observations with both theories. We find that the two-fluid theory predicts the observations better than the kinetic theory, suggesting that the small-scale KAW-like fluctuations exhibit a fluid-like behavior in the magnetosheath although the plasma is weakly collisional. We also present predictions for the KAW polarizations in the inner heliosphere that are testable with Parker Solar Probe and Solar Orbiter.

The paper develops a theoretical relationship between the polytropic index and the temperature anisotropy that may characterize space plasmas. The derivation is based on the correlation among the kinetic energies of particles with velocities described by anisotropic kappa distributions. The correlation coefficient depends on the effective dimensionality of the velocity distribution function, having its shape determined by the temperature anisotropies caused by the ambient magnetic field; on the other hand, the effective dimensionality is directly dependent on the polytropic index. This analysis leads to the connection between the correlation coefficient, effective dimensionality of velocity space, and the polytropic index, with the ratio of temperature anisotropy. Moreover, a data and statistical analysis is performed to test the application of the theoretically developed model, in the solar wind proton plasma near 1 AU. The derived theoretical relationship is in good agreement with observations, showing that the lowest and classical value of the adiabatic polytropic index occurs in the isotropic case, while higher indices characterize anisotropic plasmas. Finally, possible extensions of the theory considering (i) non-adiabatic polytropic behavior, and (ii) more general distributions, are further discussed., 23 pages, 8 figures

We develop the theoretical basis for the connection of the variety of anisotropic distributions with the statistical correlations among particles velocity components. By examining the most common anisotropic distribution, we derive the correlation coefficient among particle energies, show how this correlation is connected to the effective dimensionality of the velocity distribution, and derive the connection between anisotropy and adiabatic polytropic index. Having established the importance of correlation among particles in the formulation of anisotropic kappa distributions, we generalize these distributions within the framework of nonextensive statistical mechanics and based on the types of homogeneous or heterogeneous correlations among the particles velocity components. The formulation of the developed generalized distributions mediates the main two types of anisotropic kappa distributions, considering (a) equal correlations, or (b) zero correlations, among different velocity components. Finally, the developed anisotropic kappa distributions are expressed in terms of the energy and pitch angle in arbitrary reference frames., 46 pages, 15 figures

Energetic neutral atom (ENA) imaging techniques have become a powerful tool for remotely probing plasma environments in space. ENA imagers cover energies from 0.01 keV up to a few MeV, and they use different techniques to cover such a broad energy range. Most of them convert the ENA into a charged particle to remove the converted ENA from the initial neutral direction. In the >∼0.2 keV/nuc to 10’s of keV/nuc range, the conversion subsystem is usually an ultra-thin carbon foil. The sensitivity of ENA imagers based on charge conversion by carbon foils is driven by the ability of these foils to convert a neutral atom into an ion. The charge state distribution after the carbon foils is a strong function of the chemical and physical properties of the exit surface. In this study, we analyze the composition and structure of the surface using X-ray photoelectron spectroscopy. The surface is roughly 88% carbon and 12% oxygen, forming strong C O bonds. Annealing the foil lowers the oxygen content to about 9%. We coat the surface of the foils with Au, Al 2 O 3 , or MgO. We compare the exit charge state distributions of hydrogen prior to and post coatings. While no significant difference is observed in the exit charge state for the Au and Al 2 O 3 coatings, there is a slight decrease of the positive fraction after MgO. The annealing of the foil has the benefit of reducing the angular scattering of hydrogen by a factor of ∼1.2. This is a significant improvement that has the potential to increase sensitivity of ENA imagers.

New Horizons observed consistently and continuously the jovian magnetotail at distances up to ~2500 Jupiter Radii (Rj) during its Jupiter flyby in 2007. The Solar Wind Around Pluto (SWAP) plasma instrument on New Horizons made in situ observations of plasma ions in the energy per charge range of ~21 eV to 7.8 keV. We analyze the SWAP plasma observations and derive the bulk properties of the plasma ions in the deep jovian magnetotail for 64 intervals from ~500 to 1700 Rj, just before the spacecraft start crossing the jovian magnetopause. There is no clear evolution of the plasma parameters over this distance range and we show that the plasma is very diverse over this entire range. There are significant changes in the plasma parameters and the flow direction over times as short as a few hours, showing evidence that boundaries between different plasma structures pass over the spacecraft rapidly. We discuss in detail a few subintervals where two species are observed within the instrument׳s energy per charge range and a set of subintervals where the plasma flow rotates ~20° over just six hours. We finally discuss the mass flux during the subintervals we study and the scenario of expanding plasmoids that propagate tailward and expand and interact to fill the magnetotail. This scenario is supported by the observed plasma diversity and flow characteristics.

The first 1000 km of the ion tail of comet 67P/Churyumov-Gerasimenko were explored by the European Rosetta spacecraft, 2.7 au away from the Sun. We characterised the dynamics of both the solar wind and the cometary ions on the night-side of the comet's atmosphere. We analysed in situ ion and magnetic field measurements and compared the data to a semi-analytical model. The cometary ions are observed flowing close to radially away from the nucleus during the entire excursion. The solar wind is deflected by its interaction with the new-born cometary ions. Two concentric regions appear, an inner region dominated by the expanding cometary ions and an outer region dominated by the solar wind particles. The single night-side excursion operated by Rosetta revealed that the near radial flow of the cometary ions can be explained by the combined action of three different electric field components, resulting from the ion motion, the electron pressure gradients, and the magnetic field draping. The observed solar wind deflection is governed mostly by the motional electric field., Comment: 11 pages, 7 figures

Thin carbon foils are used in many fields of physics, including for the detection of keV particles in space plasma instrumentation. The interaction of ions or neutral atoms with these foils results in different effects: electron emission, charge exchange, angular scattering, and energy loss and straggling. We report measurements of the energy loss and straggling and angular scattering of 1–50 keV protons for different foil thicknesses (nominal 0.5–2.0 μg/cm2). We derive simple semi-empirical relationships for the energy loss and straggling as a function of incident energy and foil thickness. We also derive a semi-empirical relationship between the derived thickness and the scattering constant. This provides a simple and straightforward tool to estimate the energy loss by measuring the angular scattering of protons for a particular foil.

Graphene could be an alternative to amorphous carbon foils, in particular in space plasma instrumentation. The interaction of ions or neutral atoms with these foils results in different effects: electron emission, charge exchange, angular scattering, and energy straggling. We showed in previous studies that (1) the charge exchange properties are similar for graphene and regular carbon foils, and (2) the scattering at low energies (few keVs) is less for graphene than for one of our thinnest practical carbon foils. In this study, we report measurements of the energy loss and straggling of ∼1–50 keV H, He, C, N, and O ions in graphene. We compare graphene and a carbon foil for hydrogen. We provide simple power law fits to the average energy loss, energy straggling, and skewness of the energy distributions. We find the energy loss for ions transiting through graphene to be reduced compared to thin carbon foils but the energy straggling to be larger, which we attribute to the non-uniformity of the graphene foils used in this study.

The Solar Wind Around Pluto (SWAP) instrument on New Horizons (NH) made in situ observations of ions in Jupiter's distant magnetotail and magnetosheath during its 2007 flyby. NH observed 16 magnetopause crossings between 1654 and 2429 RJ antisunward from Jupiter. We have developed a method to calculate the bulk properties of the plasma ions (density, velocity, and temperature) based on a forward model of the SWAP instrument response. We fit the observations using both Maxwell and kappa distributions. In this paper we describe our technique, which includes accounting for the detailed and asymmetric response of the SWAP electrostatic analyzer, and present the results for the distant Jovian magnetosheath. Finally, we discuss the characteristics of the derived bulk properties and compare these results to previously developed gas dynamic models for magnetospheres of giant planets.

This paper presents the misestimation of temperature when observations from a kappa distributed plasma are analyzed as a Maxwellian. One common method to calculate the space plasma parameters is by fitting the observed distributions using known analytical forms. More often, the distribution function is included in a forward model of the instrument's response, which is used to reproduce the observed energy spectrograms for a given set of plasma parameters. In both cases, the modeled plasma distribution fits the measurements to estimate the plasma parameters. The distribution function is often considered to be Maxwellian even though in many cases the plasma is better described by a kappa distribution. In this work we show that if the plasma is described by a kappa distribution, the derived temperature assuming Maxwell distribution can be significantly off. More specifically, we derive the plasma temperature by fitting a Maxwell distribution to pseudo-data produced by a kappa distribution, and then examine the difference of the derived temperature as a function of the kappa index. We further consider the concept of using a forward model of a typical plasma instrument to fit its observations. We find that the relative error of the derived temperature is highly depended on the kappa index and occasionally on the instrument's field of view and response., 7 Figures and 19 pages The article is currently in press by "Astrophysics and Space Science" journal The articles number is ASTR-D-16-00131R4

Spacecraft data from the OMNI database were used to calculate the value of the polytropic index of the solar wind by fitting the logarithms of proton density and temperature in selected time intervals from 1 January 1995 to 30 June 2012. Bernoulli’s integral and the correlation coefficient were used to filter the results. An alternative method based on the maximization of the correlation coefficient was employed to confirm our results. The long-term behavior of the polytropic index we obtained is found to be virtually identical for both methods. We noticed a characteristic behavior of the estimated polytropic index values, particularly from 1995 to 2006, which tends to have a periodicity of about one year. The distribution of the polytropic index is best described by a κ-Gaussian distribution with mean ≈ 1.8 and standard deviation ≈ 2.4. We finally examined the possible correlation between the polytropic index values and solar activity.

The Solar Wind Around Pluto (SWAP) instrument on NASA's New Horizon Pluto mission has collected solar wind observations en route from Earth to Pluto, and these observations continue beyond Pluto. Few missions have explored the solar wind in the outer heliosphere making this dataset a critical addition to the field. We created a forward model of SWAP count rates, which includes a comprehensive instrument response function based on laboratory and flight calibrations. By fitting the count rates with this model, the proton density (n), speed (V), and temperature (T) parameters are determined. Comparisons between SWAP parameters and both propagated 1 AU observations and prior Voyager 2 observations indicate consistency in both the range and mean wind values. These comparisons as well as our additional findings confirm that small and midsized solar wind structures are worn down with increasing distance due to dynamic interaction of parcels of wind with different speed. For instance, the T-V relationship steepens, as the range in V is limited more than the range in T with distance. At times the T-V correlation clearly breaks down beyond 20 AU, which may indicate wind currently expanding and cooling may have an elevated T reflecting prior heating and compression in the inner heliosphere. The power of wind parameters at shorter periodicities decreases with distance as the longer periodicities strengthen. The solar rotation periodicity is present in temperature beyond 20 AU indicating the observed parcel temperature may reflect not only current heating or cooling, but also heating occurring closer to the Sun., Comment: 55 pages, 29 Figures, accepted for publication in The Astrophysical Journal Supplements (ApJS)

Radiolytic production has been proposed as a potential source for the molecular oxygen observed in comet 67P/Churyumov–Gerasimenko. Radiolysis can be exogenic or endogenic, the latter due to radionuclides present in the dust constitutive of the comet nucleus. We investigated the possibility of forming a significant amount of molecular oxygen through endogenic radiolysis. We applied a model of radiolytic production, developed for an Earth rock–water mixture, and improved it to account for the effect of the size of a radionuclide-bearing grain on the net radiation deposited in its ice mantle. We calculated the possible production of molecular oxygen considering the available experimental values of radiolytic yields. We found that endogenic radiolysis cannot account for the totality of the 3.8% (relative to water) O2 abundance derived from the ROSINA observations, with an end member case of our model producing at most a 1% abundance. By contrast, we predict H2O2 production leads to an abundance up to two orders of magnitude above observed values.

Aims. Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma. We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in the terminator plane. Methods. We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly using plasma moments). We propose a statistical approach over a period of one month. Results. On average, two populations were observed, separated in phase space. The motion of the first is governed by its interaction with the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatible with an ambipolar electric field. Both populations display a consistent anti-sunward velocity component. Conclusions. Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative to the nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometary ion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. There is no corresponding signature in the average magnetic field strength.

We examine a semi-analytical non-magnetic model of the termination shock location previously developed by Exarhos & Moussas. In their study, the plasma flow beyond the shock is considered incompressible and irrotational, thus the flow potential is analytically derived from the Laplace equation. Here we examine the characteristics of the downstream flow in the heliosheath in order to resolve several inconsistencies existing in the Exarhos & Moussas model. In particular, the model is modified in order to be consistent with the Rankine– Hugoniot jump conditions and the geometry of the termination shock. It is shown that a shock compression ratio varying along the latitude can lead to physically correct results. We describe the new model and present several simplified examples for a nearly spherical, strong termination shock. Under those simplifications, the upstream plasma is nearly adiabatic for large (∼100 AU) heliosheath thickness.

We present experimental results for the angular scattering of ~1-50 keV H, He, C, O, N, Ne, and Ar ions transiting through graphene foils and compare them with scattering through nominal ~0.5 μg cm(-2) carbon foils. Thin carbon foils play a critical role in time-of-flight ion mass spectrometers and energetic neutral atom sensors in space. These instruments take advantage of the charge exchange and secondary electron emission produced as ions or neutral atoms transit these foils. This interaction also produces angular scattering and energy straggling for the incident ion or neutral atom that acts to decrease the performance of a given instrument. Our results show that the angular scattering of ions through graphene is less pronounced than through the state-of-the-art 0.5 μg cm(-2) carbon foils used in space-based particle detectors. At energies less than 50 keV, the scattering angle half width at half maximum, ψ(1/2), for ~3-5 atoms thick graphene is up to a factor of 3.5 smaller than for 0.5 μg cm(-2) (~20 atoms thick) carbon foils. Thus, graphene foils have the potential to improve the performance of space-based plasma instruments for energies below ~50 keV.

d University of Colorado, Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, Colorado 80303 Abstract. Carbon foils have been used reliably for many decades in space plasma instrumentation to detect ions and energetic neutral atoms (ENA). When these particles pass through a foil, secondary electrons are emitted on both sides. Those electrons are used for coincidence detection and/or timing signals. Ultrathin carbon foils are also used to convert an ENA into an ion for further analysis. The interaction of particles with carbon foils also includes unwanted effects such as energy straggling and angular scattering, both of which scale with foil thick- ness. Therefore, it has always been a goal to use foils as thin as practically possible. The foils used in space are usually made of amorphous carbon of roughly a hundred atomic layers. Graphene can be made much thinner, even down to a single atomic layer, and is therefore a natural candidate for this kind of application. We evaluate one aspect of the interaction of particles with foils: charge exchange. We show the first measurements of exit charge state distributions of ∼1 to 50 keV ions passing through self-supported graphene foils. We compare the charge state fraction of exiting particles with state-of-the-art amorphous carbon foils. © The Authors. Published by SPIE